Extreme weather events such as droughts are expected to increase in severity and frequency as the climate changes; it is imperative that land managers be able to monitor associated changes in vegetation health efficiently and across large scales in order to mitigate or prepare for these events. This need motivated deeper study of the die‐off of bishop pine (Pinus muricata) on California's Santa Cruz Island during the 2012–2016 drought. These pines play a keystone role within the island's ecosystem and have experienced two severe droughts and associated mass die‐offs in the past 40 years. In an effort to compare these events, we used meteorological data to track changes in drought severity from 1985 to 2018 coupled with novel methods for forest monitoring to reveal dynamics not detectable by shorter‐duration studies. Leveraging 34 years of 30 m resolution Landsat imagery, we compared vegetation mortality between the two most severe droughts of that time period: 1987–1991 and 2012–2016. We used the slope of decline in the annual median value of three different Vegetation Indices (VIs; NBR, NDMI and NDVI) to compare mortality between the two drought events and to reveal spatial patterns of mortality within the bishop pine forests. Our results indicated that the 2012–2016 drought was the island's harshest in over a century and that it resulted in greater and more widespread mortality of vegetation within bishop pine stands than the 1987–1991 drought. The average VI decline was significantly greater during the 2012–2016 drought than the 1987–1991 drought by a factor of 1.89, 2.09 and 1.84 for NBR, NDMI and NDVI, respectively. Our results aligned with projections of increasing drought severity and associated tree mortality across the region. The temporal monitoring methods developed here can be adapted to study similar landscape scale changes over multiple decades in other forest ecosystems facing similar threats.

中文翻译：

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干燥的松树：对加利福尼亚圣克鲁斯岛上的两次主教松树（Pinus muricata）多年干旱和相关死亡率的定量比较

随着气候变化，干旱等极端天气事件的严重性和频率预计将增加；至关重要的是，土地管理者必须能够有效，大规模地监测植被健康的相关变化，以减轻或为这些事件做准备。这项需求促使人们对松树主教（Pinus muricata）的死亡进行更深入的研究。）在2012–2016年干旱期间在加利福尼亚州的圣克鲁斯岛上进行。这些松树在岛上的生态系统中起着关键作用，在过去的40年中经历了两次严重的干旱和相关的大规模死亡。为了比较这些事件，我们使用气象数据来跟踪1985年至2018年干旱严重程度的变化，并结合新颖的森林监测方法来揭示短期研究无法发现的动态。利用34年的30 m分辨率Landsat影像，我们比较了该时期两次最严重的干旱：1987–1991年和2012–2016年之间的植被死亡。我们使用了三种不同植被指数（VIs; NBR，NDMI和NDVI）以比较两次干旱事件之间的死亡率，并揭示主教松树林内的死亡率空间格局。我们的结果表明，2012-2016年的干旱是该岛一个多世纪以来最严重的干旱，与1987-1991年的干旱相比，它导致主教松林内植被的死亡人数越来越大。对于NBR，NDMI和NDVI，2012-2016年干旱期间的平均VI下降幅度明显大于1987-1991年干旱，分别为1.89、2.09和1.84倍。我们的结果与该地区干旱严重程度和相关树木死亡率不断增加的预测相吻合。这里开发的时间监测方法可以适应研究其他面临类似威胁的森林生态系统数十年来类似的景观尺度变化。