Ecologist/Data Analyst USDA Forest Service, RMRS Reno, Nevada, United States
In drylands, water-limited regions that cover ~40% of the global land surface, ecosystems are primarily controlled by access to soil moisture and exposure to simultaneously hot and dry conditions. However, these ecological drought patterns emerge from complex interactions among vegetation, climate and soil conditions that are not accurately represented by climate conditions alone. As a result, dryland climate impact assessments can be strengthened by incorporating ecologically-relevant drought metrics. Here, we introduce metrics that quantify variation in moisture and temperature conditions for dryland vegetation. We utilized a daily time step, multiple soil layer ecosystem water balance model to quantify these metrics across dryland ecosystems of the western United States. Expanding on the concept of growing degree days, we derived wet degree days (when soil is wet) and dry degree days (when soil is dry) limited to snow-free periods. We estimated soil water availability and climatic water deficit and used the daily results to quantify metrics that represent: overall growth potential, seasonal variability of drought conditions, seasonal timing of moisture and exposure to extreme drought (both short-term hot-dry events and chronic drought). We also developed metrics of potential suitability for recruitment of generalized dryland plants from seed.
Dry degree days and climatic water deficit were greatest in the Mojave desert, while wet degree days and soil water availability were highest in the southeastern Great Plains. The magnitude of seasonal variation in drought differed among metrics; seasonal fluctuation in soil water availability was largest in the southwest (Mojave desert) while fluctuation in climatic water deficit was largest in the northwest (northern Great Basin and Columbia Plateau). Seasonal timing of moisture also differed among metrics: timing of wet degree days, soil water availability and climatic water deficit were only weakly related to seasonal precipitation timing. Plant recruitment metrics varied strongly across western drylands. In the Great Plains, recruitment events occurred more frequently and lasted longer than in the intermountain regions, where recruitment events were comparatively rare and short. These ecological drought metrics provide new insight into patterns of soil moisture and temperature that shape dryland ecosystems. The metrics help us understand observed dynamics in dryland ecosystem structure and function, potentially through comparison with remotely sensed data. Likewise, the metrics will be useful for assessing the potential impact of climate change on dryland ecosystems and developing adaptive resource management strategies to sustain dryland ecosystem services in a changing world.