Modifying the ‘pulse–reserve’ paradigm for deserts of North America: precipitation pulses, soil water, and plant responses

Abstract

The 'pulse-reserve' conceptual model--arguably one of the most-cited paradigms in aridland ecology--depicts a simple, direct relationship between rainfall, which triggers pulses of plant growth, and reserves of carbon and energy. While the heuristics of 'pulses', 'triggers' and 'reserves' are intuitive and thus appealing, the value of the paradigm is limited, both as a conceptual model of how pulsed water inputs are translated into primary production and as a framework for developing quantitative models. To overcome these limitations, we propose a revision of the pulse-reserve model that emphasizes the following: (1) what explicitly constitutes a biologically significant 'rainfall pulse', (2) how do rainfall pulses translate into usable 'soil moisture pulses', and (3) how are soil moisture pulses differentially utilized by various plant functional types (FTs) in terms of growth? We explore these questions using the patch arid lands simulation (PALS) model for sites in the Mojave, Sonoran, and Chihuahuan deserts of North America. Our analyses indicate that rainfall variability is best understood in terms of sequences of rainfall events that produce biologically-significant 'pulses' of soil moisture recharge, as opposed to individual rain events. In the desert regions investigated, biologically significant pulses of soil moisture occur in either winter (October-March) or summer (July-September), as determined by the period of activity of the plant FTs. Nevertheless, it is difficult to make generalizations regarding specific growth responses to moisture pulses, because of the strong effects of and interactions between precipitation, antecedent soil moisture, and plant FT responses, all of which vary among deserts and seasons. Our results further suggest that, in most soil types and in most seasons, there is little separation of soil water with depth. Thus, coexistence of plant FTs in a single patch as examined in this PALS study is likely to be fostered by factors that promote: (1) separation of water use over time (seasonal differences in growth), (2) relative differences in the utilization of water in the upper soil layers, or (3) separation in the responses of plant FTs as a function of preceding conditions, i.e., the physiological and morphological readiness of the plant for water-uptake and growth. Finally, the high seasonal and annual variability in soil water recharge and plant growth, which result from the complex interactions that occur as a result of rainfall variability, antecedent soil moisture conditions, nutrient availability, and plant FT composition and cover, call into question the use of simplified vegetation models in forecasting potential impacts of climate change in the arid zones in North America.

Timing of climate variability and grassland productivity

“…However, quantifying precipitation patterns in an ecologically meaningful manner has been difficult (19), and we know little about how these patterns affect productivity outside of experimental conditions (20). We used stream discharge data from a US Geological Survey gauging station on a Konza stream (21) as an index of precipitation pattern.…”

It is getting hotter in here: determining and projecting the impacts of global environmental change on drylands

“…The size and frequency of rainfall events modulate processes such as soil and ecosystem respiration [54,55], microbial activity [56] and plant physiology and primary productivity [57]. Thus, modifications in precipitation patterns with climate change will largely affect ecosystem functioning in drylands [58], although some of these changes may not be necessarily negative (see [59] for a review). For example, increases in above-ground net primary productivity (ANPP) with increases in rainfall variability (less but more intense rainfall events) have been observed in semi-arid steppes from North America [36].…”

Sap flow in response to rainfall pulses for two shrub species in the semiarid Chinese Loess Plateau

“…Sap flow significantly accelerated, and plant transpiration increased, after plants extract the water provided by large rainfall events (Du et al, 2011;Liu et al, 2012), and this triggers a cascade of responses that effects plant growth, reproduction, and net ecosystem productivity (Moran et al, 2009;Ogle and Reynolds, 2004;Rana et al, 2005). Consequently, these events alter the carbon and water balance in the semi-arid ecosystem (Kigalu, 2007), and rainfall pulses could significantly drive the evolution of the structure and function of semi-arid ecosystems (Liu and Zhao, 2008;Reynolds et al, 2004). Understanding the responses of sap flow to rainfall pulses is therefore the basis for comprehending the physiological responses of plants and how these responses determine the ecophysiological patterns of adaptation of plants to their habitats (Klein et al, 2014).…”