The scale of ecological drought in dryland restoration success

Authors: O'CONNER, R - Us Geological Survey (USGS); GERMINO, M - Us Geological Survey (USGS); Barnard, David; ANDREWS, C - Us Geological Survey (USGS); PILLIOD, D - Us Geological Survey (USGS); ARKLE, R - Us Geological Survey (USGS); SHRIVER, R - Us Geological Survey (USGS)

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/17/2020
Publication Date: 3/13/2020
Citation: O'Conner, R.C., Germino, M.J., Barnard, D.M., Andrews, C.M., Pilliod, D.S., Arkle, R.S., Shriver, R.K. 2020. The scale of ecological drought in dryland restoration success. Global Change Biology. https://doi.org/10.1088/1748-9326/ab79e4.
DOI: https://doi.org/10.1088/1748-9326/ab79e4

Interpretive Summary: Ecological drought is defined as drought events of sufficient length or severity to alter ecosystem structure or function. As arid lands experience increasing levels of drought, the need to understand the impacts on ecosystem services is of increasing importance to resource managers tasked with sustaining threatened ecosystems and restoring areas that have been degraded by disturbance. In this study, we focused on areas that had been burned by wildfire, and seeded with big sagebrush, during the second half of the 20th century and used records of weather variability and site-level measurements of soil and vegetation properties to model soil moisture availability at >600 plots across the Great Basin USA. Modeled estimates of soil moisture were compared to observations of sagebrush presence at all sites and it was determined that the probability of sagebrush reestablishment was improved significantly when the number of wet and above freezing days in March were greater than 7. Given the need for rapid reestablishment of sagebrush after fire, the identification of a specific time period and meteorological thresholds for seedling establishment will be of great importance for resource managers delegating resources and for improving the understanding of demographic dynamics of arid shrublands.

Technical Abstract: Ecological droughts are deficits in soil-water availability that induce threshold-like ecosystem responses, such as causing altered or degraded plant-community conditions, which can be exceedingly difficult to reverse. However, ‘ecological drought’ can be difficult to define, let alone to quantify, especially at spatial and temporal scales relevant to land managers. This is despite a growing need to integrate drought-related factors into management decisions as climate changes result in precipitation instability in many semi-arid ecosystems. We asked whether success in restoration seedings of the foundational species big sagebrush (Artemisia tridentata) was related to estimated water deficit, using the SoilWat2 model and data from >600 plots located in previously burned areas in the western United States. Water deficit was characterized by: 1) the standardized precipitation-evapotranspiration index (SPEI), a coarse-scale drought index, and 2) the number of days with wet and warm conditions in the near-surface soil, where seeds and seedlings germinate and emerge (i.e. days with 0-5 cm deep soil water potential > -2.5 MPa and temperature above 0 °C). SPEI, a widely used drought index, was not predictive of whether sagebrush had reestablished. In contrast, wet-warm days elicited a critical drought threshold response, with successfully reestablished sites having experienced 7 more wet-warm days than unsuccessful sites during the first March following summer wildfire and restoration. Thus, seemingly small-scale and short-term changes in water availability and temperature can contribute to major ecosystem shifts, as many of these sites remained shrubless two decades later. These findings help clarify the definition of ecological drought for a foundational species and its imperiled semi-arid ecosystem. Further, they demonstrate that soil-water deficits do not need to be long-term to constitute ecological drought.