Huang Kaicheng1、2、3，Yi Chuixiang4、5，Wu Donghai6、7，Zhou Tao1、2、3，Zhao Xiang6、7，Blanford Williamj. 4，Wei Suhua4，Wu Hao1、2、3，Ling Du1、2、3，Li Zheng1、2、3
1. Beijing Normal Univ, State Key Lab Earth Surface Proc & Resource Ecol, Beijing 100875, Peoples R China.
2. Minist Civil Affairs, Acad Disaster Reduct & Emergency Management, Beijing 100875, Peoples R China.
3. Minist Educ, Beijing 100875, Peoples R China.
4. CUNY Queens Coll, Sch Earth & Environm Sci, Flushing, NY 11367 USA.
5. Stockholm Univ, Bert Bolin Ctr Climate Res, Dept Meteorol, S-10691 Stockholm, Sweden.
6. Beijing Normal Univ, Sch Geog, Beijing 100875, Peoples R China.
7. Beijing Normal Univ, Coll Global Change & Earth Syst Sci, Beijing 100875, Peoples R China.
Abstract: Drought-induced tree mortality has recently received considerable attention. Questions have arisen over the necessary intensity and duration thresholds of droughts that are sufficient to trigger rapid forest declines. The values of such tipping points leading to forest declines due to drought are presently unknown. In this study, we have evaluated the potential relationship between the level of tree growth and concurrent drought conditions with data of the tree growth-related ring width index (RWI) of the two dominant conifer species (Pinus edulis and Pinus ponderosa) in the Southwestern United States (SWUS) and the meteorological drought-related standardized precipitation evapotranspiration index (SPEI). In this effort, we determined the binned averages of RWI and the 11 month SPEI within the month of July within each bin of 30 of RWI in the range of 0-3000. We found a significant correlation between the binned averages of RWI and SPEI at the regional-scale under dryer conditions. The tipping point of forest declines to drought is predicted by the regression model as SPEItp = -1.64 and RWItp = 0, that is, persistence of the water deficit (11 month) with intensity of -1.64 leading to negligible growth for the conifer species. When climate conditions are wetter, the correlation between the binned averages of RWI and SPEI is weaker which we believe is most likely due to soil water and atmospheric moisture levels no longer being the dominant factor limiting tree growth. We also illustrate a potential application of the derived tipping point (SPEItp = -1.64) through an examination of the 2002 extreme drought event in the SWUS conifer forest regions. Distinguished differences in remote-sensing based NDVI anomalies were found between the two regions partitioned by the derived tipping point.
Published in ENVIRONMENTAL RESEARCH LETTERS, doi: 10.1088/1748-9326/10/2/024011
