Supporting data for changing climate reallocates the carbon debt of frequent-fire forests

<p>Ongoing climate change will likely alter the carbon carrying capacity of forests as they adjust to climatic extremes and changing disturbance regimes. Increasing drought frequency and severity are already causing widespread tree mortality events, which can exacerbate the carbon debt that has developed as a result of fire-exclusion. Reducing tree density and surface fuels decreases the risk of high-severity wildfire and may also limit drought-induced mortality by reducing competition. We utilized a long-term thinning and burning experiment in a mixed-conifer forest to investigate the effects of the 2012-2015 California drought on forest carbon dynamics, including the carbon emissions from a second-entry prescribed fire that followed the drought. We assessed differences in carbon stability and drought survival across treatments, with the expectation that both carbon stability and survival probability would increase with increasing treatment intensity (decreasing basal area).  Additionally, we analyzed the effects of drought- mortality on second-entry burn emissions and compared emissions for the first and second-entry burns. We did not find a linear relationship between treatment intensity and carbon stability, which was in part driven by varying relationships between growing space and survival across treatments. Drought mortality also increased dead tree and surface fuel carbon in all treatments, which contributed to an increase in second-entry burn emissions for two of the three burn treatments. Our findings suggest that restoration treatments will not serve as a panacea for ongoing climate change and that the carbon debt of these forests will increase as the carbon carrying capacity adjusts to severe drought events. Managing this additional carbon debt with prescribed fire will help reduce the risk of additional mortality from wildfire, but at an increasing carbon cost for forest management. </p>