Long‐term climate and competition explain forest mortality patterns under extreme drought

Young, Derek J. N.; Stevens, Jens T.; Earles, J. Mason; Moore, Jeffrey; Ellis, Adam; Jirka, A.; Latimer, Andrew M.

doi:10.1111/ele.12711

Cited by 344 publications

(335 citation statements)

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“…Although periodic droughts, warming, and the increasing evaporative demand of the atmosphere are clearly implicated in many reports of increasing tree mortality [26,[66][67][68], considerable uncertainty exists in the magnitude of interactions between external hydrological conditions and competition and between external hydrological conditions, tree size, and age [26,32,[69][70][71][72][73]. If one assumes that global change factors have increased tree growth, then a proportion of recent evidence indicating increasing time trends in tree mortality may reflect negative feedback responses to growth enhancement.…”

Section: Discussionmentioning

confidence: 99%

Low Tree-Growth Elasticity of Forest Biomass Indicated by an Individual-Based Model

Hember

Kurz

2018

Forests

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Environmental conditions and silviculture fundamentally alter the metabolism of individual trees and, therefore, need to be studied at that scale. However, changes in forest biomass density (Mg C ha −1 ) may be decoupled from changes in growth (kg C year −1 ) when the latter also accelerates the life cycle of trees and strains access to light, nutrients, and water. In this study, we refer to an individual-based model of forest biomass dynamics to constrain the magnitude of system feedbacks associated with ontogeny and competition and estimate the scaling relationship between changes in tree growth and forest biomass density. The model was driven by fitted equations of annual aboveground biomass growth (G ag ), probability of recruitment (P r ), and probability of mortality (P m ) parameterized against field observations of black spruce (Picea mariana (Mill.) BSP), interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco), and western hemlock (Tsuga heterophylla (Raf.) Sarg.). A hypothetical positive step-change in mean tree growth was imposed half way through the simulations and landscape-scale responses were then evaluated by comparing pre-and post-stimulus periods. Imposing a 100% increase in tree growth above calibrated predictions (i.e., contemporary rates) only translated into 36% to 41% increases in forest biomass density. This corresponded with a tree-growth elasticity of forest biomass (ε G,SB ) ranging from 0.33 to 0.55. The inelastic nature of stand biomass density was attributed to the dependence of mortality on intensity of competition and tree size, which decreased stand density by 353 to 495 trees ha −1 , and decreased biomass residence time by 10 to 23 years. Values of ε G,SB depended on the magnitude of the stimulus. For example, a retrospective scenario in which tree growth increased from 50% below contemporary rates up to contemporary rates indicated values of ε G,SB ranging from 0.66 to 0.75. We conclude that: (1) effects of warming and increasing atmospheric concentrations of carbon dioxide and reactive nitrogen on biomass production are greatly diminished, but not entirely precluded, scaling up from individual trees to forest landscapes; (2) the magnitude of decoupling is greater for a contemporary baseline than it is for a pre-industrial baseline; and (3) differences in the magnitude of decoupling among species were relatively small. To advance beyond these estimates, studies must test the unverified assumptions that effects of tree size and stand competition on rates of recruitment, mortality, and growth are independent of climate change and atmospheric concentrations of carbon dioxide and nitrogen.

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Section: Discussionmentioning

confidence: 99%

Low Tree-Growth Elasticity of Forest Biomass Indicated by an Individual-Based Model

Hember

Kurz

2018

Forests

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“…The USFS classifies TPA values less than 1 as the background mortality, while other studies consider background mortality to include up to three dead trees per acre [19]. Here, we classified TPA values less than 5 as the background mortality rate due to the moderate spatial resolution of our analysis (See supplementary materials).…”

Section: Figurementioning

confidence: 99%

“…Baseline VI metrics were then calculated as the long-term mean of summer VI during the 2001-2011 water years, when California experienced normal interannual fluctuations in precipitation before the onset of hotter drought conditions in 2012 [7,19]. For each of the recent four drought years (2013-2016), we quantified the departures of summer VIs from normal conditions using summer z-scores:…”

Section: Remote Sensing Data and Preprocessingmentioning

confidence: 99%

“…Trees need moisture to ward off infestations from insects, thus intensified drought stress makes trees more vulnerable to insect attacks. Bark beetle (Scolytinae) populations, for example, are found to be more successful in denser, drought-stressed forests where they can disperse easily and kill weakened trees [15,[17][18][19]59,83].…”

Section: Ecological Vulnerabilitymentioning

confidence: 99%

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Detecting Drought-Induced Tree Mortality in Sierra Nevada Forests with Time Series of Satellite Data

Byer

Jin

2017

Remote Sensing

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Abstract:A five-year drought in California led to a significant increase in tree mortality in the Sierra Nevada forests from 2012 to 2016. Landscape level monitoring of forest health and tree dieback is critical for vegetation and disaster management strategies. We examined the capability of multispectral imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS) in detecting and explaining the impacts of the recent severe drought in Sierra Nevada forests. Remote sensing metrics were developed to represent baseline forest health conditions and drought stress using time series of MODIS vegetation indices (VIs) and a water index. We used Random Forest algorithms, trained with forest aerial detection surveys data, to detect tree mortality based on the remote sensing metrics and topographical variables. Map estimates of tree mortality demonstrated that our two-stage Random Forest models were capable of detecting the spatial patterns and severity of tree mortality, with an overall producer's accuracy of 96.3% for the classification Random Forest (CRF) and a RMSE of 7.19 dead trees per acre for the regression Random Forest (RRF). The overall omission errors of the CRF ranged from 19% for the severe mortality class to 27% for the low mortality class. Interpretations of the models revealed that forests with higher productivity preceding the onset of drought were more vulnerable to drought stress and, consequently, more likely to experience tree mortality. This method highlights the importance of incorporating baseline forest health data and measurements of drought stress in understanding forest response to severe drought.

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“…Observational studies of drought-induced woody plant mortality suggest that drier locations may be more vulnerable to drought (Zhang et al 2017), and that dieoffs are often most severe in the driest parts of species' ranges (Allen and Breshears 1998, Clifford et al 2013, Young et al 2017, Benito Garz on et al 2018. Local and regional patterns of mortality have been linked to site characteristics that influence water availability, including soils and topography (Allred 1941, Gitlin et al 2006, Fensham et al 2015, and stand density and competition (Gitlin et al 2006, Bradford and Bell 2017, Young et al 2017.…”

Section: Introductionmentioning

confidence: 99%

Soil and stand structure explain shrub mortality patterns following global change–type drought and extreme precipitation

Renne

Schlaepfer

Palmquist

et al. 2019

Ecology

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2019. Soil and stand structure explain shrub mortality patterns following global change-type drought and extreme precipitation. Ecology 100(12):Abstract. The probability of extreme weather events is increasing, with the potential for widespread impacts to plants, plant communities, and ecosystems. Reports of drought-related tree mortality are becoming more frequent, and there is increasing evidence that drought accompanied by high temperatures is especially detrimental. Simultaneously, extreme large precipitation events have become more frequent over the past century. Water-limited ecosystems may be more vulnerable to these extreme events than other ecosystems, especially when pushed outside of their historical range of variability. However, drought-related mortality of shrubs-an important component of dryland vegetation-remains understudied relative to tree mortality. In 2014, a landscape-scale die-off of the widespread shrub, big sagebrush (Artemisia tridentata Nutt.), was reported in southwest Wyoming, following extreme hot and dry conditions in 2012 and extremely high precipitation in September of 2013. Here we examine how severe drought, extreme precipitation, soil texture and salinity, and shrub-stand characteristics contributed to this die-off event. At 98 plots within and around the die-off, we quantified big sagebrush mortality, characterized soil texture and salinity, and simulated soil-water conditions from 1916 to 2016 using an ecosystem water-balance model. We found that the extreme weather conditions alone did not explain patterns of big sagebrush mortality and did not result in extreme (historically unprecedented) soil-water conditions during the drought. Instead, plots with chronically dry soil conditions experienced greatest mortality following the global change-type (hot) drought in 2012. Furthermore, mortality was greater in locations with high potential run-on and low potential run-off where saturated soil conditions were simulated in September 2013, suggesting that extreme precipitation also played an important role in the die-off in these locations. In locations where drought alone contributed to mortality, stem density negatively impacted big sagebrush. In locations that may have been affected by both drought and saturation, however, mortality was greatest where stem density was lowest, suggesting that these locations may have already been less favorable to big sagebrush. Paradoxically, vulnerability to both extreme events (drought and saturation) was associated with finertextured soils, and our results highlight the importance of soils in determining local variation of the vulnerability of dryland plants to extreme events.

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Long‐term climate and competition explain forest mortality patterns under extreme drought

Cited by 344 publications

References 44 publications

Low Tree-Growth Elasticity of Forest Biomass Indicated by an Individual-Based Model

Low Tree-Growth Elasticity of Forest Biomass Indicated by an Individual-Based Model

Detecting Drought-Induced Tree Mortality in Sierra Nevada Forests with Time Series of Satellite Data

Soil and stand structure explain shrub mortality patterns following global change–type drought and extreme precipitation

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