Contrasting Responses of Planted and Natural Forests to Drought Intensity in Yunnan, China

Abstract

Abstract:In recent decades, the area and proportion of planted forests have increased; thus, understanding the responses of planted and natural forests to drought are crucial because it forms the basis for forest risk assessments and management strategies. In this study, we combined the moderate-resolution imaging spectroradiometer (MODIS) enhanced vegetation index (EVI), meteorological aridity indices, and standardized precipitation evapotranspiration indices (SPEI) to identify the drought responses of planted and natural forests. In particular, we used the EVI standard anomaly (ESA) as a physiological drought indicator and analyzed the applicability of SPEIs at time scales of 1-30 months, thereby determining the optimal time scale for the SPEI (SPEI opt ), i.e., the SPEI that best represents the drought responses of forests in Yunnan. Next, we employed the optimal SPEI and the ESA as indices to statistically analyze the response characteristics of planted and natural forests under different drought intensities. The results indicated the following: (1) The SPEI in June and a time scale of five months (i.e., SPEI Jun,5 ) comprise the optimal meteorological aridity indicator for forests in Yunnan Province, which had the strongest correlation with the EVI standard anomaly (ESA Jun ). (2) All forest types were affected by drought in Yunnan, but their responses varied according to the forest type, elevation, and drought intensity. In general, natural forests are more vulnerable and sensitive to drought than planted forests, especially natural coniferous forests at low (0-2000 m) and moderate (2000-4000 m) altitudes, and natural mixed forest at low altitudes (0-2000 m). (3) The remote sensing-based ESA (ESA Jun ) is sensitive to the intensity of water stress, which makes it a good indicator for drought monitoring. In addition, the forests' inventory survey revealed that 8.05% of forests were affected by drought; thus, we used this as a guide to estimate an approximate threshold to map forest responses to drought across the region. Below this approximate threshold (i.e., ESA Jun <´3.85), severe drought-induced effects on forests may occur. Given that natural forests are more vulnerable and sensitive to drought than the planted forests, natural forests need more careful management, especially in the context of projected increases in extreme drought events in the future.

Stock Volume Dependency of Forest Drought Responses in Yunnan, China

“…Based on the data mentioned above, the research found that Yunnan Province, with a mean annual precipitation (MAP) of 1098.6 mm and a mean annual temperature (MAT) of 15.7 • C, is warm and humid in general. In addition, the precipitation and temperature are concentrated in summer (June-August) [10], and therefore it is a typical high-coverage region in China, where the forest cover is greater than 50.0% [39]; forests with various stock volume densities are widely distributed ( Figure 1) [40], and the best status of forests grow mainly in summer (especially in July) [10]. From 2001 to 2014, large areas of Yunnan suffered many drought events (2005)(2006)(2009)(2010)(2011)(2012)(2013)(2014) with varying intensities and duration, which significantly affected the forest ecosystem [10,41,42].…”

“…In addition, the precipitation and temperature are concentrated in summer (June-August) [10], and therefore it is a typical high-coverage region in China, where the forest cover is greater than 50.0% [39]; forests with various stock volume densities are widely distributed ( Figure 1) [40], and the best status of forests grow mainly in summer (especially in July) [10]. From 2001 to 2014, large areas of Yunnan suffered many drought events (2005)(2006)(2009)(2010)(2011)(2012)(2013)(2014) with varying intensities and duration, which significantly affected the forest ecosystem [10,41,42]. Typical drought events in Yunnan include the mild drought of 2005 and the persistent drought in winter of 2009 to spring of 2010 [10,41].…”

“…From 2001 to 2014, large areas of Yunnan suffered many drought events (2005)(2006)(2009)(2010)(2011)(2012)(2013)(2014) with varying intensities and duration, which significantly affected the forest ecosystem [10,41,42]. Typical drought events in Yunnan include the mild drought of 2005 and the persistent drought in winter of 2009 to spring of 2010 [10,41]. Yunnan Province is currently a focus study region for exploring how the forest responds to drought as it has a high forest cover rate and has suffered several drought events [43,44], especially from 2001-2014.…”

“…Based on the correlations between meteorological data, the annual tree rings, and remote sensing vegetation index, Wu et al reported that forests (trees and shrubs) took more than two years to respond to drought in the temperate zone of the northern hemisphere [27]. Thus, if the differences in response characteristics (e.g., the length of hysteresis) of forests with different stock volume densities are fully taken into account, the response of forests to drought can be evaluated more reliably [10,28,29].…”

“…In recent years, drought has been more frequent, more severe, and longer [7,8]. It has affected forest ecosystems to varying degrees [6,9,10], and has threatened forest health and affected ecosystem services, so it is important to manage forests efficiently through informed decision-making that reliably evaluates and predicts the effects of drought on forest growth [11].…”