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Abstract 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 (“2ndDrought 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 the2ndDrought Onlysample experienced severe die‐off associated with extreme 4‐year Pr‐ET overdraft in 2012–2015. Stands in theBoth Droughtssample 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.
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Changes in tree drought sensitivity provided early warning signals to the California drought and forest mortality event
Abstract Climate warming in recent decades has negatively impacted forest health in the western United States. Here, we report on potential early warning signals (EWS) for drought‐related mortality derived from measurements of tree‐ring growth (ring width index; RWI) and carbon isotope discrimination (∆13C), primarily focused on ponderosa pine (Pinus ponderosa). Sampling was conducted in the southern Sierra Nevada Mountains, near the epicenter of drought severity and mortality associated with the 2012–2015 California drought and concurrent outbreak of western pine beetle (Dendroctonus brevicomis). At this site, we found that widespread mortality was presaged by five decades of increasing sensitivity (i.e., increased explained variation) of both tree growth and ∆13C to Palmer Drought Severity Index (PDSI). We hypothesized that increasing sensitivity of tree growth and ∆13C to hydroclimate constitute EWS that indicate an increased likelihood of widespread forest mortality caused by direct and indirect effects of drought. We then tested these EWS in additional ponderosa pine‐dominated forests that experienced varying mortality rates associated with the same California drought event. In general, drier sites showed increasing sensitivity of RWI to PDSI over the last century, as well as higher mortality following the California drought event compared to wetter sites. Two sites displayed evidence that thinning or fire events that reduced stand basal area effectively reversed the trend of increasing hydroclimate sensitivity. These comparisons indicate that reducing competition for soil water and/or decreasing bark beetle host tree density via forest management—particularly in drier regions—may buffer these forests against drought stress and associated mortality risk. EWS such as these could provide land managers more time to mitigate the extent or severity of forest mortality in advance of droughts. Substantial efforts at deploying additional dendrochronological research in concert with remote sensing and forest modeling will aid in forecasting of forest responses to continued climate warming.
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- Award ID(s):
- 1903721
- PAR ID:
- 10366564
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Global Change Biology
- Volume:
- 28
- Issue:
- 3
- ISSN:
- 1354-1013
- Format(s):
- Medium: X Size: p. 1119-1132
- Size(s):
- p. 1119-1132
- Sponsoring Org:
- National Science Foundation
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