Fire management impacts on carbon storage in Southwest ponderosa pine forests

Background/Question/Methods

Forest carbon storage is increasingly used as a tool to offset greenhouse gas emissions; but many forests evolved with frequent fire, an integral ecological process, and a source of carbon emissions. Over the last century, fire has been suppressed in many dry, frequent-fire forests, leading to altered forest structure as stem density increased in these historically open forests.  Fire suppressed forests may store more carbon than forests maintained with regular fire, but they are high risk carbon sinks.  Fuel accumulation increases the likelihood of a stand-replacing wildfire, and severe wildfire can convert forests from carbon sinks to sources for decades. Wildfire risk reduction treatments including prescribed fire and thinning can reduce high-severity fire risk, but they also remove carbon from the forest, leading to tradeoffs between carbon stock size and stability.   We used the Forest Vegetation Simulator (FVS) to compare carbon storage over 100 years in no management (control), prescribed burning, thinning, and a combination of burning and thinning treatments for 15 ponderosa pine (Pinus ponderosa) stands at Camp Navajo, Arizona. These stands are managed to reduce wildfire risk and to provide habitat for the federally threatened Mexican spotted owl (Strix occidentalis lucida). 

Results/Conclusions

Restoration of forest structure and low-intensity surface fire typical of Arizona ponderosa pine forests comes with an initial carbon cost. Thinning treatments removed approximately 11.4 Mg C ha ^-1, and prescribed fire treatments applied every ten years emitted between 3.0 and 7.9 Mg C ha^-1 yr^-1.  Thinning moderated prescribed fire effects and reduced emissions on average 1 Mg C ha^-1 fire^-1 when compared to the burn only treatment. Carbon stocks in treated stands increased over the 100-year simulation period and the inclusion of prescribed burning was a significant influence on the projected carbon stock size.  Over the 100-year simulation carbon stocks increased by 99.0, 22.6, 67.4, and 28.0 Mg C ha^-1 in the control, burn, thin, and thin and burn, respectively.  Although carbon storage is maximized by fire suppression, a larger proportion of the carbon stored in the fire suppressed forest is at risk of loss to severe wildfire.  This reduction in carbon stock stability is especially important given the projected increasing trend for large wildfires due to climate change in the Southwest.  Ponderosa pine forests managed at lower densities with regular fire are at lower risk of severe wildfire, while continuing to provide a carbon sink.