Warming increased bark beetle‐induced tree mortality by 30% during an extreme drought in California

Robbins, Zachary; Xu, Chonggang; Aukema, Brian H.; Buotte, Polly C.; Chitra‐Tarak, Rutuja; Fettig, Christopher J.; Goulden, Michael L.; Goodsman, Devin W.; Hall, A. D.; Koven, Charles D.; Kueppers, Lara M.; Madakumbura, Gavin D.; Mortenson, Leif A.; Powell, James A.; Scheller, Robert M.

doi:10.1111/gcb.15927

Cited by 52 publications

(38 citation statements)

References 65 publications

(110 reference statements)

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“…We found that while most mortality prior to the drought was concentrated in smaller size classes of overstory trees, drought and beetle-related mortality shifted patterns of tree mortality to be more evenly distributed across size classes and concentrated in species of pine (Fig. 2); these findings align with several other studies reporting patterns of the recent California tree mortality event (Fettig et al 2019;Koontz et al 2021;Robbins et al 2022). We observed substantial changes to Quadratic mean diameter (cm) 2.7 ± 0.4 1.9 ± 0.3 surface fuel loading, stand density metrics, canopy fuel loads, and potential wildfire emissions over a sampling period of just four years beyond when most tree mortality occurred.…”

Section: Discussionsupporting

confidence: 90%

Fuels change quickly after California drought and bark beetle outbreaks with implications for potential fire behavior and emissions

et al. 2023

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Background An extreme drought from 2012–2016 and concurrent bark beetle outbreaks in California, USA resulted in widespread tree mortality. We followed changes in tree mortality, stand structure, and surface and canopy fuels over four years after the peak of mortality in Sierra mixed conifer and pinyon pine (Pinus monophylla) forests to examine patterns of mortality, needle retention after death, and snag fall across tree species. We then investigated how the tree mortality event affected surface and canopy fuel loading and potential impacts on fire hazard and emissions. Results Drought and beetle-related tree mortality shifted mortality patterns to be more evenly distributed across size classes and concentrated in pines. Substantial changes to surface fuel loading, stand density, canopy fuel loads, and potential wildfire emissions occurred within four years following peak levels of tree mortality, with the largest changes related to increases in coarse woody debris. Nearly complete needle fall occurred within four years of mortality for all species except red fir (Abies magnifica). Pine species and incense cedar (Calocedrus decurrens) snags fell more quickly than fir species. Potential fire behavior modelling suggested that crowning and torching hazard decreased as trees dropped dead needles and fell, but as canopy fuels were transferred to surface fuels, potential for smoldering combustion increased, causing greater emissions. Conclusions Our study increases understanding of how extreme tree mortality events caused by concurrent disturbances alter canopy and surface fuel loading and have the potential to affect fire behavior and emissions in two compositionally different seasonally dry forest types. After a major tree mortality event, high canopy fuel flammability may only last a few years, but surface fuels can increase considerably over the same time period in these forest types. The accumulation of coarse woody surface fuels resulting from multi-year drought and concurrent bark beetle outbreaks combined with the increasing frequency of drought in the western U.S. have the potential to lead to heavy and dry fuel loads that under certain weather conditions may result in more extreme fire behavior and severe effects, particularly in forest types where decades of successful fire suppression has caused forest densification.

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Section: Discussionsupporting

confidence: 90%

Fuels change quickly after California drought and bark beetle outbreaks with implications for potential fire behavior and emissions

et al. 2023

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“…Our analysis does not distinguish abiotic disturbances such as windthrow and frost damage that have no linkage to droughts; however, these types of disturbance are substantially less frequent compared to fire, insects and harvest and therefore the bias should be relatively small. The drought-associated carbon loss could be partially linked to potential weakening of tree defense against insects and diseases during drought (Anderegg et al, 2015;Huang et al, 2020;Robbins et al, 2022). To test this hypothesis, we conducted additional analyses to assess whether drought played a role in insect/disease-caused disturbances by comparing the mean ESI values of pixels affected by as insect/disease that were defined by the US IDS database to pixels within a region.…”

Section: Discussionmentioning

confidence: 99%

Multi-scale quantification of anthropogenic, fire, and drought-associated forest disturbances across the continental U.S., 2000–2014

Wang

Johnson

et al. 2022

Front. For. Glob. Change

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Our understanding of broad-scale forest disturbances under climatic extremes remains incomplete. Drought, as a typical extreme event, is a key driver of forest mortality but there have been no reports on continental-scale quantification of its impact on forest mortality or how it compares to other natural or anthropogenic drivers. Thus, our ability to understand and predict broad-scale carbon cycling in response to changing climate and extreme events is limited. In this study, we applied an attribution approach based on different sources of data to quantify the area and potential carbon loss/transfer in continental U.S. (CONUS) from four types of disturbance: (1) anthropogenic (especially timber harvest); (2) fire; (3) drought-associated; and (4) other from 2000 to 2014. Our results showed that anthropogenic disturbances, fire, drought-associated disturbances, and other disturbances accounted for 54.3, 10.7, 12.7, and 22.3% of total canopy area loss, respectively. Anthropogenic disturbance was the most important driver contributing to 58.1% potential carbon loss/transfer in CONUS for 2000–2014. The potential carbon loss/transfer from natural disturbances (fire, drought, and other) for the same study period accounted for approximately 41.9% of the total loss/transfer from all agents, suggesting that natural disturbances also played a very important role in forest carbon turnover. Potential carbon loss/transfer associated with drought accounted for approximately 11.6% of the total loss/transfer in CONUS, which was of similar magnitude to potential carbon loss/transfer from fire (∼11.0%). The other natural disturbance accounted for 19.3% of potential carbon loss/transfer. Our results demonstrated the importance of the impacts of various disturbances on forest carbon stocks at the continental scale, and the drought-associated carbon loss/transfer data developed here could be used for evaluating the performance of predictive models of tree mortality under droughts.

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“…Many studies have documented adverse effects on vegetation communities as a result of the changing climate and changing wildfire regimes in California (Williams et al 2019, Huesca et al 2021. Nonetheless, most of the studies were confined to specific vegetation groups or few species, specific drought incidents, or analysis over a relatively short annual time period (Baguskas et al 2014, Taylor et al 2019, Baeza et al 2021, Wayman and Safford 2021, Robbins et al 2022; thus, limiting our understanding of the broadscale effects on tree cover as well as the spatiotemporal variability of effects among ecoregions across California. In this study, we used multiple annual land cover and land surface change products from the U.S. Geological Survey's (USGS) Land Change Monitoring, Assessment and Projection (LCMAP) product suite to assess patterns in tree cover loss in eight tree-dominated ecoregions in California from 1986 to 2019.…”

Section: Introductionmentioning

confidence: 99%

Trends in tree cover change over three decades related to interannual climate variability and wildfire in California

Dwomoh

Auch

Brown

et al. 2023

Environ. Res. Lett.

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The U.S. State of California has experienced frequent drought events, hotter temperatures and other disruptions to the climate system whose effects on ecosystems have been widely reported in recent decades. Studies primarily confined to specific vegetation communities or species, individual drought incidents, or analysis over a relatively short intervals, has limited our understanding of the broad-scale effects on tree cover and the spatiotemporal variability of effects across broader regions. We focused analysis on multi-annual land cover and land surface change to assess patterns and trends in tree cover loss in tree-dominated Californian ecoregions from 1986 to 2019. The top three years of total tree cover loss for the state were 2018 (1,901 km2), 2015 (1,556 km2), and 2008 (1,549 km2). Overall, annual tree cover loss had upward trends. Tree cover loss rapidly surged later in the study period and was apparently driven by climate stress and wildfires. Underlying geographic variability was apparent in both non-fire and fire-related tree cover loss that sharply increased during hotter multi-year droughts. The increasingly hotter and drier climate conditions were associated with significant increases in fire-induced mortality. Our findings indicate that a possible effect of future hotter and drier climate would lead to further tree cover loss, thereby endangering California’s ecosystem goods and services. Geographic variability in tree cover trends indicates that ecoregion-specific mitigation and adaptation strategies would be useful to conserve the region’s forest resources. Such strategies may benefit from consideration of historical disturbances, ecoregion’s sensitivity to disturbance types, as well as potential ecoregion-specific climate-vegetation-fire feedbacks.

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Warming increased bark beetle‐induced tree mortality by 30% during an extreme drought in California

Cited by 52 publications

References 65 publications

Fuels change quickly after California drought and bark beetle outbreaks with implications for potential fire behavior and emissions

Fuels change quickly after California drought and bark beetle outbreaks with implications for potential fire behavior and emissions

Multi-scale quantification of anthropogenic, fire, and drought-associated forest disturbances across the continental U.S., 2000–2014

Trends in tree cover change over three decades related to interannual climate variability and wildfire in California

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