Stock Volume Dependency of Forest Drought Responses in Yunnan, China

Abstract

Abstract:Revealing forest drought response characteristics and the potential impact factors is quite an important scientific issue against the background of global climate change, which is the foundation to reliably evaluate and predict the effects of future drought. Due to the high spatial heterogeneity of forest properties such as biomass, forest age, and height, and the distinct differences in drought stress in terms of frequency, intensity, and duration, current studies still contain many uncertainties. In this research, we used the forests in Yunnan Province in Southwest China as an example and aimed to reveal the potential impacts of forest properties (i.e., stock volume) on drought response characteristics. Specifically, we divided the forest into five groups of stock volume density values and then analyzed their drought response differences. To depict forest response to drought intensity, the standardized precipitation evapotranspiration index (SPEI) was chosen as the explanatory variable, and the change in remote sensing-based enhanced vegetation index (deficit of MODIS-EVI, dEVI) was chosen as the response variable of drought stress. Given that the SPEI has different time scales, we first analyzed the statistical dependency of SPEIs with different time scales (1 to 36 months) to the response variable (i.e., dEVI). The optimal time scale of SPEI (SPEI opt ) to interpret the maximum variation of dEVI (R-square) was then chosen to build the ultimate statistical models for the five groups of stock volume density. The main findings were as follows: (1) the impacts of drought showed hysteresis and cumulative effects, and the length of the hysteresis increased with stock volume densities; (2) forests with high stock volume densities required more soil water and were therefore more sensitive to the changes in water deficit; (3) compared with the optimal time scale of SPEI (SPEI opt ), the SPEI with the commonly used time scale (e.g., 1, 6, and 12 months) could not well reflect the impacts of drought on forests and the simulation error of dEVI increased with stock volume densities; and (4) forests with higher stock volume densities were likely to experience a greater risk of degradation following higher atmospheric concentrations of greenhouse gases (Representative Concentration Pathway (RCP) 8.5). As a result, both the time scale of the meteorological drought index and the spatial difference in forest stock volumes should be considered when evaluating forest drought responses at regional and global scales.

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

“…Our results showed that regional mean RWI was reduced from an average of 1,033 to 970 (Figure 3d), which is comparable to the studies of interannual legacy effects from Anderegg et al (2015) and Wu et al (2017). Previous studies reported that correlations between vegetation indices (e.g., RWI, NDVI) and drought indices (e.g., SPEI) were tested significant at a timescale over 24 months Luo et al, 2018;Vicente-Serrano et al, 2014). Nevertheless, even when the previous hydrological year was under extreme drought conditions, its impact on current year RWIs were much less than the impact from drought in the current hydrological…”

“…(). Previous studies reported that correlations between vegetation indices (e.g., RWI, NDVI) and drought indices (e.g., SPEI) were tested significant at a timescale over 24 months (Li et al., ; Luo et al., ; Vicente‐Serrano et al., ). Nevertheless, even when the previous hydrological year was under extreme drought conditions, its impact on current year RWIs were much less than the impact from drought in the current hydrological year (Figure c).…”

“…Legacy effects of trees responses to drought have been found to vary significantly at the global scale, some studies reported that forest ecosystems did not show apparent legacy effects or legacy effects <3 months (Wu et al., ), some reported around 5 months (Luo et al., ), some reported 8–10 months in semiarid and subhumid regions and shorter in arid and humid regions (Vicente‐Serrano et al., ), some reported over 24 months (Li et al., ; Luo et al., ; Vicente‐Serrano et al., ; Wu et al., ). These discrepancies were then attributed to diverse climate characteristics and forest types.…”

“…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.…”

Impacts of Water Stress on Forest Recovery and Its Interaction with Canopy Height

“…As climate change becomes unavoidable, more attention and effort are called for reducing the impact of drought on forest ecosystems. Our results showed that forest responses to climate were also affected by tree size [ 51 , 52 , 53 ], which sheds new light on managing forests under the changing climate. Although the economic benefits and ecosystem services of trees increase with their size over time, the risk of mortality or dieback under drought stress also increases with time.…”