Because some semi-deserts comprise diverse mixtures of plant growth forms and physiologies, changes in temperature, precipitation or the concentration of atmospheric C02 have the potential to alter photosynthetic performance and the relative abundance of C3, C4 and CAM pathway plants (Johnson et at. 1993). Alterations of the amount, the predictability and the seasonality of rainfall will have significant impacts on the nature of arid land ecosystems.
For example, Striegl el ai (1992) pointed out that increased moisture availability in arid land soils should significantly increase the sink for atmospheric methane. Unfortunately, most GCM predictions are not yet capable of resolving the magnitude or even the direction of these alterations for many arid regions. Most models show overall long-term increases in precipitation after a transient (ca. two centuries) drying (e.g. Rind et al. 1990).
As discussed above, many desert organisms function as ephemerals, becoming active and visible only during episodes of environmental favor-ability. The difficulties thus caused for any sampling or monitoring program arc exacerbated when one considers the temporal element necessary for assessing response to long-term changes. That is, climatic change would result only in a change in the temporal distribution of favorable periods, and thus in the frequency of appearance of particular taxa. Sampling thus becomes a statistical assessment of frequency, not some simple determination of presence or absence. Dregne and Tucker (1988) also emphasized the difficulty of assessing directional changes in production (e.g. desertification) when intcr-annual variation is so large in semi-arid regions.
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