Comparative patterns of phenology and growth form diversity in two winter rainfall deserts: the Succulent Karoo and Mojave Desert ecosystems

A comparative study of community structure and seasonal growth dynamics in the arid winter rainfall regions of the Succulent Karoo in South Africa and the Mojave Desert of the United States suggests that remarkably divergent patterns of resource use and resultant growth form diversity exist in regio ns with outwardly similar climatic regimes. An understanding of these diver gent patterns in the two winter rainfall deserts allows predictions to be m ade on vegetation response to global change. Above-ground plant growth in t he Succulent Karoo begins with the first significant rains in late summer a nd continues through winter because moderate minimum temperatures allow con tinued growth. These communities have low structural diversity above-ground , but also below-ground, where root systems commonly do not exceed 20 cm in depth. These shallow root systems harvest water from upper soil horizons s oon after rain falls, and growth declines as rainfall decreases in late spr ing. In contrast, low temperatures during winter inhibit growth in the Moja ve Desert until early spring at a time when a mean 74% of the hydrologic ye ar precipitation (July-June) has already occurred. Thus species in this str ucturally diverse system rely on deeper stores of water for growth in sprin g and early summer. A global change scenario of a 2 to 4 degrees C increase in mean annual temperature and increased summer rainfall in the Mojave des ert would be expected to produce similar conditions in the Mojave Desert to those that exist in the Succulent Karoo today. Assuming no genetic constra ints on phenotypic plasticity, this would suggest increased species diversi ty and a decline in structural diversity in the Mojave Desert over evolutio nary time. Increased summer rainfall in the Succulent Karoo would be expect ed to lead to invasions of grasses and thus increased competitive pressure reducing community diversity.