Recent tree mortality dampens semi-arid forest die-off during subsequent drought

Climate change is expected to increase drought intensity and frequency, which are commonly predicted will threaten the survival of forests. Most forest die-off projections assume that recent tree mortality will not alter die-off severity during subsequent droughts. We tested this assumption by comparing die-off in semi-arid conifer forest stands in California that were exposed to a single drought in 2012–2015 (“2nd Drought Only”) with forest stands that experienced drought in both 1999–2002 and 2012–2015 (“Both Droughts”). We quantified die-off severity as a reduction in the satellite observed Normalized Difference Moisture Index (dNDMI), and cumulative moisture deficit as negative four-year Precipitation minus Evapotranspiration (four-year Pr-ET overdraft). Here we show that recent tree morality reduces die-off severity in semi-arid conifer forests exposed to subsequent drought. Stands in the 2nd Drought Only sample experienced severe die-off associated with extreme four-year Pr-ET ove..., ,

气候变化预计将加剧干旱的强度与发生频率，而此类干旱变化普遍被认为会威胁森林的存续。现有多数森林死亡预测模型均假设，前期发生的树木死亡不会改变后续干旱期间的森林死亡严重程度。本研究通过对比加利福尼亚州两类半干旱针叶林林分的死亡情况验证上述假设：一类为仅在2012—2015年经历单次干旱的林分（记为仅第二次干旱组（2nd Drought Only）），另一类为同时在1999—2002年与2012—2015年经历两次干旱的林分（记为两次干旱组（Both Droughts））。本研究以卫星观测得到的归一化差值湿度指数（Normalized Difference Moisture Index, dNDMI）的降幅量化森林死亡严重程度，以四年期降水减蒸散的负值（即四年降水蒸散亏缺，four-year Pr-ET overdraft）表征累积水分亏缺。结果表明，在后续干旱胁迫下，半干旱针叶林的森林死亡严重程度会因前期树木死亡而降低。仅第二次干旱组林分出现了与极端四年降水蒸散亏缺相关的严重森林死亡，相关研究……