Bifurcated Response of a Regional Forest to Drought

Chuixiang Yi^1,2*, Guangwei Mu¹, George Hendrey^1,2, Sergio M Vicente-Serrano³, Wei Fang¹, Tao Zhou^4,5, Shan Gao^1,4,5 and Peipei Xu^1,4,5

¹School of Earth and Environmental Sciences, Queens College of the City University of New York, New York, 11367, USA

²Department of Earth and Environmental Sciences, the Graduate Center of the City University of New York, New York, NY 10016, USA

³Instituto Pirenaico de Ecología, Consejo Superior de Investigaciones Científicas (IPE-CSIC), Zaragoza 50080 Spain

⁴State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing 100875, China

⁵Academy of Disaster Reduction and Emergency Management, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China

*Corresponding Author : Chuixiang Yi
Professor, School of Earth and Environmental Sciences, Queens College of the City University of New York, New York, 11367, USA
Tel: 718-997-3366
E-mail: cyi@qc.cuny.edu

Received: February 27, 2018 Accepted: April 03, 2018 Published: April 06, 2018

Citation: Chuixiang Yi, Guangwei Mu, Hendrey G, Vicente-Serrano SM, Fang W, et al. (2018) Bifurcated Response of a Regional Forest to Drought. Expert Opin Environ Biol 7:2. doi: 10.4172/2325-9655.1000153

Several lines of evidence suggest that forest growth in many regions is declining as a consequence of changing climate. To predict the fate of forests in the future, a quantitative understanding of how the key climate variables (insolation, precipitation and temperature) interact with forests to cause the decline is a pressing need. Here we use a regionally-averaged tree-ring width index (RWIr) to quantify forest growth in the Southwest United States (SWUS). We show that over a period of 90 years, SWUS RWIr bifurcated into forest stands with enhanced (healthy) and reduced (declining) branches when regressed on shortwave-radiation and temperature, respectively. The reduced branch was controlled overwhelmingly by drought as measured with a regionally-averaged precipitationevapotranspiration index (SPEIr). As SPEIr approached -1.6 (previously shown as a tipping-point for SWUS conifer forest growth), RWIr approached zero and in extreme drought years, wide spread tree mortality has been observed. Modeled trends in SPEI based on four IPCC-GHG scenarios predict SWUS SPEIr falling below -1.6 more or less continuously within a few decades. With drought expanding north- and eastward over larger areas, tree mortality may become a semi-continental phenomenon with coniferous forests transitioning to more xeric ecosystems. Our results provide insights into how to differentiate functions of climate impacts on forest growth and how to identify tipping-point control parameters for forest regime transitions.