Functional Traits From Imaging Spectroscopy Inform Patterns of Forest Mortality During Sierra Nevada Drought

Abstract

California's 2012–2016 megadrought led to the mortality of over 100 million trees. In the context of extreme drought and insect outbreaks, a holistic view of plant functional traits can provide further insight into underlying physiological and abiotic drivers of the patterns of mortality. We used new maps of early-drought (pre-mortality) foliar functional traits derived from the NASA AVIRIS-Classic imaging spectrometer, along with open-access climate, topography, canopy structure, and mortality data, to assess competing influences on drought mortality at the Soaproot Saddle and Lower Teakettle NEON sites in the southern Sierra Nevada Mountains. We aimed to (1) compare mortality trends across two independently derived mortality datasets, (2) assess trait-mortality relationships across diverse sites and species, and (3) link these relationships to mechanisms of tree-level drought response. We used random forests to assess the relative importance of mortality drivers and the trends of mortality across each predictor gradient. For the lower elevation, more water-limited Soaproot Saddle site, conifer mortality was linked to taller, drier canopies while broadleaf mortality was linked to foliar traits (lower cellulose, higher sugars, and higher leaf mass per area). For the higher elevation, more energy-limited Lower Teakettle site, mortality was more strongly linked to elevation and climate, with little influence from foliar traits.