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

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.