Forests help to mitigate climate change by sequestering carbon from the atmosphere. In fire-prone forests, burn events lead to direct and indirect carbon emissions through biomass consumption and overstory tree mortality. Increasing temperatures coupled with longer, drier fire seasons and a legacy of fire suppression increase the likelihood that fire-prone forests will burn at high-severity, resulting in a transition from carbon sink to source. Thinning and prescribed burning are commonly used to reduce fire severity, but require carbon removal and loss. The expectation is the initial treatment to restore surface fire will have the largest carbon cost because of mechanical thinning and combustive losses from a prolonged accumulation of surface fuels and the carbon losses from the second-entry prescribed burn will decrease. We sought to quantify the carbon dynamics of first- and second-entry prescribed fire on both thinned and unthinned stands. We quantified the pre- and post-treatment carbon stocks from a full-factorial experiment involving three levels of thinning and two levels of burning at the Teakettle Experimental Forest in California’s Sierra Nevada. Prescribed burns were implemented in 2001 and 2017 to approximate the historic mean fire return interval for the site.
Results/Conclusions
Contrary to the expectation of lower emissions with the second-entry burn, we found higher carbon emissions from the second burn in both the Burn Only and Burn/Understory Thin treatments. In these treatments, first-entry burn emissions ranged from 25 to 32 Mg C ha-1 and second-entry from 44 to 56 Mg C ha-1. In the Burn/Overstory Thin treatment, second-entry emissions decreased from 48 Mg C to 22 Mg C ha-1. The 2012-2015 drought caused substantial tree mortality in the treatments with higher live tree basal area. The mortality increased dead tree carbon by 14 to 76 Mg C ha-1 and coarse woody debris by 8 to 14 Mg C ha-1, adding considerable fire-available fuel to the system. The second-entry burn was effective at reducing these additional fuels and returning fuel loads to pre-drought conditions. However, the drought-mortality inputs to surface fuels in the unburned plots contribute to the fire hazard. Our results indicate that while the expectation of reduced emissions with second-entry burns may hold in some locations, exogenous factors, such as drought-mortality, can significantly increase surface fuel pools and cause higher second-entry emissions.