On the frequent lack of response of plants to rainfall events in arid areas

摘要

Arid environments are those in which biological processes are limited by water. Thus, in principle it would not make much sense to find that a sizeable volume of rainfall water is apparently ignored by plants. However, this is precisely what seems to happen in many instances, and this note intends to draw attention to such data at two levels of organization: the plant-patch level and the ecosystem level. My purpose here is to show that these are not exceptions or special cases, but quite the opposite, since most seem to have explanations that are part of our current understanding of arid environments. Three decades ago, Imanuel Noy-Meir (1973) formalized what came to be known as the Pulse-Reserve Paradigm of arid-ecosystem functioning, according to which precipitation events trigger "pulses" of net primary production (NPP) that in turn are translated into "reserves" of carbon and energy which can be stored until the next rainfall episode. This conceptual model, attributable to unpublished work from Mark Westoby and Ken Bridges, has been generally interpreted in terms of water-input pulses (cf. Figs, 1a and b in Reynolds et al., 2004), and it is also used here in this widespread modified sense. Mainly depending on their duration, these pulses are able to produce (on plants and other organisms) a variety of ecophysiological effects, ranging from the subcellular to the ecosystem-wide level. These effects are best understood as a nested hierarchy of responses (Schwinning and Sala, 2004), among which a measurable NPP or other reserve-producing processes may or may not be present (e.g. Snyder et al., 2004).