The Vascular Flora of the United Arab Emirates

Abstract

This chapter discusses the flora of the United Arab Emirates (UAE), focusing on various aspects of the individual plant species. A brief overview of salient features of the flora is given in terms of species number and families, followed by a short discussion regarding some of the taxonomic problems surrounding the naming and identification of species. With respect to biogeographical aspects, it is emphasised that the current flora of the nation represents a distinct snapshot in time that has been shaped by a diversity of events in the past, all of which continue to operate on different spatio-temporal scales. After a brief discussion of keystone and foundation species in the UAE desert, autecological aspects are examined, as these are fundamental to understanding the response of plants to a changing environment. Following on from this topic, the concepts of life forms, plant functional groups and plant strategies are touched upon. In the final section, some remarkable features of the reproductive biology of desert plants are described. Chapter 5 examines the typical plant communities and also looks at the main threats to the flora and vegetation of the country.

1 Vascular Plant Species of the UAE

This chapter discusses the flora of the United Arab Emirates (UAE), i.e. various aspects of the individual plant species. Chapter 5, on the other hand, examines many of the typical plant assemblages (i.e. recurring combinations of different plant species) in the country and the underlying environmental factors determining their occurrence. Chapter 6 deals with specific issues relating to the mountain ecosystems.

Vascular plants comprise three major systematic groups, the angiosperms (‘flowering plants’), the gymnosperms (evergreen plants that do not produce flowers or fruits) and the pteridophytes (ferns). Ferns are non-flowering plants that reproduce by spores, rather than seeds, but like the angiosperms and gymnosperms, they possess roots, stems and complex leaves.

Given the lack of English common names for most plant species of the Emirates, the scientific nomenclature follows that of Jongbloed et al. (2003) to allow non-specialist readers the opportunity to check plants in that widely used field guide.

Jongbloed et al. (2003) give an outline of 668 vascular plant species that are known from the UAE. In a few cases, species are included that were recorded from the immediately adjacent areas of Oman, but not necessarily from the UAE itself. In addition, the authors mention eight freshwater species, thus yielding a total of 676 species. This figure contrasts markedly with the ca. 500 species known just 15 years previously (see Western 1989). Although a number of additional species have been recorded in the meantime, many of these are casual weeds that occasionally put in a fleeting appearance (e.g. Aspinall 2006; Shahid 2014; Shahid and Rao 2014a, b; Gairola et al. 2015; Mahmoud et al. 2015a, b, 2016; Sakkir et al. 2020). This means that the work by the earlier authors can be regarded as remarkably comprehensive and represents a powerful testimony to their dedication, especially as this was often undertaken on a voluntary basis by largely non-professional botanists. According to Shahid and Rao (2015), the total number of species found in the UAE, including casual weeds, stood at 809. However, at a recent workshop, the total number regarded as native species (defined as those present prior to the commencement of traditional agriculture at ca. 4500 BP) was whittled down to 598 (Allen et al. 2021).

The 676 listed species by Jongbloed et al. (2003) belong to about 85 families, the most species-rich being the Poaceae (grass family). As in many other parts of the world, grasses are one of the most important families in the UAE, both ecologically and economically. The family contains widespread perennial grasses of sand sheets such as Panicum turgidum, Pennisetum divisum and Stipagrostis plumosa. Other common grass species, at least locally, include Cenchrus ciliaris, Enneapogon desvauxii, Stipagrostis hirtigluma (all fairly widespread in the mountains) and Dichanthium foveolatum (coastal habitats and mountains).

In the meantime, the Chenopodiaceae (Goosefoot family or chenopods) have been incorporated into Amaranthaceae to give a relatively large, species-rich family. Whereas the members of the Amaranthaceae s. str. were typically species of disturbed or cultivated ground in the UAE, various members of the former Chenopodiaceae are dominant in widespread natural plant communities, such as Haloxylon salicornicum, Haloxylon persicum and Salsola spp. This group contains a number of distinctly halophytic (salt tolerant) species (e.g. Arthrocnemum macrostachyum, Halocnemum strobilaceum, Halopeplis perfoliata, Salicornia sinus-persica), and ones that generally avoid saline substrates (e.g. Haloxylon salicornicum, Haloxylon persicum). Recent studies from Qatar indicate that the Salicornia species in the region is not S. europaea, as was previously assumed, but S. sinus-persica (Richer et al. 2022).

The Asteraceae (Daisy family) is a large family containing several taxonomically difficult species. Based on fairly recent work, the species referred to in Jongbloed et al. (2003) as Echinops sp. is now known to be Echinops erinaceus (Fig. 13.1; Feulner 2014). Helichrysum makranicum is now regarded belonging to the more widespread H. glumaceum, and Phagnalon viridifolium is considered a synonym of P. schweinfurthii.

Fig. 13.1

Echinops erinaceus in the Hatta Mountain Conservation Reserve. Photo credit: Gary Brown

Unlike the former Chenopodiaceae and in contrast to some adjacent regions, there are no plants communities in the UAE in which members of Asteraceae predominate, with the possible exception of Rhanterium epapposum. This dwarf shrub was once locally dominant in small patches on sand sheets in the vicinity of Dubai and also on interdunal plains, but moderate to heavy grazing has led to the decline of this highly palatable species.

Although relatively poor in species, the taxonomically difficult Zygophyllaceae are well represented in terms of community presence, with Fagonia indica/F. ovalifolia, Zygophyllum simplex (Fig. 13.2) and Z. qatarense agg. widespread in the country. These species are generally avoided by livestock, which may explain their predominance.

Fig. 13.2

Zygophyllum simplex is a widespread species in the northern part of the UAE. Photo credit: Gary Brown

The Pea family (Fabaceae) contains a number of genera that play a prominent ecological role in terrestrial habitats in the country. These include Astragalus (a species-rich genus), Crotalaria, Indigofera, Lotus, Rhynchosia, Taverniera and Tephrosia. Also often included are the native shrubs and trees Acacia tortilis (see Fig. 5.6, Chap. 5), A. ehrenbergiana and Prosopis cineraria. On account of their flower arrangement and structure, these are often placed in a separate subfamily or family (Mimosaceae). The Cyperaceae (Sedge family) is an example of a family that is extremely poor in terms of native desert species, but it does contain Cyperus conglomeratus, which is one of the most common and widespread species throughout the desert, albeit taxonomically difficult (see below). Conversely, the Brassicaceae (Mustard family) is a species-rich family that contains various perennial shrubs and desert annuals. What is striking about this family is that a number of genera contain just one (e.g. Anastatica, Eremobium, Morettia, Notoceras, Savignya, Schimpera and Zilla) or few native species (e.g. Erucaria). Eremobium aegyptiacum and to a lesser extent Savignya parviflora are two of the more widespread and common annuals of desert habitats in the country. Other species-rich families in the UAE are the Boraginaceae, Caryophyllaceae, Euphorbiaceae apart from the Poaceae mentioned above. As a result of recent studies, some families have been abandoned. Apart from the Chenopodiaceae, the gray mangrove Avicennia marina is now placed in the Acanthaceae (formerly Avicenniaceae).

Only two gymnosperms are known from the UAE, both belonging to the genus Ephedra. Whereas E. foliata is fairly widespread in the east of the country north of Al Ain, E. pachyclada is a mountain species restricted to just a few sites, predominantly in the Ru’us Al Jibal, but also on the summit ridge of Jebel Qitab, south-west of Fujairah city (Feulner 2014).

According to Jongbloed et al. (2003), the UAE is possibly home to eight species of fern. It is, however, likely that a few of these may not occur in the UAE itself but immediately over the border in the neighboring parts of Oman. Due to the extreme climate, native ferns typically have a requirement for shade and high moisture input, and so seven of the species are only found in the Hajar Mountains, where they find more amenable environmental conditions for growth. Only one species, Ophioglossum polyphyllum, is known from desert areas, especially close to the coast in the north-east of the country. Many of these sites have probably been lost to coastal development and degradation in the meantime. It has, however, also been found on abandoned agricultural terraces in one part of the Hajar Mountains (Feulner 2016), which represents a very different type of habitat. Perhaps the two most widespread mountain fern species are Adiantum capillus-veneris (Maidenhair fern), which is characteristic of freshwater habitats such as seeps and springs as well as irrigation channels, and Onychium divaricatum. The latter occurs in shaded microhabitats on mountain slopes where small pockets of soil accumulate, and it is therefore not associated with freshwater habitats.

New native species since the publication Jongbloed et al. (2003) include the annual Silene arabica on sand (Shahid and Rao 2014b). The record of this to date overlooked species is not entirely surprising as it probably one of those that only occur in small numbers in particularly wet winters in the deserts of the north-east (Ras Al Khaimah, Umm Al Quwain, Sharjah), as is the case of some other species such as the grass Cutandia dichotoma (Brown et al. 2006) and the rare Schimpera arabica (Fig. 13.3), all of which are more frequent in the northern part of the Arabian Peninsula. In the mountains, Feulner and Karki (2009) recorded the perennial grass Saccharum kajkaiense in wadis in the Hajar Mountains, and the presence of Launaea omanensis in the UAE has been confirmed (Feulner 2014, 2016). Feulner (2011) added a list of 23 species (some unidentified) that are new to the Ru’us Al Jibal, as well as another 16 published Ru’us Al Jibal records that had been overlooked in prior compilations for the UAE and Oman, but it is possible that as many as 10 of that total may not occur within the UAE section of the Ru’us al Jibal due to special circumstances (e.g. collection at the very summit of Jebel Harim or in abandoned cultivation at ca.1600 m, or at the singular spring at Ain As-Si.

Fig. 13.3

The rare Schimpera arabica is known in the UAE only from a few locations on near-coastal sands in Ajman and Umm Al Quwain. Photo credit: Gary Brown

2 Taxonomic Issues Relating to the Flora of the UAE

A sound knowledge of the organisms present in a given geographical area is pivotal to effective biodiversity conservation. The seemingly simple assignment of individuals to distinct taxonomic groups is fraught with many and complex difficulties. Despite, therefore, that the number of species in the UAE may appear rather limited, a number of those species are regarded as ‘difficult’ from a taxonomic perspective. Some of these are widespread and dominant taxa, such as Cyperus conglomeratus, Fagonia indica (Fig. 13.4) and F. ovalifolia, Heliotropium bacciferum and H. kotschyi, and Zygophyllum qatarense (Fig. 13.5). These can be referred to as ‘critical plant groups’, broadly defined as ones that are difficult to identify (Rich 2006).

Fig. 13.4

Fagonia indica (now regarded by some as Zygophyllum indicum) is a highly variable species. Photo credit: Gary Brown

Fig. 13.5

The Zygophyllum taxon that grows on limestone rock on Jebel Hafeet is morphologically distinct from the desert forms of Z. qatarense. Moreover, it shows considerable morphological variation from one year to another. Photo credit: Gary Brown

The morphological species concept continues to prevail in plant conservation biology. This means that species are recognised primarily by morphological features, rather than based on, for instance, evolutionary history, which typically relies on genetic data. Strict adherence to the traditional morphological concept can underestimate the number of species present (e.g. cryptic species are overlooked), but ‘modern’, molecular-based approaches are not without their own problems (e.g. Freudenstein et al. 2017), not least because DNA bar-coding suffers severe limitations in some cases (e.g. Piredda et al. 2011).

There are several major issues that complicate the taxonomic treatment of species more generally. The first refers to the actual identity of a certain taxon. In some cases, it is difficult to know with certainty what the original author was referring to, if, for example, the type specimen has been lost (e.g. as in the case of Cyperus conglomeratus). A second, often neglected issue is that a ‘species’ as such is an artificial concept devised by humans to create order in an extremely complex system. Different experts frequently have different concepts of how to delimit certain species. What for one expert is a ‘good’ species, may be regarded by another as ‘only’ a subspecies of a broader species group. Such problems are frequently the result of morphological variation within a group of individuals, which is typically the rule, or simply that some taxa are inherently difficult. The problem of when to differentiate between two ‘good’ species is not always easy with plants, especially as the borders are sometimes ‘fuzzy’ (Mayr 1988). Although extremely useful in many cases, the application of molecular criteria (i.e. genetic analyses) does not offer a universal solution either. In fact, in some cases, the same problems persist, especially where to draw the line to delimit individuals and categorise them as members of separate species.

A third issue pertains to the naming of species, which can be frustrating for some taxa. Although this process should be based on scientific principles, it is clear that personal interpretation and preferences also play a role. Some taxa (e.g. Caralluma and related taxa in neighbouring Oman) are notorious regarding the plethora of different names that have been applied to them. However, Caralluma arabica (Fig. 13.6), a species largely confined to the Hajar Mountains, is fairly straightforward in that it currently has just one alternative name, Desmidorchis arabica.

Fig. 13.6

Caralluma arabica is a stem succulent species restricted to the Hajar Mountains in the UAE. Photo credit: Gary Brown

The proposed splitting up of the genus Acacia into three separate genera (meaning that all Arabian species would be transferred to the genus Vachellia) elicited heated discussions at the International Botanical Congress in Vienna in 2005 and it is unclear whether the decision to approve the proposal was valid (Brummitt 2010). For compelling practical reasons of nomenclatural stability, it seems sensible to protect Acacia as a ‘conserved name’ (‘nomen conservandum’—a scientific name that has special nomenclatural protection), as suggested by Brummitt 2010.

‘Fans’ of the relatively new genus Tetraena are in for a disappointment. Based on the latest, generally accepted information (e.g. Kew: Plants of the World Online), members of this genus are once again regarded as belonging to Zygophyllum. This means that Tetraena qatarensis can be discarded and it reverts to its ‘old’ name, Zygophyllum qatarense. Not only that, but due to the lack of reliable distinguishing features, the entire genus Fagonia has been embedded in Zygophyllum (see, for example, Richer et al. 2022). The widespread Fagonia indica is therefore now referred to as Zygophyllum indicum. It is only a matter of time (typically 5–10 years) before other ‘new’—or even old—names are applied to these same taxa.

With respect to the conservation of species, it is human nature that people are usually obsessed with species—lower ranks such as subspecies often get swept under the carpet. There are some exceptions to this general observation, especially in the animal kingdom in the UAE. For instance, organisations are (rightfully) very quick to emphasise the presence of the endemic subspecies of the White Collared Kingfisher at Kalba. But in general terms and certainly with plants, anything below the rank of a ‘good’ species is ignored. This is despite the fact that the Convention on Biological Diversity (Secretariat of the Convention on Biological Diversity 2005) explicitly underlines the importance of intraspecific variation, i.e. genetic variation within species. Moreover, despite the central role that plant taxonomy plays in biodiversity conservation, there has been a constant decline of classical taxonomic and ecological expertise over the past decades, which can be assumed to be highly detrimental to the cause of biodiversity protection (e.g. Akeroyd 2006).

3 Biogeographical Aspects of the Flora of the UAE

The current flora of the UAE has been shaped by all manner of events in the past, almost all of which continue to operate. The timescale of such events varies enormously, from just years or decades (in the case of many current anthropogenic impacts) to thousands if not millions of years (e.g. gradual shifts in macroclimate). Apart from climate and the influence of man and his livestock, geological, topographical and soil factors have contributed to the establishment of the flora of the UAE and the wider region. As can be inferred from the preceding sentences, as environmental conditions continue to change, so will the flora. In other words, the current flora of the country represents a ‘snapshot’ of a much longer floral history, in that it captures the distribution of species at one particular time in relation to a set of specific environmental conditions. Although these conditions may appear to be quite similar over an area as small as the UAE, even slight differences in some factors may be large enough to exert a significant influence on species’ distribution. Therefore, the concepts of temporal and spatial scale are key to understanding the current distribution patterns of species within the UAE.

On a broader geographical scale, Zohary (1973) divided Arabia into two floral regions. Whereas the interior, and therefore much of the UAE, was assigned to the Saharo-Arabian region, the Sudanian region, represented by the Nubo-Sindian Province, occupies a narrow coastal belt. However, based on more recent information, it is clear from the examples given below that the ‘narrow coastal belt’ needs to be expanded somewhat in the UAE to include near-coastal and some inland locations.

Typical representatives of the Saharo-Arabian region that occur in the country include Anastatica hierochuntica, Haloxylon salicornicum, Helianthemum lippii, Neurada procumbens, Rhanterium epapposum, Savignya parviflora and Stipagrostis spp. Characteristic of the Sudanian region are Acacia tortilis, Calotropis procera, Lasiurus scindicus, Leptadenia pyrotechnica, Panicum turgidum and Pennisetum divisum.

A number of species are floristically affiliated to Makran, i.e. the wider coastal desert strip of southern Iran and Pakistan. Kürschner (1986) considers these to be ‘Omano-Makranian’ elements, and they typically occur in the north-east of the UAE. Due to climatic constraints, several of these species do not occur west of the Hajar Mountains and the various outliers such as the inselberg Jebel Hafeet near Al Ain. Typical representatives of this distribution pattern in the country are Physorrhynchus chamaerapistrum, Gaillonia (= Plocama) aucheri and Pseudogaillonia (= Plocama) hymenostephana. Species such as Prosopis cineraria (‘ghaf’) occur somewhat further westwards, but do not reach Abu Dhabi Island naturally (Fig. 13.7). Cornulaca aucheri, Salsola drummondii and Sphaerocoma aucheri are species of a narrow coastal strip that are found all the way the border with Saudi Arabia in the west.

Fig. 13.7

Prosopis cineraria (‘ghaf’), the national tree of the UAE, is a typical ‘Omano-Makranian’ element. Photo credit: Gary Brown

Superimposed on this broad phytogeographical distribution pattern are local factors, edaphic, climatic or a combination of the two that can severely restrict the actual occurrence of species within a given phytogeographical region. This is well-illustrated by the occurrence of Acacia tortilis and Leptadenia pyrotechnica, which are both predominantly found in the north-east of the country.

The vicinity of the Hajar Mountains is probably instrumental in determining the distribution of certain species in the country, possibly due to the enhanced water supply in the subsurface layers due to runoff. For example, Acacia tortilis is found mainly east of the Al Ain road. A handful of trees also occur on the Sila’a Peninsula, and these constitute the southern extension of the large Qatari population. Leptadenia pyrotechnica is also most common in the north-east, but scattered individuals occur elsewhere in the country, usually at the foot of dunes. It is presumably able to survive in such situations due to water seepage.

Prosopis cineraria is probably a good example of a relict species from a much wetter period in the distant past. With the general aridification of the climate over the past ca. 5000 years (see Chap. 3), this has led to a thinning out of what would have been a more continuous cover by the trees. Today’s population is able to survive in sites where there are large amounts of water stored deep under the subsurface. As a consequence, it is found predominantly on dunes or on deep sand sheets, and mainly in the north-east of the country. One of the consequences of the harshness of the climate nowadays is that establishment from seedlings is probably extremely rare. Most regeneration of ghaf trees is from root suckers, which probably explains why trees are frequently observed in distinct groups (Brown and Böer 2005a).

The UAE has no nationally endemic plant species, but seven species found in the UAE are considered endemic to the mountains of the UAE and Northern Oman (Feulner 2016). These are Echinops erinaceus (although now also presumed to occur in Saudi Arabia), Launaea omanensis, Lindenbergia arabica, Pteropyrum scoparium, Pulicaria edmondsonii, Rumex limoniastrum and Schweinfurthia imbricata. An eighth species listed by Feulner (2016), Caralluma arabica, is also known from southern Yemen, from where it was originally described (Ghazanfar 2015).

Local endemism plays a role in the mountains in particular. Feulner (2011) gives an exhaustive list of local endemics that within the region are restricted exclusively, or nearly so, to the Ru’us Al Jibal.

With respect to a constantly changing environment, plant migrations have undoubtedly played a major role in the past, which helps to explain some current distribution patterns. Some species are more effective colonisers than others, whereas other species may be poor colonisers, but more effective competitors once established. The very rare Pistacia khinjuk, known from about a dozen specimens at just one locality in the Ru’us Al Jibal (see Feulner 2011) probably belongs to latter group.

In fact, within the UAE, the mountains with their varied habitats offer an excellent opportunity to study localised distribution patterns and examine the underlying factors governing such phenomena. In some cases, the causal link is clear. For instance, the orchid Epipactis veratrifolia requires a regular input of moisture and is therefore tied to moist, shady places. Some species need a distinctly cooler winter period and are therefore restricted to the highest mountain peaks in the UAE (e.g. Gynandriris sisyrinchium and Roemeria hybrida on Jebel Jais and above Wadi Qada’ah in the UAE). These species are often common down to near sea level in the northern part of Arabia, where winters are relatively cold.

In other cases, the precise reasons for localised occurrences are less apparent. For example, several plant species such as Ehretia obtusifolia, Grewia tenax and Abutilon fruticosum are known in the UAE only from Jebel Qitab. This might suggest a preferred association with gabbro substrate, but the total number of records is small, and one must generalize with great caution. In fact, all three species are found in protected situations on carbonate substrate in the southern Ru’us Al Jibal in Wadi Khabb Shamsi, Oman, and G. tenax is associated with Olea europaea on the carbonate slopes of Jebel Ghaweel in Wilayat Mahdhah, Oman.

In addition, field work uncovered E. obtusifolia growing as a half dozen small trees (with single, individual trunks) at more than 700–1000 m along the spine of the Northern Hajar, east of Jebel Hatta, in an upper tributary of Wadi Qahfi, Oman, near the Hatta border, securely within the ultrabasic harzburgite. There the trees occupied a steep, rubble-filled gulley that evidently had advantageous hydrology, because along one edge of the gulley was a lush seep with abundant Adiantum capillus-veneris (Maidenhair Fern) and even the remains of Epipactis veratrifolia that had flowered earlier. Further up the gulley were some six dozen Wild Olives (Olea europaea) and, above ca. 1000 m, smaller numbers of Ephedra pachyclada. The latter is unknown from harzburgite rocks in the UAE, but it appears that this is not a matter of geochemistry but of elevation.

No studies to date have attempted a rigorous assessment of the effects of bedrock lithology on the distribution of plant species in the UAE mountains. However, Feulner (2011) highlights distinct physical and geochemical differences between the carbonate rocks of the Ru’us Al Jibal and the ophiolitic Hajar Mountains as a probable explanation for why many typical Hajar Mountain species are absent or very rare in the former area. Some obvious examples include Aizoon canariense, Convolvulus virgatus, Lindenbergia arabica and L. indica, Physorrhynchus chamaerapistrum, Prosopis cineraria and Pulicaria glutinosa, all of which are scarce or absent in the Ru’us Al Jibal. Conversely, it should be noted that, within the UAE, common Ru’us Al Jibal species such as Prunus arabica, Convolvulus acanthocladus, Artemisia sieberi, Astragalus fasciculifolius and Lactuca orientalis are absent from the Hajar Mountains outside the Ru’us Al Jibal. More information on these topics are found in Chap. 6.

The current distribution pattern of some species remains enigmatic. For instance, the attractive flowering tree Tecomella undulata (Arabic: ‘farfar’) is found in a few wadis in the mountains of the UAE and northern Oman. It is striking that, in some cases, the trees are clustered in the vicinity of old villages, some of them, such as near Hatta, which have been abandoned for some time (Fig. 13.8). It is therefore tempting to think that the species has been introduced from Iran or Pakistan, especially as it produces valuable timber (Desert Teak). Like Prosopis cineraria, this species grows in clusters because it reproduces primarily from root suckers in the UAE. Sadly, the oldest and most mature population in the UAE, in upper Wadi Qowr, is now in pitiful condition as a result of water abstraction, lopping for wood or forage, and apparent neglect.

Fig. 13.8

The attractive flowering tree Tecomella undulata is typically found in the vicinity of old villages, such as near Hatta. Photo credit: Gary Brown

With respect to the finer-scale distribution of plant species in the UAE therefore, much remains to be discovered. However, degradation of large parts of the desert and broader coastal belt will impede any such undertaking. Current anthropogenic impacts are far larger than naturally operating ones. As a consequence, newly insularised or fragmented populations of plants are being created far more rapidly than historically.

4 Keystone and Foundation Plant Species in the UAE Desert

A keystone species is defined as one whose impact on its community is large, and disproportionately so relative to its abundance (Power et al. 1996). The concept has frequently been applied to animal predators, less so to the effects of plants on their environment. In the case of the latter, Munzbergova and Ward (2002) consider the three species of Acacia in the Negev Desert, including A. tortilis, which is widespread in the east of the UAE, to be keystone species on the basis that they greatly improve soil conditions and increase plant species diversity under their canopies. Whether this is in keeping with the original definition is open to question, and in the meantime, possibly more appropriate terms have been introduced, as explained below. Some authors have modified the original definition of a keystone species somewhat to expand its applicability. Accordingly, Narango et al. (2020) refer to four keystone plant genera that support the majority of Lepidoptera (butterfly and moth) species in the USA and without these genera, the Lepidoptera populations would collapse. In the UAE, much more basic research is required to understand these and related issues.

Plant communities in the UAE are often characterised by certain species that predominate, at least physiognomically (e.g. Haloxylon salicornicum). Due to the extreme environmental conditions, such species rarely dominate by excluding others, as is the case in more temperate parts of the world. In fact some, including H. salicornicum (e.g. Brown and Porembski 1997), possibly facilitate the occurrence of others. Such species are referred to as ‘foundation species’ (Ellison 2019). Acacia tortilis is probably another good example of a foundation species, whereas ones that have no or few apparent beneficial effects on ecosystem functioning, typical indicators of severe degradation such as Rhazya stricta and Calotropis procera, are not.

Whereas the native tree Prosopis cineraria can be regarded an important foundation species, the alien invasive Prosopis juliflora is certainly not. Various plant species, not to mention a host of animal species, are associated with the native ghaf trees. However, the phytotoxic effects of Prosopis juliflora on plants have been well documented (Goel et al. 1989).

5 Autecology

All plants, irrespective of their environment, are subject to varying degrees of stress. Grime (2001) defines stress as the external constraints that limit the rate of dry matter production of all or part of the vegetation. In non-desert environments with abundant resources, stress is often the result of inter- and intraspecific competition for those resources. In general terms, the main stress factors to which desert plants are exposed are low, variable, and often unpredictable precipitation, frequently resulting in droughts during the hot months, high temperatures, extremely high potential evapotranspiration and high light intensities. In addition, substrate salinity may be a major issue locally.

Particularly during the hot summer months in the UAE, therefore, when there is virtually no effective rainfall, the desert environment imposes severe restrictions on plant growth and survival.

5.1 Drought Survival Strategies

How plants adapt to their environment and to changes in the underlying conditions is one of the fundamental questions in plant science. A variety of mechanisms have evolved in desert plants to help them cope with drought. At a fairly basic, but nonetheless ecologically useful level, Larcher (2003) summarised survival mechanisms of plants in dry regions according to their autecological adaptations and life-forms (see below), and this system can be applied to plants in the UAE:

• 1. Arido-active plants are ones that remain active throughout the year and delay desiccation by various mechanisms, such as deep rooting systems, succulence (storage of water), and reduction of transpirational loss (epicuticular waxes, reflective leaf surfaces, reduction of leaf size). This means that the degree of physiological activity varies enormously depending on a number of factors including, amongst others, photosynthetic pathway and water availability. Examples include Acacia tortilis, Euphorbia larica, Haloxylon salicornicum and Panicum turgidum.

• 2. Arido-passive plants spend the unfavourable season in an inactive state, either as seed (desert annuals) or as a storage organ, usually as a subterranean bulb, corm or rhizome (geophytes, which are perennial plants). Seeds do not store significant quantities of water, just nutrients and energy. This makes them highly resistant to drought and heat. Seeds are therefore strongly dependent on external water for the initiation of growth. In contrast, the storage organ of geophytes often contains a relatively large amount of water, but even so, many, if not most species in the UAE, are reliant on precipitation to initiate growth. Examples of widespread desert annuals include Arnebia hispidissima and Neurada procumbens, whereas Dipcadi biflorum and Gynandriris (= Moraea) sisyrinchium are perennials with bulbs.

• 3. Arido-tolerant plants can tolerate desiccation of their organs without suffering any apparent physiological damage and resume normal physiological activity upon rehydration. In the UAE, this group of plants is quite rare and local. It is represented primarily by cryptogams (lichens and some bryophytes), which are found mainly in the mountains. In addition, there are several species of poikilohydric ferns that occur locally in the mountains, and these typically inhabit rock clefts and crevices. Examples include Ceterach officinarum, Cheilanthes pteridioides (Fig. 13.9) and C. vellea. These ferns are characteristic ‘resurrection plants’. During periods of drought, they curl up and turn brown, entering a state of dormancy. Once water is received, they almost immediately resume normal physiological activity.

Fig. 13.9

The fern Cheilanthes pteridioides is a typical resurrection plant. Photo credit: Gary Brown

5.2 Physiological, Morphological and Anatomical Adaptations to Arid Desert Ecosystems

In the following, the focus is on individual features that confer superior fitness for coping with the harsh desert conditions.

5.2.1 Photosynthetic Pathways

A fundamental characteristic of plants is their photosynthetic pathway and the manner in which the primary fixation of CO₂ proceeds in the chloroplasts. There are three main pathways, C₃, C₄ and CAM, with the abbreviations alluding to either the first photosynthetic compound produced after initial CO₂ fixation or the main storage compound.

In desert environments such as in the UAE, C₃ plants display their main period of active growth during the cooler part of the year when water is generally more abundant and potential evapotranspiration is much lower. Although many are arido-active species, some, such as Farsetia aegyptia and Rhanterium epapposum, discard all their leaves and green tissues on the stems during the hot part of the year and reduce their physiological activity to an absolute minimum.

C₄ plant species are distinctly active during the hotter part of the year when water is a limiting factor (Larcher 2003). They display reduced physiological activity during the cooler, moister period. This pathway is more efficient at higher temperatures than the ‘normal’ C₃ pathway, allowing plants to acquire more atmospheric CO₂ for the same amount of water lost through transpiration (von Willert et al. 1992). This enables the plants to extend their growth season into the late spring months and longer. Such species include the dwarf shrub Haloxylon salicornicum and most of the perennial grasses in the country (e.g. Lasiurus scindicus, Panicum turgidum, Pennisetum divisum). In addition, the small woody succulent Zygophyllum simplex has evolved C₄ photosynthesis.

In some cases, different photosynthetic pathways have evolved within relatively small families. For instance, whereas members of the small genus Pteropyrum (Polygonaceae), which has a very limited distribution and is represented in the UAE by P. scoparium, are typical C₃ species, C₄ photosynthesis has evolved in the closely related genus Calligonum (Doostmohammadi et al. 2020).

Of particular ecophysiological interest is the fact that the seedlings of both Haloxylon persicum (Pyankov et al. 1999) and H. salicornicum (Fig. 13.10) exhibit the C₃ mode of photosynthesis, allowing them to grow rapidly during the cooler part of the year when soil moisture is more available (Brown and Al-Mazrooei 2001). Once established, the seedlings discard their leaves and become leafless, semi-succulent C₄ species. Photosynthesis is subsequently conducted in the green stems, and growth is more rapid in the late spring and early summer months.

Fig. 13.10

The seedlings of Haloxylon salicornicum exhibit the C₃ mode of photosynthesis, adult plants the C₄ mode. Photo credit: Gary Brown

CAM plants in the UAE are typically found in the ‘true’ or drought succulents in the mountains. However, it is known that the coastal annual Mesembryanthemum nodiflorum (rare in the UAE) undergoes a transition in its metabolism from C₃ to CAM photosynthesis, which is induced by salt stress (Guan et al. 2020).

As can be inferred from the above information, photosynthetic pathways can be assessed and monitored to study the impacts of climate change. Any long-term increase in temperatures could shift the competitive advantage of plants with different photosynthetic pathways, changing species distributions and community composition in ecosystems (e.g. Munroe et al. 2021).

5.2.2 Succulence

Two distinct types of succulence can be distinguished, salt succulence and drought succulence (Larcher 2003). Salt succulence is a means of coming to terms with high salinity in the substrate, as described below, and is typical of plants of coastal and some inland desert environments. In contrast, drought succulents (or true succulents) generally grow in the mountains of the UAE. The best examples are given by the stem succulents Caralluma arabica and C. flava, both of which resemble small cactus-like plants (but without the spines). Salt succulents are usually characterised by the C₃ or C₄ photosynthetic pathway. A frequent question is why there are no cacti or cacti-like plants in the deserts of the UAE, as they are often regarded as being typical of arid environments. In general terms, succulents thrive in areas with low rainfall, but precipitation needs to be ‘predictable’, i.e. occurring at fairly regular intervals (von Willert et al. 1992). In the UAE, this is not the case, except to a certain extent in the mountains. The south of Oman exhibits an abundance of succulents with the CAM pathway because the summer monsoon provides a regular and ‘predictable’ source of moisture input.

5.2.3 Reduction of Leaf Size

A typical feature of many desert plant species that survive throughout the hot summer months is that they are for all intents and purposes leafless or have small, narrow leaves that help substantially reduce transpirational water-loss (Larcher 2003). In such cases, the stems, which are less exposed to intensive sunlight, play an important role in photosynthesis. Various perennial grass species have small, narrow leaf blades (e.g. Lasiurus scindicus, Panicum turgidum, Pennisetum divisum). Shrubs and dwarf shrubs exhibit a more varied approach: for instance, there are those with small leaves (e.g. Farsetia aegyptia, Rhanterium epapposum), others that are de facto leafless (e.g. Calligonum crinitum, Haloxylon persicum—see Fig. 5.7, Chap. 5–, H. salicornicum), leafless representatives with, erect, whip-like stems (“spartinoid” growth-form) such as Leptadenia pyrotechnica (Fig. 13.11), Ochradenus arabicus, Periploca aphylla and a small group of leafless succulents (e.g. Caralluma arabica, Euphorbia larica).

Fig. 13.11

The shrub Leptadenia pyrotechnica is virtually leafless to reduce excessive water loss. The green stems are responsible for conducting photosynthesis, respiration and transpiration. Photo credit: Gary Brown

5.2.4 Rooting Depth

Plants with deeper rooting systems are able to access water stored in the subsurface layers. Plants that obtain a significant amount of water from the groundwater or the saturated capillary fringe above the water table, rather than relying on rainwater stored in the upper substrate layers, are referred to as phreatophytes. On account of its extensive rooting system, Prosopis cineraria is a good example of a phreatophyte. Ziziphus spina-christi grows primarily in wadi beds and is also regarded as a phreatophyte (Le Houérou 2009). Somewhat surprisingly, as a species of hot, arid environments, Salvadora persica has large leaves. This is possible because it is a phreatophyte, and so it can use the regular supply of water to maintain its considerable foliage throughout the hot summer period.

Citrullus colocynthis is also a phreatophyte, but it grows in situations where the groundwater is relatively close to the surface. In fact, Citrullus needs to transpire huge amounts of water to cool its large leaves, which are located on the desert surface (Althawadi and Grace 1986). Temperatures on desert surfaces can exceed 70 °C in the summer months (Nobel and Geller 1987).

Species such as Acacia tortilis are facultative phreatophytes. Whereas in the winter months their lateral roots exploit water close to the soil surface, in the summer a much deeper rooting system can extract water stored at considerable depths (down to at least 25 m, Do et al. 2008). Ross and Burt (2015) observed that populations of Acacia tortilis in the UAE have an unusual tilted canopy architecture compared with the flat or dome-shaped morphology typical throughout its pan-tropical range. The authors suggested that the conspicuous south-facing tilt is a plastic response that maximizes soil shading and reduces soil temperature around the base of the tree.

Stem succulents, such as Caralluma arabica and C. flava, are species that do not maintain an extensive rooting system, but store water in their succulent stem tissues. When it rains, they rapidly develop a shallow rooting system to profit from water temporarily stored in the upper surface layers. This is typical of many succulents (von Willert et al. 1992).

It is generally assumed that desert annuals, with their very short life span, do not invest substantial amounts of resources in producing an extensive rooting system. Nonetheless, there are marked differences in root architecture within this group. At the one end of the spectrum, there are species that develop a very shallow, fibrous rooting system. Examples from the UAE include Ifloga spicata and Schismus barbatus. In contrast, other species produce a distinctly thickened and deeper taproot with thinner lateral roots. This is the case in some species of Launaea and Anastatica hierochuntica (see Fig. 13.20).

5.2.5 Adaptations to Mobile Sand

Mobile sand constitutes a highly unfavourable medium for plants, therefore limiting the number of species able to colonise such substrates (Danin 1996). This is due to the potential exposure of roots and the threat of desiccation as well as ‘sand-blasting’, abrasion of plant tissues from wind-blown sand. In addition, mobile sand is usually characterised by pronounced nutrient deficiency.

Although there are no species that actually require sand accumulation in the UAE (as opposed to parts of Africa), several species are resistant to sand cover and removal. Calligonum comosum (and the closely related C. crinitum) is highly adapted to such situations in that it can produce numerous adventitious roots and shoots from buried stems (Danin 1996). Its well-developed lateral rooting system can extend more than 25 m. Because the main lateral roots become stout and woody, they form a useful support to protect the plant from sand deflation. The species also forms characteristic nebkhas, i.e. mounds of wind-blown sand that accumulate around the base of the plant. Cyperus conglomeratus (Fig. 13.12) is a widespread and common sedge in the sand seas of the UAE with anatomical features of the rhizome that enable it to endure mild to moderate sand movement (Danin 1996). The same applies to the perennial grass Centropodia forsskalii, although this species is less tolerant of more mobile substrates than Cyperus. Other perennials in the UAE that show some degree of resistance to sand mobility include the perennial grasses Lasiurus scindicus, Panicum turgidum, Pennisetum divisum and Stipagrostis plumosa as well as the dwarf shrubs Heliotropium digynum and Moltkiopsis ciliata.

Fig. 13.12

The sedge Cyperus conglomeratus is one of the few species that is highly tolerant of mobile sand. The roots are enclosed by a rhizosheath (see below), which provides mechanical support. Photo credit: Gary Brown

Species such as Cornulaca monacantha, Haloxylon persicum and Haloxylon salicornicum typically thrive on the more stable sand sheets. In marked contrast, the species that is often referred to as Cornulaca arabica on the megadunes in the southern part of the UAE is highly resistant to mobile sand. This species is regarded by others (e.g. Miller and Cope 1996) as conspecific with C. monacantha, which is more characteristic of stable sand sheets.

The rather delicate Eremobium aegyptiacum can be locally abundant on sand dunes after plentiful rainfall. It is one of the few desert annuals able to colonise mobile, sandy substrates. It is furnished with a dense covering of short, star-like hairs. Apart from protecting the plant from solar radiation, the hairs also trap small sand grains, reducing the potential of injury to the plant from ‘sand-blasting’. The seeds of this species are yellow and therefore well-camouflaged amongst the sand grains, thus affording them some degree of protection from various granivores.

5.2.6 Rhizosheaths

Most perennial graminoids (grasses and sedges) and some annual grass species display a remarkable adaptation that helps them come to terms with the special conditions of sandy substrates. Such species include Centropodia forsskalii, Coelachyrum piercii, Cyperus conglomeratus (Fig. 13.12), Enneapogon desvauxii (Fig. 13.13), Lasiurus scindicus, Panicum turgidum, Pennisetum divisum and Stipagrostis spp. (e.g. S. drarii, S. hirtigluma—a mountain species, S. plumosa and S. uniplumis). The roots of these species are enclosed within a rhizosheath. This is composed of root hairs and sand grains (Danin 1996) and is glued together by a mucilage produced by the roots. The ‘rhizosheath-root system’ can be regarded as a xerophytic adaptive trait (Wullstein et al. 1979).

Fig. 13.13

The small perennial grass Enneapogon desvauxii is primarily a species of mountains and sandy rocky slopes in the east of the UAE. It develops a distinctive rhizosheath. The actual roots (white) are seen in the bottom right of the image. Photo credit: Gary Brown

Not only do such structures absorb water from the surrounding sand more efficiently, they also create specific nano-environmental conditions that encourage the growth of nitrogen-fixing bacteria (Wullstein et al. 1979). In other words, the micro-environment of the roots becomes enriched with nitrogen compounds that can be utilised by the plants.

5.2.7 Adaptation of Plants to Highly Saline Substrates

Soils with elevated salinity are a common phenomenon in coastal environments in the UAE, but also locally inland where high evaporation rates can lead to an increase of salt in the surface layers (Brown 2006). Sabkha is a good example of a highly saline ecosystem that is widespread in some coastal regions and on interdunal plains. Typically, the main salt present in coastal saline soils is NaCl, but other salts also occur in smaller concentrations. Saline substrates can pose substantial physiological problems for plants, and such substrates therefore exert a strong selective influence on species able to colonise them. However, soil salinity levels fluctuate widely, especially away from the immediate coastline. After heavy rainfall, salts are leached from the surface layers allowing species to germinate or to resume growth.

Plants able to complete their life cycles on saline substrates are referred to as halophytes; those that avoid such conditions as glycophytes. The definition, however, of what precisely constitutes a halophyte is somewhat open to interpretation. In the more specialised scientific literature, plants are regarded as halophytes if they are able to survive to reproduce at NaCl concentrations exceeding 200 mM NaCl, i.e. at least a third of the NaCl concentration of sea-water (Flowers and Colmer 2008). Tolerance to salinity is not an all-or-nothing response but is presumably dependent on the concentration of salts in the substrate, at least within certain species-specific thresholds. Determining, therefore, the thresholds that deem a plant to be a halophyte is somewhat arbitrary. According to Barbour et al. (1987), most plants that are regarded as halophytes are in fact intolerant of salinity. Under controlled laboratory conditions, they often display maximum growth at low salinity and declining growth with increasing salinity. The same author appears to suggest that obligate halophytes (i.e. actually requiring excessive concentrations of salt for normal growth) are perhaps non-existent.

The most tolerant of halophytes, however defined, can thrive in soils containing NaCl concentrations equivalent to, or even higher than, those found in sea-water. This is the clearly case for a number of plant species in the UAE that inhabit coastal marshlands, for example Arthrocnemum macrostachyum, Avicennia marina, Halocnemum strobilaceum, Halopeplis perfoliata, Limonium axillare and Salicornia sinus-persica. However, it is far from clear whether these species require high levels of salt in the substrate for their survival. Whereas the chenopods amongst these species come to terms with substrate salinity through varying degrees of succulence—most prominently seen in Halopeplis perfoliata (Fig. 13.14)—Avicennia marina and Limonium axillare possess specialised salt-excreting glands. As a more unspecific response to high salinity, the shedding of leaves once an internal threshold concentration of salt has been reached is a good example.

Fig. 13.14

The salt succulent Halopeplis perfoliata is characteristic of highly saline environments. Photo credit: Gary Brown

Some plant species are able to germinate on sandy sabkha surface after heavy rainfall because the input of freshwater leads to a temporary reduction of salt concentrations in the substrate. For instance, even Zygophyllum qatarense (mildly to at best moderately halophytic) can germinate under such conditions, and once the plants have become established, they can probably survive for years in a state of dormancy by discarding their succulent leaves when salt concentrations rise above a certain threshold level. After further heavy downpours, they are able to develop new leaves and resume growth for limited periods of time (Brown 2006).

Despite its name, Haloxylon salicornicum can be assumed to be a glycophyte in the UAE and throughout Arabia because it avoids saline soils (but apparently that is not the case in other parts of its distributional range). The same applies to Haloxylon persicum.

6 Main Life Forms, Plant Functional Groups and Plant Strategies

In the above sections, basic plant ‘strategies’ were introduced to group plants of arid deserts in terms of how they deal with drought. In addition, some fundamental mechanisms were described to explain how plants survive in hot, arid environments. Although much work is still required on species in the UAE and the wider region, the general significance of distinctive anatomical, morphological and physiological features that facilitate the survival of plants in different environments has long been understood. In the following, theoretical concepts are discussed, which aim to bring together individual adaptative mechanisms and to group plants according to relevant life history patterns. These can be of high predictive value for forecasting how plants come to terms with their environment in response to various factors. Such factors include the complex issue of climate change, which is highly relevant to the UAE (see Chap. 3). Instead, therefore, of investigating how individual species react to certain changes in environmental factors, it is often more instructive to assess groups of species with similar demands, such as plant functional types (e.g. Box 1996). This topic is dealt with in brief below.

From a theoretical point of view, it is tempting to think that if it were possible to design a plant to cope with all the harsh environmental conditions encountered in the UAE desert, there would be an ‘ideal’ solution, i.e. one plant type that possesses a single set of superior characteristics enabling it to withstand even the most severe conditions. However, even a superficial examination of the flora of the country reveals that there are many approaches (or ‘strategies’) that enable plants to survive, as is the case with most ecosystems on the planet. Each ostensible beneficial trait brings with it a trade-off. A trade-off is the ‘cost’ paid in terms of fitness when a beneficial change in one trait is linked to a detrimental change in another (Stearns 1989). For example, a desert annual with its potentially high reproductive output each season has the capacity to dominate ecosystems. However, the downside of the annual growth form is that the growing season is typically short, severely limiting the potential for plant growth, and moreover, each season must commence with the germination and successful establishment of the species.

Although, therefore, the number of plant species in the UAE is relatively high, each with its own unique combination of traits, there is only a limited set of general approaches to coping with the stressful environment. In this context, desert annuals and geophytes, both arido-passive plants as explained above, are two examples of so-called ‘life-forms’.

Various systems exist to classify life-forms. Perhaps the most appealing and widely used is one that on the surface appears rather simplistic. It can, however, reveal a large amount of information on how plants come to terms with their environment. This is the system devised by Raunkiaer (1934), initially for northern Europe, but later expanded to include other parts of the world. It revolves around the position of the perennating bud (the bud that produces new shoots) in relation to the soil surface to survive the harsh season. For the vast majority of plant species in deserts, this is the hot summer period when drought occurs. For the UAE, the relevant life-forms are shown in Table 13.1 (Fig. 13.15).

Table 13.1 Relevant life-form categories of plants in the UAE

Fig. 13.15

Example of a chamaephyte, the dwarf shrub (Haloxylon salicornicum, a), and a hemicryptophyte (Corchorus depressus, b). Photo credits: Gary Brown

Such morphological, rather than taxonomic, classifications are often useful in certain contexts, for instance in terms of understanding more encompassing ‘plant functional types’, as explained below. Life forms can also be used in geographic areas where taxonomic knowledge of the species present is inadequate. In such cases, rather than attempt to identify all the species present, a functional analysis of the flora can be undertaken, from which ecologically relevant information can be gleaned.

Life-forms are principally based on a limited set of morphological and anatomical characteristics, but as outlined above, physiological traits are also indicative of certain environmental conditions. The significance of the various combinations of these features to facilitate the survival of plants in different environments has long been recognised, and the value of this information for predicting their responses to stress should not be underestimated. In this context, plant functional types (PFTs) are sets of species that exhibit similar responses to environmental conditions and having similar effects on dominant ecosystem processes. One of the major challenges is being able to identify key plant attributes to characterise relevant plant functional types. This has so far not been attempted in the UAE or the wider region, and so a massive research opportunity exists.

Examples of attributes that are undoubtedly relevant to the response of plants to climate change are, among many others: (a) type of photosynthetic pathway, (b) water-use efficiency, (c) cardinal temperatures for germination and growth, (d) degree of desiccation tolerance, (e) rooting depth, (f) reproductive rate, (g) life-form, (h) leaf area, specific leaf area (i.e. ratio of leaf area to leaf mass), etc. There are other characteristics that are much less obvious, but may be equally if not more important to some of those already listed. One such characteristic is the extent of mycorrhizal infection, especially as it has been suggested that mycorrhizal fungal–plant interactions may mitigate the effects of climate change (e.g. Bennett and Classen 2020). It is clear that some of these attributes are difficult to assess if fundamental data are lacking. Given the potential seriousness of the problem of climate change, financing programmes that aim to produce standardised data on relevant, but less obvious species attributes should be seen as a priority.

7 Interesting Features of Reproductive Biology in Desert Plants: Pollination, Dispersal and Germination

Due to the extremely wide-ranging nature of these topics, they can only be covered briefly.

7.1 Pollination

Flowers are the among the most defining structures of angiosperms, and pollination syndromes are characteristic traits of floral structures that have evolved to take advantage of different vectors involved in the pollination process. Most plant species in the UAE are either pollinated by invertebrates, primarily insects, or by the wind. Water pollination is typical of seagrasses (see Chap. 9). Self-pollination is another mechanism to ensure fertilisation of the ovary, but this topic is not dealt with here.

Wind pollination is highly typical of the graminoids (Poaceae—Fig. 13.16a–, Cyperaceae, Juncaceae), but it also occurs in various other families, including in chenopods such as Cornulaca monacantha (Fig. 13.16b), Haloxylon salicornicum and H. persicum. A typical feature of wind-pollinated plants is that obvious flowering structures such as petals are greatly reduced. Often the number of flowers is increased to produce large amounts of pollen due to the unspecific nature of the pollen transfer mechanism.

Fig. 13.16

Wind pollination is highly typical of the grasses such as (a) Panicum turgidum, and also of many chenopods (now Amaranthaceae) such as (b) Cornulaca monacantha. Photo credits: Gary Brown

With respect to animal pollination, plant-pollinator interactions belong to the classical pollination syndromes and are an excellent example of co-evolution (Olesen and Jordano 2002). Plants have adapted in varying degrees to their pollinator, whilst in turn pollinators have adapted to plants. With animal pollination, the necessity to produce lots of pollen is greatly reduced, as the pollinator typically assumes the role of transporting the pollen quite specifically from one flower another. The most common groups involved in animal pollination in the UAE are bees and wasps (Hymenoptera), beetles (Coleoptera), flies (Diptera) and butterflies/moths (Lepidoptera).

Insect pollination is dealt with in more detail in Chap. 17, and so for the present, several interesting phenomena are discussed.

In many parts of the world, trees are typically wind-pollinated, but this is not necessarily the case in the UAE. For instance, Adgaba et al. (2016) found that most visitors to the inflorescences of Acacia tortilis and A. ehrenbergiana were members of the Hymenoptera, which were attracted by significant amounts of nectar secreted by both species.

Perhaps one of the more remarkable examples of co-evolution is found in members of the fig family. Figs, including the native species such as Ficus salicifolia, possess a unique pollination system that involves tiny, highly specific fig wasps (Agaonidae). The pollination mechanism, including in Ficus salicifolia, has been described by Nefdt and Compton (1996).

On account of a number of striking features, the genus Silene has been used as a model system for studies in insect pollination and evolution for some time, in fact dating back to the genetic and ecological work of Mendel and Darwin (Bernasconi et al. 2009). Typically, two contrasting flower phenotypes have been described in Silene, nocturnal and diurnal (Prieto-Benítez et al. 2016). Diurnal species usually have pink or red petals, and flowers usually remain open during the day and night. Nocturnal species possess white or pale-coloured flowers that often close during the day. Native species belonging to the latter group are present in the UAE, including Silene arenosa, S. austro-iranica and S. villosa.

Hawkmoth pollination is prominent in Rhazya stricta, and this involves several diurnal species, including Silver-striped Hawk Moths (Hippotion celerio), which can be locally abundant (Fig. 13.17).

Fig. 13.17

Hawkmoth pollination, here with Hippotion celerio, is prominent in Rhazya stricta. Photo credit: Gary Brown

Fly pollination is a characteristic feature of the two stapeliads known from the mountains of the UAE, Caralluma arabica and C. flava. It is also found in Periploca aphylla (Fig. 13.18) and Cynomorium coccineum. These insects are attracted by the smell or colour of the flowers.

Fig. 13.18

Fly pollination is a characteristic feature of Periploca aphylla. Photo credit: Gary Brown

Aphidophagous hoverflies (Syrphidae) are possibly involved in deceptive pollination in the UAE’s only native orchid, Epipactis veratrifolia. This and other members of the genus are frequently infested with black aphids on their vegetative organs. The complex flowers of the orchid produce small black swellings imitating aphids that dupe the hoverflies to visit specific regions of the flower, thus facilitating the transport of pollinaria (a cohesive mass of pollen grains typical of the Orchidaceae). This phenomenon has been described in detail by Jin et al. (2014) from the Eastern Himalayas.

Bird pollination systems are often dominated by specialist nectarivores. In the UAE, bird pollination is very poorly represented. However, Purple Sunbird is thought to play a role in the pollination of Aloe vera, and the same bird species is also known to occasionally visit the flowers of Lycium shawii.

7.2 Seed Dispersal

Seed dispersal and germination of desert plants, in particular desert annuals, are topics that have been the focus of much research work throughout the world, especially on account of the more fascinating solutions that have evolved in some species. These processes are fundamental to maintaining populations of many species. Desert annuals in particular are heavily reliant on germinating in the right place and at the right time. In the following, the term ‘seed’ is often used loosely to denote the diaspore, i.e. the unit of dispersal. In the flora of the UAE, diaspores vary from being an individual seed, often a fruit, and in one case (Brassica tournefortii), the entire plant.

As sedentary organisms, plants face a certain challenge to ensure that their seeds are dispersed to sites that have the potential to facilitate germination and establishment. Often, such sites are located in the immediate vicinity of the mother plant, but in others, dispersal away from the adult plant is promoted to limit competition. Seed dispersal therefore involves various mechanisms, including (1) ones that promote long-distance dispersal (telechory), (2) no obvious features to promote dispersal (atelechory), and (3) mechanisms that actively suppress dispersal (antitelechory). Seed dispersal can be achieved by abiotic factors (e.g. wind), mediated through organisms (zoochory) or the plant itself assumes responsibility (autochory). All of these mechanisms are represented in the flora of the UAE. As indicated above, timing of germination is of crucial importance to desert annuals in particular. Navarro et al. (2021) provided an outline of species with delayed seed dispersal in the UAE, where the seeds are retained in maternal plant structures for varying lengths of time, and a few prominent examples are given below.

With respect to telechoric species, although specific adaptations to extreme long-distance dispersal may be rare or even absent in the flora of the UAE, dispersal away from the mother plant for considerable distances is achieved by various means.

Wind dispersal (anemochory) is a common phenomenon, and this can be facilitated for example by small, light seeds (e.g. Diplotaxis harra, Schismus barbatus) or by seeds that have an appendage to help them fly. Some species possess a pappus (i.e. modified calyx characteristic of the Asteraceae—e.g. Senecio glaucus), as typically found in the Asteraceae, others an equivalent structure (e.g. the perennial shrubs Calotropis procera, Leptadenia pyrotechnica, Periploca aphylla or the grass Imperata cylindrica). Yet other species have winged diaspores, such as the perennials Haloxylon salicornicum, H. persicum, Pteropyrum scoparium (Fig. 13.19a) and Salsola drummondii (Fig. 13.19b).

Fig. 13.19

(a) Winged fruits of Pteropyrum scoparium and (b) Salsola drummondii. Photo credits: Gary Brown

Wind dispersal is also a characteristic feature of Brassica tournefortii (Brassicaceae), the most prominent tumbleweed in the UAE. This annual species is restricted to agricultural environments in the far east of the country. When the seeds are mature, the entire dry plant breaks off from a predefined weak point in the stem just above the soil surface. The plant is then blown across the desert by gusts of winds, with seeds being spread at the same time. To a certain extent, the Brassicaceae Physorrhynchus chamaerapistrum can also behave as a tumbleweed in that the upper parts of the plant break off and blow around in mountain wadis.

Balloon fruits can be dispersed by wind and gravity. Such fruits are produced by various perennials in the UAE, including A. fasciculifolius and Pseudogaillonia hymenostephana. The fruits of Citrullus colocynthis are also probably best assigned to the balloon type category. Although they are initially very heavy, on drying the fruits become extremely light and brittle, and are then easily blown across the surface, breaking up and scattering seeds in the process. A common desert annual in the mountains of the UAE with balloon fruits is Rumex vesicarius.

A particularly interesting dispersal mechanism is found in amphicarpic species. Amphicarpy is a form of heterocarpy, i.e. the plant produces two markedly different types of diaspore, typically to facilitate both telechory and atelechory (Gutterman 1993). With amphicarpic species, one type of diaspore is located below-ground. These underground fruits are heavy and antitelechoric. In contrast, the aerial diaspores are typically light and are more adapted for short-distance dispersal. Two amphicarpic species are characteristic of the region, at least locally, Emex spinosa and Gymnarrhena micrantha. Emex is locally abundant in desert areas of the north-east of the country, whereas Gymnarrhena is extremely rare in the UAE, so far known only from Jebel Hafeet. In addition, a third, potentially amphicarpic species, Enneapogon desvauxii, occurs in the mountains of the UAE. This is usually a distinctly small perennial/facultative annual grass. Studies conducted by Stopp (1958) indicate that it is amphicarpic in other parts of its distributional range (e.g. Africa). Strictly speaking, however, it is not an amphicarpic species as the antitelechoric diaspores develop in the densely packed basal leaf axils at the surface of the ground (rather than below-ground). Nonetheless, it is unclear whether the plants in the UAE, which are distinctly smaller than in other parts of its distributional range, are amphicarpic as we have not observed the phenomenon in material in the UAE so far.

The retention of seeds in the canopy of the mother plant to delay seed dispersal is referred to as bradyspory (van Rheede van Oudtshoorn and van Rooyen 1999), and this phenomenon is not generally widespread in desert plants. However, in the UAE, there are several classic examples of species with bradyspory, which enable the plants to carefully regulate seed dispersal to occur gradually over an extended period of time.

Asteriscus hierochunticus (= Asteriscus pygmaeus) is such a species with an aerial seedbank. It is typically associated with rocky habitats and silty-gravelly wadis in the UAE. Details of seed dispersal and germination, which are tightly regulated by rainfall long after the mother plant has died and dried out, have been summarised in Gutterman (1993). During development of the fruits, the involucral bracts (the leaves that enclose the flowerheads) gradually become woody and completely enclose the capitulum (flowerhead), protecting the seeds from herbivores. The woody involucral bracts are sensitive to rainwater: within minutes of wetting, they open, and fruits, beginning with the peripheral whorls, are released. The action of raindrops bouncing on the achenes (small fruits) helps to dislodge them from the capitulum. Following periods of moisture, the bracts close again, returning to their original shape. This process of opening and closing can continue for up to 20 years, with the seeds remaining viable throughout (Gutterman 1994).

A variation of this theme is found in the Brassicaceae Anastatica hierochuntica (Fig. 13.20a). It is one of the few annuals (as with Asteriscus hierochunticus) that is known to become lignified (Danin 1983). The fruits remain attached to the dead mother plant. On drying, the woody branches become incurved, forming a spherical structure (Fig. 13.20b). This is anchored in the ground by a strong, woody taproot. The fruits (silicula) are enclosed in the centre of the structure and are well-protected from herbivores by the thick, densely interwoven branches. After rainfall, the branches rapidly expand to open, and wetting weakens the sutures of the silicula. Some fruits are torn open, especially if continued rainfall occurs, and a few seeds are released. On drying, the plant returns to its original spherical structure. Only a handful of seeds are released in any one season, so that seed dispersal takes place over a number of years (Danin 1983). Apart from ensuring that seeds are dispersed at a favourable time of the year to facilitate seedling establishment, this mechanism minimises the exposure of seeds to granivores. As with Asteriscus, this species is typically associated with rocky habitats and silty-gravelly wadis in the east of the country.

Fig. 13.20

(a) The Brassicaceae Anastatica hierochuntica is a typical annual species when flowering. (b) Aerial seed bank on the in-curled, lignified branches. Photo credits: Gary Brown

Blepharis ciliaris (Fig. 13.21) is a cushion plant that is primarily associated with the Hajar Mountains in the UAE. The species also possesses an aerial seedbank with the seeds remaining on the dead maternal plant. Gutterman (1994) describes in detail the remarkable ‘double safety autochorous mechanism’ involving both the dried calyx and capsule, which ensures that only a limited number of seeds will be dispersed during any one precipitation event of sufficient duration to facilitate germination. A certain amount of continuous wetting is required to trigger the autonomous, ‘ballistic’ dispersal mechanism. The exploding capsule can eject seeds for distances of up to a metre and possibly more.

Fig. 13.21

Blepharis ciliaris is a cushion plant that possesses an aerial seedbank and a ‘ballistic’ seed dispersal mechanism. Photo credit: Gary Brown

Like many other members of the genus (Webster 1994), Euphorbia larica, a species of the Hajar Mountains, is presumably autochorous due to explosion of the ballistic fruits. The same applies to various members of the Geraniaceae in the UAE.

Zoochory refers to the dispersal of seeds by animals, and this phenomenon is one of the prime examples of animal-plant interactions. The major advantage of zoochory is that it facilitates long-distance dispersal. Zoochory is a fairly prevalent phenomenon in desert ecosystems. As a broad categorisation, it is possible to distinguish between endozoochory, i.e. transported within an animal after ingestion and deposited later, epizoochory (transported externally on an animal) and myrmecochory (dispersed by ants).

Although there are no specific studies from the UAE, the results of Rohner and Ward (1999) strongly suggest that endozoochory is probably widespread in Acacia tortilis. This species frequently regenerates from seed in the east of the UAE (as opposed to Prosopis cineraria). Camels eat large quantities of the pods. Not only are the seeds dispersed in the dung of the animals, but sterilisation of the seeds that survive passage through the digestive tract takes place. This is important because almost all pods of A. tortilis are infested with bruchid beetles, whose larvae damage a large percentage of the seeds. Seedlings emerge after rainfall, often from camel droppings. However, camels eat most of the seedlings, meaning that very few become established. Endozoochory possibly also plays a role in mountain plant species that develop fleshy fruits, such as Ochradenus arabicus. These are then eaten, and the seeds dispersed by birds. Frugivorous lizards are known to be involved in the dispersal of Capparis spinosa seeds in other parts of the world (e.g. Fici and Lo Valvo 2004), and the same almost certainly applies to the UAE.

Epizoochory is conspicuous in a few species. Neurada procumbens is a common desert annual in the UAE, occurring primarily on sandy substrates. The fruit is disc-shaped and asymmetrical (Fig. 13.22), with spiny processes on the upper surface that become very sharp as the fruit dries out. The lower surface is flat and non-spinose. The dry fruits remain on the desert surface and become attached to the feet of animals or the tyres of vehicles. In this way, they can be transported for considerable distances before falling loose.

Fig. 13.22

The fruits of the common desert annual Neurada procumbens are disc-shaped and readily attach to the hooves of larger animals. Photo credit: Gary Brown

With Rhanterium epapposum, the dried capitulum is the unit of dispersal (which contains several to many seeds). This is either blown for short distances across the desert surface or transported over longer distances in the fur of animals. Thalen (1979) lists sheep, goats, camels and hares (Lepus capensis) as being instrumental in long-distance dispersal in this species. Hooked diaspores are typical of Medicago laciniata (Fig. 13.23a, left) and the alien Cenchrus echinatus (Fig. 13.23b, right).

Fig. 13.23

(a) The fruits of the desert annual Medicago laciniata and (b) the alien Cenchrus echinatus are readily transported by unsuspecting animals. Photo credits: Gary Brown

Myrmecochory is probably a fairly widespread phenomenon in the UAE, but it has not been reported so far. In Kuwait, the role of harvester ants in seed dispersal has been described in detail by Brown et al. (2012) in an intact Rhanterium epapposum community.

7.3 Germination

A widespread mechanism that prevents germination occurring at the wrong time of the year in desert ecosystems is dormancy (Gutterman 1993). This mechanism stops seeds, which are usually produced at the end of the spring before the onset of the summer drought, from germinating in response to isolated rainfall events in the summer period. Any seedlings germinating then would inevitably perish. Many desert annuals, belonging to a diversity of plant families, have seeds with physiological dormancy (i.e. the embryo itself is incapable of developing), and this is gradually broken over the hot, dry season (Baskin and Baskin 1998).

Predictive germination, defined as germination that is directly sensitive to environmental factors associated with conditions favourable for immediate seedling growth (Smith et al. 2000), is an important strategy that allows plant populations to thrive in highly variable environments. Gutterman (1993) indicates that the amount and temporal distribution of available soil moisture are primary environmental variables on which predictive germination is likely to be based. Brown and Al-Mazrooei (2001) showed that the seeds of the perennial Haloxylon salicornicum, which are produced to coincide the start of the following rainy season, did not require dormancy but could germinate at very high percentages immediately. In this study, highest germination rates were below 20 °C, but above 30 °C, germination was severely retarded. This is in agreement with the general statement concerning many desert annuals, namely that high temperatures result in thermal-inhibition of germination, even though in this group of plants, high temperatures are required to first break dormancy.

A second important mechanism that enables desert plants to persist in their harsh environment is fractional (= delayed) germination (Baskin and Baskin 1998). With fractional germination, only a certain proportion of germinable seeds will actually germinate, even under ideal conditions. This strategy buffers a population from the consequences of near or complete reproductive failure in unfavourable years, and has been well-documented from American desert annuals (Venable and Lawlor 1980). Fractional germination is also known from plants of the UAE, such as in Spergularia diandra (Gutterman 1996). It is also probably a widespread mechanism in plants of the region, as has been demonstrated for species such as Plantago boissieri by Brown (2001).

Fractional germination is generally regarded as a bet-hedging strategy (Baskin and Baskin 1998). This means that in a ‘bad’ year, only a certain proportion of the seeds will germinate, but the non-germinating fraction will be carried over until another season. In this manner, overall seedling mortality is reduced. However, in a ‘good’ year, not all seeds that would have been capable of producing offspring germinate either. Fractional germination has therefore been characterised as a ‘cautious’ germination strategy (Gutterman 1993).

8 Recommended Readings

Readers interested in the plant species occurring in the United Arab Emirates should refer to Jongbloed et al. (2003). Brown and Böer (2005b) give an overview of various aspects of plants in the country (e.g. adaptations, traditional use, etc.). Danin (1996) provides exhaustive details of how plants come to terms with sand dune environments in arid zones.