Extreme drought impacts have been underestimated in grasslands and shrublands globally

Abstract

Climate change is increasing the frequency and severity of short-term (~1 y) drought events-the most common duration of drought-globally. Yet the impact of this intensification of drought on ecosystem functioning remains poorly resolved. This is due in part to the widely disparate approaches ecologists have employed to study drought, variation in the severity and duration of drought studied, and differences among ecosystems in vegetation, edaphic and climatic attributes that can mediate drought impacts. To overcome these problems and better identify the factors that modulate drought responses, we used a coordinated distributed experiment to quantify the impact of short-term drought on grassland and shrubland ecosystems. With a standardized approach, we imposed ~a single year of drought at 100 sites on six continents. Here we show that loss of a foundational ecosystem function-aboveground net primary production (ANPP)-was 60% greater at sites that experienced statistically extreme drought (1-in-100-y event) vs. those sites where drought was nominal (historically more common) in magnitude (35% vs. 21%, respectively). This reduction in a key carbon cycle process with a single year of extreme drought greatly exceeds previously reported losses for grasslands and shrublands. Our global experiment also revealed high variability in drought response but that relative reductions in ANPP were greater in drier ecosystems and those with fewer plant species. Overall, our results demonstrate with unprecedented rigor that the global impacts of projected increases in drought severity have been significantly underestimated and that drier and less diverse sites are likely to be most vulnerable to extreme drought.

Keywords: Drought-Net; International Drought Experiment; climate extreme; productivity.

Fig. 1.

Geographic extent and climate space encompassed by the IDE. The 100 grasslands (green circles) and shrublands (brown triangles) included in the analysis spanned six continents (A) and broad gradients of mean annual temperature and MAP (B). Closed symbols denote sites (n = 44) that experienced statistically extreme 1-in-100-y drought (i.e., below average annual precipitation during the experiment year). Open symbols denote IDE sites (n = 56) that experienced nominal (not statistically extreme) drought (i.e., average or above-average annual precipitation during the experiment year). Photos: Shown are drought shelters at representative sites on each continent. Drought shelters were designed to exclude a fixed proportion of each rainfall event from the plots below. The proportion excluded was selected to impose a 1-in-100-y drought for each site during years with average annual precipitation (based on long-term precipitation records, see Materials and Methods for details; see SI Appendix, Table S2 for site codes).

Fig. 2.

Response of ANPP to a standardized 1-y drought for 100 IDE sites. (A) Mean drought response for sites that experienced extreme drought or nominal conditions for: all sites, grasslands, and shrublands (SI Appendix, Tables S3 and S4). The gray bar indicates the range of ANPP loss from −19 to −12.6% found in Song et al. and Wang et al. (20, 21), respectively. Drought response is calculated as: ln(average ANPP_DROUGHT/average ANPP_CONTROL); 0 (black dashed line) represents no effect of drought, and negative numbers indicate less ANPP in drought vs. control plots. (B) Mean drought response for each site ordered from negative (Top) to positive (Bottom). Site codes and corresponding site information are listed in SI Appendix, Table S2. Shown are 95% CIs for mean site-level drought responses. *Indicates site with CI that was omitted for clarity because it exceeds the x axis scale.

Fig. 3.

Effect of abiotic and biotic factor on drought response. Relationships between 1-y drought responses across IDE sites and (A) MAP, (B) previous year’s precipitation (relativized by MAP), (C) interannual percent coefficient of variation (CV) of MAP, (D) natural log of the AI, (E) percent sand, (F) average proportion of graminoids, and (G) average richness of plant species. Information on abiotic and biotic characteristics for each site can be found in SI Appendix, Table S5. Model results are summarized in SI Appendix, Table S6. Drought response is calculated as: ln(average ANPP_DROUGHT/average ANPP_CONTROL); 0 (black dashed line) represents no effect of drought, and negative numbers indicate less ANPP in drought vs. control plots. Lines are shown only for significant relationships. Shaded area represents the 95% CI. *Previous year’s precipitation included the precipitation in the 365 to 730 d preceding the biomass collection date and was relativized by MAP.

Fig. 4.

Relationship between drought severity and drought response. A linear mixed effects model found strong evidence for a negative effect of increasing drought severity (becoming more negative) on drought response for 1 y of drought across all sites (intercept = −0.11; slope = 0.53; P = 0.009; adjusted R² = 0.06). Model results for the effects of drought severity on drought response for extreme and nominal sites are summarized in the Main Text. Shaded area represents the 95% CI. Drought response is calculated as: ln(average ANPP_DROUGHT/average ANPP_CONTROL). For drought responses, 0 represents no effect of drought, negative numbers indicate less ANPP in drought vs. control plots. Drought severity is calculated as: (Precip_DROUGHT-MAP)/MAP; MAP = mean annual precipitation. Because ambient precipitation during the experiment year determines the severity of the imposed drought, positive drought severity can occur during anomalously wet years when plots beneath drought shelters also experience above average precipitation. The open symbols denote those IDE sites (n = 56) where ambient precipitation was above average, and thus the imposed drought was not statistically extreme (1-in-100 y). Closed symbols denote those sites (n = 44) with average or below average annual precipitation during the experiment year. All of these IDE sites experienced statistically extreme drought. The filled orange square denotes the mean drought response for sites experiencing extreme drought whereas the open orange square is the mean for sites that experienced less severe drought. Note that there was no relationship between drought severity and drought response when only those sites that experienced extreme drought are considered. The red dashed lines provide visual guides for 50% and 75% reductions in ANPP.