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

Authors: WINKLER, DANIEL - Us Geological Survey (USGS); BELNAP, JAYNE - Us Geological Survey (USGS); Hoover, David; REED, SASHA - Us Geological Survey (USGS); DUNIWAY, MICHAEL - Us Geological Survey (USGS)

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/23/2019
Publication Date: 6/10/2019
Citation: Winkler, D., Belnap, J., Hoover, D.L., Reed, S., Duniway, M. 2019. Shrub persistence and increased grass mortality in response to drought in dryland systems. Global Change Biology. 25:3121-3135. https://doi.org/10.1111/gcb.14667.
DOI: https://doi.org/10.1111/gcb.14667

Interpretive Summary: Droughts in the southwestern US are getting worse with climate change and understanding which plants are most vulnerable is key to predicting the future of these dryland ecosystems. We examined the effects of drought on plant mortality using experimental rainout shelters over a 8 year period. We found that the grasses were most vulnerable to drought while shrubs were resistant. Furthermore, we found that mortality increased in the presence of invasive plants. These results provided insight into the 'new normal' of climate change in this region.

Technical Abstract: Droughts in the Southwest U.S. have led to major forest and grassland die-off events in recent decades, suggesting plant community and ecosystem shifts are imminent as native perennial 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 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. To capture variability among community and soil types, we distributed plots across numerous plant, soil, and parent material types, resulting in 40 distinct sites across a 4500 km2 region of the Colorado Plateau Desert. For eight years we tracked ca. 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 drought. Importantly, drought-induced mortality increased in the presence of invasive species, and native plant die-off occurred even under ambient conditions, suggesting that recent climate changes are already negatively impacting dominant species. 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.