Time to burn: landscape drivers of fuel trait variability and fire regime in savanna ecosystems

Aim The historical variability of fire regimes must be understood in the context of drivers of the occurrence of fire operating at a range of spatial scales from local site conditions to broad‐scale climatic variation. In the present study we examine fire history and variations in the fire regime at multiple spatial and temporal scales for subalpine forests of Engelmann spruce–subalpine fir (Picea engelmannii,Abies lasiocarpa) and lodgepole pine (Pinus contorta) of the southern Rocky Mountains.Location The study area is the subalpine zone of spruce–fir and lodgepole pine forests in the southern sector of Rocky Mountain National Park (ROMO), Colorado, USA, which straddles the continental divide of the northern Colorado Front Range (40°20′ N and 105°40′ W).Methods We used a combination of dendroecological and Geographic Information System methods to reconstruct fire history, including fire year, severity and extent at the forest patch level, forc. 30,000 ha of subalpine forest. We aggregated fire history information at appropriate spatial scales to test for drivers of the fire regime at local, meso, and regional scales.Results The fire histories coveredc. 30,000 ha of forest and were based on a total of 676 partial cross‐sections of fire‐scarred trees and 6152 tree‐core age samples. The subalpine forest fire regime of ROMO is dominated by infrequent, extensive, stand‐replacing fire events, whereas surface fires affected only 1–3% of the forested area.Main conclusions Local‐scale influences on fire regimes are reflected by differences in the relative proportions of stands of different ages between the lodgepole pine and spruce–fir forest types. Lodgepole pine stands all originated following fires in the last 400 years; in contrast, large areas of spruce–fir forests consisted of stands not affected by fire in the past 400 years. Meso‐scale influences on fire regimes are reflected by fewer but larger fires on the west vs. east side of the continental divide. These differences appear to be explained by less frequent and severe drought on the west side, and by the spread of fires from lower‐elevation mixed‐conifer montane forests on the east side. Regional‐scale climatic variation is the primary driver of infrequent, large fire events, but its effects are modulated by local‐ and meso‐scale abiotic and biotic factors. The low incidence of fire during the period of fire‐suppression policy in the twentieth century is not unique in comparison with the previous 300 years of fire history. There is no evidence that fire suppression has resulted in either the fire regime or current forest conditions being outside their historic ranges of variability during the past 400 years. Furthermore, in the context of fuel treatments to reduce fire hazard, regardless of restoration goals, the association of extremely large and severe fires with infrequent and exceptional drought calls into question the future effectiveness of tree thinning to mitigate fire hazard in the subalpine zone.

Background The Lupande Game Management Area (GMA) and the adjacent South Luangwa National Park (NP) in Zambia allow comparison of fire regimes in African savannas with different human densities. Aims To investigate humans’ effects on fire regimes within a sub-Saharan savanna ecosystem. Methods We delineated burned areas for the Lupande GMA and South Luangwa NP using 156 Landsat images from 1989 to 2017. We performed comparisons of fire regimes between the Lupande GMA and South Luangwa NP using various burned area variables and assessed their association with precipitation. Key results Overall, and compared with the South Luangwa NP, the Lupande GMA had a greater extent of burned area and a higher frequency of repeat burns. The Lupande GMA experienced fires earlier in the fire season, which are typically less damaging to woody vegetation. We observed a significant positive relationship between precipitation and burned area trends in South Luangwa NP but not in the Lupande GMA, suggesting that precipitation increases burned area in South Luangwa NP. Conclusions Results support the theory that human fire management mitigates climate’s effect, particularly rainfall, on interannual burned area variation. Implications This study shows that human-dominated fire regimes in savannas can alter the influence of precipitation.

Our objective was to infer the controls of spatial variation in historical fire regimes. We reconstructed a multicentury history of fire frequency, size, season, and severity from fire scars and establishment dates of 1426 trees sampled on grids in four watersheds (∼64 plots, over ∼1620 ha each) representative of the Blue Mountains, Oregon and Washington, USA. The influence of regional climate, a top-down control, was inferred from among-watershed variation in fire regimes, while the influence of local topography, a bottom-up control, was inferred from within-watershed variation. Before about 1900, fire regimes varied among and within watersheds, suggesting that both top-down and bottom-up controls were important. At the regional scale, dry forests (dominated by ponderosa pine), burned twice as frequently and earlier in the growing season in southern watersheds than in northern watersheds, consistent with longer and drier fire seasons to the south. Mesic forests (dominated by subalpine fir or grand fir) probably also burned more frequently to the south. At the local scale, fire frequency varied with different parameters of topography in watersheds with steep terrain, but not in the watershed with gentle terrain. Frequency varied with aspect in watersheds where topographic facets are separated by significant barriers to fire spread, but not in watersheds where such facets interfinger without fire barriers. Frequency varied with elevation where elevation and aspect interact to create gradients in snow-cover duration and also where steep talus interrupts fuel continuity. Frequency did not vary with slope within any watershed. The presence of both regional-scale and local-scale variation in the Blue Mountains suggests that top-down and bottom-up controls were both important and acted simultaneously to influence fire regimes in the past. However, an abrupt decline in fire frequency around 1900 was much greater than any regional or local variation in the previous several centuries and indicates that 20th-century fire regimes in these watersheds were dramatically affected by additional controls such as livestock grazing and fire suppression. Our results demonstrate the usefulness of examining spatial variation in historical fire regimes across scales as a means for inferring their controls.

Fire occurrence influences the distribution of plant species, and dynamics of plant populations, either independently from other factors or in interaction with them. Numerous studies have identified the effects of components of fire regimes (frequency, intensity and season of occurrence) on the population dynamics of individual plant species and the floristic composition of plant communities, both in Australia and in other fire-prone countries. Nevertheless, there has been considerably less research into understanding the causes of spatial variation in fire regimes and this will likely result in a major obstacle for the development of vegetation theory. Research into spatial and temporal patterns of fire regime and determining the extent to which this type of variation results in variation in the occurrence of plant species and hence the composition of plant communities, can help overcome this obstacle. Two hypotheses are constructed and addressed. They are:- (i) that because of the influence of a sites' neighbourhood, variation in fire regimes will exist for any particular set of sites that occupy a particular part of environmental space and are therefore otherwise similar, and (ii) that this variation in fire regimes will result in patterns of plant species occurrence and hence demonstrate that landscape induced pattems of fire regime are a fundamentally important component in determining the realised niche of plant species in spatially complex landscapes. These hypotheses are examined for a spatially complex landscape in the Australian Capital Territory region, Australia. Several distinct phases of research led to the conclusion that both hypotheses should be accepted for the study region. Firstly, a review of the available literature found that empirical approaches, and related statistical models, for determining long-term fire regimes provided data that was neither of sufficient length and accuracy nor of appropriate spatial resolution for examining landscape dependent patterns in fire regimes as outlined in hypothesis one. On the other hand, theoretical approaches which synthesise landscape patterns from the well­ understood processes that affect fire occurrence and behaviour proved to be a useful methodology, provided that the inadequacies in existing models, including their reliance on the North American approach to modelling fire spread, were addressed. These provisions were addressed by the construction of a new landscape-level process­ based dynamic simulation model known as FIRESCAPE. The development of the model involved parameterising and testing the Richardson weather generator for fire danger modelling and a re-assessment of the McRae lightning ignition model. The approach used in FIRESCAPE combines these models with existing models of terrain, solar radiation budgets, fuel moisture, soil moisture, fuel accumulation and fire spread to model spatial variation in fire regimes by the accumulation of information from spatially and temporally distinct fire events. Various analyses indicated that…

Peatlands contain one-third to one-half of global soil carbon, and disturbances, specifically fire, directly influence these carbon stocks. Despite this, historical variability of peatland fire regimes is largely unknown. This gap in knowledge partly stems from reconstructions of peatland fire regimes with methods limited to evaluating infrequent, severe fire events and not capturing frequent, low-severity events. Furthermore, variability in fire regimes is likely higher in heterogenous landscapes like the hemiboreal subzone, the transition between boreal and temperate biomes, where peatlands are embedded in landscapes including forests with high proportions of fire-dependent species, such as red pine (Pinus resinosa), that are well adapted to frequent low-severity fires. Here, we sought to evaluate the role of low- and moderate-severity fires within hemiboreal peatlands in central North America to better understand historical variability in fire regimes. We reconstructed historical fire regimes using fine-scale (temporal and spatial) dendrochronology methods to estimate frequency of low- and moderate-severity fires, identify synchronous fire events among forested uplands within and surrounding individual peatlands as well as among sites, and assess fire-climate relationships. We collected 220 cross-sections or partial-tree sections within three poor fen peatlands across the Great Lakes Region. Using standard dendrochronological techniques, we crossdated 129 samples, assigning dates to 414 fire scars (128 unique fire years) comprising a 500-year tree-ring record (1520–2019). Prior to the mid-1900s, fire events were frequent and widespread within peatlands we evaluated, with mean fire return intervals (MFRI) ranging from 7 to 31 years. Fire events were also synchronous among forested uplands within and surrounding peatlands. Fires predominantly occurred in the dormant and latewood (growing season) positions and during regionally dry conditions corresponding to mild and moderate drought (Palmer Drought Severity Index ≥ −2.99) but interestingly not during regionally severe drought (Palmer Drought Severity Index ≤ −3.00). While large-scale, high-severity fires are important to the ecology of peatlands and to changing climate-fire interactions, our results suggest that widespread low- to moderate-severity fires were historically frequent in hemiboreal peatlands and likely central to their development and maintenance. Evaluating whether peatlands will continue to be carbon sinks or become carbon sources due to climate change requires an understanding of the inherent variability in fire regimes, especially in hemiboreal systems.

The potential productivity of forests is an important parameter in the evaluation of vegetation as a carbon sink. At the same time, potential productivity can be considered as an indicator of growth conditions and also as a measure of available fuel loads, which, in Mediterranean-type ecosystems, are a main factor of regional fire incidence. The present work deals with the relationship between an estimation of forest potential productivity and the fire incidence registered in peninsular Spain. Fire incidence was characterized by means of several fire regime variables. In order to contrast the patterns obtained, a similar analysis of the relationship between fire regime and human population density was also carried out. The results show that higher fire incidence was registered in more productive areas. Potential productivity was correlated to variables related to the number of fires and to the area burned, whereas the population density was also correlated to the number of fire variables and to the area burned, but with lower correlation coefficients. Although it is difficult to establish cause-and-effect relationships between complex phenomena that depend on a large number of factors, finding statistically significant relationships between fire incidence and the estimation of potential forest productivity used over a long time period is considered very relevant. These relationships make it necessary to take into account the fire regime when evaluating both forests and other terrestrial ecosystems as carbon sinks so as to meet the demands of the Kyoto Protocol.

Fire is a major ecological process in many ecosystems worldwide. We sought to identify which attributes of fire regimes affect temporal change in the presence and abundance of Australian native mammals. Our detailed study was underpinned by time series data on 11 mammal species at 97 long-term sites in southeastern Australia between 2003 and 2013. We explored how temporal aspects of fire regimes influenced the presence and conditional abundance of species. The key fire regime components examined were: (1) severity of a major fire in 2003, (2) interval between the last major fire (2003) and the fire prior to that, and (3) number of past fires. Our long-term data set enabled quantification of the interactions between survey year and each fire regime variable: an ecological relationship missing from temporally restricted studies. We found no evidence of any appreciable departures from the assumption of independence of the sites. Multiple aspects of fire regimes influenced temporal variation in the presence and abundance of mammals. The best models indicated that six of the 11 species responded to two or more fire regime variables, with two species influenced by all three fire regime attributes. Almost all species responded to time since fire, either as an interaction with survey year or as a main effect. Fire severity or its interaction with survey year was important for most terrestrial rodents. The number of fires at a site was significant for terrestrial rodents and several other species. Our findings contain evidence of the effects on native mammals of heterogeneity in fire regimes. Temporal response patterns of mammal species were influenced by multiple fire regime attributes, often in conjunction with survey year. This underscores the critical importance of long-term studies of biota that are coupled with data sets characterized by carefully documented fire history, severity, and frequency. Long-term studies are essential to predict animal responses to fires and guide management of when and where (prescribed) fire or, conversely, long-unburned vegetation is needed. The complexity of observed responses highlights the need for large reserves in which patterns of heterogeneity in fire regimes can be sustained in space and over time.

Bat activity response to fire regime depends on species, vegetation conditions, and behavior

A chronosequence approach was used to investigate the influence of fire regimes on mammal assemblages and habitat characteristics. Nineteen lowland Forest sites,last burnt between 2 and 75 years ago, via either wildfire or prescribed burning and with fire frequencies ranging from one to three fires in the previous 30 years, were sampled over a period of 3 years. The fire history variables: time since fire; type of fire and frequency of fire were explored both in combination, as a set of fire history categories, and individually, to determine how accurately they described the habitat structure and faunal assemblages at each site. The results of the study are considered in the context of identifying suitable release sites for translocated animals, such as those released following rehabilitation after injury. Such animals are regularly released into novel locations, which are usually selected by wildlife rehabilitators. The movement of wildlife to novel locations (translocation) is often associated with low survivorship. Habitat quality and the presence of conspecifics at release sites have been identified as the most influential factors affecting survival of wildlife following translocation. Accurately measuring these parameters may be beyond the scope of many wildlife rehabilitators due to limitations around capacity, experience, available time and funds. A means of simplifying the selection of high quality release sites, while reducing the need for hands on exploration of each site, was a main focus of this study. In addition to the on ground measurements, the current practices and priorities of Victorian wildlife rehabilitators relating to release of wildlife and the selection of release sites were explored via surveys. The fire history categories and the individual fire regime variables showed some relationships with faunal assemblages and with habitat composition. Habitat variables associated with arboreal fauna; such as the amount of canopy cover and tree condition were most strongly related to fire history. Swamp wallaby distributions were reasonably well described by fire frequency and severity;and other species demonstrated some spatial trends related to fire regime variables. However these variables did not provide accurate surrogate measures for habitat condition, or for the abundance or richness of fauna, across all sites. The distributions of some faunal species, such as common ringtail possum, were more accurately described by habitat structures, and did not appear to be related to fire history. Further work is needed to identify accurate, easily measured, indicators of the suitability of release sites for translocated wildlife. The wildlife rehabilitators identified habitat and the presence of conspecifics as their main priorities in selecting suitable release sites. These are also identified as important in translocation research.. A very low priority was placed on fire history. Cues to determine release readiness for hand-reared wildlife and preferences around release locations varied between respondents. Many respondents reported they release wildlife from their own properties. This practice is not aligned with operating guidelines for wildlife rehabilitators in Victoria. Although some respondents actively monitored wildlife post-release, the majority did not; therefore outcomes for released animals are largely unknown. The broad variation in release locations and techniques, as well as considerable degrees of non-compliance with wildlife rehabilitation guidelines, suggest further educational opportunities may be necessary and could improve outcomes for wildlife.

Species distribution and abundance patterns in the southern Cascades are influenced by both environmental gradients and fire regimes. Little is known about fire regimes and variation in fire regimes may not be independent of environmental gradients or vegetation patterns. In this study, we analyze variation in fire regime parameters (i.e., return interval, season, size, severity, and rotation period) with respect to forest composition, elevation, and potential soil moisture in a 2042 ha area of montane forest in the southern Cascades in the Thousand Lakes Wilderness (TLW). Fire regime parameters varied with forest composition, elevation, and potential soil moisture. Median composite and point fire return intervals were shorter (4-9 yr, 14-24 yr) in low elevation and more xeric white fir (Abies concolor)-sugar pine (Pinus lambertiana) and white fir-Jeffrey pine (P. jeffreyi) and longest (20-37 yr, 20-47 yr) in mesic high elevation lodgepole pine (Pinus contorta) and red fir (Abies magnifica)-mountain hemlock (Tsuga mertensiana) forests. Values for mid-elevation red fir-white fir forests were intermediate. The pattern for fire rotation lengths across gradients was the same as for fire return intervals. The percentage of fires that occurred during the growing season was inversely related to elevation and potential soil moisture. Mean fire sizes were larger in lodgepole pine forests (405 ha) than in other forest groups (103-151 ha). In contrast to other parameters, fire severity did not vary across environmental and compositional gradients and >50% of all forests burned at high severity with most of the remainder burning at moderate severity. Since 1905, fire regimes have become similar at all gradient positions because of a policy of suppressing fire and fire regime modification will lead to shifts in landscape scale vegetation patterns.

Abstract: Approaches to fire management in the savanna ecosystems of the 2‐million ha Kruger National Park, South Africa, have changed several times over the past six decades. These approaches have included regular and flexible prescribed burning on fixed areas and a policy that sought to establish a lightning‐dominated fire regime. We sought to establish whether changes in management induced the desired variability in fire regimes over a large area. We used a spatial database of information on all fires in the park between 1957 and 2002 to determine elements of the fire regime associated with each management policy. The area that burned in any given year was independent of the management approach and was strongly related to rainfall (and therefore grass fuels) in the preceding 2 years. On the other hand, management did affect the spatial heterogeneity of fires and their seasonal distribution. Heterogeneity was higher at all scales during the era of prescribed burning, compared with the lightning‐fire interval. The lightning‐fire interval also resulted in a greater proportion (72% vs. 38%) of the area burning in the dry season. Mean fire‐return intervals varied between 5.6 and 7.3 years, and variability in fire‐return intervals was strongly influenced by the sequencing of annual rainfall rather than by management. The attempt at creating a lightning‐dominated fire regime failed because most fires were ignited by humans, and the policy has been replaced by a more pragmatic approach that combines flexible prescribed burning with lightning‐ignited fires.

Fire is a fundamental driving force in the dynamics of fynbos. This chapter discusses the role of fire in fynbos shrublands but also covers other fire-prone or fire-sensitive vegetation types (renosterveld, strandveld, and forest) in the greater Cape Floristic Region. The biophysical determinants of fire, namely fuels, weather, and ignitions (with emphasis on lightning) are explored in relation to geographic and climatic gradients, and potential long-term changes in fire danger weather indices. Fire histories in protected areas are reviewed and recent fire regimes are characterized in terms of fire frequency, season, intensity, severity, size, cause, and variability in these elements. The considerable variation in fire regimes across the region are depicted spatially, as well as quantified. Abiotic responses to variability in elements of fire regimes are assessed, including that of soil, water, and nutrients. Information on faunal responses to fire regimes is summarized and plant fire survival strategies discussed in more detail, with emphasis on reproduction and persistence traits and trends in post-fire succession. The chapter also touches on the role of fire in determining vegetation boundaries (particularly forest – fynbos and thicket – fynbos boundaries); interactions between fire and herbivory in renosterveld; and the relation between fire and the invasion of fynbos by alien plants. Lastly, an overview is given of the evolution of fire management policies since the early twentieth century, providing context for observed fire regimes and the challenges experienced in contemporary fire management.

Statistical descriptions of reconstructed fire regimes are often extrapolated from a composite of small forest stands to represent extensive geographical areas. However, statistical properties of fire regimes are scale‐dependent, thus causing some extrapolations from fine scale to coarse scale and comparisons between fire‐scar‐based reconstructions to be inappropriate. We assessed landscape fire regimes of the Sacramento Mountains, in southern New Mexico, using dendrochronological methods and a variety of fire statistics and analysis filters. We reconstructed historical and recent fire regimes for the Mescalero Apache Tribal Lands (MATL) at tree and site scale (25 ha). We then estimated the Sacramento Mountains historical and recent fire regimes by combining paleo fire data from this study with published data from the adjacent Lincoln National Forest (LNF). We applied filters to provide fire statistics that are relatively unbiased to the different spatial measurement extent of the studies. This is the first study to assess fire regime in the MATL over multiple spatial and temporal scales. The results show that frequent surface fires occurred at all scales in the Sacramento Mountains until fire was excluded from the landscape in the early 1900s. Historical fires were found to be synchronous with drought years, typically La Niña events, and often preceded by wet years. We did not find evidence supporting differences in fire regimes between the MATL and the LNF, suggesting that fire cessation following intensive Euro‐American settlement was widespread. The interruption of frequent surface fires, together with other changes in forest structure and climate, pose a significant threat to sustainability of forest ecosystems on Native American tribal lands.

Resistance of Drakensberg grasslands to compositional change depends on the influence of fire-return interval and grassland structure on richness and spatial turnover

This paper analyses the fire regime over 41 years (1956 to 1996) in Hluhluwe-Umfolozi Park, a mesic savanna area in South Africa. The study focuses at the landscape scale of tens of kilometres and at the medium term temporal scale of decades. Variation in fire regime was analysed in relation to variation in annual rainfall and the influence of management. The average annual area of the park that burnt was 26% and the average area of individual fires was 9.1 km2. Statistics relating to the percentage of area burnt annually, the average size of an individual burn, the mean and median fire return periods and the influence of management philosophy relative to the spatial and temporal variation in rainfall conditions are presented. Sources of ignition and implications for management are discussed.