Woody plant expansion (WPE) into grasslands is a well-documented phenomenon yet the effects on ecosystem biogeochemical cycling remain poorly understood. For example, effects of WPE on ecosystem carbon (C) storage and associated soil cycling of nitrogen (N) and phosphorus (P) remain poorly constrained. Thus, recently observed trends in the increase of WPE and ecosystem productivity in the Northern Great Plains (NGP) have important considerations for 1. determining the trajectory of ecosystem carbon storage, 2. constraints on further WPE and 3. effects on future nutrient availability. Furthermore, fire is an important disturbance in the maintenance of temperate savanna structure and function, but recent widespread fire suppression has had ecological consequence. The introduction of prescribed fire to these landscapes presents a possible pathway to reduce WPE, despite limited knowledge of effects on C, N and P cycling. To investigate cumulative effects of fire and WPE on ecosystem N, C, and P pools in the NGP, we established 25 long-term monitoring plots in Central Montana, with known WPE establishment dates from tree core analysis. We employ both a space-for-time and pre/post/control experimental design across a grassland-forest gradient to evaluate short and long-term ecological effects of WPE during a BLM-lead prescribed fire in 2018.
Results/Conclusions
Our analyses integrate data from across multiple temporal and spatial scales that combine soil, foliar, tree core, and aerial imagery to address both long-term ecosystem effects of WPE and short-term effects from prescribed fire. Since the 1950s, we report a near doubling in tree cover across our 4,500 ha study area. Further, we note increases in aboveground and soil C pools with increasing tree cover. Conversely, we report large declines in soil N and P pools in response to WPE. These findings suggest that increases in ecosystem carbon storage may come at the cost of declining nutrient availability and potentially constrain further WPE. Examination of the above-and-below ground effects one-year post fire revealed 32% mortality in woody cover and 98% mortality of saplings. Further, we find the fire generated increases to mineral soil pools of C, N and P. Preliminary results show a parallel increase in the production of chemically stable pyrogenic carbon in-step with increasing fire severity, creating a heterogenous mosaic of mineral soil carbon stability across the burn zone. Together these results are important when considering prescribed fire for management of future projected WPE, and their cumulative and interactive effect on ecosystem nutrient cycling and C sequestration.