Abstract
Resilience and resistance concepts have broad application to ecology and society. Resilience is an emergent property that reflects the amount of disruption a system can withstand before its structure or organization uncharacteristically shift. Resistance is a component of resilience. Before the advent of intensive forest management and fire suppression, western North American forests exhibited a naturally occurring resilience to wildfires and other disturbances. Using evidence from ten ecoregions, spanning forests from Canada to Mexico, we review the properties of these forests that reinforced those qualities. We show examples of multi-level landscape resilience, of feedbacks within and among levels, and how conditions have changed under climatic and management influences. We highlight geographic similarities and differences in the structure and organization of historical landscapes, their forest types, and in the conditions that have changed resilience and resistance to abrupt or large-scale disruptions. We discuss the regional climates’ role in episodically or abruptly reorganizing plant and animal biogeography, and forest resilience and resistance to disturbances. We give clear examples of these changes and suggest that managing for resilient forests is a construct that is strongly dependent on scale and social values. It involves human community adaptations that work with the ecosystems they depend on and the processes that shape them. It entails actively managing factors and exploiting mechanisms that drive dynamics at each level as means of adapting landscapes, species, and human communities to climate change, and maintaining core ecosystem functions, processes, and services. Finally, it compels us to prioritize management that incorporates ongoing disturbances and anticipated effects of climatic changes, to support dynamically shifting patchworks of forest and nonforest. Doing so will make these shifting forest conditions and wildfire regimes more gradual and less disruptive to individuals and society.
Highlights
The concepts of resilience and resistance broadly apply to ecological systems; they reflect the allied capacities of systems to regain and retain their fundamental structure, organization, and processes when impacted by stresses or disturbances (Holling, 1973)
We examine the properties of dry, moist, cold, and boreal forests of the Western United States (US), Mexico (MX), and British Columbia (BC), Canada that make them resilient and resistant to wildfires and other stressors
Across western North American ecoregions, we find that a strong core of emergent properties historically conferred forest resilience and resistance to disturbances and climatic changes
Summary
The concepts of resilience and resistance broadly apply to ecological systems; they reflect the allied capacities of systems to regain and retain their fundamental structure, organization, and processes when impacted by stresses or disturbances (Holling, 1973). Thickbarked tree species (e.g., ponderosa pine-Pinus ponderosa, Jeffrey pine-P. jeffreyi, Douglas-fir-Pseudotsuga menziesii, and western larch-Larix occidentalis), and certain fire-adapted understory vegetation (e.g., bunchgrasses-Festuca spp., Agropyron spp., Poa spp., Koelaria spp., pinegrasses-Calamagrostis spp., buckbrushCeanothus spp., sagebrush-Artemisia spp., and bitterbrushPurshia spp.) exhibited resistance to surface fires, surviving, or resprouting from roots or buried seeds in the weeks to years following fire Between this simplified dichotomy of climate- and fuellimited are so-called “hybrid” systems (McKenzie and Littell, 2017), and they include a variety of mixed-conifer forests. For those hybrid ecosystems that characteristically supported moderate-severity fire, and in forests where high tree densities reflect natural postfire cohorts (Schoennagel et al, 2004), increased moisture deficits could lead to increasing fire severity, especially where prior land use and fire suppression have contributed to fuel ladders and elevated surface fuels These ecosystems are vulnerable to wildfires, as species traits that historically conferred resistance to low- and moderate-severity fires neither provide resistance nor resilience to crownfires
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