Long-term Decline in Grassland Productivity Driven by Increasing Aridity

Abstract:

The ability of terrestrial ecosystems to increase net primary production in response to rising CO₂ requires that the availability of water and other resources keep pace with increasing evaporative demand due to warming. While climate models project increasing aridity and drought severity for many areas, our understanding of the potential effects of drying on ecosystems is uncertain and largely based on satellite observations and short-term experiments. With limited long-term field data it is difficult both to distinguish effects of transient departures from climatological norms (drought) from monotonic changes to background aridity and to separate the effects of drying from other factors (e.g., carbon dioxide and nitrogen (N)) that might ameliorate responses to increased evaporative demand. Here, we use a long-term harvest record to document a >50% reduction in primary production of a native subalpine grassland over the last four decades and link this to increasing regional aridity. Declines in plant productivity occurred across C3 grasses and non-graminoid annual forbs but were most severe in forbs. Using the statistical method of convergent cross mapping (CCM) in combination with a long-term snow-addition experiment and climatological and biogeochemical data, we assign the timing and mode of forcing to increasing late summer dryness. Analysis of long-term patterns of carbon and nitrogen stable isotopes of plants and regional patterns of atmospheric N deposition suggest that increasing water use efficiency may have slightly ameliorated the decline in grasses but not forbs and that N availability had no effects. Our results demonstrate a sustained climate-induced decline in the productivity of a grassland ecosystem and underscore the importance of long-term monitoring.