Peter Z. Fulé’s research while affiliated with Northern Arizona University and other places

Fire regimes are changing across the globe, with new wildfire behaviour phenomena and increasing impacts felt, especially in ecosystems without clear adaptations to wildfire. These trends pose significant challenges to the scientific community in understanding and communicating these changes and their implications, particularly where we lack underlying scientific evidence to inform decision-making. Here, we present a perspective on priority directions for wildfire science research—through the lens of academic and government wildfire scientists from a historically wildfire-prone (USA) and emerging wildfire-prone (UK) country. Key topic areas outlined during a series of workshops in 2023 were as follows: (A) understanding and predicting fire occurrence, fire behaviour and fire impacts; (B) increasing human and ecosystem resilience to fire; and (C) understanding the atmospheric and climate impacts of fire. Participants agreed on focused research questions that were seen as priority scientific research gaps. Fire behaviour was identified as a central connecting theme that would allow critical advances to be made across all topic areas. These findings provide one group of perspectives to feed into a more transdisciplinary outline of wildfire research priorities across the diversity of knowledge bases and perspectives that are critical in addressing wildfire research challenges under changing fire regimes. This article is part of the theme issue ‘Novel fire regimes under climate changes and human influences: impacts, ecosystem responses and feedbacks’.

The frequency and severity of drought events are predicted to increase due to anthropogenic climate change, with cascading effects across forested ecosystems. Management activities such as forest thinning and prescribed burning, which are often intended to mitigate fire hazard and restore ecosystem processes, may also help promote tree resistance to drought. However, it is unclear whether these treatments remain effective during the most severe drought conditions or whether their impacts differ across environmental gradients. We used tree‐ring data from a system of replicated, long‐term (>20 years) experiments in the southwestern United States to evaluate the effects of forest restoration treatments (i.e., evidence‐based thinning and burning) on annual growth rates (i.e., basal area increment; BAI) of ponderosa pine (Pinus ponderosa), a broadly distributed and heavily managed species in western North America. The study sites were established at the onset of the most extreme drought event in at least 1200 years and span much of the climatic niche of Rocky Mountain ponderosa pine. Across sites, tree‐level BAI increased due to treatment, where trees in treated units grew 133.1% faster than trees in paired, untreated units. Likewise, trees in treated units grew an average of 85.6% faster than their pre‐treatment baseline levels (1985 to ca. 2000), despite warm, dry conditions in the post‐treatment period (ca. 2000–2018). Variation in the local competitive environment promoted variation in BAI, and larger trees were the fastest‐growing individuals, irrespective of treatment. Tree thinning and prescribed fire altered the climatic constraints on growth, decreasing the effects of belowground moisture availability and increasing the effects of atmospheric evaporative demand over multi‐year timescales. Our results illustrate that restoration treatments can enhance tree‐level growth across sites spanning ponderosa pine's climatic niche, even during recent, extreme drought events. However, shifting climatic constraints, combined with predicted increases in evaporative demand in the southwestern United States, suggest that the beneficial effects of such treatments on tree growth may wane over the upcoming decades.

Background Prescribed fire is vital for fuel reduction and ecological restoration, but the effectiveness and fine-scale interactions are poorly understood. Aims We developed methods for processing uncrewed aircraft systems (UAS) imagery into spatially explicit pyrometrics, including measurements of fuel consumption, rate of spread, and residence time to quantitatively measure three prescribed fires. Methods We collected infrared (IR) imagery continuously (0.2 Hz) over prescribed burns and one experimental calibration burn, capturing fire progression and combustion for multiple hours. Key results Pyrometrics were successfully extracted from UAS-IR imagery with sufficient spatiotemporal resolution to effectively measure and differentiate between fires. UAS-IR fuel consumption correlated with weight-based measurements of 10 1-m² experimental burn plots, validating our approach to estimating consumption with a cost-effective UAS-IR sensor (R² = 0.99; RMSE = 0.38 kg m⁻²). Conclusions Our findings demonstrate UAS-IR pyrometrics are an accurate approach to monitoring fire behaviour and effects, such as measurements of consumption. Prescribed fire is a fine-scale process; a ground sampling distance of <2.3 m² is recommended. Additional research is needed to validate other derived measurements. Implications Refined fire monitoring coupled with refined objectives will be pivotal in informing fire management of best practices, justifying the use of prescribed fire and providing quantitative feedback in an uncertain environment.

Changes in forest structure and shifts in tree species composition have occurred globally due to climate change and altered disturbance regimes. With climate trending toward warmer and drier conditions, these altered forest communities may reorganize in diverse and unpredictable ways. This is especially true in mountain environments where a range of vegetation types and abiotic conditions coexist. In this study, we used long-term permanent plot data from a site spanning broad environmental gradients to assess regeneration and mortality patterns in populations of aspen (Populus tremuloides). The study site, located on the San Francisco Peaks, Ari-zona, USA, is near the hot, dry edge of the species' range and has experienced compounding pressure from extreme drought, chronic ungulate browsing, and wildfire in the past two decades. Over a 20-year study period, spanning one of the most prolonged drought periods in at least 1200 years, aspen overstory mortality averaged 42 % and was most common in smaller, younger trees and at lower elevations. Aspen regeneration density increased 13 % and was found in a greater proportion of study sites. However, we observed a noticeable lack of stems in the tallest regeneration size class (>200 cm) and the smaller tree size class (2.5-15 cm in diameter), potentially indicating a demographic bottleneck whereby few trees are recruiting into the overstory. Likewise, prolific aspen suckering occurred after a 2001 wildfire, although regeneration density eventually decreased to pre-fire levels, with <1 % of individuals reaching heights >200 cm. Aspen regeneration densities showed the greatest increases in cool, wet sites and beneath open forest canopies. Disturbances function as catalysts for aspen regeneration, but persistence of aspen stands depends on recruitment of stems into overstory size classes, a process that is limited, particularly on lower and more exposed sites.

The frequency and severity of drought events are predicted to increase due to anthropogenic climate change, with cascading effects across forested ecosystems. Management activities such as forest thinning and prescribed burning, which are often intended to mitigate fire hazard and restore ecosystem processes, may also help promote tree resistance to drought. However, it is unclear if these treatments remain effective during the most severe drought conditions or if their impacts differ across environmental gradients. We used tree-ring data from a system of replicated, long-term (> 20 years) experiments in the southwestern United States (US) to evaluate the effects of forest restoration treatments (i.e., evidence-based thinning and burning) on annual growth rates (i.e., basal area increment; BAI) of ponderosa pine (Pinus ponderosa), a broadly distributed and heavily managed species in western North America. The study sites were established at the onset of the most extreme drought event in at least 1,200 years and span much of the climatic niche of Rocky Mountain ponderosa pine. Across sites, tree-level BAI increased due to treatment, where trees in treated units grew 133.1% faster than trees in paired, untreated units. Likewise, trees in treated units grew an average of 85.6% faster than their pre-treatment baseline levels (1985 to ca. 2000), despite warm, dry conditions in the post-treatment period (ca. 2000 to 2018). Variation in the local competitive environment promoted variation in BAI, and larger trees were the fastest-growing individuals, irrespective of treatment. Tree thinning and prescribed fire altered the climatic constraints on growth, decreasing the effects of belowground moisture availability and increasing the effects of atmospheric evaporative demand over multi-year timescales. Our results illustrate that restoration treatments can enhance tree-level growth across sites spanning ponderosa pine's climatic niche, even during recent, extreme drought events. However, shifting climatic constraints, combined with predicted increases in evaporative demand in the southwestern US, suggest that the beneficial effects of such treatments on tree growth may wane over the upcoming decades.

The 21st century’s warming climate threatens aspen (Populus tremuloides) growth in the southern Rocky Mountains (western US), endangering ecosystem diversity, functionality, and associated services. This study linked aspen growth with temperature and moisture variations, assessing how warmer climates and seasonal changes affect ring widths. Sampling 211 aspen trees from 12 latitudinally distributed sites, we compiled three regional aspen chronologies spanning 1913–2018. We investigated climate-growth associations using correlation and modeling (climwin) techniques. Results revealed that aspen at the trailing southern edge of the Rocky Mountains are vulnerable to drought, where elevated temperatures and diminished precipitation emerge as primary factors contributing to their reduced growth. In contrast, aspens in the northern region of the gradient exhibited a positive growth response to rising temperatures, potentially linked to the alleviation of growth-limiting cold temperatures. However, while pre-2000s droughts increased growth, recent droughts elicited growth reductions in the North; suggesting that with continued warming, northern populations will increasingly face sensitivity to droughts akin to their southern counterparts. These findings emphasize the increased vulnerability of southern Rocky Mountain aspen populations to climate change-induced growth constraints, particularly in the anticipated warmer and drier conditions of the 21st century. This study is crucial to understanding aspen responses to climate fluctuations in the region.