CEFE, Univ Montpellier, CNRS, EPHE, INRAE, IRD, Montpellier, France.
Instituto Nacional de Investigación Agropecuaria (INIA), Área de Pasturas y Forrajes, Estación Experimental INIA Treinta y Tres, Treinta y Tres, Uruguay.
Ecology. 2024 Sep;105(9):e4383. doi: 10.1002/ecy.4383. Epub 2024 Jul 26.
Mediterranean ecosystems are predicted to undergo longer and more intense summer droughts. The mechanisms underlying the response of herbaceous communities to such drier environments should be investigated to identify the resilience thresholds of Mediterranean rangelands. A 5-year experiment was conducted in deep and shallow soil rangelands of southern France. A rainout shelter for 75 days in summer imposed drier and warmer conditions. Total soil water content was measured monthly to model available daily soil water. Aboveground net primary production (ANPP), forage quality, and the proportion of graminoids in ANPP were measured in spring and autumn. Plant senescence and plant cover were assessed in summer and spring, respectively. The experimental years were among the driest ever recorded at the site. Therefore, manipulated summer droughts were drier than long-term ambient conditions. Interactions between treatment, community type, and experimental year were found for most variables. In shallow soil communities, spring plant cover decreased markedly with time. This legacy effect, driven by summer plant mortality and the loss of perennial graminoids, led to an abrupt loss of resilience when the extreme water stress index exceeded 37 mm 10 day, characterized by a reduction of spring plant cover below 50% and a decreased ANPP in rainy years. Conversely, the ANPP of deep soil communities remained unaffected by increased summer drought, although the presence of graminoids increased and forage nutritive value decreased. This study highlights the role of the soil water reserve of Mediterranean plant communities in modulating ecosystem responses to chronically intensified summer drought. Communities on deep soils were resilient, but communities on shallow soils showed a progressive, rapid, and intense degradation associated with a loss of resilience capacity. Notably, indexes of extreme stress were a better indicator of tipping points than indexes of integrated annual stress. Considering the role of soil water availability in other herbaceous ecosystems should improve the ability to predict the resilience of plant communities under climate change.
地中海生态系统预计将经历更长时间和更强烈的夏季干旱。应该研究草本群落对这种干燥环境的响应机制，以确定地中海牧场的恢复力阈值。在法国南部的深土和浅土牧场进行了为期 5 年的实验。在夏季，遮阳棚覆盖 75 天，造成更干燥和温暖的条件。每月测量总土壤含水量，以模拟可用的每日土壤含水量。春季和秋季测量地上净初级生产力 (ANPP)、饲料质量和 ANPP 中禾本科植物的比例。夏季和春季分别评估植物衰老和植物盖度。实验年份是该地点有记录以来最干旱的年份之一。因此，人为夏季干旱比长期环境条件更干燥。大多数变量都发现处理、群落类型和实验年份之间存在相互作用。在浅土群落中，春季植物盖度随时间明显下降。这种由夏季植物死亡率和多年生禾本科植物的损失引起的遗留效应，导致当极端水分胁迫指数超过 37 毫米 10 天时，恢复力突然丧失，其特征是春季植物盖度降至 50%以下，在多雨年份 ANPP 降低。相反，深土群落的 ANPP 不受夏季干旱增加的影响，尽管禾本科植物的存在增加，饲料营养价值降低。本研究强调了地中海植物群落土壤水分储备在调节生态系统对长期强化夏季干旱的响应中的作用。深土群落具有恢复力，但浅土群落表现出与恢复力丧失能力相关的渐进、快速和强烈的退化。值得注意的是，极端压力指数是临界点的更好指标，而综合年度压力指数则不是。考虑土壤水分可利用性在其他草本生态系统中的作用，应该能够提高预测气候变化下植物群落恢复力的能力。
