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Climate warming is expected to drive upward and poleward shifts at the leading edge of tree species ranges. Disturbance has the potential to accelerate these shifts by altering biotic and abiotic conditions, though this potential is likely to vary by disturbance type. In this study, we assessed whether recent wildfires and spruce beetle outbreaks promoted upward range expansion of trembling aspen.

The San Juan Mountains of southern Colorado, USA (37°34′–37°50′N, 106°49′–107°21′W).

Populus tremuloides.

We used aerial imagery to determine the upper elevational limit of adult aspen and conducted seedling surveys at and above this upper limit in burned and unburned areas, which had already incurred high canopy mortality due to spruce bark beetle (Dendroctonus rufipennis) outbreaks. We compared characteristics of burned versus unburned bark beetle‐killed sites and assessed microsite conditions related to aspen seedling establishment using generalized linear models and interaction indices.

Aspen seedling establishment occurred upslope of its previous range within burns, but not in unburned areas, despite severe beetle‐driven canopy mortality across all sites before the fire. Aspen seedling establishment was associated more with the light and mineral soil created by fire than the presence of nearby seed sources. Aspen seedlings were associated with nurse objects suchmore »as logs and rocks at the highest elevations, where these objects may ameliorate a range of stressors associated with the high elevation range boundary.

Not all disturbance types are equal in promoting tree species migrations at the leading edge. Range shifts can be highly localized, and microsites are important for driving local range expansions in transitional environments. The mosaic of future disturbances across the landscape will drive forest compositional shifts, depending on the disturbance types and the species they promote.

Climate warming is increasing fire activity in many of Earth’s forested ecosystems. Because fire is a catalyst for change, investigation of post‐fire vegetation response is critical to understanding the potential for future conversions from forest to non‐forest vegetation types. We characterized the influences of climate and terrain on post‐fire tree regeneration and assessed how these biophysical factors might shape future vulnerability to wildfire‐driven forest conversion.

Montane forests, Rocky Mountains, USA.

1981–2099.

Pinus ponderosa;Pseudotsuga menziesii.

We developed a database of dendrochronological samples (n = 717) and plots (n = 1,301) in post‐fire environments spanning a range of topoclimatic settings. We then used statistical models to predict annual post‐fire seedling establishment suitability and total post‐fire seedling abundance from a suite of biophysical correlates. Finally, we reconstructed recent trends in post‐fire recovery and projected future dynamics using three general circulation models (GCMs) under moderate and extreme CO₂emission scenarios.

Though growing season (April–September) precipitation during the recent period (1981–2015) was positively associated with suitability for post‐fire tree seedling establishment, future (2021–2099) trends in precipitation were widely variable among GCMs, leading to mixed projections of future establishment suitability. In contrast, climatic water deficit (CWD), which is indicative of warm, dry conditions, was negatively associated with post‐firemore »seedling abundance during the recent period and was projected to increase throughout the southern Rocky Mountains in the future. Our findings suggest that future increases in CWD and an increased frequency of extreme drought years will substantially reduce post‐fire seedling densities.

This study highlights the key roles of warming and drying in declining forest resilience to wildfire. Moisture stress, driven by macroclimate and topographic setting, will interact with wildfire activity to shape future vegetation patterns throughout the southern Rocky Mountains, USA.