Trends and Tipping Points of Drought-induced Tree Mortality

Abstract:

Drought-induced tree mortality worldwide has been recently reported in a review of the literature by Allen et al. (2010). However, a quantitative relationship between widespread loss of forest from mortality and drought is still a key knowledge gap. Specifically, the field lacks quantitative knowledge of tipping point in trees when coping with water stress, which inhibits the assessments of how climate change affects the forest ecosystem. We investigate the statistical relationships for different (seven) conifer species between Ring Width Index (RWI) and Standardized Precipitation Evapotranspiration Index (SPEI), based on 411 chronologies from the International Tree-Ring Data Bank across 11 states of the western United States. We found robust species-specific relationships between RWI and SPEI for all seven conifer species at dry condition. The regression models show that the RWI decreases with SPEI decreasing (drying) and more than 76% variation of tree growth (RWI) can be explained by the drought index (SPEI). However, when soil water is sufficient (i.e., SPEI>SPEI^u), soil water is no longer a restrictive factor for tree growth and, therefore, the RWI shows a weak correlation with SPEI. Based on the statistical models, we derived the tipping point of SPEI (SPEI^tp) where the RWI equals 0, which means the carbon efflux by tree respiration equals carbon influx by tree photosynthesis. When the severity of drought exceeds this tipping point(i.e. SPEI<SPEI^tp), trees might not be able to sustain their lives as the carbon assimilated by photosynthesis could not suffice the lowest need of trees maintain respiration. The ranges of the tipping points for seven species-specific trees vary between -2.45 and -1.40. The lower value of a tipping point represents the stronger ability to endure drought. The predicted tipping points can be used as reference of tree mortality for assessment of forest mortality risk under climate change.

This work was supported by the Fund for Creative Research Groups of National Natural Science Foundation of China (No. 41321001), the National Basic Research Program of China (No. 2012CB955401), the New Century Excellent Talents in University (No. NCET-10-0251), U.S. PSC-CUNY Award (PSC-CUNY-ENHC-44-83) and the High Technology Research and Development Program of China (No. 2013AA122801).