Soil and stand structure explain shrub mortality patterns following global change–type drought and extreme precipitation

Abstract

2019. Soil and stand structure explain shrub mortality patterns following global change-type drought and extreme precipitation. Ecology 100(12):Abstract. The probability of extreme weather events is increasing, with the potential for widespread impacts to plants, plant communities, and ecosystems. Reports of drought-related tree mortality are becoming more frequent, and there is increasing evidence that drought accompanied by high temperatures is especially detrimental. Simultaneously, extreme large precipitation events have become more frequent over the past century. Water-limited ecosystems may be more vulnerable to these extreme events than other ecosystems, especially when pushed outside of their historical range of variability. However, drought-related mortality of shrubs-an important component of dryland vegetation-remains understudied relative to tree mortality. In 2014, a landscape-scale die-off of the widespread shrub, big sagebrush (Artemisia tridentata Nutt.), was reported in southwest Wyoming, following extreme hot and dry conditions in 2012 and extremely high precipitation in September of 2013. Here we examine how severe drought, extreme precipitation, soil texture and salinity, and shrub-stand characteristics contributed to this die-off event. At 98 plots within and around the die-off, we quantified big sagebrush mortality, characterized soil texture and salinity, and simulated soil-water conditions from 1916 to 2016 using an ecosystem water-balance model. We found that the extreme weather conditions alone did not explain patterns of big sagebrush mortality and did not result in extreme (historically unprecedented) soil-water conditions during the drought. Instead, plots with chronically dry soil conditions experienced greatest mortality following the global change-type (hot) drought in 2012. Furthermore, mortality was greater in locations with high potential run-on and low potential run-off where saturated soil conditions were simulated in September 2013, suggesting that extreme precipitation also played an important role in the die-off in these locations. In locations where drought alone contributed to mortality, stem density negatively impacted big sagebrush. In locations that may have been affected by both drought and saturation, however, mortality was greatest where stem density was lowest, suggesting that these locations may have already been less favorable to big sagebrush. Paradoxically, vulnerability to both extreme events (drought and saturation) was associated with finertextured soils, and our results highlight the importance of soils in determining local variation of the vulnerability of dryland plants to extreme events.

Robust ecological drought projections for drylands in the 21st century

“…Our results indicate substantial potential for rapidly increasing hot‐dry stress, particularly in the northern portions of these drylands. Stressful combinations of high temperature and low soil moisture have been shown to drive mortality of trees (Breshears et al, 2018), and may also apply to other plants (Renne et al, 2019). Our results reinforce other studies anticipating the importance of increasing heat stress for dryland plants (McDowell et al, 2016), and our focus on periods of co‐occurring extreme heat and low soil moisture provides new perspectives about patterns in future hot‐dry stress.…”

“…Barcharts show the proportion of each ecoregion with robust signal (>90% of models agreeing) of positive or negative changes in monthly SWA under the four future scenarios (colors consistent with lines) Our results indicate substantial potential for rapidly increasing hot-dry stress, particularly in the northern portions of these drylands. Stressful combinations of high temperature and low soil moisture have been shown to drive mortality of trees (Breshears et al, 2018), and may also apply to other plants (Renne et al, 2019).…”

“…Dryland ecosystems are tightly linked to water availability, so future shifts in overall aridity may substantially alter dryland plant communities and the ecosystem services they provide (Bestelmeyer et al, 2015; Reynolds et al, 2007). Vegetation dynamics in dryland regions are driven primarily by availability of soil moisture, which shapes the structure and function of plant communities by affecting photosynthesis, biomass production, plant mortality, and regeneration dynamics (Gremer, Bradford, Munson, & Duniway, 2015; Renne et al, 2019; Schlaepfer et al, 2015; Shriver et al, 2018; Thoma, Munson, & Witwicki, 2018; Witwicki, Munson, & Thoma, 2016). Drought stress can leave ecosystems vulnerable to rapid species turnover and altered structure by reducing plant growth and regeneration.…”

Water ‘on the rocks’: a summer drink for thirsty trees?

“…A frequently overlooked determinant of ecosystem hydrology is the rock component, including bedrock features (Schwinning, 2010; Dawson et al ., 2020). Several studies investigating forest responses to drought were focused on habitats with relatively deep soils that contribute to the largest share of water available to plants over the growing season (Elliott & Swank, 1994; Fensham & Fairfax, 1997; Renne et al ., 2019). However, large portions of terrestrial habitats are dominated by forests on shallow soils overlaying substrates that strongly limit root growth and water storage.…”

Climate-Driven Shifts in Soil Temperature and Moisture Regimes Suggest Opportunities to Enhance Assessments of Dryland Resilience and Resistance

“…For example, severe drought events can cause dryland plant mortality and decrease productivity in surviving individuals during subsequent years (Bigler et al, 2007;Bradford and Bell, 2017). At the other extreme, unusual wet conditions can interact with soil and stand characteristics to contribute to dryland plant mortality (Renne et al, 2019) as well as facilitate regeneration of perennial plants (Shriver et al, 2018), a notoriously episodic process (Schlaepfer et al, 2014;Petrie et al, 2016). Because the frequency and severity of extreme events can influence an ecosystem's composition, structure and susceptibility to biological invaders (Bradley et al, 2010;Reichstein et al, 2013;Ummenhofer and Meehl, 2017), incorporating metrics that represent ecologically relevant extreme drought conditions may improve assessments of resistance to invasion.…”