Estimating Individual Tree Mortality in the Sierra Nevada Using Lidar and Multispectral Reflectance Data

Abstract

Widespread tree mortality events occur during periods of severe drought in temperate conifer forests and are expected to become more frequent in many areas due to climate change. Improved mapping of individual tree mortality is needed to identify risk factors and design effective conservation strategies. In this study, we used National Ecological Observatory Network (NEON) lidar and multispectral reflectance airborne observations to map individual tree mortality over a 160 km2 area during and after the 2012–2016 drought for two sites in California's Sierra National Forest. We used NEON lidar to derive tree locations and crown perimeters and multispectral data to map tree mortality for more than 1 million trees. We found that 25.4% of the trees in our study area died between 2013 and 2017, with considerably higher mortality at the lower‐elevation Soaproot Saddle site. Between 2017 and 2019, an additional 2.0%–2.8% of the trees died each year. Two wildfires in 2020 and 2021 increased tree mortality within burned area perimeters by 49%–89% between 2019 and 2021. Consistent with previous work, we found that tree mortality risk increased as a function of tree height. Tree mortality was positively associated with distance from rivers, trees per hectare, and decreasing slope at the lower elevation site. In contrast, increasing slope was positively associated with tree mortality at the higher elevation site. Our approach and dataset provide a means to study the combined effects of drought and wildfire on tree mortality and may improve projections of forest resilience under a changing climate.

Potential of Earth Observation to Assess the Impact of Climate Change and Extreme Weather Events in Temperate Forests—A Review

“…However, the largest group (n = 27) of studies with an exclusive focus on droughts refer to the mortality of a tree, stand, or forest. The focus of the studies on Germany [87,105,[127][128][129][130][131] and the USA [132][133][134][135][136][137][138] is noticeable. With seven study areas in each country, this proportion is distinctly higher than the overall distribution.…”

“…Studies in the USA have focused on the Sierra Nevada region [133][134][135]137]. Again, several variables were shown to influence vulnerability to drought.…”

“…Again, several variables were shown to influence vulnerability to drought. The comprehensive study by Hemming-Schroeder et al [135] used AHS and ALS data to show that mortality risk increases with tree height, forest density, and distance to the nearest river. The US studies also showed that tree species are particularly important.…”

“…Using a combination of dendroecology and remote sensing, it was shown that proximity to the forest edge is negatively related to mortality and positively related to growth. Hemming-Schroeder et al [135] used ALS and AHS data in their study of conifer mortality in the Sierra Nevada to show, among other things, that mortality increases with distance from a river.…”

Placing 21st Century Warming in Southern California, USA in a Multi-Century Historical Context

“…In California, which has the largest population [2], highest GDP [3], and highest value of crop sales [4] of all US states, the effects of warming have been extensive [5]. These warming impacts include increased drought risk [5], altered elevational distribution ranges of forest fires, greater burn frequency in the Sierra Nevada [6,7], increased extreme autumnal fires [8], lengthened fire season and area burned [9], reduced snowpack [10], earlier snowpack melt and runoff [11], and decreased (increased) growth of low-elevation (high-elevation) conifers [12,13]. Considerable spatial variability exists within California regarding the extent of 21st century warming effects, with the largest increases occurring in the southern third, and most populous region, of the state [14].…”