Verification of a threshold concept of ecologically effective precipitation pulse: From plant individuals to ecosystem

Abstract

In water-limited ecosystems, an ecologically significant rainfall pulse was defined as a rainfall event that altered both soil water status and plant physiological activity. We developed a new threshold concept of an ecologically effective precipitation pulse (EEPP) applicable to both plant individual and ecosystem scales. The concept was tested in a typical steppe on Inner Mongolia plateau. Two EEPPs, single 3-mm rainfall and 5-mm rainfall, were applied to investigate their effects on soil and plant water status, CO2 assimilation of five species (four C3 plants and one C4 plant), whole-plot soil respiration (Rs), and net ecosystem CO2 exchange (NEE) on 1 June and 28 July 2009, respectively. Both EEPPs increased leaf water potential (Ψl) of all the species, which peaked 1–3days after rainfall pulses. Soil water content (SWC) in two depths (5cm and 20cm) significantly increased after the two EEPPs for 1–3days. Soil water potential (Ψs) within 20‐cm soil layer in EEPP treatments significantly differed (p<0.05) from control. Net assimilation rates (Anet) of all C3 plants had a slight increase at the next day after two EEPPs, in contrast to the C4 species. Rs elevated and peaked 1–3days later after water supply. Ecosystem net CO2 absorption rate rose to maximum value 3days after the 5-mm pulse on 28 July, higher than the response to 3-mm pulse on June 1. The grassland turned to net emission of CO2 after 3-mm pulse on 28 July. The results supported that there was an ecosystem level threshold for EEPP, and the threshold was temporally variable. It also highlighted the necessity of considering the response threshold of EEPP in rainfall manipulative experiment. In addition, effective rainfall amount was more approriate than total rainfall amount in modeling ecosystem carbon balance.