Functional response of U.S. grasslands to the early 21st‐century drought

Abstract

Grasslands across the United States play a key role in regional livelihood and national food security. Yet, it is still unclear how this important resource will respond to the prolonged warm droughts and more intense rainfall events predicted with climate change. The early 21st-century drought in the southwestern United States resulted in hydroclimatic conditions that are similar to those expected with future climate change. We investigated the impact of the early 21st-century drought on aboveground net primary production (ANPP) of six desert and plains grasslands dominated by C4 (warm season) grasses in terms of significant deviations between observed and expected ANPP. In desert grasslands, drought-induced grass mortality led to shifts in the functional response to annual total precipitation (P(T)), and in some cases, new species assemblages occurred that included invasive species. In contrast, the ANPP in plains grasslands exhibited a strong linear function of the current-year P(T) and the previous-year ANPP, despite prolonged warm drought. We used these results to disentangle the impacts of interannual total precipitation, intra-annual precipitation patterns, and grassland abundance on ANPP, and thus generalize the functional response of C4 grasslands to predicted climate change. This will allow managers to plan for predictable shifts in resources associated with climate change related to fire risk, loss of forage, and ecosystem services.

Discerning “Flavors” of Drought Using Climate Extremes Indices

“…Holden et al (2007) found the timing and intensity of precipitation events rather than precipitation totals to be the best predictors of historical variability in wildfire severity in a southwestern U.S. forest ecosystem. Similar results have been found in semiarid grassland ecosystems where metrics measuring precipitation timing, intensity, and frequency were all better predictors of overall soil moisture status and plant growth at multiple time scales (Loik et al 2004;Lauenroth and Bradford 2012;Zhang et al 2013;Hottenstein et al 2015;Moran et al 2014;Gremer et al 2015;Barnes et al 2016). Soil moisture levels are often cited as the best indicator of potential drought status given their direct connection to plant health in ecosystems and agricultural systems, fuel moisture levels, and interactions with surface and subsurface hydrological conditions, but observations are very limited, and new networks are expensive to install and maintain (Legates et al 2011;Quiring et al 2016).…”

“…These types of soil moisture-related drought impacts would be more akin to ''agricultural'' droughts, which occur on the order of months to seasons, even though the time scale of analysis is at the annual scale. The lack of systematically collected drought-impact data for the study period in this region precluded a rigorous evaluation of the type and time scale of drought impacts to which each index best relates, but studies referenced previously using similar indices lend credence that they relate to soil moisture-related impacts such as seasonal vegetation production and stress, wildfire activity, and the effectiveness of precipitation at recharging and maintaining soil moisture levels (Holden et al 2007;Dettinger et al 2011;Zhang et al 2013;Hottenstein et al 2015;Moran et al 2014). …”

“…Drought indices typically use monthly-scale data and often focus on moving windows like in the standardized precipitation index (SPI; McKee et al 1995) or integrate conditions over seasonal and longer time scales like in the Palmer drought severity index (PDSI; Heim 2002). The seasonal timing, frequency, and intensity of precipitation events can often be as important as or more important than precipitation totals tracked in typical indices relative to different types of drought impacts on ecosystems and hydrological conditions (Holden et al 2007;Dettinger et al 2011;Hottenstein et al 2015;Moran et al 2014). Holden et al (2007) found the timing and intensity of precipitation events rather than precipitation totals to be the best predictors of historical variability in wildfire severity in a southwestern U.S. forest ecosystem.…”

“…The IPCC (2012) notes that increasing extremes are pronounced in modeling projections and may lead to shifts in hydroclimatic variability such as increasing numbers of dry days and longer dry spells in some regions, including the western United States. Some of these indices have been incorporated into recent ecosystem studies as well, as it has been noted that precipitation extremes perform better at explaining ecosystem productivity and emergent drought impacts (Zhang et al 2013;Hottenstein et al 2015;Moran et al 2014). Even though these indices to date have been largely applied in assessments of regional climate change and in modeling-output studies, they have direct utility in drought monitoring and as complementary drought indices to track subtle shifts in intra-annual hydroclimatic variability.…”

Integrating precipitation, grazing, past effects and interactions in long-term vegetation change

“…Another factor that may be important are lag effects (Walker, 1993;Wiegand and Milton, 1996). Lags at short time scales, such as the effect of the previous year's precipitation, have been shown to affect plant diversity (Adler and Levine, 2007), while cover for shrubs and grasses were shown to be correlated with precipitation from two to four years previous (Anderson and Inouye, 2001) and productivity of desert grasses can take 2e4 years to recover to expected levels after drought (Moran et al, 2014).…”