Plant Diversity In Drylands

The ability to withstand long periods of moisture shortage is not a prerogative of any one plant group or life-form. In trees, shrubs, grasses, ferns, mosses and lichens there are species which are drought tolerant (Figs. 2.21-2.22) and others which are restricted to areas of plentiful water. This evolution of drought-resistant species in all the major life-forms of plants is sometimes overlooked as it is easy to be deceived by the dramatic changes in vegetation that take place when passing from areas of high to low rainfall. In particular, the reduction in tree cover which can make a very vivid impression on arriving in an arid zone can lead to an underestimation of the range of drought tolerance in certain tree species.

The absence of trees in many dry areas is often as much due to man and his animals as to a reduction in precipitation. When passing into a rain shadow area from one of plentiful precipitation, reduction in water supply produces a greater change on the species composition of the forest than on the existence of the tree form. Tree species native to arid regions do not necessarily consume more water than grassland. In the cool rainforests of Chile and western Patagonia rainfall of 2500-3800 mm is needed to support the growth of the evergreen southern beech (Nothofagus dombeyii).

Fig. 2.20 Fire and the Cerrado. (a) Geographical location ofthe Cerrado. (b) Incidence offire in South America in September 2003 as recorded from satellite detection of night-time thermal spots (source: World Fire Atlas - European Space Agency web page: http://dup. eserin.esa.int/ionia/. (c) Burning cerrado vegetation (Photo Dr P. E. Gibbs). (d) Post-fire cerrado - note the twisted blackened stems from recent fires from which new shoots will emerge.

Fig. 2.20 Fire and the Cerrado. (a) Geographical location ofthe Cerrado. (b) Incidence offire in South America in September 2003 as recorded from satellite detection of night-time thermal spots (source: World Fire Atlas - European Space Agency web page: http://dup. eserin.esa.int/ionia/. (c) Burning cerrado vegetation (Photo Dr P. E. Gibbs). (d) Post-fire cerrado - note the twisted blackened stems from recent fires from which new shoots will emerge.

Prosopis Juliflora Tree

Fig. 2.21 An ancient specimen of mesquite (Prosopis juliflora) in the desert at Bahrain (Persian Gulf). This remarkable specimen is known locally as the tree oflife. The species is native to Central America and the southern states ofthe USA but is now widespread in other arid lands. In some areas, as in the USA, it is considered an invasive bush that is taking over former grasslands as a result of overgrazing and increasing temperatures. In other places, as here in the Persian Gulf, it is valued for its sand-binding properties, shade, edible fruits, and as a source of honey.

Fig. 2.21 An ancient specimen of mesquite (Prosopis juliflora) in the desert at Bahrain (Persian Gulf). This remarkable specimen is known locally as the tree oflife. The species is native to Central America and the southern states ofthe USA but is now widespread in other arid lands. In some areas, as in the USA, it is considered an invasive bush that is taking over former grasslands as a result of overgrazing and increasing temperatures. In other places, as here in the Persian Gulf, it is valued for its sand-binding properties, shade, edible fruits, and as a source of honey.

Sixty kilometres to the east the rain shadow of the Andes has reduced the rainfall to less than 350 mm, yet in spite of this trees still survive in areas that are not grazed.

The succulents are one of the most characteristic plant groups of arid lands. Despite their restriction to a limited number of families, and a homogeneity of form in adopting the capacity to carry out crassulacean acid metabolism (CAM), they are nevertheless surprisingly variable in their morphology and have growth forms that can be trees as well as shrubs. The well-known Joshua tree (Yucca brevifolia) is one of the larger species to show CAM metabolism (Fig. 2.22). As long as there is some moisture, even if it is only coastal fog, there is nearly always a flora adapted to exploit an ecological window, however short. Hot deserts are usually ancient habitats and the plants and animals that live there have long histories of drought adaptation. Consequently, the biodiversity of arid lands at low latitudes should not be a surprise.

Fig. 2.22 Variation in tree forms in drought-prone habitats. (Top) The Cordilleran cypress (Austrocedrus chilensis) in the western edge of the Patagonian Desert (Argentina). (Above left) The Joshua tree (Yucca brevifolia) in the Mohave Desert — a dendroid member of the Liliaceae and one of the larger species to possess crassulacean acid metabolism (CAM). (Above right) Ceiba chodatii (Bombaceae) in Paraguay, a water-storing bottle-tree (Photo Dr P. E. Gibbs.)

Fig. 2.22 Variation in tree forms in drought-prone habitats. (Top) The Cordilleran cypress (Austrocedrus chilensis) in the western edge of the Patagonian Desert (Argentina). (Above left) The Joshua tree (Yucca brevifolia) in the Mohave Desert — a dendroid member of the Liliaceae and one of the larger species to possess crassulacean acid metabolism (CAM). (Above right) Ceiba chodatii (Bombaceae) in Paraguay, a water-storing bottle-tree (Photo Dr P. E. Gibbs.)

Although deserts can be climatically grouped as arid areas there is great variation in the periodicity and duration of the drought period. Examination of any one major desert area reveals considerable climatic variations. The Atacama Desert in Chile and Peru is one of the driest in the world. At Antofagasta measurable rainfall can sometimes be detected in less than 6 years out of 20 and then the precipitation does not exceed 4-6 mm per annum. However, from May to November a dense cold mist, the Garua (Peru) or Camancha (Chile), rolls in with the sea breeze from the cold Humboldt Current. It is particularly dense at night but is sufficiently thick by day to obscure the tropical sun for days at a time. In the month of August some areas exposed to this mist have only 36 hours of sunshine and an average temperature of 13 °C while less than 800 m up-slope above the fog the temperature rises sharply to 24 °C. This mist supports a lichen-dominated vegetation over extensive areas of the fog-shrouded hills facing the sea.

In certain particularly favoured spots bordering the Atacama Desert there exists a unique forest vegetation, the lomas. The precipitation under the trees can be eight times that which is condensed in the open. Typical tree species are Carica candens, various species of Eugenia, Caesalpina tinctoria and Schinus molle, and near Lachay, Acacia macrocantha (Hueck, 1966). The lomas have a high proportion of endemic species together with a rich flora of bromeliad species.

Also striking in the Atacama Desert are the 14 species of Tillandsia with both epiphytic and unrooted, terrestrial representatives (Rundel & Dillon, 1998). All the Tillandsia species listed by Rundel and Dillon are epiphytic in the broad sense, but in addition to growing on plants they also grow on rocks, and six species (T. purpurea, T. latifolia, T. capillaris, T. marconae, T. werdermanii and T. landbeckii) have all evolved the ability to survive unrooted on sand (Fig. 2.23). As noted by Rundel, nowhere in the world are bromeliads more dominant or have more biomass than in these coastal species growing on sand. Many of these species grow at the absolute limits of vascular plant tolerance. It is possible to find an entire community consisting of a single Tillandsia species.

The Sahara occupies an area of almost nine million square kilometres in Africa and lies in a zone with less than 100 mm mean annual rainfall. Nevertheless, despite this general overall aridity there exists considerable bioclimatic diversity. Between the Mediterranean Sahara in the north, and the Tropical Sahara in the south, there are also the Central Plains Sahara, the Montane Sahara and an Oceanic Sahara, all of which have their own distinctive climatic patterns and biological diversity. Distribution patterns of plants and animals are closely linked with the climatic parameters, particularly in the amount and seasonality of rainfall and temperature. The latter may play as important a role as the former in controlling animal and plant distribution, since Mediterranean species can dominate in communities at higher elevations under tropical rainfall regimes, whereas tropical species intrude into the Mediterranean rainfall regions wherever winter temperatures are sufficiently warm (Lehouerou, 1995).

African deserts also provide astonishing examples of just how little rainfall is necessary to support a species-rich flora, provided the precipitation is regular in its occurrence. The sandy coastal belt between Port Nolloth and Alexander Bay on the north-west coast of Namaqualand in South Africa is part of the biologically diverse Succulent Karoo biome. In an area of 750 km2, 300 plant species occur with 24% endemicity (Desmet & Cowling, 1999).

Egypt's Sinai Peninsula supports a flora of about 1285 species. The southern region is particularly species rich, supporting 800 species (including infraspecific taxa) with approximately 4.3% endemic species. Beta diversity between different landforms in the St Catherine area reflects a large biotic change between slopes and terraces on the one hand and between terraces and ridges on the other (Ayyad et al., 2000). As yet, molecular genetic studies in deserts are few but those that have been carried out reveal considerable diversity between populations. In some cases this may be associated with the longevity of many desert perennials. An investigation of the genetic substructure of the insect-pollinated desert species Alkanna orientalis in the St Catherine desert revealed that the subpopulations growing in three different steep-sided wadis and a central plain area were genetically distinct from each other, but nevertheless showed evidence of gene flow, particularly between two of the wadis and the adjacent plain (Wolff et al., 1997). The high mountain ridges between the wadis of the St Catherine desert make movement by bees across the ridges unlikely, nor do bees forage far beyond the range of a few plants at each wadi. Seed transport by flash floods therefore appears to be the more

Fig. 2.23 Tillandsia latifolia (Bromeliaceae) in the Atacama Desert. This is a free-living bromeliad species that survives unrooted and moves about the desert blown by the wind.

likely cause of gene flow between populations and illustrates again that even in an environmentally stressed habitat some benefit can accrue from disturbance. Hot deserts despite their apparent spatial and sometimes temporal emptiness can contain floras that are both species rich and varied and illustrate yet again that marginal areas can have significant biodiversity.

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