Khoury Sacha, Coomes David A
Forest Ecology and Conservation Group, Department of Plant Sciences, University of Cambridge Conservation Research Institute, Cambridge, UK.
Glob Chang Biol. 2020 Dec;26(12):7079-7098. doi: 10.1111/gcb.15268. Epub 2020 Sep 7.
A widespread increase in forest cover is underway in northern Mediterranean forests because of land abandonment and decreased wood demand, but the resilience of these successional forests to climate change remains unresolved. Here we use 18-year time series of canopy greenness derived from satellite imagery (NDVI) to evaluate the impacts of climate change on Spain's forests. Specifically, we analyzed how NDVI was influenced by the climatic water balance (i.e. Standardized Precipitation-Evapotranspiration Index, SPEI), using monthly time-series extracted from 3,100 pixels of forest, categorized into ten forest types. The forests increased in leaf area index by 0.01 per year on average (from 1.7 in 2000 to 1.9 in 2017) but there was enormous variation among years related to climatic water balance. Forest types varied in response to drought events: those dominated by drought-avoiding species showed strong covariance between greenness and SPEI, while those dominated by drought-tolerant species showed weak covariance. Native forests usually recovered more than 80% of greenness within the 18 months and the remainder within 5 years, but plantations of Eucalyptus were less resilient. Management to increase the resilience of forests-a key goal of forestry in the Mediterranean region-appears to have had a positive effect: canopy greenness within protected forests was more resilient to drought than within non-protected forests. In conclusion, many of Spain's successional forests have been resilient to drought over the past 18 years, from the perspective of space. Future studies will need to combine remote sensing with field-based analyses of physiological tolerances and mortality processes to understand how Mediterranean forests will respond to the rapid climate change predicted for this region in the coming decades.
由于土地撂荒和木材需求下降，地中海北部森林的森林覆盖面积正在普遍增加，但这些演替森林对气候变化的恢复力仍未得到解决。在这里，我们使用从卫星图像（归一化植被指数，NDVI）得出的18年树冠绿度时间序列来评估气候变化对西班牙森林的影响。具体而言，我们分析了NDVI如何受到气候水分平衡（即标准化降水-蒸散指数，SPEI）的影响，使用从3100个森林像素中提取的月度时间序列，这些像素被分为十种森林类型。森林的叶面积指数平均每年增加0.01（从2000年的1.7增加到2017年的1.9），但与气候水分平衡相关的年份之间存在巨大差异。不同森林类型对干旱事件的反应不同：那些以避旱物种为主的森林，其绿度与SPEI之间表现出很强的协方差，而那些以耐旱物种为主的森林，其协方差较弱。原生森林通常在18个月内恢复超过80%的绿度，其余部分在5年内恢复，但桉树种植园的恢复力较弱。旨在提高森林恢复力的管理——地中海地区林业的一个关键目标——似乎产生了积极影响：受保护森林内的树冠绿度比未受保护森林内的更能抵御干旱。总之，从空间角度来看，过去18年中西班牙的许多演替森林对干旱具有恢复力。未来的研究需要将遥感与基于实地的生理耐受性和死亡率过程分析相结合，以了解地中海森林将如何应对该地区未来几十年预计的快速气候变化。
