Large drought-induced aboveground live biomass losses in southern Rocky Mountain aspen forests

Widespread drought-induced forest mortality has been documented across multiple tree species in North America in recent decades, but it is a poorly understood component in terrestrial carbon (C) budgets. Recent severe drought in concert with elevated temperature likely triggered widespread forest mortality of trembling aspen (Populus tremuloides), the most widely distributed tree species in North America. The impact on the regional C budgets and spatial pattern of this drought-induced tree mortality, which has been termed sudden aspen decline (SAD), is not well known and could contribute to increased regional C emissions, an amplifying feedback to climate change. We conducted a regional assessment of drought-induced live aboveground biomass (AGB) loss from SAD across 915km2 of southwestern Colorado, USA, and investigated the influence of topography on the severity of mortality by combining field measures, remotely sensed nonphotosynthetically active vegetation and a digital elevation model. Mean [+/- standard deviation (SD)] remote sensing estimate of live AGB loss was 60.3 +/- 37.3Mgha-1, which was 30.7% of field measured AGB, totaling 2.7Tg of potential C emissions from this dieback event. Aspen forest health could be generally categorized as healthy (030% field measured canopy dieback), intermediate (3150%), and SAD (51100%), with the remote sensing estimated mean (+/- SD) live AGB losses of 26.4 +/- 15.1, 64.5 +/- 9.2, and 108.5 +/- 24.0Mgha-1, respectively. There was a pronounced clustering pattern of SAD on south-facing slopes due to relatively drier and warmer conditions, but no apparent spatial gradient was found for elevation and slope. This study demonstrates the feasibility of utilizing remote sensing to assess the ramification of climate-induced forest mortality on ecosystems and suggests promising opportunities for systematic large-scale C dynamics monitoring of tree dieback, which would improve estimates of C budgets of North America with climate change.