Postdoctoral Scholar LTER Network, National Center for Ecological Analysis and Synthesis, United States
Climate is known to drive grassland aboveground net primary production (ANPP), and the global climate is known to be changing in multiple ways as atmospheric CO2 concentrations increase. However, the net effect of increasing CO2 and changing climatic conditions on grassland ANPP remains unresolved. Aboveground net primary production (ANPP) is an integrated indicator of overall ecosystem function and a key component of ecosystem carbon cycling which can feedback to atmospheric drivers of climate change (CO2). Understanding ANPP, its drivers, and the driver–ANPP relationship is therefore important for understanding and predicting the impacts of climate change. We used long-term (~40 year) records of climate and ANPP to determine what specific aspects of temperature and precipitation drive ANPP as well as how those drivers, ANPP, and the relationship between ANPP and its drivers have changed over time. Because different types of grasslands and different topographic positions within grasslands have been shown to respond differently to precipitation, we used datasets from two topographic positions (upland and lowland) at two different grasslands (mesic tallgrass prairie and semi-arid shortgrass steppe) at opposite ends of the US Great Plains precipitation gradient.
Our analysis confirms that precipitation amount is a strong driver of ANPP at both topographic positions in both grasslands. Model selection indicated that total precipitation and average dry period length during the growing season best explained inter-annual variation in ANPP at all sites (except the shortgrass steppe lowlands, where total precipitation was replaced by the number of large precipitation events in the best-fit model). Further, we found that total precipitation, average precipitation event size, the number of large precipitation events, and the daily minimum temperature during the growing season have increased at the tallgrass prairie over the last ~40 years while the number of precipitation events during the growing season decreased at the shortgrass steppe. Over the same time, ANPP increased in the tallgrass prairie lowlands and the shortgrass steppe uplands, leading to divergence between topographic positions at the tallgrass prairie and convergence between topographic positions at the shortgrass steppe. This suggests a role of factors other than climate in determining shortgrass steppe upland ANPP, which did not respond as expected based on our best-fit climate model. Finally, we found no evidence that the relationship between precipitation and ANPP has changed over time at either topographic position at either grassland.