• Energy Research

SummaryShrub encroachment, forest decline and wildfires have caused large‐scale changes in semi‐arid vegetation over the past 50 years. Climate is a primary determinant of plant growth in semi‐arid ecosystems, yet it remains difficult to forecast large‐scale vegetation shifts (i.e. biome shifts) in response to climate change. We highlight recent advances from four conceptual perspectives that are improving forecasts of semi‐arid biome shifts. Moving from small to large scales, first, tree‐level models that simulate the carbon costs of drought‐induced plant hydraulic failure are improving predictions of delayed‐mortality responses to drought. Second, tracer‐informed water flow models are improving predictions of species coexistence as a function of climate. Third, new applications of ecohydrological models are beginning to simulate small‐scale water movement processes at large scales. Fourth, remotely‐sensed measurements of plant traits such as relative canopy moisture are providing early‐warning signals that predict forest mortality more than a year in advance. We suggest that a community of researchers using modeling approaches (e.g. machine learning) that can integrate these perspectives will rapidly improve forecasts of semi‐arid biome shifts. Better forecasts can be expected to help prevent catastrophic changes in vegetation states by identifying improved monitoring approaches and by prioritizing high‐risk areas for management.

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.

Simulated responses of big sagebrush ecosystems to increased precipitation intensity. Here we use an individual plant-based ecohydrological model (STEPWAT2) to simulate water cycling and shrub, grass, and forb growth for 200 sites across the western United States, and test the effects of simulations conducted with 25%, 50% and 100% increases in precipitation event sizes without changing annual precipitation amounts. Simulations were also performed for 3 °C and 5 °C warming, and four soil textures. The files included in this repository are summarized output from model simulations. The include mean biomass responses for each sites (and treatment combination). Additionally, for each site and treatment values of numerous ecohydrological values are provided (e.g. transpiration, drainage, evaporation). Where relevant these include values for each of 8 soil layers. In the case of transpiration, values for are also provided for each plant functional type. Analyses using these data show that: Precipitation intensification generally increased shrub growth in arid and semi-arid sites, but not mesic sites, by decreasing evaporation and ‘pushing’ water deeper into the soil. This effect partially counteracted the mostly negative effects of warming on shrub growth. In contrast, forbs and grasses did not exhibit consistent responses to precipitation intensification. Overall precipitation intensification provided a competitive advantage to shrub growth under a wide range of aridity, soil textures, and temperatures and can be expected to contribute to the woody plant encroachment that has been observed around the world in the past 50 years.