Persistent changes in tree mortality rates can alter forest structure, composition, and ecosystem services such as carbon sequestration. Our analyses of longitudinal data from unmanaged old forests in the western United States showed that background (noncatastrophic) mortality rates have increased rapidly in recent decades, with doubling periods ranging from 17 to 29 years among regions. Increases were also pervasive across elevations, tree sizes, dominant genera, and past fire histories. Forest density and basal area declined slightly, which suggests that increasing mortality was not caused by endogenous increases in competition. Because mortality increased in small trees, the overall increase in mortality rates cannot be attributed solely to aging of large trees. Regional warming and consequent increases in water deficits are likely contributors to the increases in tree mortality rates.

Fire is a powerful ecological and evolutionary force that regulates organismal traits, population sizes, species interactions, community composition, carbon and nutrient cycling and ecosystem function. It also presents a rapidly growing societal challenge, due to both increasingly destructive wildfires and fire exclusion in fire‐dependent ecosystems. As an ecological process, fire integrates complex feedbacks among biological, social and geophysical processes, requiring coordination across several fields and scales of study. Here, we describe the diversity of ways in which fire operates as a fundamental ecological and evolutionary process on Earth. We explore research priorities in six categories of fire ecology: (a) characteristics of fire regimes, (b) changing fire regimes, (c) fire effects on above‐ground ecology, (d) fire effects on below‐ground ecology, (e) fire behaviour and (f) fire ecology modelling. We identify three emergent themes: the need to study fire across temporal scales, to assess the mechanisms underlying a variety of ecological feedbacks involving fire and to improve representation of fire in a range of modelling contexts. Synthesis: As fire regimes and our relationships with fire continue to change, prioritizing these research areas will facilitate understanding of the ecological causes and consequences of future fires and rethinking fire management alternatives.

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.

Tree radial growth and seedling establishment of Nothofagus pumilio at alpine treeline near 40° S latitude in Chile and Argentina show time‐ and site‐dependent relationships to interannual‐and decadal‐scale climate variation. Six treelines were sampled at two spatial scales corresponding to regional and local climates. A shift in climate from cool–wet to warm–dry conditions facilitated comparison of climate–vegetation relationships during two distinct periods: 1957–1976 and 1977–1996. For each treeline, tree radial growth and seedling establishment were correlated against monthly and seasonal temperature, precipitation, moisture availability, and two indices of El Niño–Southern Oscillation (ENSO): southern oscillation index (SOI) and sea surface temperature (SST). Four key aspects of climatic influences on N. pumilio radial growth and seedling establishment were as follows. (1) The relationship between krummholz radial growth and temperature variation was nonlinear. (2) Moisture availability was the dominant climatic factor influencing seedling establishment, although temperature–precipitation interactions resulted in variability among study areas. (3) Climate conditions that facilitated Nothofagus pumilio seedling establishment were distinct, and often opposite, from those that enhanced radial growth. (4) The relationships of radial growth and seedling demography with climate and ENSO differed among study areas and have been unstable over the past 40 years. Observed spatial and temporal instabilities in vegetation–climate relationships demonstrate the complexity of treeline dynamics in northern Patagonia under a changing climate. We conclude that a directional increase in temperature, as predicted by current global climate scenarios, will not necessarily result in an upslope expansion of the N. pumilio forests growing at altitudinal treeline in northern Patagonia.

Implementation of wildfire-and climate-adaptation strategies in seasonally dry forests of western North America is impeded by numerous constraints and uncertainties. After more than a century of resource and land use change, some question the need for proactive management, particularly given novel social, ecological, and climatic conditions. To address this question, we first provide a framework for assessing changes in landscape conditions and fire regimes. Using this framework, we then evaluate evidence of change and lack of change in contemporary conditions relative to those maintained by active fire regimes, i.e., those uninterrupted by a century or more of human-induced fire exclusion. The cumulative results of more than a century of research document a persistent and substantial fire deficit and widespread alterations to ecological structures and functions. These changes are not necessarily apparent at all spatial scales or in all dimensions of fire regimes and forest and nonforest conditions. Nonetheless, loss of the once abundant influence of low-and moderate-KEYWORDS

Abstract. We inferred climate drivers of regionally synchronous surface fires from 1651 to 1900 at 15 sites with existing annually accurate fire-scar chronologies from forests dominated by ponderosa pine or Douglas-fir in the inland Northwest (interior Oregon, Washington and southern British Columbia). Years with widespread fires (35 years with fire at 7 to 11 sites) had warm spring-summers and warm-dry summers, whereas years with no fires at any site (18 years) had the opposite conditions. Spring climate likely affected the length of the fire season via the effects of snowmelt on soil and fuel moisture, whereas summer climate influenced fuel moisture during the fire season. Climate in prior years was not a significant driver of regionally synchronous surface fires, likely because fuels were generally sufficient for the ignition and spread of such fires in these forests. Fires occurred significantly more often than expected by chance when the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) were both warm phase and less often when they were both cool phase. Interactions between large-scale climate patterns influenced fire synchrony in the inland Northwest because phases of ENSO and PDO were associated with changes in the frequency of warm-dry v. cool-wet spring-summer climate.

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For over 100 years, mountain treelines have been the subject of varied research endeavors and remain a strong area of investigation. The purpose of this paper is to examine aspects of the epistemology of mountain treeline research-that is, to investigate how knowledge on treelines has been acquired and the changes in knowledge acquisition over time, through a review of fundamental questions and approaches. The questions treeline researchers have raised and continue to raise have undoubtedly directed the current state of knowledge. A continuing, fundamental emphasis has centered on seeking the general cause of mountain treelines, thus seeking an answer to the question, ''What causes treeline?'' with a primary emphasis on searching for ecophysiological mechanisms of low-temperature limitation for tree growth and regeneration. However, treeline research today also includes a rich literature that seeks local, landscape-scale causes of treelines and reasons why treelines vary so widely in three-dimensional patterns from one location to the next, and this approach and some of its consequences are elaborated here. In recent years, both lines of research have been motivated greatly by global climate change. Given the current state of knowledge, we propose that future research directions focused on a spatial approach should specifically address cross-scale hypotheses using statistics and simulations designed for nested hierarchies; these analyses will benefit from geographic extension of treeline research.