We evaluated response patterns to drought stress in forest ecosystems in the Sierra Nevada Mountains, California adapted to extended summer drought using leaf and airborne imaging spectroscopy. We chose to use imaging spectrometry data and field measured leaf reflectance to determine what specific information about stress conditions could be detected over the multiyear drought in California from spring 2013 to fall 2015. Plant stresses can be detected in predictable ways using airborne spectroscopy, a type of remote sensing with numerous bands (typically hundreds) that measure the reflected solar spectrum, ranging from visible wavelengths through the shortwave-infrared (~400nm to 2500nm). We evaluated leaf reflectance for biophysical indicators of stress including declines in concentrations of photosynthetic pigments, changes in chlorophyll to carotenoid ratio, decline in foliar water content, increasing leaf mass area (mass/area), and increases in the proportion of dry leaf residues in the canopy among other changes. We also evaluated longer duration indicators of the progressive drought, including evidence for early leaf senescence and lower leaf area.
Results/Conclusions
We followed the changes in leaf and canopy properties for forest species in the Sierra Nevada Mountains, California as a severe drought progressed from spring 2013 to fall 2015. We show increasing evidence of drought stress over this period in broadleaf and conifer forest species, detected from spectral reflectance changes related to biophysical variables in these species at both leaf and canopy scales. Changes depended on the species and its topographic situation, from mild to tree mortality, in some cases, by winter of 2015-2016.