Ponderosa pine ecosystems are widespread in western North America and overlap with regions expected to see future increases in wildfire frequency, severity, and extent. Regeneration in ponderosa pine forests is episodic and depends on nearby live canopy seed sources. Therefore, wildfires that produce large continuous patches with high burn severity could instigate an ecosystem transition from ponderosa pine to shrubland or grassland. We examined the likelihood and the carbon (C) cycle implications of such a transition by measuring tree regeneration and soil respiration from 2014 to 2016 in three wildfires (4, 14, and 20 years old) that burned during the last 20 years in the Front Range of the Rocky Mountains in Colorado, USA.
Results/Conclusions
Tree regeneration, >10 years following wildfire, in areas burned at high severity was extremely low (118 seedlings ha-1) and declined with distance from the forest edge (P<0.0001). Both distance from a seed source and elevation were significant predictors of regeneration density. Beyond 100 m from a seed source, regeneration dropped to 70-80 seedlings ha-1 and very few seedlings were observed beyond 200 m. Burn severity mattered: regeneration and respiration trends in stands that experienced only a surface fire were more similar to unburned forests than to areas that had experienced crown fire. Severity (P=0.07) and time since fire (P=0.09) both negatively influenced soil respiration, but responses were highly variable. This was because soil respiration in burned areas was positively related to both live tree basal area and productivity of understory vegetation (r2=0.27 on multiple regression), which exhibited high spatial variability. However, post-fire productivity rates of understory vegetation (mean ANPP of 1.2 Mg C ha-1y-1) are likely too low in the 4-yr old burn to compensate for losses of C via soil respiration (~ 4 to 8 Mg C ha-1y-1). In addition, the lack of tree regeneration in these areas reduces the potential for longer-term C sequestration into wood.
Scaling these results based on landscape patterns of burn severity and regeneration suggests that large expanses of crown fire in ponderosa pine forests will have long-lasting impacts on regional C cycle budgets. We urge that variability in regeneration strategies and rates among different forest types, as well as spatially resolved burn severity should be incorporated into ecosystem modeling efforts to more completely understand the implications of climate-driven changes in fire regimes.