(by Michael Gallagher, science & technology committee)

Prescribed burning is a common technique used to reduce fuels and mitigate risk to public safety or ecosystem services. Fuel reduction operations inherently affect forest carbon storage, through the consumption or redistribution of carbon rich aboveground biomass, or by influencing rates of carbon uptake and release. Short and long-term net effects of fuel treatments on carbon storage vary with forest type, treatment technique, and environmental factors, however understanding impacts of different techniques can provide context for forest management planning and the fine tuning of specific techniques.

Prescribed burn in the NJ pine barrens

photo courtesy of USDA Forest Service

Recent work, published by the US Forest Service in the Journal of Sustainable Forestry, quantified the effects of prescribe burning treatments of carbon dynamics and fuel loading in oak-pine, pine-oak, and pine-scrub oak stands in the New Jersey Pine Barrens, where prescribed burning treatments have been used by New Jersey Forest Fire Service to manage fire risk for roughly 100 years. Researchers evaluated long term trends in forest floor and shrub layer fuel loading, trends in net ecosystem CO2 exchange of treated and untreated stands, and amount of carbon in forest floor and shrub layer fuels. Shrub and canopy leaf area recovery was also evaluated post fire.

A survey of stands burned 2-45 years earlier found fine fuel and shrub fuel loading to track closely with time since burn. For instance, forest floor biomass in a fuel protection strip burned on a 2-3 year rotation averaged 530 grams per square meter, whereas forest floor biomass in a stand not burned since 1963 averaged 2338 grams per square meter. Shrub leaf mass, however, was found to maximize within 1-2 years post-burn. This was further confirmed by a rapid recovery leaf area (>95% of pre-burn conditions) within the first season of burning, and suggests a rapid recovery of productivity in this strata of the forest. Shrub stem mass accumulated steadily for 14 years following burns, before slowing down in rate. Shrub stem mass averaged 1174 grams per square meter in the oldest stand in the study (45 years). Downed woody material was poorly related to time-since fire.

Canopy leaf area was unevenly affected by prescribed fire, with some areas scorched and others relatively unaffected. Leaf area in more affected areas recovered to 79% of post burn conditions by the August following the burn. Ecosystem respiration of carbon was reduced compared to normal conditions in the first growing season post-fire, and seasonal expansion of new needles contributed to the recovery of the canopy’s photosynthetic capacity, especially where scorch was not a problem. Net accumulation of carbon for burned stands achieved C neutrality within in 3-4 years post burn (i.e. lost carbon was regained).

In the short term, prescribe burns burn off forest biomass and therefore carbon.   The amount burned off in a single burn is strongly related to pre-burn fuel loading, and can be estimated using simple equations prior to burning. Other equations can be used to estimate forest floor and shrub layer fuel loading from time since burn data. While shrub stem biomass and visible structure may take a long time to maximize, shrub leaf area recovers within the first season or two following treatments. Integrating the results of this study over the typical 5-8 year interval between burn treatments, stands accumulated an estimated 33-58% of the carbon accumulated by their unburned upland counterparts in this time frame. Related recent studies employing modeling techniques to simulate current and enhanced management practices, with respect to prescribed burning, wildfire, and climate change, suggest that achieving the same acreage currently burned or even 50% more area would likely still result in a net gain of carbon in upland forests when integrated over a 100 year period. Recovery of carbon uptake was most rapid where canopy scorch from fire was minimized, therefore, if fire management is being balanced with carbon storage and ecosystem productivity goals, minimizing canopy damage will decrease stand recovery time.

Full Text Article:  Clark, Kenneth L., Nicholas Skowronski, and Michael Gallagher. “Fire Management and Carbon Sequestration in Pine Barren Forests.” Journal of Sustainable Forestry 34 (2014): 125-146. http://www.tandfonline.com/doi/abs/10.1080/10549811.2014.973607#.VN0jOkYcRes