Temperate Forest Health in an Era of Emerging Megadisturbance. Millar, C. I. and Stephenson, N. L. Science, 349(6250):823–826, August, 2015.
doi  abstract   bibtex   
Although disturbances such as fire and native insects can contribute to natural dynamics of forest health, exceptional droughts, directly and in combination with other disturbance factors, are pushing some temperate forests beyond thresholds of sustainability. Interactions from increasing temperatures, drought, native insects and pathogens, and uncharacteristically severe wildfire are resulting in forest mortality beyond the levels of 20th-century experience. Additional anthropogenic stressors, such as atmospheric pollution and invasive species, further weaken trees in some regions. Although continuing climate change will likely drive many areas of temperate forest toward large-scale transformations, management actions can help ease transitions and minimize losses of socially valued ecosystem services. [Excerpt] Forests not only are essential components of the natural environment but also offer profound spiritual and material benefits to humanity. After centuries of exploitation, there is much to celebrate in the resilience (ability to rebound after perturbation) of temperate forests. Broad swaths of forest that were cut in recent centuries continue to regrow vigorously, absorbing a substantial proportion of anthropogenic carbon-dioxide emissions (1). Despite deeply concerning declines of ancient trees in forests worldwide (2), large trees are increasingly abundant in areas of temperate forests that are regrowing after logging (3). In other regions, air-quality regulations have reduced acidic deposition and other air-pollution effects on forests, providing improved conditions for forest growth and sustainability (4). [] Despite some encouraging trends, 21st-century forests still face grave threats. For millennia, the main threat to forests was overexploitation, but recent research has identified a range of emerging challenges to forest persistence and health. We focus on emerging '' megadisturbances'' that are capable of driving abrupt tree mortality of a spatial extent, severity, and frequency surpassing that recorded during recent human history. Where these occur, effects to ecosystems and society follow. Thus, while acknowledging the resilience of many forests, we highlight here the nature and consequences of changing environmental conditions that increasingly threaten widespread regions of temperate forest. In particular, we describe the rise of an especially potent threat to forest health that has only recently begun to receive broad attention, that is, persistent and recurring drought combined with high temperatures (see Fig. 1). [...] [Forests of the future: Easing transitions] What do these changes portend for temperate forests through the 21st century? In the short term, some forests will likely continue to absorb or rebound from disturbances, sustain a diversity of ecological functions, and deliver ecosystem services similar to those of past decades (Fig. 2). Over the longer term, however, most temperate forests are likely to change in condition (49), with megadisturbances frequently catalyzing these effects (14). The changes could range from minor shifts in forest structure (e.g., tree density and ages) and species compositions to major transformation of vegetation types, some resulting in novel ecosystems relative to recent centuries. In many cases, drought-hardy species, species with physiological plasticity capable of coping with compound stresses, and species with shorter statures might outcompete current species (50, 51). Native insects and pathogens may effectively act as invasive exotics as they move beyond their historic ranges (52). [] Minimizing the effects to society from these transitions is emerging as a primary goal for forest management today (Fig. 4). A challenge to research will be to develop tools to assess the sensitivity of forests to thresholds from cumulative disturbances and evaluate their vulnerability to transformation. If we can identify in advance the most vulnerable forests, in some cases management intervention might be able to ease the transition to new and better-adapted forest states, minimizing losses of ecosystem services. Because the scope of the challenge is vast, triage exercises will almost certainly be necessary to identify the highest-priority forests and those where management action might have the greatest effect. [] Success will depend on far more integrated and coordinated efforts by institutions, agencies, and governments than presently exists (53). Distributed monitoring systems that observe changes on multiple scales of forest health are essential; these will become increasingly reliant on remote methods. Climate adaptation will likely move from compartmentalized to comprehensive strategies, with attention to proactive methods (54). Although thresholds are likely to be approached in the future, and changes are inevitable, the actions we take now in temperate forests can ease and guide transitions, diminishing effects to forest ecosystems and human societies.
@article{millarTemperateForestHealth2015,
  title = {Temperate Forest Health in an Era of Emerging Megadisturbance},
  author = {Millar, Constance I. and Stephenson, Nathan L.},
  year = {2015},
  month = aug,
  volume = {349},
  pages = {823--826},
  issn = {1095-9203},
  doi = {10.1126/science.aaa9933},
  abstract = {Although disturbances such as fire and native insects can contribute to natural dynamics of forest health, exceptional droughts, directly and in combination with other disturbance factors, are pushing some temperate forests beyond thresholds of sustainability. Interactions from increasing temperatures, drought, native insects and pathogens, and uncharacteristically severe wildfire are resulting in forest mortality beyond the levels of 20th-century experience. Additional anthropogenic stressors, such as atmospheric pollution and invasive species, further weaken trees in some regions. Although continuing climate change will likely drive many areas of temperate forest toward large-scale transformations, management actions can help ease transitions and minimize losses of socially valued ecosystem services.

[Excerpt] Forests not only are essential components of the natural environment but also offer profound spiritual and material benefits to humanity. After centuries of exploitation, there is much to celebrate in the resilience (ability to rebound after perturbation) of temperate forests. Broad swaths of forest that were cut in recent centuries continue to regrow vigorously, absorbing a substantial proportion of anthropogenic carbon-dioxide emissions (1). Despite deeply concerning declines of ancient trees in forests worldwide (2), large trees are increasingly abundant in areas of temperate forests that are regrowing after logging (3). In other regions, air-quality regulations have reduced acidic deposition and other air-pollution effects on forests, providing improved conditions for forest growth and sustainability (4).

[] Despite some encouraging trends, 21st-century forests still face grave threats. For millennia, the main threat to forests was overexploitation, but recent research has identified a range of emerging challenges to forest persistence and health. We focus on emerging '' megadisturbances'' that are capable of driving abrupt tree mortality of a spatial extent, severity, and frequency surpassing that recorded during recent human history. Where these occur, effects to ecosystems and society follow. Thus, while acknowledging the resilience of many forests, we highlight here the nature and consequences of changing environmental conditions that increasingly threaten widespread regions of temperate forest. In particular, we describe the rise of an especially potent threat to forest health that has only recently begun to receive broad attention, that is, persistent and recurring drought combined with high temperatures (see Fig. 1). [...]

[Forests of the future: Easing transitions]

What do these changes portend for temperate forests through the 21st century? In the short term, some forests will likely continue to absorb or rebound from disturbances, sustain a diversity of ecological functions, and deliver ecosystem services similar to those of past decades (Fig. 2). Over the longer term, however, most temperate forests are likely to change in condition (49), with megadisturbances frequently catalyzing these effects (14). The changes could range from minor shifts in forest structure (e.g., tree density and ages) and species compositions to major transformation of vegetation types, some resulting in novel ecosystems relative to recent centuries. In many cases, drought-hardy species, species with physiological plasticity capable of coping with compound stresses, and species with shorter statures might outcompete current species (50, 51). Native insects and pathogens may effectively act as invasive exotics as they move beyond their historic ranges (52).

[] Minimizing the effects to society from these transitions is emerging as a primary goal for forest management today (Fig. 4). A challenge to research will be to develop tools to assess the sensitivity of forests to thresholds from cumulative disturbances and evaluate their vulnerability to transformation. If we can identify in advance the most vulnerable forests, in some cases management intervention might be able to ease the transition to new and better-adapted forest states, minimizing losses of ecosystem services. Because the scope of the challenge is vast, triage exercises will almost certainly be necessary to identify the highest-priority forests and those where management action might have the greatest effect. 

[] Success will depend on far more integrated and coordinated efforts by institutions, agencies, and governments than presently exists (53). Distributed monitoring systems that observe changes on multiple scales of forest health are essential; these will become increasingly reliant on remote methods. Climate adaptation will likely move from compartmentalized to comprehensive strategies, with attention to proactive methods (54). Although thresholds are likely to be approached in the future, and changes are inevitable, the actions we take now in temperate forests can ease and guide transitions, diminishing effects to forest ecosystems and human societies.},
  journal = {Science},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-13708352,~to-add-doi-URL,adaptation,air-pollution,climate-change,disturbances,droughts,ecosystem-services,featured-publication,forest-management,forest-pests,integrated-natural-resources-modelling-and-management,invasive-species,pressures,sustainability,temperate-forests,wildfires},
  lccn = {INRMM-MiD:c-13708352},
  number = {6250}
}
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