Recent Tree Mortality Dampens Semi-Arid Forest Die-Off During Subsequent Drought

Climate change is expected to increase drought intensity and frequency, which are commonly predicted will threaten the survival of forests. Most forest die-off projections assume that recent tree mortality will not alter die-off severity during subsequent droughts. We tested this assumption by comparing die-off in semi-arid conifer forest stands in California that were exposed to a single drought in 2012-2015 ("2nd Drought Only") with forest stands that experienced drought in both 1999-2002 and 2012-2015 ("Both Droughts"). We quantified die-off severity as a reduction in the satellite observed Normalized Difference Moisture Index, and cumulative moisture deficit as negative 4-year Precipitation minus Evapotranspiration (4-year Pr-ET overdraft). Here we show that recent tree morality reduces die-off severity in semi-arid conifer forests exposed to subsequent drought. Stands in the 2nd Drought Only sample experienced severe die-off associated with extreme 4-year Pr-ET overdraft in 2012-2015. Stands in the Both Droughts sample experienced severe die-off and 4-year Pr-ET overdraft in 1999-2002, but comparatively little 2012-2015 die-off despite continued 4-year Pr-ET overdraft. We interpret this as a dampening effect, where prior tree mortality reduces forest die-off severity during subsequent drought exposure. As forests continue to experience disturbances linked to climate change, dampening effects will impose a transient, and perhaps long-term, constraint on the impact of repeated drought.