When I entered the field fifty years ago, we assumed that the future yields and composition of forests could be predicted based on long-term empirical studies. Those assumptions are a lot harder to make today with unparalleled outbreaks of fire, insects, and disease under a changing climate. What underlying changes have occurred to set forests into transition? What changes in growth and composition might we expect across the Pacific Northwest and beyond? My presentation illustrates an approach using process-based models, field surveys, and remote sensing designed to help chart the future of forests in different parts of western North America. The key to recognizing shifts in the productive capacity of forests is to follow trends in the layers of leaves that can be supported. Which species are most vulnerable? Which species are most likely to persist or to migrate? Vulnerability, persistence, and migration can be predicted based on a species’ differential response to drought, humidity deficits, and temperature variation throughout the year. The potential exists to decrease the vulnerability of forests to fire, insects, and disease through management, using techniques of selective thinning, shorter rotations, and by increasing biodiversity. The approach will be illustrated with examples.
The need to manage for heterogeneity at multiple scales is increasingly being recognized in forest management. Transitioning from a stand-centered to a multi-scale approach has proven challenging, however. Methods to incorporate targets for spatial pattern into treatments are not well developed or widely used. In this lecture, I present a framework for multi-scale management that integrates concepts of scale, patch hierarchy, and pattern-process dynamics from landscape ecology with silviculture. The framework has four basic components. First, the desired pattern of forest structure and composition must be quantified in terms of ranges, which are derived from reference conditions or functional objectives such as fire behavior or wildlife habitat. Second, current patterns of structure and composition are evaluated against targets. Third, prescriptions are developed that include specific, operational guidance to move current pattern towards the desire ranges. Finally, a monitoring system is developed that assesses both desired pattern and process over time.
I present three examples where this framework has been applied in eastern Oregon and Washington. The first is a watershed scale (10,000-20,000 ha) landscape evaluation and prioritization using methods developed by Dr. Paul Hessburg. I then scale down to a stand neighborhood (500-2000 ha) and report results from a project seeking to restore patterns associated with mixed severity fire. Lastly, I present a stand-level approach that quantifies within-stand pattern in terms of widely spaced individual trees, tree clumps, and openings. I summarize reference data from over 30 x 4ha plots and show how this information is being used to guide ongoing treatments across multiple National Forests.
We are on the cusp of significant changes in forest ecosystems in some regions of North America, mediated primarily by ecological disturbance. Because climatic extremes and their secondary effects (wildfire, insect attacks, etc.) will drive changes in ecosystems in future decades, climate change adaptation should focus primarily on functionality and processes across large landscapes, rather than on distribution and abundance of species. Concepts such as potential vegetation, historic range of variation, and restoration will need to be redefined for planning as we transition to a non-analog climate. When large disturbances occur, they will be opportunities to facilitate this transition and modify the occurrence of future disturbances. Climate-smart thinking needs to be quickly mainstreamed to ensure that viable options are available for the long-term experimental approach required by adaptive management. In the western United States, federal agencies are leading the way in building resilience in terrestrial and aquatic systems through active forest management and revised guidelines for planning and management.
California forests face major changes over the next century, but the extent, intensity, and type of change is likely to be variable among different ecosystems. This variability in response will be driven by factors like the ecological tolerances of component species, site histories, the rate and nature of future environmental change, and management policies. In the Sierra Nevada of California, a major ecotone occurs between lower and upper montane forests, at approximately the altitude of deepest winter snowpack and the average freezing elevation in winter storms. Below this line, the Sierra Nevada is dominated by yellow pine and mixed conifer forests, which are mostly moisture-limited systems historically dominated by highly frequent fire; above this line the range is dominated by red fir and subalpine forests, which are mostly energy-limited systems dominated historically by relatively infrequent fire. Additionally, below this line most of the Sierra Nevada is comprised of “working forest”, while above it much of the range is included in roadless and wilderness areas and National Parks. I contrast these two different environments with respect to their historical, current, and likely future conditions, focusing on the impacts of three classes of environmental stressors: climate change, wildfire, and invasive species. Vulnerabilities to these stressors differ appreciably between lower montane and upper montane forests, but climate warming and human population growth, among other things, will likely introduce many lower montane afflictions to the upper montane zone in the not-too-distant future. I discuss what sorts of management actions, both active and passive, might be employed in these different environments to increase ecosystem resilience to future change. At the same time, I caution that human expectations and assumptions about these ecosystems, their permanence and the services they provide must eventually be reconciled with the likelihood that the Sierra Nevada of one-hundred years hence may look little like the Sierra Nevada of today.
One of the greatest challenges forest ecologists face is forecasting how global climate change will influence forest community structure. Locally, forests are expected lose cold-adapted trees while warm-adapted trees increase in abundance. However, potential lags due to slow-growing and long-lived trees paired with species-specific climatic sensitivities could add complexity to compositional changes. We use extensive demographic measurements across a large climatic gradient at Mt. Rainier National Park (WA, USA) to ask 1) how rapid recruitment and mortality occurs in these forests; and 2) whether differences among tree species in their climatic sensitivities are likely to result in idiosyncratic changes to forest communities. We found that successful seedling recruitment is rare, sapling growth rates in forest understories are extremely slow, and mortality of adult trees is low, implying forest community composition may not change rapidly with future climate change. Consistent with this finding is the surprisingly low compositional change in forest communities over the last 35 years, despite significant turnover of individuals. However, tree species differ significantly in recruitment, growth and mortality rates. Moreover, tree growth and seedling recruitment of high-elevation trees is especially sensitive to high snowpack and short growing seasons, suggesting the potential for rapid population growth with warming for these species. Additionally, mortality rates may increase precipitously, especially if climate change alters disturbance regimes. In all, we conclude that future climate change could have large and unpredictable impacts on forest composition at Mt. Rainier, despite the resilience showing by forest communities thus far.
When forest ecosystems develop over millennia, trees live five centuries, and mortality unfolds over decades, direct repeated observation – longitudinal data – may be the only way to understand the fate of organisms. Longitudinal data sets have contributed greatly to our understanding of forests, complementing experimental, modeling, and chronosequence approaches. In western forests, longitudinal data have provided insights into the mechanisms of early seral colonization, tree mortality, and relationships between overstory and understory. Changing climate is changing forests – particularly through altered mortality – and the elusive nature of integrated mechanistic understanding requires refinements to historically productive protocols. The combination of the longitudinal protocols developed by the Smithsonian Center for Tropical Forest Science – originally for examination of tropical species diversity – and those developed by the US Geological Survey for annual tree mortality assessment allows investigation of climate-mediated temperate forest change. The etiologies of the different factors contributing to tree mortality are unique, and only some are expected to respond to climate variability and change. Because tree mortality rates are low (1-5%), large numbers of individuals (≥10,000) must be tracked to understand changing mortality rates, particularly those of less common species or important large-diameter sub-populations. And because mortality factors can be spatially aggregated and density-dependent, the causes and rates of tree mortality depend on the specific relationships between climate and forest spatial structure. If spatial heterogeneity within forests creates pockets of instability where trees experience elevated sensitivity to climate-induced mortality, local spatial structure would modulate the effects of interannual climatic variation on tree mortality rates and causes.
It is hypothesized that climate variability and change impacts forest mosaics through ecological disturbances such as wildfires. However, climate-fire research has primarily focused on understanding drivers of fire frequency and area burned, primarily due to scale mismatches and limited availability of data and computing power. Recent new datasets, however, allow for the characterization of ecological patch metrics cross regions broad enough to investigate climate linkages. One area of particular interest is the occurrence of fire refugia within wildfire perimeters. While much recent research emphasis has been placed on high severity patches within wildfires, it is the unburned patches and low severity refugia that provide critical remnant habitat and serve as seed sources to initiate colonization and succession in recently burned landscapes. These patches also may yield insights into approaches for developing fire resilient landscapes by forest managers and comunities seeking to reduce wildfire hazard. Recent efforts to begin characterizing fire refugia across landscapes have yielded some surprising results, including the absence of a temporal trend in unburned proportion even as research and anecdotal information suggests fires are becoming larger and more severe. These efforts also reveal challenges related to classifying refugia from remotely-sensed data. Here, I present results from nascent efforts to characterize climate drivers and patterns of fire refugia across recently-burned forests in the western US, and contextualize these results in a suggested framework for addressing climate change impacts on forest pattern more broadly.
Private forest managers are often charged with responsibility for maintaining structural complexity, biological diversity, and ecosystem services on their lands. However, little consideration has been given to potential geographic variation in the nature of relationships between elements of forest structure and biodiversity. Decomposition rates of snags and coarse woody debris are mediated by available energy, and the state of decay in snags and coarse woody debris affects their use by birds and other wildlife. Accordingly, the value of stand level structural components to wildlife species may depend on the amount of energy available at the local scale. We sampled montane avian communities across an energy gradient in the Cascades Mountains of Oregon and Washington, USA. Available energy refers to factors influencing vegetation growth including light, heat and precipitation. We refer to Gross Primary Productivity, a satellite derived index representing a collection of those factors. We predicted avian functional diversity would be positively associated with large snag density in a high energy landscape in Oregon and decrease in a low energy landscape in Washington. We also predicted abundance and richness of species within ground foraging guilds would increase with increasing coarse woody debris in higher energy settings, but that coarse woody debris would not be a significant factor in lower energy locations. We fit a Bayesian multispecies site occupancy model to estimate species level covariate effects as well as population level measures of occupancy, including species richness. For abundance data, we fit a Bayesian multispecies version of the N-mixture model. We found nearly all foraging guilds were represented in each stand. As a result, the strength of association between avian functional diversity, snag and coarse woody debris density was small in both high and low energy landscapes. A large and precise negative response of key foraging guild bird abundance to large snag density was observed in the lower energy site. 95 percent posterior credibility intervals for the community-level hyper-parameters for snag density at the low energy landscape did not include zero, indicating a defined overall population-level trend. Large but less precise negative responses of primary guild bird abundance to coarse woody debris density were observed at both high and low energy landscapes. In this case, 95 percent posterior intervals for the community hyper-parameters included zero at both landscapes, indicating little overall population-level trend with respect to coarse woody debris. We discuss how residual forest structure can be managed across a gradient in available energy to conserve avian diversity in forested settings.
Historically, conservation strategies to preserve forested ecosystems included designating lands into a protected area network, such as the National Wilderness Preservation System. As conservation science has developed, protected areas have repeatedly been shown to effectively sustain forest values including native biological diversity, large old trees and old-growth stands, wildlife habitat, and clean water. However, invasive species, atmospheric deposition, and altered disturbance regimes are increasingly recognized as threatening the biodiversity and ecological processes we value from protected areas. Given these compounding impacts, as well as the predicted exacerbating effects of climate change, some conservation scientists have begun to question the appropriateness of protected areas, such as wilderness, in such a profoundly altered world. In some cases, human trammeling in the form or restoration or innovative management may be required to sustain ecological values of forests we hope to protect and convey into the future. Uncertainty about how forested ecosystems will respond to synergistic impacts of the “Anthropocene” precludes our ability to know which strategy will best sustain ecological values on federally-managed lands. Conservation scientists at The Wilderness Society and elsewhere are increasingly viewing this uncertainty as a problem of risk management that requires a portfolio approach where diverse strategies are implemented and adaptive learning adjusts future decisions. I will discuss how this portfolio approach to conservation can be used to guide both management and science in forested ecosystems, while also discussing the ever-relevant role wilderness will play alongside other active management strategies in the age of global change.