Research Ecologist U.S. Geological Survey Moab, UT, United States
Background/Question/Methods Global climate warming has led to an increase in the intensity and frequency of extreme drought events with major implications for ecosystem structure and function. Yet, our knowledge of the sensitivity of ecosystem productivity, i.e., gross primary productivity (GPP), to drought extremes remains limited. This knowledge gap is especially large across highly spatial and temporal variability drylands, which are under-represented by long-term, continuous field measurements. During summer and autumn 2020, the US Southwest experienced a widespread extreme drought event, with record-low precipitation and record-high temperature across much of the region. Here, we evaluate the extent and intensity of GPP extremes resulting from this extreme drought event from the site to the regional scale. First, we evaluated site-level observations of multiple remotely sensed GPP proxies including solar-induced chlorophyll fluorescence (SIF), photochemical reflectivity index (PRI), and near infrared reflectance (NIRv) in a wet year (2019) and an extreme drought year (2020) at the semi-arid Kendall grassland eddy covariance flux tower site (US-WKG). Next, we scale up and evaluate remotely sensed GPP proxies across a representative synthesis of Southwest flux tower sites and for the full Southwest region.
Results/Conclusions At the site level, we found that tower-based SIF most accurately capture seasonal transitions in GPP dynamics during both the wet and extreme dry year. Scaling up to the regional level, we found that SIF and NIRv were the best performing satellite GPP proxies and captured complementary aspects of seasonal GPP dynamics across dryland ecosystem types. Next, using a novel combination of satellite proxies, we estimate that the 2020 US Southwest drought results in a 140 Tg C (~27%) reduction in GPP below the mean, by far the lowest regional GPP over the evaluated satellite record. We finally present evidence that satellite SIF offers improved ability to track seasonal GPP dynamics by capturing near instantaneous changes in plant physiological function from diurnal to seasonal timescales.