COS 6-3 - Conifer expansion in semi-arid grasslands increases ecosystem productivity and exacerbates nitrogen limitation and episodic carbon and nitrogen losses from fire

Abstract: Extensive areas of semi-arid grasslands have been increasing in productivity and declining in foliar nitrogen over the last several decades. While much of this greening is due to CO2 fertilization and climate change, less is understood about how woody plant expansion (WPE) has contributed to this trend. Tree cover is thought to have increased largely due to fire suppression but the ecosystem level consequences of reintroducing fire versus wildfire for plant-soil C, N, and P cycles are poorly understood. Here, we combine remote sensing and field-based measures to examine the biogeochemical effects of coniferous WPE and its interaction with fire in the Northern Great Plains (NGP) of North America. We leveraged an extensive plot network distributed across moisture gradients and chronosequences of prescribed burns and wildfires to examine the effects of WPE and fire on ecosystem C, N, and P storage and cycling. We developed statistical and process-based models constrained by dendrochronology, plant and soil chemistry, and natural abundance isotope distributions to evaluate the past, present, and future alternative ecosystem states in semi-arid grasslands of the NGP.

We find that >100 years of conifer expansion into semi-arid grasslands has resulted in large increases in biomass stores of C, N, and P and declines in soil N, but mixed results for soil C and P. Mass balance calculations indicate that much of decline in soil N could be explained by sequestration into slow-turnover wood and foliar N pools. This effect was disproportionately driven by relatively N-rich conifer species. Reintroducing fire via experimental burning after >60 years of fire absence resulted in large volatization losses of aboveground C and N, but significant increase in surface soil C and nutrients, while repeated burning and wildfires resulted in long-term declines in soil C and N relative to unburned controls and grasslands. C and N stable isotope distributions among species and between foliage and soil further support that conifer expansion has fundamentally altered N availability across the landscape. Our biogeochemical measures and long-term records of individual stem growth indicate strong co-limitation of plant productivity by water and N availability. While precipitation has been increasing in recent decades, warming and increasing drought severity suggest increasing probability of wildfire with increasing conifers, which may exacerbate N limitation and lead to alternative ecosystem states.