As temperatures continue rising, the direction, magnitude, and tempo of change in disturbance-prone forests remain unresolved. Even forests long resilient to stand-replacing fire face uncertain futures, and efforts to project changes in forest structure and composition are sorely needed to anticipate future forest trajectories. We simulated fire (incorporating fuels feedbacks) and forest dynamics on five landscapes spanning the Greater Yellowstone Ecosystem (GYE) to ask: (1) How and where are forest landscapes likely to change with 21 st -century warming and fire activity? (2) Are future forest changes gradual or abrupt, and do forest attributes change synchronously or sequentially? (3) Can forest declines be averted by mid-21 stcentury stabilization of atmospheric greenhouse gas (GHG) concentrations? We used the spatially explicit individual-based forest model iLand to track multiple attributes (forest extent, stand age, tree density, basal area, aboveground carbon stocks, dominant forest types, species occupancy) through 2100 for six climate scenarios. Hot-dry climate scenarios led to more fire, but stand-replacing fire peaked in mid-century and then declined even as annual area burned continued to rise. Where forest cover persisted, previously dense forests were converted to sparse young woodlands. Increased aridity and fire drove a ratchet of successive abrupt declines (i.e., multiple annual landscape-level changes ≥ 20%) in tree density, basal area and extent of older (>150 yr) forests, whereas declines in carbon stocks and mean stand age were always gradual.Forest changes were asynchronous across landscapes, but declines in stand structure always preceded reductions in forest extent and carbon stocks. Forest decline was most likely in less topographically complex landscapes dominated by fire-sensitive tree species (Picea engelmannii, Abies lasiocarpa, Pinus contorta var. latifolia) and where fire resisters (Pseudotsuga menziesii var. glauca) were not already prevalent. If current GHG emissions continue unabated (RCP 8.5) and aridity increases, a suite of forest changes would transform the GYE, with cascading effects on biodiversity and myriad ecosystem services. However, stabilizing GHG concentrations by mid-century (RCP 4.5) would slow the ratchet, moderating fire activity and dampening the magnitude and rate of forest change. Monitoring changes in forest structure may serve as an operational early warning indicator of impending forest decline.

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Boreal forest and tundra biomes are key components of the Earth system because the mobilization of large carbon stocks and changes in energy balance could act as positive feedbacks to ongoing climate change. In Alaska, wildfire is a primary driver of ecosystem structure and function, and a key mechanism coupling high‐latitude ecosystems to global climate. Paleoecological records reveal sensitivity of fire regimes to climatic and vegetation change over centennial–millennial time scales, highlighting increased burning concurrent with warming or elevated landscape flammability. To quantify spatiotemporal patterns in fire‐regime variability, we synthesized 27 published sediment‐charcoal records from four Alaskan ecoregions, and compared patterns to paleoclimate and paleovegetation records. Biomass burning and fire frequency increased significantly in boreal forest ecoregions with the expansion of black spruce, ca. 6,000–4,000 years before present (yr BP). Biomass burning also increased during warm periods, particularly in the Yukon Flats ecoregion from ca. 1,000 to 500 yr BP. Increases in biomass burning concurrent with constant fire return intervals suggest increases in average fire severity (i.e., more biomass burning per fire) during warm periods. Results also indicate increases in biomass burning over the last century across much of Alaska that exceed Holocene maxima, providing important context for ongoing change. Our analysis documents the sensitivity of fire activity to broad‐scale environmental change, including climate warming and biome‐scale shifts in vegetation. The lack of widespread, prolonged fire synchrony suggests regional heterogeneity limited simultaneous fire‐regime change across our study areas during the Holocene. This finding implies broad‐scale resilience of the boreal forest to extensive fire activity, but does not preclude novel responses to 21st‐century changes. If projected increases in fire activity over the 21st century are realized, they would be unprecedented in the context of the last 8,000 yr or more.

Subalpine forests that historically burned every 100–300 yr are expected to burn more frequently as climate warms, perhaps before trees reach reproductive maturity or produce a serotinous seedbank. Tree regeneration after short‐interval (<30‐yr) high‐severity fire will increasingly rely on seed dispersal from unburned trees, but how dispersal varies with age and structure of surrounding forest is poorly understood. We studied wind dispersal of three conifers (Picea engelmannii, Abies lasiocarpa, and Pinus contorta var. latifolia, which can be serotinous and nonserotinous) after a stand‐replacing fire that burned young (≤30 yr) and older (>100 yr) P. contorta forest in Grand Teton National Park (Wyoming, USA). We asked how propagule pressure varied with time since last fire, how seed delivery into burned forest varied with age and structure of live forest edges, what variables explained seed delivery into burned forest, and how spatial patterns of delivery across the burned area could vary with alternate patterns of surrounding live forest age. Seeds were collected in traps along 100‐m transects (n = 18) extending from live forest edges of varying age (18, 30, and >100 yr) into areas of recent (2‐yr) high‐severity fire, and along transects in live forests to measure propagule pressure. Propagule pressure was low in 18‐yr‐old stands (~8 seeds/m2) and similarly greater in 30‐ and 100‐yr‐old stands (~32 seeds/m2). Mean dispersal distance was lowest from 18‐yr‐old edges and greatest from >100‐yr‐old edges. Seed delivery into burned forest declined with increasing distance and increased with height of trees at live forest edges, and was consistently higher for P. contorta than for other conifers. Empirical dispersal kernels revealed that seed delivery from 18‐yr‐old edges was very low (≤2.4 seeds/m2) and concentrated within 10 m of the live edge, whereas seed delivery from >100‐yr‐old edges was >4.9 seeds/m2 out to 80 m. When extrapolated throughout the burned landscape, estimated seed delivery was low (<49,400 seeds/ha) in >70% of areas that burned in short‐interval fire (<30 yr). As fire frequency increases, immaturity risk will be compounded in short‐interval fires because seed dispersal from surrounding young trees is limited.

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