Plants in deserts

Abstract

There is no single definition of "desert," but it is widely agreed that deserts are arid because they receive little precipitation and experience high evaporation annually. These factors result in low soil water availability that severely limits plant productivity. Thus, another feature of deserts is low vegetation cover. Although all deserts are dry, there is extreme abiotic and biotic variability among the world's deserts - perhaps more so than for any other biome. This arises in part from the varied causes of desert formation, their disjunct distributions, and their independent floral histories. High spatial and temporal variability of the abiotic environment present challenges to desert life that has important implications at both the ecological and evolutionary scales. Besides limited water, other abiotic factors play important roles in desert ecosystems. Temperatures can be extremely high, but in some deserts low temperatures also constrain productivity. Resources, such as nitrogen, are also generally low in deserts, so that even when water is available, plant productivity may be relatively constrained. Most if not all life forms are present in desert ecosystems, regardless of the classification scheme used. Perennial shrubs dominate most desert landscapes, but in any single habitat trees, grasses, annuals, stem succulents, or leaf succulents may be the dominant form. From studies of desert plants, researchers have identified many adaptive functions at the ecophysiological level. These emerge from a plant's need to grow and survive through extreme drought, high solar radiation, and high temperatures, as well as through wide fluctuations in all of these abiotic factors. Plants exhibiting the succulent syndrome (which includes water storage, extensive surficial roots, and often CAM photosynthesis) are well adapted for life in warm arid ecosystems. Succulent plants are key components of many desert communities but they are rarely the dominant life form and are entirely absent from some deserts. Nutrients usually limit desert productivity during periods when water is available. Low external nutrient input results in decomposition by both abiotic and biotic processes playing a major role in nutrient availability. Other biotic-mediated processes, such as microbial nitrogen fixation and fungal root associations, are critical to maintaining favorable nutrient balance in desert plants. In spite of low productivity, deserts have surprisingly high biodiversity and endemism. Climate variability, geographic isolation, geologic history, and edaphic anomalies are among the primary drivers for greater-than-expected plant biodiversity. Biotic interactions were once thought to be rare in deserts and thus not important in desert community dynamics. In recent decades however, intra- and interspecific competition and facilitation have been clearly identified as important drivers in shaping desert plant communities. Arid and semiarid ecosystems are now widely used to test theories about the interplay between competition and facilitation. Deserts have always been susceptible to soil disturbances by nonnative ungulates and human activities. The profound effects on soil and nutrient losses are difficult to restore. In contrast, deserts were once considered relatively resistant to alien plant invasion, but recent spread of nonnatives has led to altered biogeochemical cycles and increased fire disturbances. In some cases, the changes have led to type conversion of vegetation. New pressures, such as renewable energy development, underscore the need for a solid scientific understanding of plant functions and ecosystem processes in arid and semiarid ecosystems.