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 (dNDMI), and cumulative moisture deficit as negative four-year Precipitation minus Evapotranspiration (four-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 four-year Pr-ET overdraft in 2012–2015. Stands in the Both Droughts sample experienced severe die-off and four-year Pr-ET overdraft in 1999–2002, but comparatively little 2012–2015 die-off despite continued four-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.Funding provided by: University of California Laboratory Fees Research Program*Crossref Funder Registry ID: Award Number: LFR-18-542511Funding provided by: California Strategic Growth Council*Crossref Funder Registry ID: Award Number: CCR20021Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: DGE-173504