Ecosystem resilience to invasion and drought: Insights after 24 years in a rare never-grazed grassland

Authors: DUNIWAY, MICHAEL - Us Geological Survey (USGS); FINGER-HIGGINS, REBECCA - Us Geological Survey (USGS); GEIGER, ERIKA - Us Geological Survey (USGS); Hoover, David; PFENNIGWERTH, ALIX - Us Geological Survey (USGS); KNIGHT, ANNA - Us Geological Survey (USGS); VAN SCOYOC, MATT - National Park Service; MILLER, MARK - National Park Service

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2023
Publication Date: N/A
Citation: N/A

Interpretive Summary: Drylands are threated by climate change through increased aridifcation and drought. In this study, we examined plant community dynamics over time (1997-2021) in a unique, semi-arid grassland that has never been grazed. This allowed us to examine the effects of two global change drivers (climate change and invasion) in the absence of grazing, which is the dominant land use in this region. We observed plant community temporal fluctuations, likely responding to regional climate variability, and a remarkable resilience of native perennial grasses to climate and invasion. However, we observed recent declines in native plant cover, with ongoing megadrought, leaving questions regarding the resiliency of these grasslands to ongoing aridification.

Technical Abstract: Our understanding of the potential resilience of many plant communities globally is hampered by the large footprint and complex novel drivers of change characteristic of the Anthropocene. This is particularly true for drylands of the western US, where an abrupt and widespread alteration of disturbance regimes and spread of invasive non-native species occurred with westward expansion during the 1800s, including disturbance and grazing by domestic livestock and spread of Bromus tectorum, an invasive non-native annual grass. In addition, the western US is in a 20-year long ‘megadrought,’ not seen for 1200 years. We present 24 years of twice-annual monitoring (1997-2021) from a semi-arid grassland that was never grazed by domestic livestock but was subject to a patchy invasion of B. tectorum just prior to the study and compare our findings to surveys done in 1967. The study area has experienced a strong and significant warming trend, particularly in the summer and fall, with more than 75% of study year temperatures being warmer than average. We observed plant community responses to climate variability, with native perennial grasses showing a remarkable resilience to climate forcings with cover values like those in 1967. In invaded patches, B. tectorum cover was greatest in the early years (~20-40%) but was subsequently constrained by climate and subtle variation in soils, with limited evidence of long-term impacts to native vegetation, contradicting earlier studies. Our ability to predict year-to-year variation in functional group and species cover with climate metrics varied, with a 12-month integrated climate index and fall and winter patterns appearing most important. However, precipitous declines in cover in recent years in response to regional drought intensification leave questions regarding the resiliency of intact grasslands to ongoing aridification and whether the vegetation observations reported here may be a lagging indicator of impending change in this protected ecosystem.