California Rangelands: Impacts of Drought on Net Primary Productivity (NPP)

Jeremy James, California Polytechnic State University, SLO Department of Natural Resources and Environmental Sciences

Jack Alexander, California Polytechnic State University, SLO

Mary McCafferty, California Polytechnic State University, SLO 

Andrew Fricker, California Polytechnic State University, SLO

Oak woodlands and related rangeland cover types comprise a large proportion of California’s land area and therefore play a key role in the state’s overall carbon budget.  Increasing variation in precipitation input and an overall drying climate, however, pose a serious threat to the ability of these ecosystems to store carbon.  In this study we quantified spatial variation in mean NPP and asymmetry of NPP response to periods of anomalously low and high water availability. Using Generalized Boosted Models (GBMs), we compared NPP with several biotic and abiotic predictors to identify variables strongly associated with mean NPP and NPP asymmetry at three different spatial scales (4 km, 270 m, and 30 m). We then incorporate those variables into a linear mixed-effects model to compare NPP drivers across 7 key rangeland vegetation cover types in California. Our preliminary results indicate that spatial variation in mean cumulative NPP across our 7 vegetation cover types was most highly correlated with mean early Spring precipitation, mean annual maximum temperature, and mean mid-Spring maximum temperature. Temporal variation in annual cumulative NPP was most highly correlated with annual maximum temperature, south-facing exposure, and precipitation variance from the mean.  Our 7 vegetation cover types exhibited substantial variation in NPP asymmetry, with increases in NPP during high water availability years greater in magnitude than decreases in NPP during low water availability years in Grassland areas. In other systems such as Oak Woodlands, however, NPP decreases during low water availability years were greater in magnitude than NPP increases during high water availability years.  Our results are consistent with previous findings that precipitation and maximum temperature are the primary drivers of NPP dynamics, but we also explore important fine scale variation in the patterns within vegetation cover types.  Additionally, we show that ecosystem carbon gain during anomalous wet years across the state are unlikely to balance carbon loss during anomalous dry years.