Indigenous fire management and cross-scale fire-climate relationships in the Southwest United States from 1500 to 1900 CE

The influence of changing fire regimes on specialized plant-animal interactions

Ecological effects of changing fire regimes are well documented for plant and animal populations, but less is known about how fire influences, and is influenced by, specialized plant-animal interactions. In this review, we identified mutualistic (pollination, seed dispersal and food provision), commensal (habitat provision) and antagonistic (seed predation, herbivory and parasitism) plant-animal interactions from fire-prone ecosystems. We focused on specialized interactions where a single genus depended on one to two genera in a single family of plant or animal. We categorized the plant partner's post-fire reproductive mode to assess the likely outcome of changing fire regimes on ecological functions provided by these interactions. Traits underlying specialization in fire-prone ecosystems for plants were: post-fire reproductive mode, time to maturity, morphology and phenology; and, for animals: dispersal, specialized organs, nesting and egg deposition substrates, plant consumption behaviours and pollinator behaviours. Finally, we identified a number of cases where stabilizing feedbacks maintained plant-animal interactions under natural fire regimes. Potential reinforcing feedbacks were also identified, but were more likely to happen abruptly and result in collapse of the plant-animal partnership, or partner switching. Our synthesis reveals how fire regime changes impact fire-dependent specialist plant-animal interactions and potentially drive eco-evolutionary dynamics in fire-prone ecosystems globally.This article is part of the theme issue 'Novel fire regimes under climate changes and human influences: impacts, ecosystem responses and feedbacks'.

Integrating the physical harvesting of dead wood into fuel treatments to reduce wildfire hazards and enhance carbon benefits

A century-long fire suppression policy, coupled with global warming and severe drought, has led to increased incidence of devastating wildfires in the forests of the western United States that deviate from historical fire regimes. Forest managers commonly use mechanical treatments combined with prescribed burning (Rx burn) to reduce fuels, but the intensity of treatment efforts and efficacy of Rx burning in particular for reducing extreme fire risks and emissions, restoring historic fire regimes, and enhancing ecosystem services such as carbon sequestration is increasingly questioned in the context of global warming and wildland-urban interface enhancements of extreme fire risk. Here, we aim to examine the effectiveness of integrating physical harvesting - a surface fuel removal approach, minus subsequent combustion, equivalent to historic firewood collection practices by pre-Columbian Indigenous Peoples - as a fuel treatment alternative to minimize wildfire hazard and enhance carbon benefits from reduced emission and increased sequestration potentials. We utilized field plots data from Sierra Nevada range, California, USA, mixed conifer forests to simulate the outcomes of various forest management alternatives involving one or a combination of fuel reduction treatments on wildfire behavior and carbon pool using the Fire and Fuel Extension of Forest Vegetation Simulator (FFE-FVS). Carbon benefits were computed using standard conversion and substitution factors from published literature. Forest management alternatives involving a combination of fuel reduction treatments such as thinning, physical harvesting, and Rx burning (ThPyRx) and thinning and physical harvesting (ThPy) were effective in terms of; a) lowered probability of torching; b) reduced wildfire risk levels; c) lowered tree crowning percentage and crown fire potential; and d) minimum tree basal area killed. Although the average stand level carbon pool was not significantly different among treatments, forest managements involving physical harvesting (Py), alone and in combination with thinning (ThPy), were the two lowest pyrogenic carbon emitters. Moreover, the ThPy alternative not only had the highest live vs dead carbon ratio but also resulted in the greatest carbon sequestration prospects via biochar conversion and innovative wood use. Overall, integrating physical harvesting with existing fuel treatments shows promise for reducing wildfire hazards and improving carbon credits, offering a non-intrusive, carbon-beneficial alternative approach to dead fuel reduction. However, longer-term research, both simulation modeling and field experiments, will be helpful for testing its efficacy temporally with repeated treatments and in multiple forest types, especially its potential in restoring historic wildfire regimes for healthy and resilient forests in the western US.

Extreme Fire Spread Events Burn More Severely and Homogenize Postfire Landscapes in the Southwestern United States

Extreme fire spread events rapidly burn large areas with disproportionate impacts on people and ecosystems. Such events are associated with warmer and drier fire seasons and are expected to increase in the future. Our understanding of the landscape outcomes of extreme events is limited, particularly regarding whether they burn more severely or produce spatial patterns less conducive to ecosystem recovery. To assess relationships between fire spread rates and landscape burn severity patterns, we used satellite fire detections to create day-of-burning maps for 623 fires comprising 4267 single-day events within forested ecoregions of the southwestern United States. We related satellite-measured burn severity and a suite of high-severity patch metrics to daily area burned. Extreme fire spread events (defined here as burning > 4900 ha/day) exhibited higher mean burn severity, a greater proportion of area burned severely, and increased like adjacencies between high-severity pixels. Furthermore, increasing daily area burned also resulted in greater distances within high-severity patches to live tree seed sources. High-severity patch size and total high-severity core area were substantially higher for fires containing one or more extreme spread events than for fires without an extreme event. Larger and more homogenous high-severity patches produced during extreme events can limit tree regeneration and set the stage for protracted forest conversion. These landscape outcomes are expected to be magnified under future climate scenarios, accelerating fire-driven forest loss and long-term ecological change.

Multidecadal vegetation transformations of a New Mexico ponderosa pine landscape after severe fires and aerial seeding

Wildfires and climate change increasingly are transforming vegetation composition and structure, and postfire management may have long-lasting effects on ecosystem reorganization. Postfire aerial seeding treatments are commonly used to reduce runoff and soil erosion, but little is known about how seeding treatments affect native vegetation recovery over long periods of time, particularly in type-converted forests that have been dramatically transformed by the effects of repeated, high-severity fire. In this study, we analyze and report on a rare long-term (23-year) dataset that documents vegetation dynamics following a 1996 post-fire aerial seeding treatment and a subsequent 2011 high-severity reburn in a dry conifer landscape of northern New Mexico, USA. Repeated surveys between 1997 and 2019 of 49 permanent transects were analyzed for differences in vegetation cover, richness, and diversity between seeded and unseeded areas, and to characterize the development of seeded and unseeded vegetation communities through time and across gradients of burn severity, elevation, and soil-available water capacity. Seeded plots showed no significant difference in bare ground cover during the initial years postfire relative to unseeded plots. Postfire seeding led to a clear and sustained divergence in herbaceous community composition. Seeded plots had a much higher cover of non-native graminoids, primarily Bromus inermis, a likely contaminant in the seed mix. High-severity reburning of all plots in 2011 reduced native graminoid cover by half at seeded plots compared with both prefire levels and with plots that were unseeded following the initial 1996 fire. In addition, higher fire severity was associated with increased non-native graminoid cover and reduced native graminoid cover. This study documents fire-driven ecosystem transformation from conifer forest into a shrub-and-grass-dominated system, reinforced by aerial seeding of grasses and high-severity reburning. This unique long-term dataset illustrates that post-fire seeding carries significant risks of unwanted non-native species invasions that persist through subsequent fires-thus alternative postfire management actions merit consideration to better support native ecosystem resilience given emergent climate change and increasing disturbance. This study also highlights the importance of long-term monitoring of postfire vegetation dynamics, as short-term assessments miss key elements of complex ecosystem responses to fire and postfire management actions.

Building references for nature conservation

Conservation references have long been used in conservation biology to compare current biodiversity processes and states with past conditions. However, beyond the paucity of data for the construction of ancient, even prehuman, references, the relevance of these ancient references for studying ecosystems radically modified by human activities is questionable, particularly when the notions of conservation references and conservation objectives are confused and when several conservation ethics coexist that require distinct references. Because of this implicit heterogeneity in the nature of the references and their temporal baseline, conservation references not only have different meanings, but also deliver different messages. I propose establishing a common framework for conservation references to approach past biological systems and build comparable references between studies and projects. The selection of these references (distinct from conservation objectives) should be an early, explicit, standardized, and transparent milestone in any conservation process and these references should be based on state, pressure, or process dynamics, rather than fixed states. Finally, the importance of the diversity of temporal baselines used to build conservation references and to measure anthropogenic impacts should be recognized to understand the biodiversity crisis in its entirety.

Paleoecology provides context for conserving culturally and ecologically important pine forest and barrens communities

In fire-prone ecosystems, knowledge of vegetation-fire-climate relationships and the history of fire suppression and Indigenous cultural burning can inform discussions of how to use fire as a management tool, particularly as climate continues to change rapidly. On Wiisaakodewan-minis/Stockton Island in the Apostle Islands National Lakeshore of Wisconsin, USA, structural changes in a pine-dominated natural area containing a globally rare barrens community occurred after the cessation of cultural burning by the Indigenous Ojibwe people and the imposition of fire-suppression policies, leading to questions about the historical role of fire in this culturally and ecologically important area. To help understand better the ecological context needed to steward these pine forest and barrens communities, we developed palaeoecological records of vegetation, fire, and hydrological change using pollen, charcoal, and testate amoebae preserved in peat and sediment cores collected from bog and lagoon sediments within the pine-dominated landscape. Results indicated that fire has been an integral part of Stockton Island ecology for at least 6000 years. Logging in the early 1900s led to persistent changes in island vegetation, and post-logging fires of the 1920s and 1930s were anomalous in the context of the past millennium, likely reflecting more severe and/or extensive burning than in the past. Before that, the composition and structure of pine forest and barrens had changed little, perhaps due to regular low-severity surface fires, which may have occurred with a frequency consistent with Indigenous oral histories (~4-8 years). Higher severity fire episodes, indicated by large charcoal peaks above background levels in the records, occurred predominantly during droughts, suggesting that more frequent or more intense droughts in the future may increase fire frequency and severity. The persistence of pine forest and barrens vegetation through past periods of climatic change indicates considerable ecological resistance and resilience. Future persistence in the face of climate changes outside this historical range of variability may depend in part on returning fire to these systems.