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.