Detecting Drought-induced Tree Mortality in Sierra Nevada Forests
Detecting Drought-induced Tree Mortality in Sierra Nevada Forests

Author: Sarah Ann Byer

Publisher:

Published: 2017

Total Pages:

ISBN-13: 9780355461763

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A five-year drought in California has 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 express 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.

Modeling Environmental Factors Related to Drought Induced Tree Mortality Based on Lidar and Hyperspectral Imagery
Modeling Environmental Factors Related to Drought Induced Tree Mortality Based on Lidar and Hyperspectral Imagery

Author:

Publisher:

Published: 2021

Total Pages: 0

ISBN-13:

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Climate change is projected to bring more frequent and prolonged droughts, causing widespread forest die-off. Identifying tree mortality over large spatial extents in response to the most recent California drought will help forest managers and conservationists understand where there may be a greater likelihood of future die-offs. In order to find more at-risk areas, this study evaluated how interacting site-specific topographic, climate, substrate, and stand characteristics mediated tree mortality in the Central Sierra Nevada during the 2012-2016 drought. The author used lidar and hyperspectral imagery provided by the National Ecological Observatory Network to identify individual dead trees using the Random Forest classification method and created a Random Forest Regression model to assess site-specific environmental variables that had a greater influence on tree mortality. The results show that the most influential variables were tree height, density, and elevation. Results also found higher mortality rates in pines and oaks, meaning further widespread die-off of these trees could reduce forest productivity, increase fire hazard risk, and drive a shift in community composition over the long-term. This study provides a finer resolution mapping of tree mortality over the research area than was reported by the USFS Aerial Detection Survey. Due to the confounding evidence regarding the relative influence of environmental factors on tree mortality during droughts, these results provide robust information to help maintain these changing forests in a climate-informed manner. Because this study is site-specific, more research is needed to assess how environmental factors mediate drought-induced mortality in other regions also projected to have more intense droughts as a result of climate change.

Effects of Drought on Forests and Rangelands in the United States
Effects of Drought on Forests and Rangelands in the United States

Author: James M. Vose

Publisher:

Published: 2016

Total Pages: 302

ISBN-13:

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This assessment provides input to the reauthorized National Integrated Drought Information System (NIDIS) and the National Climate Assessment (NCA), and it establishes the scientific foundation needed to manage for drought resilience and adaptation. Focal areas include drought characterization; drought impacts on forest processes and disturbances such as insect outbreaks and wildfire; and consequences for forest and rangeland values. Drought can be a severe natural disaster with substantial social and economic consequences. Drought becomes most obvious when large-scale changes are observed; however, even moderate drought can have long-lasting impacts on the structure and function of forests and rangelands without these obvious large-scale changes. Large, stand-level impacts of drought are already underway in the West, but all U.S. forests are vulnerable to drought. Drought-associated forest disturbances are expected to increase with climatic change. Management actions can either mitigate or exacerbate the effects of drought. A first principal for increasing resilience and adaptation is to avoid management actions that exacerbate the effects of current or future drought. Options to mitigate drought include altering structural or functional components of vegetation, minimizing drought-mediated disturbance such as wildfire or insect outbreaks, and managing for reliable flow of water.

Drought, Tree Mortality, and Regeneration in Northen California
Drought, Tree Mortality, and Regeneration in Northen California

Author: Sophia Lemmo

Publisher:

Published: 2022

Total Pages: 0

ISBN-13:

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The 2012-2016 California drought was the most severe in the state’s recorded history, contributing to the death of millions of trees. While the effects of this drought on forests are relatively well studied in the central and southern Sierra Nevada, less is known about its effects on the heavily timbered and diverse forests of northern California. Through sampling 54 0.25 ha plots in northern California, this study compared tree mortality and regeneration patterns before, during, and after California’s most recent record-setting drought. This study evaluated 1) the influence of habitat and competitive covariates on mortality and regeneration trends using ridge regression analysis; and 2) tree death and seedling/sapling establishment dates using dendrochronology and Superposed Epoch Analysis to explore the influence of climate on forest demographics. Montane drought-induced tree mortality occurred primarily in trees smaller than 40 cm diameter at breast height (DBH), with no coastal drought-related mortality in trees with DBH greater than 80 cm. The highest rates of overstory mortality across all sites were observed in Abies grandis (51%), Pinus lambertiana (43%), and Pinus monticola (37%). Picea breweriana (6%) and Picea sitchensis (9%) had the lowest average mortality rates. In montane environments, years with high rates of mortality were positively associated with climatic water deficit (CWD; drier than expected conditions) in the 1-2 years preceding and during tree death dates. Pre-drought montane mortality was greater at wet sites than dry sites, and recent montane mortality (~2013-2020) was positively related with canopy openness. In coastal environments, recent tree mortality was positively associated with maximum temperature and topographic position. Regeneration was dominated by advanced regeneration (median age of 32 years) of shade-tolerant species. In montane environments, regeneration dates were significantly associated with lower-than-average CWD the year proceeding. In coastal environments, regeneration was greater at dry sites than wet sites, and was positively associated with stand density and maximum temperature. These data demonstrate that these forests are not actively perpetuating as diversely into the future, especially in montane environments where more mortality is found in white pine species (Pinus lambertiana and P. monticola) and where the regeneration is weighted towards advanced regeneration of shade-tolerant fir species. This work indicates a need to implement targeted management aimed at generating disturbances to foster balanced and responsive regeneration. This management should preferentially retain medium to larger trees, as these size classes seem to be the least vulnerable to mortality. Such management would be promising for supporting the resilience and diversity of northern California landscapes.

Developing a Tool for the Early Detection of Drought-induced Forest Stress and Mortality in British Columbia
Developing a Tool for the Early Detection of Drought-induced Forest Stress and Mortality in British Columbia

Author: Craig DeLong

Publisher:

Published: 2022

Total Pages: 0

ISBN-13: 9780772680839

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We tested components of a drought mortality decision tool for application in British Columbia forests. The previously estimated drought thresholds for lodge-pole pine and western larch were tested and revised based on the field assessment of visible foliar drought indicators in relation to estimated soil moisture regimes using the ratio of actual evapotranspiration to potential evapotranspiration (AET/PET). Post-processed photography from drones flown over sites with known drought mortality was very successful at mapping dead trees. Using a supervised image classification, dead trees identified in the field were composed of 75% pixels identified as dead. Only 1% of the dead trees were composed of > 50% live pixels based on image classification. In combination, the drought assessment tool using AET/PET estimates, along with image-mapping tools, should be effective at identifying and preparing treatment maps for forests impacted by drought. Furthermore, these tools have the potential to map drought risk, allowing appropriate management planning.

Synthesis of Research Into the Long-term Outlook for Sierra Nevada Forests Following the Current Bark Beetle Epidemic
Synthesis of Research Into the Long-term Outlook for Sierra Nevada Forests Following the Current Bark Beetle Epidemic

Author:

Publisher:

Published: 2019

Total Pages: 27

ISBN-13:

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This paper summarizes the 2012-2017 bark beetle epidemic in the Sierra Nevada and its implications for long-term changes in tree species composition and forest structure. Preliminary plot and landscape-scale data are reviewed, showing higher levels of mortality for pine species and greater impacts in the southern Sierra Nevada compared to the northern portions of the range. The federal government owns approximately three quarters of the forested area impacted by high levels of tree morality, with the remainder of the land controlled by nonindustrial (18%) and industrial (6%) ownerships. The accumulation of dead and downed fuel and standing dead trees is expected to increase fire intensity and severity, and pose significant hazards for fire control efforts. Potential long-term changes in Sierra Nevada forest composition were explored with a GIS analysis conducted for the Sierra National Forest, located in the southern Sierra. GIS layers included very high fire threat, aspect, high tree mortality, topographic position classification, and climatic exposure. A factor of one was assigned to each parameter (i.e., no weighting for any of the variables). The modeling showed that 4% of the Sierra National Forest is at very high risk for type conversion from mixed conifer to shrublands, and 12% is at high risk. This information can inform landowners regarding the general locations where successful reforestation will be most challenging, as well as illustrate the scale of concern for one national forest in the southern Sierra Nevada. Changes to disturbance regimes, continuing land use changes, and climate change with associated species shifts pose significant challenges for maintaining healthy and resilient forests in the Sierra Nevada. Significant unknowns exist regarding the future species composition for vast portions of this region, but type conversions from mixed conifer to shrublands or oak/grass/woodland appear likely for some areas. Recommended best management practices focus on reducing tree densities, achieving successful reforestation, and using adaptive management in the face of currently unknown future changes in growing conditions. With the exception of the bark beetle epidemic in southern California in the early 2000s, lessons learned from other locations in western North America that have had sustained bark beetle epidemics in the past decade are not directly applicable to Sierra Nevada, with its Mediterranean climate, complex topography, and mixed-conifer forests. For these reasons, ongoing research efforts to characterize and understand tree mortality drivers and changes in forest structure and composition in the Sierra Nevada are extremely important.

Ecosystem Collapse and Climate Change
Ecosystem Collapse and Climate Change

Author: Josep G. Canadell

Publisher: Springer Nature

Published: 2021-06-19

Total Pages: 365

ISBN-13: 303071330X

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Human-driven greenhouse emissions are increasing the velocity of climate change and the frequency and intensity of climate extremes far above historical levels. These changes, along with other human-perturbations, are setting the conditions for more rapid and abrupt ecosystem dynamics and collapse. This book presents new evidence on the rapid emergence of ecosystem collapse in response to the progression of anthropogenic climate change dynamics that are expected to intensify as the climate continues to warm. Discussing implications for biodiversity conservation, the chapters provide examples of such dynamics globally covering polar and boreal ecosystems, temperate and semi-arid ecosystems, as well as tropical and temperate coastal ecosystems. Given its scope, the volume appeals to scientists in the fields of general ecology, terrestrial and coastal ecology, climate change impacts, and biodiversity conservation.