Drought, Tree Mortality, and Wildfire in Forests Adapted to Frequent Fire

Stephens, Scott L.; Collins, Brandon M.; Fettig, Christopher J.; Finney, Mark A.; Hoffman, Chad; Knapp, Eric E.; North, Malcolm P.; Safford, Hugh D.; Wayman, Rebecca B.

doi:10.1093/biosci/bix146

Cited by 295 publications

(213 citation statements)

References 73 publications

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“…Frequent and severe hot extremes and droughts are associated with lower ecosystem respiration and primary productivity of terrestrial ecosystems (Ciais et al., ). Hot and dry weather has also been found to drive forest wildfire, which can have severe ecological impacts on carbon sequestration (Reichstein et al., ) and wildlife habitat availability (Stephens et al., ). For marine realms, the Tropical Atlantic, Indo‐Pacific, and Tropical eastern Pacific are particularly vulnerable to heat extremes, while high‐latitude oceans are projected to experience new precipitation extreme regimes.…”

Section: Discussionmentioning

confidence: 99%

Projected timing of perceivable changes in climate extremes for terrestrial and marine ecosystems

Tan

Gan

Horton

2018

Global Change Biology

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Human and natural systems have adapted to and evolved within historical climatic conditions. Anthropogenic climate change has the potential to alter these conditions such that onset of unprecedented climatic extremes will outpace evolutionary and adaptive capabilities. To assess whether and when future climate extremes exceed their historical windows of variability within impact-relevant socioeconomic, geopolitical, and ecological domains, we investigate the timing of perceivable changes (time of emergence; TOE) for 18 magnitude-, frequency-, and severity-based extreme temperature (10) and precipitation (8) indices using both multimodel and single-model multirealization ensembles. Under a high-emission scenario, we find that the signal of frequency- and severity-based temperature extremes is projected to rise above historical noise earliest in midlatitudes, whereas magnitude-based temperature extremes emerge first in low and high latitudes. Precipitation extremes demonstrate different emergence patterns, with severity-based indices first emerging over midlatitudes, and magnitude- and frequency-based indices emerging earliest in low and high latitudes. Applied to impact-relevant domains, simulated TOE patterns suggest (a) unprecedented consecutive dry day occurrence in >50% of 14 terrestrial biomes and 12 marine realms prior to 2100, (b) earlier perceivable changes in climate extremes in countries with lower per capita GDP, and (c) emergence of severe and frequent heat extremes well-before 2030 for the 590 most populous urban centers. Elucidating extreme-metric and domain-type TOE heterogeneities highlights the challenges adaptation planners face in confronting the consequences of elevated twenty-first century radiative forcing.

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Section: Discussionmentioning

confidence: 99%

Projected timing of perceivable changes in climate extremes for terrestrial and marine ecosystems

Tan

Gan

Horton

2018

Global Change Biology

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“… , , Stephens et al. ). In the future, large‐scale and coordinated management efforts will likely be necessary to increase ecosystem patchiness and reduce the probability of synchronous disturbances that could push whole landscapes over ecological thresholds from which there may be no return (Betancourt ).…”

Section: Management Implicationsmentioning

confidence: 99%

“…, Stephens et al. ). The potential combination of drought‐induced tree mortality and large, high‐severity fires could precipitate major changes to ecosystem composition, structure, and function (McKenzie et al.…”

Section: Introductionmentioning

confidence: 99%

Forest structure and climate mediate drought‐induced tree mortality in forests of the Sierra Nevada,USA

Restaino

Young

Estes

et al. 2019

Ecological Applications

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103

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Extreme drought stress and associated bark beetle population growth contributed to an extensive tree mortality event in California, USA, resulting in more than 129 million trees dying between 2012 and 2016. Although drought is an important driver of this mortality event, past and ongoing fire suppression and the consequent densification of forests may have contributed. In some areas, land management agencies have worked to reduce stand density through mechanical treatments and prescribed fire to restore forests to less dense, more open conditions that are presumably more resilient to disturbance and changing climate. Here, we evaluate if stand structural conditions associated with treated (e.g., thinned and prescribed burned) forests in the Sierra Nevada of California conferred more resistance to the bark beetle epidemic and drought event of 2012–2016. We found that, compared to untreated units, treated units had lower stand densities, larger average tree diameters, and greater dominance of pines (Pinus), the historically dominant trees. For all tree species studied, mortality was substantially greater in climatically drier areas (i.e., lower elevations and latitudes). Both pine species studied (ponderosa pine [Pinus ponderosa] and sugar pine [Pinus lambertiana]) had greater mortality in areas where their diameters were larger, suggesting a size preference for their insect mortality agents. For ponderosa pine, the tree species experiencing greatest mortality, individual‐tree mortality probability (for a given tree diameter) was significantly lower in treated stands. Ponderosa pine mortality was also positively related to density of medium‐ to large‐sized conspecific trees, especially in areas with lower precipitation, suggesting that abundance of nearby host trees for insect mortality agents was an important determinant of pine mortality. Mortality of incense cedar (Calocedrus decurrens) and white fir (Abies concolor) was positively associated with basal area, suggesting sensitivity to competition during drought, but overall mortality was lower, likely because the most prevalent and effective mortality agents (the bark beetles Dendroctonus brevicomis and D. ponderosae) are associated specifically with pine species within our study region. Our findings suggest that forest thinning treatments are effective in reducing drought‐related tree mortality in forests, and they underscore the important interaction between water and forest density in mediating bark beetle‐caused mortality.

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“…, Stephens et al. ). Active restoration of fire‐dependent forests can create conditions that foster low‐severity fire and counter the successional effects of past management (Fulé et al.…”

Section: Introductionmentioning

confidence: 99%

Vegetation dynamics following compound disturbance in a dry pine forest: fuel treatment then bark beetle outbreak

Crotteau

Keyes

Hood

et al. 2019

Ecological Applications

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In the western United States, restoration of forests with historically frequent, low‐severity fire regimes often includes fuel reduction that reestablish open, early‐seral conditions while reducing fuel continuity and loading. Between 2001 and 2016, fuel reduction (e.g., thinning, prescribed burning, etc.) was implemented on over 26 million hectares of federal lands alone in the United States, reflecting the urgency to mitigate risk from high‐severity wildfire. However, between 2001 and 2012, nearly 20 million hectares in the United States were impacted by mountain pine beetle (MPB; Dendroctonus ponderosae), compounding restoration effects in wildfire‐hazard‐treated stands. Knowledge of the effects of treatments followed by natural disturbance on long‐term forest structure and communities is needed, especially considering that fuel treatments are increasingly being implemented and warming climate is predicted to exacerbate disturbance frequency and severity. We tested the interacting effects of treatments designed to reduce high‐severity wildfire hazard in stands subsequently challenged by MPB outbreak on vegetation dynamics using a factorial experimental design (control, thin only, burn only, thin + burn) in a ponderosa pine (Pinus ponderosa)‐dominated forest. Stands were treated by 2002, then impacted by MPB outbreak from 2005 to 2012. We assessed change in overstory and understory forest community structure, composition, and diversity over time. There were distinct thinning, burning, and year effects. Thinning immediately reduced overstory density; pine density then declined 4.5 times more in unthinned than thinned treatments due to MPB. Burning immediately reduced graminoid, shrub, and total understory cover by as much as 52%, resulting in greater species evenness than unburned treatments, but differences disappeared by 2016 due to growth and MPB outbreak. Similarly, multivariate analyses indicated forest communities were starkly different after treatment but became more similar over time, though key understory and overstory attributes still distinguish control and thin + burn. This study shows the value of long‐term silvicultural experiments to evaluate treatment longevity and the compounded effects of treatment and natural disturbance. We demonstrate the homogenizing effects of treatment‐induced growth coupled with MPB‐caused tree mortality on management strategies that just treat the overstory (thinning) or understory (burning), showing that only combined treatments can provide the unique structural and compositional outcomes expected of restoration.

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Drought, Tree Mortality, and Wildfire in Forests Adapted to Frequent Fire

Cited by 295 publications

References 73 publications

Projected timing of perceivable changes in climate extremes for terrestrial and marine ecosystems

Projected timing of perceivable changes in climate extremes for terrestrial and marine ecosystems

Forest structure and climate mediate drought‐induced tree mortality in forests of the Sierra Nevada,USA

Vegetation dynamics following compound disturbance in a dry pine forest: fuel treatment then bark beetle outbreak

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