Assessing drought-induced change in a pinon-juniper woodland with Landsat: a multiple endmember spectral mixture analysis approach

Pinon-juniper communities exist on mid-elevation mountain ranges throughout the Southwestern USA. These species are drought adapted, and have lived with climatic stochasticity since the end of the Pleistocene. Rising temperatures and drought within the past two decades have stressed much of this community beyond its adaptive limits. With increased drought-induced stress, pinon show greater vulnerability to die-off than juniper. Widespread pinon die-off occurred from extreme drought in northern New Mexico (2002-2004) with minimal juniper die-off. This study assessed the feasibility of quantifying differential pinon and juniper mortality following a recent drought at a site in central New Mexico by performing multiple endmember spectral mixture analysis on six Landsat images from 2009 through 2015 using field-based spectra collected throughout 2015. An ideal spectral separability time of year was identified to maximize separation between constituent land-cover classes by calculating vegetation indices for the five dominant land-cover classes at the site (juniper, pinon, dead pinon, herbaceous, and soil). Ideal separability periods were also determined by evaluating precipitation and temperature data. Peak separabililty between land-cover classes within the study period was determined to occur during the pre-monsoon season between late spring/early summer (May) when minimal spectral overlap occurred between classes (sigma = 1). The field-based spectra were then used to unmix each image in the study period. Results indicate a 24.6% decline of pinon across the study period with a comparable 23.8% increase of dead pinon. Accuracy assessment using high spatial (5-8 cm) resolution imagery for 2014 and 2015 showed strong correlation with modelled fractional cover results for 2014: live pinon and juniper (R-2 = 0.72), and dead pinon (R-2 = 0.79), and 2015: live pinon and juniper (R-2 = 0.77), dead pinon (R-2 = 0.65). Results demonstrate the potential of MESMA to monitor and accurately quantify the differential die-off of pinon and juniper at a regional scale as climate change-induced drought and higher temperatures are projected to continue in the Southwest.