Seasonal drought treatments impact plant and microbial uptake of nitrogen in a mixed shrub grassland on the Colorado Plateau.

For many drylands, both long- and short-term drought conditions can accentuate landscape heterogeneity at both temporal (e.g., role of seasonal patterns) and spatial (e.g., patchy plant cover) scales. Furthermore, short-term drought conditions occurring over one season can exacerbate long-term, multidecadal droughts or aridification, by limiting soil water recharge, decreasing plant growth, and altering biogeochemical cycles. Here, we examine how experimentally altered seasonal precipitation regimes in a mixed shrub grassland on the Colorado Plateau impact soil moisture, vegetation, and carbon and nitrogen cycling. The experiment was conducted from 2015 to 2019, during a regional multidecadal drought event, and consisted of three precipitation treatments, which were implemented with removable drought shelters intercepting ~66% of incoming precipitation including: control (ambient precipitation conditions, no shelter), warm season drought (sheltered April-October), and cool season drought (sheltered November-March). To track changes in vegetation, we measured biomass of the dominant shrub, Ephedra viridis, and estimated perennial plant and ground cover in the spring and the fall. Soil moisture dynamics suggested that warm season experimental drought had longer and more consistent drought legacy effects (occurring two out of the four drought cycles) than either cool season drought or ambient conditions, even during the driest years. We also found that E. viridis biomass remained consistent across treatments, while bunchgrass cover declined by 25% by 2019 across all treatments, with the earliest declines noticeable in the warm season drought plots. Extractable dissolved inorganic nitrogen and microbial biomass nitrogen concentrations appeared sensitive to seasonal drought conditions, with dissolved inorganic nitrogen increasing and microbial biomass nitrogen decreasing with reduced soil volumetric water content. Carbon stocks were not sensitive to drought but were greater under E. viridis patches. Additionally, we found that under E. viridis, there was a negative relationship between dissolved inorganic nitrogen and microbial biomass nitrogen, suggesting that drought-induced increases in dissolved inorganic nitrogen may be due to declines in nitrogen uptake from microbes and plants alike. This work suggests that perennial grass plant-soil feedbacks are more vulnerable to both short-term (seasonal) and long-term (multiyear) drought events than shrubs, which can impact the future trajectory of dryland mixed shrub grassland ecosystems as drought frequency and intensity will likely continue to increase with ongoing climate change.

Finger-Higgens R ，Hoover DL ，Knight AC ，Wilson SL ，Bishop TBB ，Reibold R ，Reed SC ，Duniway MC ... -  《-》

Droughting a megadrought: Ecological consequences of a decade of experimental drought atop aridification on the Colorado Plateau.

Global dryland vegetation communities will likely change as ongoing drought conditions shift regional climates towards a more arid future. Additional aridification of drylands can impact plant and ground cover, biogeochemical cycles, and plant-soil feedbacks, yet how and when these crucial ecosystem components will respond to drought intensification requires further investigation. Using a long-term precipitation reduction experiment (35% reduction) conducted across the Colorado Plateau and spanning 10 years into a 20+ year regional megadrought, we explored how vegetation cover, soil conditions, and growing season nitrogen (N) availability are impacted by drying climate conditions. We observed large declines for all dominant plant functional types (C3 and C4 grasses and C3 and C4 shrubs) across measurement period, both in the drought treatment and control plots, likely due to ongoing regional megadrought conditions. In experimental drought plots, we observed less plant cover, less biological soil crust cover, warmer and drier soil conditions, and more soil resin-extractable N compared to the control plots. Observed increases in soil N availability were best explained by a negative correlation with plant cover regardless of treatment, suggesting that declines in vegetation N uptake may be driving increases in available soil N. However, in ecosystems experiencing long-term aridification, increased N availability may ultimately result in N losses if soil moisture is consistently too dry to support plant and microbial N immobilization and ecosystem recovery. These results show dramatic, worrisome declines in plant cover with long-term drought. Additionally, this study highlights that more plant cover losses are possible with further drought intensification and underscore that, in addition to large drought effects on aboveground communities, drying trends drive significant changes to critical soil resources such as N availability, all of which could have long-term ecosystem impacts for drylands.

Finger-Higgens R ，Bishop TBB ，Belnap J ，Geiger EL ，Grote EE ，Hoover DL ，Reed SC ，Duniway MC ... -  《-》

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

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.

Winkler DE ，Belnap J ，Hoover D ，Reed SC ，Duniway MC ... -  《-》

High land-use intensity exacerbates shifts in grassland vegetation composition after severe experimental drought.

Climate change projections anticipate increased frequency and intensity of drought stress, but grassland responses to severe droughts and their potential to recover are poorly understood. In many grasslands, high land-use intensity has enhanced productivity and promoted resource-acquisitive species at the expense of resource-conservative ones. Such changes in plant functional composition could affect the resistance to drought and the recovery after drought of grassland ecosystems with consequences for feed productivity resilience and environmental stewardship. In a 12-site precipitation exclusion experiment in upland grassland ecosystems across Switzerland, we imposed severe edaphic drought in plots under rainout shelters and compared them with plots under ambient conditions. We used soil water potentials to scale drought stress across sites. Impacts of precipitation exclusion and drought legacy effects were examined along a gradient of land-use intensity to determine how grasslands resisted to, and recovered after drought. In the year of precipitation exclusion, aboveground net primary productivity (ANPP) in plots under rainout shelters was -15% to -56% lower than in control plots. Drought effects on ANPP increased with drought severity, specified as duration of topsoil water potential ψ < -100 kPa, irrespective of land-use intensity. In the year after drought, ANPP had completely recovered, but total species diversity had declined by -10%. Perennial species showed elevated mortality, but species richness of annuals showed a small increase due to enhanced recruitment. In general, the more resource-acquisitive grasses increased at the expense of the deeper-rooted forbs after drought, suggesting that community reorganization was driven by competition rather than plant mortality. The negative effects of precipitation exclusion on forbs increased with land-use intensity. Our study suggests a synergistic impact of land-use intensification and climate change on grassland vegetation composition, and implies that biomass recovery after drought may occur at the expense of biodiversity maintenance.

Stampfli A ，Bloor JMG ，Fischer M ，Zeiter M ... -  《-》

Response of dominant grass and shrub species to water manipulation: an ecophysiological basis for shrub invasion in a Chihuahuan Desert grassland.

Throop HL ，Reichmann LG ，Sala OE ，Archer SR ... -  《-》