Schmid College of Science and Technology, Chapman University Orange, California, United States
Evidence is accumulating that drought not only reduces the total amount of ecosystem productivity, but also alters its timing. This may have unexpected consequences for ecosystem structure and functioning. In particular, the seasonality of drought impacts on ecosystem productivity, and the generality of these impacts, remains unclear. Such effects may be especially pronounced in grasslands, which are highly sensitive to changes in water availability. In this study, we asked: How are the intra-annual dynamics of grassland primary productivity impacted by drought? We used remote sensing datasets of MODIS-based gross primary productivity and weather (Daymet) across two expansive grassland ecoregions of the western US: the C4-dominated shortgrass steppe and the C3-dominated northern prairies. We focused on understanding when drought, defined as the driest year in the long-term record, alters productivity during the growing season. To this end, we quantified changes to the temporal patterns of gross primary productivity during the growing season across each ecoregion.
We find widespread evidence that drought consistently confines gross primary productivity to the early growing season and reduces productivity primarily during the mid-to-late growing season. Analysis of the temporal patterns of cumulative gross primary productivity during drought indicate that productivity actually keeps pace with average years during the early growing season and even increases productivity in the northern prairies. However, productivity then slows during drought in the mid- and late parts of the growing season, presumably when water limitation reaches a maximum. Across 90% of the sites (pixels), the day at which 50% of total productivity was accumulated was reached earlier during drought, with this growth milestone occurring earlier across the shortgrass steppe (median = 22 days) than the northern prairies (median = 11 days). Qualitatively similar results were observed for the day at which 25% of growth occurs. In total, we find that 1) drought-induced reductions in productivity are largest during the middle and late growing season and 2) drought advances the day at which key growth milestones are achieved across US grasslands. The consistent seasonal signature in the effects of drought on grassland functioning has implications for time-sensitive management decisions across increasingly drought-stricken western US grasslands and rangelands.