Bifurcated Response of a Regional Forest to Drought

Abstract

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 (RWI r) to quantify forest growth in the Southwest United States (SWUS). We show that over a period of 100 years, SWUS RWI r 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 (SPEI r). As SPEI r approached-1.6 (previously shown as a tipping-point for SWUS conifer forest growth), RWI r 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 SPEI r 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.

Dynamic responses of tree‐ring growth to multiple dimensions of drought

“…Among all vegetation types, forests are potentially the most susceptible to drought based on the hydraulic corollary of vascular plant physiology (McDowell & Allen, ). Increasing focus has been placed on widespread drought‐induced tree mortality and forest declines that cause a significant reduction in carbon flux from the atmosphere to the land (Wei, Yi, Fang, & Hendrey, ; Yi, Pendall, & Ciais, ; Yi et al., ), which have been investigated from diverse perspectives and on different scales using a wide variety of approaches (Adams et al., ; Allen et al., ; Barbeta et al., ; Bhuyan, Zang, Vicente‐Serrano, & Menzel, ; Dobbertin, ; Dorman, Svoray, Perevolotsky, & Sarris, ; González‐Cásares, Pompa‐García, & Camarero, ; He et al., ; Huang et al., ; Luo et al., ; Xu et al., ; Yi et al., ). However, detailed and locally specific conclusions and hypothetical mechanisms derived from in situ experiments or event‐based observations are difficult to be applied at the regional to global scales, while regional‐ to global‐scaled studies tend to simplify or abstract the processes of climate–vegetation interplay, which may cause uncertainties or even paradoxes.…”

The forest recovery path after drought dependence on forest type and stock volume

“…Drought events are likely to strongly influence a forest's structure, function, and the ecosystem services it provides [5,7], and can cause the death of trees by hydraulic failure and/or carbon starvation [4,[8][9][10][11][12]. Previous studies have demonstrated that the drought-induced forest mortality can cause the vegetation system to go from a net carbon sink to a significant carbon source, creating sizable positive feedback to global warming [4,5,11,13,14]. Thus, understanding forest drought response characteristics is crucial for forest management, and will contribute to reducing drought risk and slowing global warming.…”

Spatial Upscaling of Tree-Ring-Based Forest Response to Drought with Satellite Data

“…The study area is located in the Southwestern United States (SWUS), including the states of Utah, Colorado, Arizona, and New Mexico (Figure 1). As forests in the SWUS are mainly limited by water deficits, it is a typical area for researching vegetation index and tree-ring response to drought [5,7,40]. It provides us with a natural experiment by which we can develop a link between satellite observation and field measurements.…”