Plant traits and soil fertility mediate productivity losses under extreme drought in C3 grasslands

Extreme drought decreases aboveground net primary production (ANPP) in most grasslands, but the magnitude of ANPP reductions varies especially in C3-dominated grasslands. Because the mechanisms underlying such differential ecosystem responses to drought are not well-resolved, we experimentally imposed an extreme 4-year drought (2015-2018) in two C3 grasslands that differed in aridity. These sites had similar annual precipitation and dominant grass species (Leymus chinensis) but different annual temperatures and thus water availability. Drought treatments differentially affected these two semiarid grasslands, with ANPP of the drier site reduced more than at the wetter site. Structural equation modeling revealed that community-weighted means for some traits modified relationships between soil moisture and ANPP, often due to intraspecific variation. Specifically, drought reduced community mean plant height at both sites, resulting in a reduction in ANPP beyond that attributable to reduced soil moisture alone. Higher community mean leaf carbon content enhanced the negative effects of drought on ANPP at the drier site, and ANPP-soil moisture relationships were influenced by soil C:N ratio at the wetter site. Importantly, neither species richness nor functional dispersion were significantly correlated with ANPP at either site. Overall, as expected, soil moisture was a dominant, direct driver of ANPP response to drought, but differential sensitivity to drought in these two grasslands was also related to soil fertility and plant traits.

极端干旱会降低多数草地的地上净初级生产力（aboveground net primary production, ANPP），但ANPP降幅存在差异，在以C3植物为主的草地中这一现象尤为明显。由于这类生态系统对干旱的差异化响应机制尚未得到充分阐明，本研究在两个干旱度存在差异的C3草地中开展了为期4年（2015-2018年）的极端干旱控制实验。两个研究站点的年降水量与建群草本植物羊草（Leymus chinensis）均一致，但年温存在差异，进而导致可利用水量不同。干旱处理对这两个半干旱草地的影响存在差异，干旱程度更高的站点其ANPP降幅大于湿度更高的站点。结构方程模型（Structural equation modeling）分析显示，部分性状的群落加权均值会改变土壤水分与ANPP之间的关联，这一现象通常源于种内变异。具体而言，干旱在两个站点均降低了群落平均株高，由此导致的ANPP降幅超出了仅由土壤水分减少所能解释的范围。在干旱程度更高的站点，群落平均叶片碳含量越高，干旱对ANPP的负向影响越强；而在湿度更高的站点，ANPP与土壤水分的关联则受土壤C:N比调控。值得注意的是，两个站点的物种丰富度与功能分散度均与ANPP无显著相关性。总的来说，正如预期，土壤水分是调控干旱下ANPP响应的主导直接驱动因子，但这两个草地对干旱的差异化敏感性还与土壤肥力及植物性状相关。