Climate, Environment, and Disturbance History Govern Resilience of Western North American Forests

Abstract

Before the advent of intensive forest management and fire suppression, western North American forests exhibited a naturally occurring resistance and resilience to wildfires and other disturbances. Resilience, which encompasses resistance, reflects the amount of disruption an ecosystem can withstand before its structure or organization qualitatively shift to a different basin of attraction. In fire-maintained forests, resilience to disturbance events arose primarily from vegetation pattern-disturbance process interactions at several levels of organization. Using evidence from 15 ecoregions, spanning forests from Canada to Mexico, we review the properties of forests that reinforced qualities of resilience and resistance. 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 important 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 Hessburg et al. Resilience in North American Forests disruptions. We discuss the role of the regional climate 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 strongly depends on scale and human social values. It involves human communities actively working with the ecosystems they depend on, and the processes that shape them, to adapt landscapes, species, and human communities to climate change while maintaining core ecosystem processes and services. Finally, it compels us to embrace management approaches that incorporate ongoing disturbances and anticipated effects of climatic changes, and to support dynamically shifting patchworks of forest and non-forest. Doing so could make these shifting forest conditions and wildfire regimes less disruptive to individuals and society.

Widespread severe wildfires under climate change lead to increased forest homogeneity in dry mixed‐conifer forests

“…This loss of heterogeneity can have detrimental effects. Landscapes containing multiple species and forest types are more resilient to disturbance through functional redundancy (Oliver 2015, van der Plas 2016, variable response to a disturbance among forest patches (Hessburg et al 2019), and variable responses to different disturbances (e.g., windthrow, insect infestations, disease; Folke et al 2004). They also provide a variety of wildlife habitats, natural resources, and recreation environments that produce a broad range of ecosystem services.…”

Local forest structure variability increases resilience to wildfire in dry western U.S. coniferous forests

“…Forests are essential components of the biosphere, and ensuring their persistence is of high management priority given their large carbon stores and other valued ecosystem services (Trumbore et al ; Higuera et al ). Modern forests are subject to disturbances that are increasingly frequent, intense and entangled with human society, which may compromise their resilience and their ability to persist (Millar & Stephenson ; Seidl et al ; Schoennagel et al ; Hessburg et al ; McWethy et al ). A resilient forest can absorb disturbances and may reorganise, but is unlikely to transition to an alternate vegetation type in the long run (Holling ; Walker et al ).…”

Fine‐scale fire patterns mediate forest structure in frequent‐fire ecosystems

“…Forest resilience is the capacity for a system to regain its structure, function, and feedbacks following a disturbance (Holling 1973, Hessburg et al 2019). Forest resistance is often considered a core part of resilient systems (Ingrisch and Bahn 2018) and is defined as the difficulty or ease of changing the state, function, and pattern‐process linkages of an ecosystem (Holling 1973).…”