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Global climate change in the Mojave Desert will likely result in a greater intensity of summer (monsoon) rain events and greater N deposition. The nitrogen cycle has already been significantly altered by human activities to the extent that anthropogenically released N now equals natural terrestrial biological fixation (Vitousek et al. 1997; Galloway 1998). Because most bryophytes receive the bulk of their nutrients from direct atmospheric deposition (Bates 2000), this influx of N can affect the productivity of individual species and thus may alter bryophyte community structure and function. In addition to N deposition, global change models for the southwestern USA predict significant increases in summer precipitation in the northern Mojave Desert (Taylor & Penner 1994; Higgins & Shi 2001). The interaction between increased N deposition and an increased monsoon effect on bryophytes in the arid southwestern USA is largely unknown. Although growth rates of desert bryophytes are relatively low compared with bryophytes in more mesic ecosystems, the contribution of biological soil crusts (a community of cyanobacteria, mosses, lichens, algae, and fungi) to the global cycling of trace gases can be significant in regard to global budgets (Zaady et al. 2000).
Most field studies have found a rapid negative effect of N fertilization on the growth and productivity of mosses, with nutrient uptake a function of desiccation regime, temperature, and light. For several bryophyte species, high experimental N deposition rates decreased biomass production except in a widely tolerant species of Sphagnum (Jauhiainen et al. 1998).
Bryophytes, because they descend from the earliest branching events in the phylogeny of land plants, hold an important position in our investigations into the mechanisms by which plants respond to dehydration and by what paths such mechanisms have evolved. This is true regardless of what aspect of plant responses to dehydration one is interested in; whether it be mild water deficit stress as seen in most plants including those of agronomic importance, or desiccation as seen in orthodox seeds or in the leaves of desiccation-tolerant (or resurrection) plants. It is quite possible that the mechanisms by which bryophytes tolerate dehydration closely reflect the way that the first land plants coped with the rigors of a drying atmosphere as they began their colonization of the land. In a recent phylogenetic synthesis of the evolution of desiccation tolerance within the land plants (Oliver et al. 2000), it was postulated that vegetative desiccation tolerance was required for plants to transition from an aqueous environment to the dry land. In the initial ventures into dehydrating atmospheres, plants were of a very simple architecture and had yet to evolve the complex strategies to prevent water loss that we see in modern day plants. Once the cells of these plants were no longer surrounded by liquid water they would rapidly lose water and dry.