Tyler L. McIntosh

Background: Wildfire activity is increasing across western United States forests, leading to more frequent reburns, or areas that burn multiple times within short intervals (approximately two decades). Prior fire can reduce subsequent burn severity through fuel limitation, but this effect is not consistent across landscapes or climate conditions. Understanding when reburns burn less severely and when they do not is critical for predicting future fire behavior and informing management. Here, we evaluated how reburn severity varies with fire history, time since fire, and atmospheric drying across reburn events in 23 EPA Level III ecoregions in the western U.S. Results: Using 755 small and large forested reburn events from 2001–2024 and hierarchical Bayesian models of modeled burn severity (Composite Burn Index, CBI, range 0–3), we found that predicted severity was about 15% higher in previously unburned areas than in reburned areas. However, this effect was structured by interactions between fuel history and atmospheric drying (vapor pressure deficit, VPD). Within reburned areas, reburn severity increased with time since fire and atmospheric drying, with both effects depending on initial fire severity. Predicted severity ranged from ~0.6 CBI under low VPD and shorter intervals to ~1.7 CBI under high VPD and longer intervals of the observed time range. Atmospheric drying had a stronger effect following low-severity initial fires, whereas time since fire had a stronger effect following higher-severity initial fires, indicating a shift from climate-mediated to fuel-mediated controls. Across ecoregions, these interactions reveal multiple pathways to elevated reburn severity, with interval effects strongest in more productive systems and weaker in drier interior regions. Conclusions: Reburn severity is governed by interacting effects of fire history, time since fire, and atmospheric drying, rather than prior fire alone. While prior fire often reduces subsequent burn severity, this buffering effect weakens over time and under extreme fire weather. The moderating effect of prior fire therefore diminishes over time and is contingent on environmental conditions. Strategies that rely on recent fire, such as managed wildfire or prescribed burning, to reduce future severity may provide short-term benefits but are unlikely to remain reliable as fire weather becomes increasingly warm and dry.

Weather and climate extremes are transforming ecosystems in ways that threaten society through the disruption of nature’s contributions to people (NCP) from food and water security to disaster mitigation and mental well-being. While scientists and policy makers increasingly recognize the need to account for nature-related losses from climate-driven weather extremes, major research gaps remain, particularly in how climate-altered disturbance regimes will impact NCP and associated human well-being. Addressing this gap is critical for climate mitigation and adaptation planning, because the loss of NCP amplifies social-ecological vulnerability and exacerbates climate risks. However, this cross-systems dynamic remains underrepresented in most climate risk models. Climate risk modeling offers a probabilistic view of evolving hazard regimes under climate change, while ecological research provides mechanistic insight into ecosystem responses to disturbance. We argue that bridging these fields is essential to increasing societal resilience to climate change and propose an interdisciplinary research agenda to integrate social-ecological dynamics with a climate risk perspective. Advancing this agenda is critical to project, anticipate, and plan for ecosystem-related impacts of weather and climate extremes on society.

Mountains comprise ∼30% of the Earth's surface, but mercury (Hg) cycling in these regions remains understudied, particularly in the semi‐arid western U.S. where strong climatic and ecological gradients in mountainous landscapes influence Hg deposition, retention, and bioaccumulation. In this study, we quantified growing season inputs, storage, and bioaccumulation of Hg along a ∼2,000 m elevation gradient in the Colorado Rocky Mountains, spanning the plains to the alpine. We measured Hg in atmospheric deposition, vegetation, soil, and 12‐day‐old chickadees. Accounting for percent canopy cover, open precipitation was the largest component of atmospheric deposition at all elevations, followed by throughfall and litterfall fluxes. Atmospheric Hg fluxes peaked at mid‐elevations, likely due to cloud‐cap dynamics and denser canopy cover. Total gaseous Hg and precipitation fluxes were highest at low elevations, likely reflecting local emissions and meteorological pooling. Surface soil Hg storage was more strongly predicted by organic matter content ( R ² = 0.49; p < 0.01) and water retention ( R ² = 0.45; p < 0.01) than by elevation ( R ² = 0.21; p < 0.05). Alpine soils (66.3 ± 25.3 ng g ⁻¹ ) had significantly higher total Hg concentrations than lower elevations (−1 ; p < 0.01), likely reflecting slower organic matter turnover. Soils on north‐facing slopes also retained significantly higher pools of Hg in surface soils compared with south‐ and east‐facing slopes. Vegetation Hg pools were greatest in the alpine region, likely due to long‐lived plant species. Methylmercury (MeHg) concentrations in chickadee feathers peaked at mid‐elevations (205 ± 155 ng g ⁻¹ ), corresponding to higher ecosystem Hg inputs via throughfall. Our results show that deposition, canopy cover, and meteorological conditions—not elevation alone—predict Hg retention and bioaccumulation.

Wildfires are integral for western US forests that have evolved with fire. Here we define “good wildfire” as areas that burn in an ecologically beneficial way, with a severity and return interval analogous to their historical fire regimes prior to European settlement. When severities match what an ecosystem historically experienced they can regulate forest structure while promoting regeneration, even in a warming climate ¹ . We quantified the amount of forested area (i.e., deciduous, conifer, or mixed forest types) burned with a severity and frequency matching its regime, and compared that to the area of prescribed burns in forests (2010-2020). Of forests that burned in the western US, 49% of the area burned as low-moderate severity good wildfire. High severity good wildfire (in systems that historically experienced this type of fire) represented an additional 9% of forest area burned, bringing the total area of good wildfire to 58% of forested area burned. The low-moderate severity good wildfires burned 3.1 million forest ha (N = 18,061 events), more than double the 1.4 million ha of prescribed burning (N = 24,817 events on federal land) over the same period. Knowing that fires are likely going to increase in frequency and area with warming ² , our key challenge will be promoting good wildfire while still protecting lives and property.

The most destructive and deadly wildfires in US history were also fast. Using satellite data, we analyzed the daily growth rates of more than 60,000 fires from 2001 to 2020 across the contiguous US. Nearly half of the ecoregions experienced destructive fast fires that grew more than 1620 hectares in 1 day. These fires accounted for 78% of structures destroyed and 61% of suppression costs ($18.9 billion). From 2001 to 2020, the average peak daily growth rate for these fires more than doubled (+249% relative to 2001) in the Western US. Nearly 3 million structures were within 4 kilometers of a fast fire during this period across the US. Given recent devastating wildfires, understanding fast fires is crucial for improving firefighting strategies and community preparedness.