Fire Return Intervals Research Articles

The mid- and Late Holocene history of forest fires in Central Evenkia inferred from macrocharcoal analysis

The paper presents the reconstruction of the Mid- and Late Holocene frequency of forest fires in the Lower Tunguska River basin in the vicinity of the settlement of Tura (central Evenkia) revealed from plant macro remains and macroscopic charcoal analysis of peat cores from two peatlands. The results of analysis showed similar patterns of charcoal accumulation rates in peat deposits from both bogs during the Holocene and their significant temporal variability. According to the data obtained, the stages of increased frequency and intensity of fires in Central Evenkiya occurred during the periods 7800-6500 cal years BP and 3800-2900 cal years BP and were influenced by climatic factors. The period of increasing fire activity, which has been observed from the first half of the XVII century to the present day, was caused by a combination of anthropogenic and natural factors. A decline in the charcoal accumulation rate in peatlands and an increase in the length of fire return intervals were characteristic for periods 6500-3800 cal years BP and 2900-600 cal years BP. Perhaps the cooling climate created unfavorable conditions for the occurrence and spread of forest fires.

Human-induced fires and land-use change in Lubusz Land (western Poland) derived from Lake Lubińskie sedimentary record

Sediments of Lake Lubińskie were investigated to track human-induced vegetation and fire activity changes in Lubusz Land (western Poland). Geochemical data, as well as pollen, macrocharcoal, and statistical analyses (CONISS, CharAnalysis, PCA, BINCOR) allowed us to recognize the major phases of the human impact. At the beginning of the investigation period, 3000 years ago, the area had already experienced disturbances from humans caused by the Lusatian and Przeworsk cultures, followed by western Slavs, and modern society. Land cover changes were mostly related to deforestation and gaining arable lands. The forest communities shrank substantially also due to timber demand from the growing society. However, the pollen record shows that within the last ~50 years the area experienced forest regeneration. Wildfires played an important role in shaping the landscape in the area of Lake Lubińskie. Overall, 23 fires were identified within the profile showing relatively high fire frequencies (up to 4.8 fires per 500 years) between 600 −38/+36 BCE and 1 ± 206 CE as well as between 1065 ± 98 CE and 1380 ± 92 CE. The highest fire frequency was observed between the 1000s and mid-1400s, showing decreased fire return interval (FRI) and increased charcoal flux as well as progressive deforestation. These changes were related to the establishment of villages (e.g., Lubin village) and the presence of the Joannites. Fire was most commonly used as a land management tool to gain patches of land for agrarian purposes. PCA and BINOCR analyses suggest that most of the fires were set by humans, but natural causes of ignition cannot be excluded. The land-use changes affected the vegetation in the region, causing increased erosion and shifts in the geochemistry of the lake reflected in better bottom waters ventilation and progressive eutrophication.

Resource objective wildfire leveraged to restore old growth forest structure while stabilizing carbon stocks in the southwestern United States

Wildfire futures and aboveground carbon (C) dynamics associated with forest restoration programs that integrate resource objective wildfire as part of a larger treatment strategy are not well understood. Using simulation modeling, we examined alternative forest and fuel management strategies on a 237,218-ha study area within a 778,000-ha landscape that is a high priority target for federal restoration programs. We simulated two wildfire management scenarios combined with three levels of conventional forest restoration treatments over 64 years using a detailed landscape disturbance and succession model developed in prior work. We found accelerated forest restoration used in concert with resource objective wildfire was the most effective at returning old growth forest structure, while stabilizing aboveground C stocks and restoring the fire return interval to its historic range of variation. In scenarios without forest restoration, the continued practice of resource objective wildfires during shoulder fire seasons reduced summer emissions in a negative feedback loop. In the short term, scenarios without forest restoration increased live tree C, but also increased the likelihood of C loss during wildfire activity driven by extreme fire weather. We found scenarios most effective at restoring fire-excluded pine forests to their historical old growth conditions came at a short-term cost of lost C, but with the long-term benefit of substantially increasing fire-resistant live tree C. Our results inform how local decision making can best balance competing goals of sequestering C, and stabilizing C stocks in frequent-fire pine forests using the principles of local fire ecology to restore and maintain old growth forest structure.

Functional Trait Responses of C4 Bunchgrasses to Fire Return Intervals in the Semi-Arid Savanna of South Africa

C4 grasses coevolved with fires, employing specialized adaptive traits to recover from recurrent fires of varying regimes, thereby maintaining plant diversity and plant population stability. However, the knowledge of how C4 bunchgrasses recover from varying fire return intervals (FRIs) is limited. Biomass, tillering, flowering, and growth-related traits of Digitaria eriantha, Themeda triandra, Sporobolus fimbriatus, and Cymbopogon plurinodis were assessed in 0- (unburned), 1-, 2-, and 4-year FRIs, each applied in two 0.5 ± 0.01 ha plots from 1980–2022 at the University of Fort Hare research farm, South Africa. FRIs and grass species interacted significantly on biomass production, crown size, tiller production, and reproductive tillers, with responses varying interspecifically depending on the FRI. Cymbopogon plurinodis attained higher total biomass in 1-year FRI, whereas T. triandra produced relatively low biomass in all FRIs compared to 0-year FRI. Nonetheless, T. triandra attained nearly two to three-fold more tillers per plant and three to five-fold more reproductive tillers in 2- and 4-year FRIs compared to other FRIs. Similarly, S. fimbriatus had two-fold more reproductive tillers in 2-year FRIs compared to 0- and 1-year FRIs. We deduce that C4 bunchgrasses respond differentially under recurrent fires depending on the fire return interval, with 2- and 4-year FRIs promoting vegetative and sexual regeneration by enhancing tillering and flowering.

Exceptional variability in historical fire regimes across a western Cascades landscape, Oregon, USA

AbstractDetailed information about the historical range of variability in wildfire activity informs adaptation to future climate and disturbance regimes. Here, we describe one of the first annually resolved reconstructions of historical (1500–1900 ce) fire occurrence in coast Douglas‐fir dominated forests of the west slope of the Cascade Range in western Oregon. Mean fire return intervals (MFRIs) across 16 sites within our study area ranged from 6 to 165 years. Variability in MFRIs was strongly associated with average maximum summer vapor pressure deficit. Fire occurred infrequently in Douglas‐fir forest stands seral to mountain hemlock or silver fir, but fire frequency was much shorter than predicted by theory in other forest types. MFRIs within Douglas‐fir stands seral to western hemlock or grand fir ranged from 19 to 45 years, and MFRIs in stands seral to Douglas‐fir ranged from 6 to 11 years. There was little synchrony in fire occurrence or tree establishment across 16 sites separated by 4 km. The lack of synchrony in fire suggests that large, wind‐driven fire events that are often considered to be characteristic of coast Douglas‐fir forests were not an important driver of succession in our study area during the last ~400–500 years. Climate was more arid than normal during fire years in most forest types, but historical fire in stands seral to Douglas‐fir was strongly associated with antecedent moisture and less strongly associated with drought. We interpret the extraordinary tempo of fire we observed in stands seral to Douglas‐fir and the unique climate pattern associated with fire in these stands to be indicative of Indigenous fire stewardship. This study provides evidence of far more frequent historical fire in coast Douglas‐fir forests than assumed by managers or scientists—including some of the most frequent fire return intervals documented in the Pacific Northwest. We recommend additional research across the western Cascades to create a comprehensive account of historical fire in highly productive forests with significant cultural, economic, and ecological importance.

Modeling fuel moisture dynamics under climate change in Spain’s forests

BackgroundCurrent assessments of the effects of climate change on future wildfire risk are based on either empirical approaches or fire weather indices. No study has yet used process-based models over national scales to understand how and where will increases in climate aridity affect the likelihood of fire activity through changes in the moisture content of live (LFMC) and of dead (DFMC) fuels. Here, we used process-based models to forecast changes in LFMC and DFMC under the 21st century climatic conditions projected from moderate and high greenhouse gas emission scenarios (RCP4.5 and RCP8.5). Predictions were performed across broad productivity gradients in peninsular Spain to understand how productivity mediates the effects of climate change on fuel moisture dynamics.ResultsLFMC and DFMC were predicted to decline under the climatic conditions projected for the coming decades. Increases in the annual frequency of days with fuel moisture content below wildfire occurrence thresholds were predicted to extend fire season lengths by 20 days under RCP4.5 and by 50 days under RCP8.5. The effects of climate change on LFMC and DFMC varied linearly and negatively with productivity (stronger fuel moisture decreases in least productive environments). Although we observed a significant mitigation effect from rising CO2 (via increases in water-use efficiency), it was not enough to offset LFMC declining trends induced by increased temperature and aridity.ConclusionsWe predicted that the warmer and more arid climatic conditions projected for the 21st century will lead to generalized declines in fuel moisture, lengthening fire seasons, and increasing wildfire danger. The use of process-based models to forecast LFMC dynamics allowed the consideration of plant species capabilities to buffer climate change impacts. Significant increases in the fire season length predicted in the most productive environments, currently with large fire return intervals, would pose an increase of fire danger in major Spanish carbon sinks. Finally, the CO2 mitigation effect would not be enough to offset climate change-driven declines in seasonal LFMC levels.

Fuel treatments in shrublands experiencing pinyon and juniper expansion result in trade-offs between desired vegetation and increased fire behavior

BackgroundNative pinyon (Pinus spp.) and juniper (Juniperus spp.) trees are expanding into shrubland communities across the Western United States. These trees often outcompete with native sagebrush (Artemisia spp.) associated species, resulting in increased canopy fuels and reduced surface fuels. Woodland expansion often results in longer fire return intervals with potential for high severity crown fire. Fuel treatments are commonly used to prevent continued tree infilling and growth and reduce fire risk, increase ecological resilience, improve forage quality and quantity, and/or improve wildlife habitat. Treatments may present a trade-off; they restore shrub and herbaceous cover and decrease risk of canopy fire but may increase surface fuel load and surface fire potential. We measured the accumulation of surface and canopy fuels over 10 years from ten sites across the Intermountain West in the Sagebrush Steppe Treatment Evaluation Project woodland network (www.SageSTEP.org), which received prescribed fire or mechanical (cut and drop) tree reduction treatments. We used the field data and the Fuel Characteristic Classification System (FCCS) in the Fuel and Fire Tools (FFT) application to estimate surface and canopy fire behavior in treated and control plots in tree expansion phases I, II, and III.ResultsIncreased herbaceous surface fuel following prescribed fire treatments increased the modeled rate of surface fire spread (ROS) 21-fold and nearly tripled flame length (FL) by year ten post-treatment across all expansion phases. In mechanical treatments, modeled ROS increased 15-fold, FL increased 3.8-fold, and reaction intensity roughly doubled in year ten post-treatment compared to pretreatment and untreated controls. Treatment effects were most pronounced at 97th percentile windspeeds, with modeled ROS up to 82 m min−1 in mechanical and 106 m min−1 in prescribed fire treatments by 10 years post-treatment compared to 5 m min−1 in untreated controls. Crown fire transmissivity risk was eliminated by both fuel treatments.ConclusionsWhile prescribed fire and mechanical treatments in shrublands experiencing tree expansion restored understory vegetation and prevented continued juniper and pinyon infilling and growth, these fuel treatments also increased modeled surface fire behavior. Thus, management tradeoffs occur between desired future vegetation and wildfire risk after fuel treatments.

Rapid fuel recovery after stand-replacing fire in closed-cone pine forests and implications for short-interval severe reburns

Accelerating disturbance activity under a warming climate increases the potential for multiple disturbances to overlap and produce compound effects that erode ecosystem resilience — the capacity to experience disturbance without transitioning to an alternative state. A key concern is the potential for amplifying or attenuating feedbacks via interactions among successive, linked disturbance events. Following severe wildfires, fuel limitation is a negative feedback that may reduce the likelihood of subsequent fire. However, the duration of, and pre-fire vegetation effects on fuel limitation remain uncertain. To address this knowledge gap, we characterized fuel profiles over a 35-year post-fire chronosequence in California closed-cone pine forests, a system with stand dynamics comparable to stand-replacing fire regimes in other temperate and boreal conifer forests with much longer fire return intervals. We used this system to ask: 1) How do fuel profiles change with time since recent stand-replacing fire? 2) How do trajectories of fuel profiles differ between forests established from different pre-fire vegetation legacies — specifically areas that were forest vs. non-forest pre-fire? We also assessed fuel profiles that supported stand-replacing fire during a large (>90,000 ha) wildfire that burned through our study area shortly after data collection and compared with fuel profiles across the chronosequence. Overall, live and dead surface fuels accumulated quickly, reaching levels comparable to those capable of supporting severe fire after 10–15 years. Canopy fuels peaked at 20 years post-fire but were at levels comparable to those capable of supporting severe fire from 15 years onwards. Pre-fire vegetation legacy drove divergence in post-fire dead surface fuel trajectories, but effects on live surface and canopy fuels were minor. Fuel profiles in California closed-cone pine forests can support subsequent severe fire within 10 years, a fraction of the historical minimum interval between stand-replacing fires (≥30 years) that supports robust post-fire regeneration, but potential for severe fire remains lower for an extended period in areas where pre-fire vegetation was non-forest. Our findings show that the negative feedback of fuel limitation disappears quickly following fire, suggesting that these systems are vulnerable to short-interval severe reburns that could erode forest resilience.

Postglacial establishment and expansion of marginal populations of sugar maple in western Québec, Canada: Palynological detection and interactions with fire, climate and successional processes

An isolated sugar maple ( Acer saccharum Marsh.) stand is located in the boreal forest of Abitibi, about 75 km beyond its present northern range limit. When did this relatively thermophilous tree species establish after ice retreat? Were its populations more abundant than now sometimes in the past? If so, when and how did they expand then retracted? How did the species persist in boreal forest over time? What could have been the role of fire on this stand? To answer those questions, we reconstructed the postglacial fire and vegetation history from three lacustrine sediment sequences distributed along a c. 180 km latitudinal transect from southern boreal forests to the northern portion of deciduous forests. From north to south, those are lakes Labelle, Chasseur and Fur. We explored a procedure based on pollen accumulation rates in order to detect the probable presence of sugar maple within the lakes’ watershed. The procedure successfully indicates a sugar maple establishment c. 7800–5100 cal. BP at Fur, 5500–4400 cal. BP at Chasseur and c. 4000–2700 cal. BP at Labelle, in the north. At Fur, the subsequent sugar maple expansion happened 1 to 2 thousand years after establishment, during colder and moister climatic conditions favoring Pinus strobus L. replacement by Betula spp. c. 6000–5000 cal. BP. Sugar maple establishment, persistence or expansion is apparently not linked to a change in fire activity at Fur and Chasseur, but at Labelle, the species was more abundant during periods of shorter fire return intervals from 2000 to 500 years ago. Our study suggests that northern (Chasseur and Labelle) sugar maple establishment and possible expansion was probably more controlled by a complex interaction of inhibition and facilitation dynamics than by climate alone, a process reliant on the dominant vegetation’s composition and structure.

The recent fire regimes of Luengue-Luiana and Mavinga national parks, Angola

The adjoining Luengue-Luiana and Mavinga national parks in southeastern Angola are two of the largest though least actively managed protected areas in Africa. The parks are home to a diversity of wildlife and are managed as a single unit. The area is characterised as a fire-dependent savanna, where fires may be naturally ignited (such as by lightning), but where the fires each year are frequently set by people. As a result of the lack of resource investment, uncontrolled fires dominate the region, potentially negatively affecting biodiversity. The past fire regimes (2001–2021) of the two parks were investigated with use of the MODIS and VIIRS remote-sensing instrument. Extremely short fire return intervals dominated the landscape, with almost half (47.8%) of the area experiencing a subsequent fire within a year of the previous fire, and an additional 30% of the area burning within two years after a preceding fire. Only 2.4% of the landscape was shown to remain fire-free for a period of ten years or more. Overall, the fire intensities were highest from August to October, with the largest areas burnt between August and September. Fire frequency, seasonality and intensity were influenced by the land-cover type, and a comparison of the different satellite products was made. This information is the first of its kind for the region and is intended to provide a basis from which fire management activities can be coordinated.

Contemporary wildfires are more severe compared to the historical reference period in western US dry conifer forests

Dry conifer forests in the western US historically experienced frequent fire prior to European American colonization. Mean fire return interval ranged from about 5–35 years, with the majority of fires burning at low-to-moderate severity. The arrival of European Americans initiated notable reductions in fire frequency through the cessation of Indigenous burning, livestock grazing, and fire suppression. These activities effectively excluded fire from most dry conifer forests, thereby allowing live and dead fuels to accumulate. There are concerns that this fuel accumulation is causing contemporary fires (since 1985) to burn more severely compared to pre-colonization time periods, thereby increasing potential for enduring fire-catalyzed conversions to non-forest. Here, we used satellite-derived fire severity data to characterize the prevalence of contemporary (1985–2020) stand-replacing fire in dry conifer forests within four broad ecoregions for all fires, non-wilderness fires, wilderness fires, and prescribed fires. Our results indicate, that as a proportion of area burned, contemporary fires experienced 2.9 to 13.6 times more stand-replacing fire (depending on the ecoregion) compared to the pre-colonization period. Non-wilderness areas exhibit somewhat higher prevalence of stand-replacing fire, relative to the historical fire regime, than wilderness areas (where logging is prohibited). The relatively small difference between non-wilderness and wilderness suggests that fuel accumulation resulting from fire exclusion has played a larger role than historical logging activities on the prevalence of contemporary stand-replacing fire. Prescribed fires do not exhibit a higher prevalence of stand-replacing fire compared to the historical fire regime. We conducted a parallel analysis in the Gila Wilderness in New Mexico, USA, is it is arguably the best example of an unrestrained fire regime (many fires are not suppressed) in western US dry conifer forests. The Gila Wilderness proportionally experienced only 1.8 times more stand-replacing fire compared to the historical reference period; in contrast, the southwest ecoregion experienced 5.9 times more stand-replacing fire. Fire will inevitably burn a given forest no matter how much money and personnel we commit to fire prevention and suppression, and because of the high prevalence of stand-replacing fire, the long-term persistence of dry conifer forests is jeopardized. Unless forest managers are provided the social license and enabling conditions to substantially accelerate efforts to restore conditions resistant to stand-replacing fire, we anticipate that the extent of dry conifer forests will continue to erode as fire-catalyzed conversions increasingly unfold across the western US.

Microbial Community Responses to Alterations in Historical Fire Regimes in Montane Grasslands

The influence of fire regimes on soil microbial diversity in montane grasslands is a relatively unexplored area of interest. Understanding the belowground diversity is a crucial stepping-stone toward unravelling community dynamics, nutrient sequestration, and overall ecosystem stability. In this study, metabarcoding was used to unravel the impact of fire disturbance regimes on bacterial and arbuscular mycorrhizal fungal community structures in South African montane grasslands that have been subjected to an intermediate (up to five years) term experimental fire-return interval gradient. Bacterial communities in this study exhibited a shift in composition in soils subjected to annual and biennial fires compared to the controls, with carbon and nitrogen identified as significant potential chemical drivers of bacterial communities. Shifts in relative abundances of dominant fungal operational taxonomic units were noted, with Glomeromycota as the dominant arbuscular mycorrhiza observed across the fire-return gradient. A reduction in mycorrhizal root colonisation was also observed in frequently burnt autumnal grassland plots in this study. Furthermore, evidence of significant mutualistic interactions between bacteria and fungi that may act as drivers of the observed community structure were detected. Through this pilot study, we can show that fire regime strongly impacts bacterial and fungal communities in southern African montane grasslands, and that changes to their usually resilient structure are mediated by seasonal burn patterns, chemical drivers, and mutualistic interactions between these two groups.

The implications of seasonal climatic effects for managing disturbance dependent populations under a changing climate

Abstract The frequency of ecological disturbances, such as fires, is changing due to changing land use and climatic conditions. Disturbance‐adapted species may thus require the manipulation of disturbance regimes to persist. However, the effects of changes in other abiotic factors, such as climatic conditions, are frequently disregarded in studies of such systems. Where climatic effects are included, relatively simple approaches that disregard seasonal variation in the effects are typically used. We compare predictions of population persistence using different fire return intervals (FRIs) under recent and predicted future climatic conditions for the rare fire‐dependent herb Eryngium cuneifolium. We used functional linear models (FLMs) to estimate the cumulative effect of climatic variables across the annual cycle, allowing the strength and direction of the climatic impacts to differ over the year. We then estimated extinction probabilities and minimum population sizes under past and forecasted future climatic conditions and a range of FRIs. Under forecasted climate change, E. cuneifolium is predicted to persist under a much broader range of FRIs, because increasing temperatures are associated with faster individual growth. Climatic impacts on fecundity do not result in a temporal trend in this vital rate due to antagonistic seasonal effects operating through winter and summer temperatures. These antagonistic seasonal climatic effects highlight the importance of capturing the seasonal dependence of climatic effects when forecasting their future fate. Synthesis. Awareness of the potential effects of climate change on disturbance‐adapted species is necessary for developing suitable management strategies for future environmental conditions. However, our results suggest that widely used simple methods for modelling climate impacts, that disregard seasonality in such effects, may produce misleading inferences.

Peeking under the canopy: anomalously short fire-return intervals alter subalpine forest understory plant communities.

Climate change is driving changes in disturbance regimes world-wide. In forests adapted to infrequent, high-severity fires, recent anomalously short fire-return intervals (FRIs) have resulted in greatly reduced postfire tree regeneration. However, effects on understory plant communities remain unexplored. Understory plant communities were sampled in 31 plot pairs across Greater Yellowstone (Wyoming, USA). Each pair included one plot burned at high severity twice in < 30 yr and one plot burned in the same most recent fire but not burned previously for > 125 yr. Understory communities following short-interval fires were also compared with those following the previous long-interval fire. Species capable of growing in drier conditions and in lower vegetation zones became more abundant and regional differences in plant communities declined following short-interval fire. Dissimilarity between plot pairs increased in mesic settings and decreased with time since fire and postfire winter snowfall. Reduced postfire tree density following short-interval fire rather than FRI per se affected the occurrence of most plant species. Anomalously short FRIs altered understory plant communities in space and time, with some indications of community thermophilization and regional homogenization. These and other shifts in understory plant communities may continue with ongoing changes in climate and fire across temperate forests.

Persistent, viable seedbank buffers serotinous bishop pine over a broad fire return interval

BackgroundIn ecosystems where fire has been excluded, pyrosilviculture can restore some processes historically maintained by fire while mitigating risk where fire is inevitable. Pyrosilviculture in crown fire-adapted forests is, however, limited by insight into the temporal window of fire return matching canopy seedbank development. Here, we characterized demographic responses to fire and non-native pine pitch canker infection in a chronosequence of serotinous bishop pine stands burned at high-severity to quantify (1) temporal patterns of seedbank development given seed viability and density, cone production, and tree density, and (2) pine pitch canker incidence across tree ages and sizes to assess how infection might impact stand and seedbank development. We use our findings to elucidate pyrosilvicuture as a means of restoring fire given practical challenges of reintroducing high-severity fire in crown-fire adapted forests embedded within wildland urban interfaces (WUI).ResultsBishop pine produces an abundant, viable seedbank within eight years that persists across developmental stages and age classes. Seed abundance and viability are exceptionally high at even the earliest age (median > 600,000 seeds ha− 1 and 97% viability at 6 years) and remain high, with the oldest stands (36 years) maintaining median densities of > 500,000 seeds ha− 1 and viability of 95%. We additionally learned that pine pitch canker infection is most severe during the sapling stage (8–10 years post-fire), likely altering stand development trajectories as well as potentially limiting recruitment, and thus the aerial seedbank, into the canopy.ConclusionsIn bishop pine and equally-fecund serotinous species, pyrosilviculture appears a viable management tool across a broad fire return window given the early development and persistence of a robust, viable seedbank, allowing managers flexibility in restoring fire to promote forest persistence while simultaneously mitigating wildfire risk. Moreover, pyrosilviculture in pine pitch canker infected stands may also provide disease mitigation. Although the long-term effects of pine pitch canker infection remain unknown, bishop pines’ viable, persistent seedbank suggests that managers can ignite prescribed fire across a broad return interval — as short as eight years and as long as several decades — to promote bishop pine persistence, mitigate disease infection rates, and reduce wildfire risk in WUI-adjacent ecosystems.

Why woody plant modularity through time and space must be integrated in fire research?

Different ecosystems evolved and are maintained by fire, with their vegetation hosting species with a wide diversity of persistence strategies allowing them to insulate their body and resprout new branches after fire disturbance. Changes in fire regime are predicted due to climate change, either by promoting more frequent and/or severe fires or by reducing the number of fire events due to the limitation of fuel load. Predicting the future of fire-driven ecosystems is a complex task as species' survival depends on many factors that vary in space and time. Since plants are constantly experiencing new environments as they grow through meristem development, woody plant modularity, modules morpho-physiological aspects and their integration should be considered when investigating species strategies in fire-prone ecosystems: according to their position and their tissue composition, plants' modules experience fire differently and will contribute differently to other modules and the whole plant survival, with consequences cascading over the overall vegetation structure. Growth modules may hold the key to understanding how fast plants can get protected from fire, ultimately helping us to predict which species will persist across changing fire regimes. We present an empirical example showing how different fire-return intervals translate into distinct pressures on the timing, protection and location of modules, and discuss how these can translate into modifications in the vegetation structure due to climate change.

Pastoral Burning and Its Contribution to the Fire Regime of Alto Minho, Portugal

Alto Minho (in northwestern Iberia) is one of the European regions most affected by fires. Many of these fires originate from rangeland management of Atlantic heathlands, and, while being illegal, often are not actively suppressed. In this study, pastoral fires (autumn-to-spring fires unrecorded by authorities), spring wildfires, and summer wildfires were independently mapped and dated from remote sensing. Alto Minho burned at a mean annual rate of 5.0% of the territory between 2001 and 2020. Pastoral burning totalled 40,788 hectares during the period, accounting for 20% of the total burnt area. Rangeland burning occurs mostly from December to April, the rainiest months that guarantee the conditions for pasture renewal and fire self-extinction. The mean fire return interval of pastoral burning is slightly higher than that of wildfires (13 years vs. 11 years), except in part of the inner mountains where it dominates fire activity. Pastoral fires are more frequent and largely prevail over wildfires in the parishes with higher livestock quantities. Conversely, the largest wildfires and higher summer burnt areas correspond with very low livestock and nearly non-existing pastoral fires. Traditional fire knowledge should not be overlooked by fire management, as it contributes to more sustainable fire regimes and ecosystems.

Effectiveness of land use and disturbance measures, compared to climate, soil, and topography, for modeling relative abundances of tree species in the eastern United States

Environmental factors control species distributions and abundances, but effectiveness of land use and disturbance variables for modeling species generally is unknown compared to climate, soil, and topography variables. Therefore, I used predictor variables from categories of 1) land use and disturbance, 2) climate, and 3) soil, topography, and wind speed to model the relative abundances (i.e., percentage of all trees) of 65 common tree species in the eastern United States, with a contrast to presence-absence models of species distributions. First, I modeled variables within each category to identify the five most important variables. Then, I combined variables from each category to isolate most important variables, based on five model combinations of input variables from each category, ranging from one (i.e., three total) to five (i.e., 15 total) variables. From the five models of combined categories for each tree species, I identified the model with the greatest R2 value. Overall, climate variables were most important for tree species models with one and two input variables from each category, but land use and disturbance variables were most important for models with three to five input variables from each category. Although a range of R2 values occurred by species and number of input model variables, 32 species had best models with greatest R2 values of 0.50 to 0.81. For all best species models, the most important variables were temperature of the warmest quarter, historical fire return interval for all fires, agricultural area during years 1850 to 1997, and precipitation of the driest month. Current land cover classes, which are accessible and the most commonly modeled land use variables, were not important for modeling tree species abundances or distributions. Climate variables were most important for modeling species distributions. Results support the concept that while climate sets soft boundaries on distributions, relative abundances within distributions are affected by other filters. Future modeling may establish other important land use and disturbance variables, or refinements within the important variables of historical fire return interval and agricultural area over time, advancing integration of both land use and climate variables into studies.

Forage quantity and protein concentration changes across a forest-savanna gradient with management implications for white-tailed deer

White-tailed deer (Odocoileus virginianus) hunting is an important economic activity associated with the management of forests and rangelands in the USA, with over $12.9 billion dollars of related annual expenditures. Reducing tree cover through thinning and prescribed fire both have the potential to increase the quantity and quality of deer forage. We evaluated the long-term impacts of eight different combinations of fire return intervals and tree harvest on forage productivity and protein content of the forage. Based on management regime, study units ranged from savanna to closed-canopy forest. Aboveground net primary production (ANPP) of six functional groups (grass, panicum, forb, legume, woody, sedge) of understory vegetation was measured in October 2019 and 2020 using destructive sampling. Samples for foliar crude protein (CP) concentration were collected in spring, summer, and fall of 2020. Total understory ANPP ranged from 2.9 to 466.3 g m−2 and was up to 566% greater in savanna systems maintained by frequent fire (return interval of three years or less) than in non-burned forest treatments. Annual burning resulted in ANPP dominated by herbaceous plants composed mostly of fire-tolerant grasses (e.g., Andropogon gerardii, Schizachyrium scoparium). Longer fire return intervals or no fire resulted in roughly equal ANPP from understory woody and herbaceous species. Crude protein concentrations were up to 45.7% greater in the woodland and forest units than in the savanna units for seven of the eleven species sampled. The greater CP in the forests was most noticeable in the summer when deer needs for quality forage are substantial. Increased protein concentrations of understory species in the forests, but greater ANPP in the savannas indicate that managing for a mix of savanna and woodland could be ideal for balancing forage quantity with increased forage protein.

Climate and fire impacts on tree recruitment in mixed conifer forests in Northwestern Mexico and California.

Frequent-fire forests were once heterogeneous at multiple spatial scales, which contributed to their resilience to severe fire. While many studies have characterized historical spatial patterns in frequent-fire forests, fewer studies have investigated their temporal dynamics. We investigated the influences of fire and climate on the timing of conifer recruitment in old-growth Jeffrey pine-mixed conifer forests in the Sierra San Pedro Martir (SSPM) and the eastern slope of Sierra Nevada. Additionally, we evaluated the impacts of fire exclusion and recent climate change on recruitment levels using statistical models with realized as well as fire suppression and climate change free counterfactual scenarios. Excessive soil drying from anthropogenic climate change resulted in diminished recruitment in the SSPM but not in the Sierra Nevada. Longer fire-free intervals attributable to fire suppression and exclusion resulted in greater rates of recruitment across all sites but was particularly pronounced in the Sierra Nevada where suppression began > 100 years ago and recruitment was 28 times higher than the historical fire return interval scenario. This demonstrates the profound impact of fire's removal on tree recruitment in Sierra Nevada forests even in the context of recent climate change. Tree recruitment at the SSPM coincided with the early Twentieth Century North American Pluvial, as well as a fire quiescent period in the late 18th and early 19th centuries. Episodic recruitment occurred in the SSPM with no 'average' recruitment over the last three centuries. We found that temporal heterogeneity, in conjunction with spatial heterogeneity, are critical components of frequent-fire adapted forests. Episodic recruitment could be a desirable characteristic of frequent-fire adapted forests and this might be more amenable to climate change impacts that forecast more variable precipitation patterns in the future. One key to this outcome would be for frequent-fire to continue to shape these forests versus continued emphasis on fire suppression in California.