Shrub persistence and increased grass mortality in response to drought in dryland systems

• Published in: Global change biology Vol. 25; no. 9; pp. 3121 - 3135
• Main Authors: Winkler, Daniel E., Belnap, Jayne, Hoover, David, Reed, Sasha C., Duniway, Michael C.
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.09.2019
• Subjects: Aridity; Bromus tectorum; cheatgrass; Climate change; Community composition; Deserts; Dominant species; Drought; Ecosystems; Environmental changes; graminoids; grass; Grasses; Grasslands; Herbivores; Indigenous plants; Indigenous species; Introduced species; Invasive plants; Invasive species; Moisture content; Mortality; parent material; perennials; Plant communities; Plateaus; populations; Precipitation; Salsola; Shrubs; Soil; Soil water; Soils; Colorado Plateau; shrubs; grass; perennials; parent material; cheatgrass; graminoids; Salsola; populations; Bromus tectorum; climate change
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.14667

Droughts in the southwest United States have led to major forest and grassland die‐off events in recent decades, suggesting plant community and ecosystem shifts are imminent as native perennial grass populations are replaced by shrub‐ and invasive plant‐dominated systems. These patterns are similar to those observed in arid and semiarid systems around the globe, but our ability to predict which species will experience increased drought‐induced mortality in response to climate change remains limited. We investigated meteorological drought‐induced mortality of nine dominant plant species in the Colorado Plateau Desert by experimentally imposing a year‐round 35% precipitation reduction for eight continuous years. We distributed experimental plots across numerous plant, soil, and parent material types, resulting in 40 distinct sites across a 4,500 km2 region of the Colorado Plateau Desert. For all 8 years, we tracked c. 400 individual plants and evaluated mortality responses to treatments within and across species, and through time. We also examined the influence of abiotic and biotic site factors in driving mortality responses. Overall, high mortality trends were driven by dominant grass species, including Achnatherum hymenoides, Pleuraphis jamesii, and Sporobolus cryptandrus. Responses varied widely from year to year and dominant shrub species were generally resistant to meteorological drought, likely due to their ability to access deeper soil water. Importantly, mortality increased in the presence of invasive species regardless of treatment, and native plant die‐off occurred even under ambient conditions, suggesting that recent climate changes are already negatively impacting dominant species in these systems. Results from this long‐term drought experiment suggest major shifts in community composition and, as a result, ecosystem function. Patterns also show that, across multiple soil and plant community types, native perennial grass species may be replaced by shrubs and invasive annuals in the Colorado Plateau Desert. A grand challenge in biology is forecasting which species will be most vulnerable to drought, given that drought severity is increasing and can vary due to the impacts of subtle interannual climate variability and species‐level tolerances. We show that long‐term experimental drought has led to major die‐offs of dominant species in the Colorado Plateau Desert. Our 8 year experiment used 40 sites distributed across climate, plant, and soil types to track the effects of subtle interannual climate variability. Ambient drought also resulted in widespread mortality, indicating that climate change is already impacting these dryland systems. Overall, results from this experiment suggest major shifts in ecosystem function, and also an increased ability to predict responses by understanding biotic–abiotic interactions.