We deconvolved local versus regional controls on multiple ecosystem responses to climate change in Pacific Northwest prairies along a 520-km latitudinal climate gradient in three sites located in southern Oregon (SOR), central Oregon (COR), and Central Washington (WA). At each site we implemented a fully-factorial treatment combination of 2.5-3oC warming and 20% added precipitation intensity. Climate treatments spanned 21-27 months. We examined (1) above- and belowground net primary productivity (NPP), phenology, soil carbon (C) pools and mineralization, soil respiration, and nutrient cycling and availability; (2) whether responses were consistent along the climate gradient or site-dependent; and (3) whether we could explicitly link belowground to aboveground responses. Aboveground NPP was measured as peak standing biomass and belowground NPP with in-growth root cores. Phenology of green biomass was estimated bi-weekly with NDVI. Nitrogen (N) and other nutrient availability was sampled seasonally with resin strips. Soil respiration was measured monthly with an infrared gas analyzer. Carbon and N pools were sampled seasonally, including total C and N content, C fractions measured through density fractionation, and seasonal labile indices including microbial biomass C through chloroform fumigation, 24 hour C mineralization rate, and water extractable C; we also assessed long-term C mineralization potentials through incubation.
Results/Conclusions
Warming dominated the effects and overall there were few effects of precipitation intensity. Warming led to significant increases in both above- and belowground NPP in all sites and years. Heated plots displayed higher peak greenness, decreased winter senescence, and earlier summer senescence. Both the NPP and phenology responses were consistent along the climate gradient. Warming effects on N and P availability were site specific, but overall plant productivity increases were likely due to changes in phenology and soil nutrient availability. In 2011, annual soil efflux increased significantly due to warming in COR and WA, though not in SOR. However, there was no effect of warming on total soil C and N stocks, N and C content of density fractions, or labile soil C pools despite the increase in total NPP. This disconnect between belowground and aboveground C inputs and outputs is likely due to the increased inputs being either too small to detect or being rapidly decomposed as litter, while losses due to efflux after ~2 yrs of warming are likely still too small to be detected in the overall soil C pools.