Socioeconomic Factors Drive Fire-Regime Variability in the Mediterranean Basin

In recent decades, fires in Mediterranean Europe have become larger and more frequent. This trend has been driven by socioeconomic changes that have generated rural depopulation and changes in traditional land use. Within the Mediterranean Basin, the most contrasting socioeconomic conditions are found by comparing southern European with North African countries, and thus our hypothesis is that this difference generates contrasting fire regimes between the two regions. Specifically, we predict that current fire regimes in Mediterranean Africa resemble past fire regimes in the Mediterranean Europe when rural activities dominated the landscape. To test our hypothesis, we compared fire statistics from the western Rif (northern Morocco, 1988–2015) and from Valencia (eastern Spain, 1880–2014). The results suggest that the Rif has a typical Mediterranean fire regime with fires occurring in the hot, dry summer season. However, fires are very small and the annual proportion of burnt area is very low, compared to the current regime in Valencia (post-1970s). The current Rif fire size class distribution matches the fire regime in Valencia prior to the 1970s before the collapse of the rural population and when fires were fuel-limited. The shift in the recent decades in fire regime observed in different countries of the Mediterranean Europe (from small, fuel-limited fires to drought-driven fires) can be identified when moving from the southern to the northern rim of the basin. That is, most spatial and temporal variability in fire regimes of the Mediterranean Basin is driven by shifts in the amounts of fuel and continuity imposed by changes in socioeconomic drivers.     

Anthropogenic modifications to fire regimes in the wider Serengeti‐Mara ecosystem

Fire is a key driver in savannah systems and widely used as a land management tool. Intensifying human land uses are leading to rapid changes in the fire regimes, with consequences for ecosystem functioning and composition. We undertake a novel analysis describing spatial patterns in the fire regime of the Serengeti‐Mara ecosystem, document multidecadal temporal changes and investigate the factors underlying these patterns. We used MODIS active fire and burned area products from 2001 to 2014 to identify individual fires; summarizing four characteristics for each detected fire: size, ignition date, time since last fire and radiative power. Using satellite imagery, we estimated the rate of change in the density of livestock bomas as a proxy for livestock density. We used these metrics to model drivers of variation in the four fire characteristics, as well as total number of fires and total area burned. Fires in the Serengeti‐Mara show high spatial variability—with number of fires and ignition date mirroring mean annual precipitation. The short‐term effect of rainfall decreases fire size and intensity but cumulative rainfall over several years leads to increased standing grass biomass and fuel loads, and, therefore, in larger and hotter fires. Our study reveals dramatic changes over time, with a reduction in total number of fires and total area burned, to the point where some areas now experience virtually no fire. We suggest that increasing livestock numbers are driving this decline, presumably by inhibiting fire spread. These temporal patterns are part of a global decline in total area burned, especially in savannahs, and we caution that ecosystem functioning may have been compromised. Land managers and policy formulators need to factor in rapid fire regime modifications to achieve management objectives and maintain the ecological function of savannah ecosystems.     

Identifying the Threshold of Dominant Controls on Fire Spread in a Boreal Forest Landscape of Northeast China

The relative importance of fuel, topography, and weather on fire spread varies at different spatial scales, but how the relative importance of these controls respond to changing spatial scales is poorly understood. We designed a “moving window” resampling technique that allowed us to quantify the relative importance of controls on fire spread at continuous spatial scales using boosted regression trees methods. This quantification allowed us to identify the threshold value for fire size at which the dominant control switches from fuel at small sizes to weather at large sizes. Topography had a fluctuating effect on fire spread across the spatial scales, explaining 20–30% of relative importance. With increasing fire size, the dominant control switched from bottom-up controls (fuel and topography) to top-down controls (weather). Our analysis suggested that there is a threshold for fire size, above which fires are driven primarily by weather and more likely lead to larger fire size. We suggest that this threshold, which may be ecosystem-specific, can be identified using our “moving window” resampling technique. Although the threshold derived from this analytical method may rely heavily on the sampling technique, our study introduced an easily implemented approach to identify scale thresholds in wildfire regimes.     

Consistent spatial scaling of high-severity wildfire can inform expected future patterns of burn severity

Increasing wildfire activity in forests worldwide has driven urgency in understanding current and future fire regimes. Spatial patterns of area burned at high severity strongly shape forest resilience and constitute a key dimension of fire regimes, yet remain difficult to predict. To characterize the range of burn severity patterns expected within contemporary fire regimes, we quantified scaling relationships relating fire size to patterns of burn severity. Using 1615 fires occurring across the Northwest United States between 1985 and 2020, we evaluated scaling relationships within fire regimes and tested whether relationships vary across space and time. Patterns of high-severity fire demonstrate consistent scaling behaviour; as fire size increases, high-severity patches consistently increase in size and homogeneity. Scaling relationships did not differ substantially across space or time at the scales considered here, suggesting that as fire-size distributions potentially shift, stationarity in patch-size scaling can be used to infer future patterns of burn severity.     

Effect of Climate on Wildfire Size: A Cross-Scale Analysis

Theory predicts that wildfires will encounter spatial thresholds where different drivers may become the dominant influence on continued fire spread. Studying these thresholds, however, is limited by a lack of sufficiently detailed data sets. To address this problem, we searched for scale thresholds in data describing wildfire size at the Avon Park Air Force Range, south-central Florida. We used power-law statistics to describe the “heavy-tail” of the fire size distribution, and quantile regression to determine how the edges of data distributions of fire size were related to climate. Power-law statistics revealed a heavy-tail, a pattern consistent with scale threshold theory, which predicts that large fires will be rare because only fires that cross all thresholds will become large. Results from quantile regression suggested that different climate conditions served as critical thresholds, influencing wildfire size at different spatial scales. Modeling at higher quantiles (≥75th) implicated drought as driving the spread of larger fires, whereas modeling at lower quantiles (≤25th) implicated that wind governed the spread of smaller fires. Fires of intermediate size were negatively associated with relative humidity. Our results are consistent with the idea that fire spread involves scale thresholds, with the small-scale drivers allowing fires to spread after ignition, but with further spread only being possible when large-scale drivers are favorable. These results suggest that other data sets that have heavy-tailed distributions may contain patterns generated by scale thresholds, and that these patterns may be revealed using quantile regression.     

Fire as a Restoration Tool: A Decision Framework for Predicting the Control or Enhancement of Plants Using Fire

Wildfires change plant communities by reducing dominance of some species while enhancing the abundance of others. Detailed habitat‐specific models have been developed to predict plant responses to fire, but these models generally ignore the breadth of fire regime characteristics that can influence plant survival such as the degree and duration of exposure to lethal temperatures. We provide a decision framework that integrates fire regime components, plant growth form, and survival attributes to predict how plants will respond to fires and how fires can be prescribed to enhance the likelihood of obtaining desired plant responses. Fires are driven by biotic and abiotic factors that dictate their temporal (seasonality and frequency), spatial (size and patchiness), and magnitude (intensity, severity, and type) components. Plant resistance and resilience to fire can be categorized by a combination of life form, size, and ability to disperse or protect seeds. We use a combination of life form and vital plant attributes along with an understanding of fire regime components to suggest a straightforward way to approach the use of fire to either reduce or enhance particular species. A framework for aiding decisions is organized by life form and plant size. Questions regarding perennating bud and seed characteristics direct restoration practitioners to fire regimes that may achieve their management objectives of either increasing or decreasing plants with specific life form characteristics.     

Buffering the savanna: fire regimes and disequilibrium ecology in West Africa

According to contemporary ecological theory, the mechanisms governing tree cover in savannas vary by precipitation level. In tropical areas with mesic rainfall levels, savannas are unstable systems in which disturbances, such as fire, determine the ratio of trees to grasses. Precipitation in these so-called “disturbance-driven savannas” is sufficient to support forest but frequent disturbances prevent transition to a closed canopy state. Building on a savanna buffering model we argue that a consistent fire regime is required to maintain savannas in mesic areas. We hypothesize that the spatiotemporal pattern of fires is highly regular and stable in these areas. Furthermore, because tree growth rates in savannas are a function of precipitation, we hypothesize that savannas with the highest rainfall levels will have the most consistent fire pattern and the most intense fires—thus the strongest buffering mechanisms. We analyzed the spatiotemporal pattern of burning over 11 years for a large subset of the West African savanna using a moderate resolution imaging spectroradiometer active fire product to document the fire regime for three savanna belts with different precipitation levels. We used LISA analysis to quantify the spatiotemporal patterns of fires, coefficient of variance to quantify differences in peak fire dates, and center or gravity pathways to characterize the spatiotemporal patterns of the fires for each area. Our analysis confirms that spatiotemporal regularity of the fire regime is greater for mesic areas that for areas where precipitation is lower and that areas with more precipitation have more regular fire regimes.     

Climatic and Landscape Influences on Fire Regimes from 1984 to 2010 in the Western United States

An improved understanding of the relative influences of climatic and landscape controls on multiple fire regime components is needed to enhance our understanding of modern fire regimes and how they will respond to future environmental change. To address this need, we analyzed the spatio-temporal patterns of fire occurrence, size, and severity of large fires (> 405 ha) in the western United States from 1984–2010. We assessed the associations of these fire regime components with environmental variables, including short-term climate anomalies, vegetation type, topography, and human influences, using boosted regression tree analysis. Results showed that large fire occurrence, size, and severity each exhibited distinctive spatial and spatio-temporal patterns, which were controlled by different sets of climate and landscape factors. Antecedent climate anomalies had the strongest influences on fire occurrence, resulting in the highest spatial synchrony. In contrast, climatic variability had weaker influences on fire size and severity and vegetation types were the most important environmental determinants of these fire regime components. Topography had moderately strong effects on both fire occurrence and severity, and human influence variables were most strongly associated with fire size. These results suggest a potential for the emergence of novel fire regimes due to the responses of fire regime components to multiple drivers at different spatial and temporal scales. Next-generation approaches for projecting future fire regimes should incorporate indirect climate effects on vegetation type changes as well as other landscape effects on multiple components of fire regimes.     

Customary Fire Regimes and Vegetation Structure in Gabon’s Bateke Plateaux

Most fires in Africa are anthropogenic yet remain understudied. Studies typically address managed fire, or the ??fire triad?? of early dry season-late dry season-suppression, and fire regimes which are annual or less, leaving unstudied the anthropogenic fire regimes that occur in the majority of African savannas. I take the case of the Bateke Plateaux area where burning today occurs both annually and semi-annually and measure the impacts of these regimes on savanna structure, measuring stem survival post fire and post fire regeneration of resprouts of the dominant savanna tree. While annual fires are hot and burn completely, semi-annual fires are cooler and patchy, favouring re-sprout survival and an escape route for small stems to mature into trees. This work extends the fire triad model to include an anthropogenic semi-annual regime which favours tree survival. The integration of local fire regimes into future studies will help increase our understanding of climate, vegetation dynamics as well as help orient policy and conservation.     

Neural Avalanches at the Critical Point between Replay and Non-Replay of Spatiotemporal Patterns

We model spontaneous cortical activity with a network of coupled spiking units, in which multiple spatio-temporal patterns are stored as dynamical attractors. We introduce an order parameter, which measures the overlap (similarity) between the activity of the network and the stored patterns. We find that, depending on the excitability of the network, different working regimes are possible. For high excitability, the dynamical attractors are stable, and a collective activity that replays one of the stored patterns emerges spontaneously, while for low excitability, no replay is induced. Between these two regimes, there is a critical region in which the dynamical attractors are unstable, and intermittent short replays are induced by noise. At the critical spiking threshold, the order parameter goes from zero to one, and its fluctuations are maximized, as expected for a phase transition (and as observed in recent experimental results in the brain). Notably, in this critical region, the avalanche size and duration distributions follow power laws. Critical exponents are consistent with a scaling relationship observed recently in neural avalanches measurements. In conclusion, our simple model suggests that avalanche power laws in cortical spontaneous activity may be the effect of a network at the critical point between the replay and non-replay of spatio-temporal patterns.     

Mediterranean forest dynamics and forest bird distribution changes in the late 20th century

Processes derived from global change such as land-use changes, climate warming or modifications in the perturbation regime may have opposite effects on forest extent and structure with still unknown consequences on forest biodiversity at large spatial scales. In the present study, we aimed at determining forest dynamics associated with global change processes (forest spread, maturation and fire) that have driven the variation in forest bird distributions in Mediterranean forest ecosystems in recent years. The study was located in Catalonia (NE Spain) and used changes in richness of specialist and generalist forest bird species in the last 20 years of the 20th century as indicators of forest biodiversity change. Forest bird distribution changes showed strong spatial patterns and appeared to be related to population processes occurring beyond sampling units (10 km × 10 km squares). Forest maturation appeared as the most important driver of such changes because most of the studied species have a non-Mediterranean origin and are associated with more mature forests. To a lower degree, forest spread also contributed to forest bird distribution changes whereas the impact of forest fires was not associated to a decrease in the richness of either group of forest species. Given the relatively coarse scale at which our study was conducted, caution should be taken when extrapolating our results to the possible future impacts of climate change on fire regime and forest bird distribution. Our results indicate that large-scale forest maturation and spread due mainly to land abandonment in Catalonia has overridden the potentially negative effects of fires on forest bird distributions and are currently driving changes in forest biodiversity patterns across the region.     

Divergent Fire Regimes in Two Contrasting Mediterranean Chestnut Forest Landscapes

Humans have historically played a critical role in the management of Mediterranean-type ecosystems (MTEs) through traditional fire use. Although chestnut forests are widespread across the Mediterranean Basin, little is known about their historical fire regimes. Our goal here is to generate testable hypotheses about the drivers of fire regime dynamics in chestnut dominated ecosystems. To examine anthropogenic fire management we selected two sites in Spain that have similar biophysical characteristics but divergent levels of economic development and fire management policies. Fire regime-landscape feedbacks were characterized through a pilot dendroecological study, official fire statistics, aerial photography and forest inventory data. Our results suggest that fire incidence in both sites has increased since the pre-industrial era but fire season, fire size, and forest structure have changed to a greater extent in the more developed site. These changes are probably driven by the decline in annual anthropogenic burning of litterfall by local communities at the more developed site during the non-vegetative season.     

Tropical and north Pacific teleconnections influence fire regimes in pine-dominated forests of north-eastern California, USA

Aim To assess the importance of drought and teleconnections from the tropical and north Pacific Ocean on historical fire regimes and vegetation dynamics in north‐eastern California. Location The 700 km² study area was on the leeward slope of the southern Cascade Mountains in north‐eastern California. Open forests of ponderosa pine (Pinus ponderosa var. ponderosa Laws.) and Jeffrey pine (P. jeffreyi Grev. & Balf) surround a network of grass and shrub‐dominated meadows that range in elevation from 1650 to 1750 m. Methods Fire regime characteristics (return interval, season and extent) were determined from crossdated fire scars and were compared with tree‐ring based reconstructions of precipitation and temperature and teleconnections for the period 1700–1849. The effect of drought on fire regimes was determined using a tree‐ring based proxy of climate from five published chronologies. The number of forest‐meadow units that burned was compared with published reconstructions of the El Niño/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Results Landscape scale fires burned every 7–49 years in meadow‐edge forests and were influenced by variation in drought, the PDO and ENSO. These widespread fires burned during years that were dryer and warmer than normal that followed wetter and cooler years. Less widespread fires were not associated with this wet, then dry climate pattern. Widespread fires occurred during El Niño years, but fire extent was mediated by the phase of the PDO. Fires were most widespread when the PDO was in a warm or normal phase. Fire return intervals, season and extent varied at decadal to multi‐decadal time scales. In particular, an anomalously cool, wet period during the early 1800s resulted in widespread fires that occurred earlier in the year than fires before or after. Main conclusions Fire regimes in north‐eastern California were strongly influenced by regional and hemispheric‐scale climate variation. Fire regimes responded to variation that occurred in both the north and tropical Pacific. Near normal modes of the PDO may influence fire regimes more than extreme conditions. The prevalence of widespread teleconnection‐driven fires in the historic record suggests that variation in the Pacific Ocean was a key regulator of fire regimes through its influence on local fuel production and successional dynamics in north‐eastern California.     

Global climate change and litter decomposition: more frequent fire slows decomposition and increases the functional importance of invertebrates

Of the many mechanisms by which global climate change may alter ecosystem processes perhaps the least known and insidious is altered disturbance regimes. We used a field-based experiment to examine the climate change scenario of more frequent fires with altered invertebrate assemblages on the decomposition of Eucalyptus leaves. Our design comprised three fire regimes [long-term fire exclusion (FE), long-term frequent burning (FB) and FE altered to FB (FEFB)] and two litter bag mesh sizes (8.0 and 0.2 mm) that either permitted or denied access to the leaf litter by most invertebrates. We found a significant interaction effect between fire regime and mesh size in losses of litter mass and net carbon (C). Compared with the regime of FE, with more frequent burning (FB and FEFB) the pace of decomposition was slowed by 41% (when access to litter by most invertebrates is not impeded). For the regime of FE, denying access to leaf litter by most invertebrates did not alter the pace of decomposition. Conversely, under regimes of frequently burning, restricting access to the litter by most invertebrates altered the pace of decomposition by 46%. Similar results were found for net C. For net losses of nitrogen (N), no interaction effects between fire regime and mesh size were detected, although both main effects were significant. Our results show that by modifying disturbance regimes such as fire frequency, global climate change has the potential to modify the mechanism by which ecosystems function. With more FB, decomposition is driven not only by fire regime induced changes in substrate quality and/or physiochemical conditions but through the interaction of disturbance regime with animal assemblages mediating ecosystem processes.     

Introduced annual grass increases regional fire activity across the arid western USA (1980–2009)

Non‐native, invasive grasses have been linked to altered grass‐fire cycles worldwide. Although a few studies have quantified resulting changes in fire activity at local scales, and many have speculated about larger scales, regional alterations to fire regimes remain poorly documented. We assessed the influence of large‐scale Bromus tectorum (hereafter cheatgrass) invasion on fire size, duration, spread rate, and interannual variability in comparison to other prominent land cover classes across the Great Basin, USA. We compared regional land cover maps to burned area measured using the Moderate Resolution Imaging Spectroradiometer (MODIS) for 2000–2009 and to fire extents recorded by the USGS registry of fires from 1980 to 2009. Cheatgrass dominates at least 6% of the central Great Basin (650 000 km²). MODIS records show that 13% of these cheatgrass‐dominated lands burned, resulting in a fire return interval of 78 years for any given location within cheatgrass. This proportion was more than double the amount burned across all other vegetation types (range: 0.5–6% burned). During the 1990s, this difference was even more extreme, with cheatgrass burning nearly four times more frequently than any native vegetation type (16% of cheatgrass burned compared to 1–5% of native vegetation). Cheatgrass was also disproportionately represented in the largest fires, comprising 24% of the land area of the 50 largest fires recorded by MODIS during the 2000s. Furthermore, multi‐date fires that burned across multiple vegetation types were significantly more likely to have started in cheatgrass. Finally, cheatgrass fires showed a strong interannual response to wet years, a trend only weakly observed in native vegetation types. These results demonstrate that cheatgrass invasion has substantially altered the regional fire regime. Although this result has been suspected by managers for decades, this study is the first to document recent cheatgrass‐driven fire regimes at a regional scale.     

Stand Age Influence on Potential Wildfire Ignition and Spread in the Boreal Forest of Northeastern Canada

Although it has long been assumed that wildfire occurrence is independent of stand age in the North American boreal forest, recent studies indicate that young forests may influence burn rates by limiting the ignition and spread of fires for several years. Wildfires not only structure the stand-age mosaic of boreal landscapes, but also alter the likelihood and behavior of subsequent fires. Using a fire simulation model, we evaluated the effect of stand age on the magnitude and spatial patterns of burn probability (BP) in the boreal forest of northeastern Canada. Specifically, we assessed the stand age effect on the two processes driving fire likelihood, ignition and spread, by simulating tens of thousands of fires under three fire regime scenarios that vary in terms of mean fire size and number of burned patches. Assuming minimal resistance to fire ignition and spread, where only the youngest stands (≤ 10 years) are resistant to burning, mean BP is reduced by 10%; in contrast, assuming maximum resistance, where stands up to 90 years old impede wildfires, mean BP can be reduced up to 85%. Although the resistance to ignition on BP is almost identical in magnitude to that of spread, it yields substantially different spatial arrangements of BP. Furthermore, stand age resistance reduces subsequent fire activity not only within but also outside the perimeter of burned patches through a shadow effect. Our results help to untangle the role of factors contributing to stand age resistance on wildfires and offer new insights for improving the spatial mapping of fire likelihood.     

Ngadju kala: Australian Aboriginal fire knowledge in the Great Western Woodlands

Indigenous fire knowledge offers significant benefits for ecosystem management and human livelihoods, but is threatened worldwide because of disruption of customary practices. In Australia, the historical prevalence and characteristics of Aboriginal burning are intensely debated, including arguments that Aboriginal burning was frequent across the continent. Frequent burning is supported by contemporary Aboriginal knowledge and practice in some regions, but in southern Australia evidence is typically limited to historical and ecological records. Towards characterizing Aboriginal fire regimes in southern Australia, we collaborated with Ngadju people from the globally significant Great Western Woodlands in south‐western Australia to document their fire knowledge. We used workshops, site visits, interviews and occupation mapping to aid knowledge sharing. Consistent with the established significance of Aboriginal fire in Australia, planned fires were important in Ngadju daily life and land management. However, Ngadju use of fire was characterized by its selectivity rather than its ubiquity. Specifically, Ngadju described only highly targeted planned burning across extensive eucalypt woodlands and sandplain shrublands. By contrast, frequent planned burning was described for resource‐rich landscape elements of more restricted extent (granite outcrop vegetation, grasslands and coastal scrub). Overall, Ngadju fires are likely to have resulted in subtle but purposeful direct effects on the vegetation and biota. However the extent to which they collectively constrained large, intense wildfires remains unclear. Ngadju demonstrated a predictive knowledge of the ecological consequences of burning, including attention to fine‐scale needs of target organisms, and application of diverse fire regimes. These are consistent with the recently proposed concept that Aboriginal burning was guided by ‘templates’ targeting different resources, although diverse regimes predominantly reflect edaphically driven vegetation patterns rather than template‐driven use of fire to create resource diversity. We conclude that Ngadju fire knowledge fills an important gap in understanding Aboriginal fire regimes in southern Australia, highlighting a novel balance between frequent and constrained use of fire.     

Using population viability analysis to predict the effect of fire on the extinction risk of an endangered shrub <Emphasis Type="Italic">Verticordia fimbrilepis</Emphasis> subsp. <Emphasis Type="Italic">fimbrilepis</Emphasis> in a fragmented landscape

The fragmentation of mediterranean climate landscapes where fire is an important landscape process may lead to unsuitable fire regimes for many species, particularly rare species that occur as small isolated populations. We investigate the influence of fire interval on the persistence of population fragments of the endangered shrub Verticordia fimbrilepis Turcz. subsp. fimbrilepis in mediterranean climate south-west of Western Australia. We studied the population biology of the species over 5 years. While the species does recruit sporadically without fire this occurs only in years with above average rainfall, so fire seems to be the main environmental factor producing extensive recruitment. Transition matrix models were constructed to describe the shrub’s population dynamics. As the species is killed by fire and relies on a seed bank stimulated to germinate by smoke, stochastic simulations to compare different fire frequencies on population viability were completed. Extinction risk increased with increasing average fire interval. Initial population size was also important, with the lowest extinction risk in the largest population. For populations in small reserves where fire is generally excluded, inevitable plant senescence will lead to local extirpation unless fires of suitable frequency can be used to stimulate regeneration. While a suitable fire regime reduces extinction risk small populations are still prone to extinction due to stochastic influences, and this will be exacerbated by a projected drying climate increasing rates of adult mortality and also seedling mortality in the post-fire environment.     

Climate change, human land use and future fires in the Amazon

There is increasing consensus that the global climate will continue to warm over the next century. The biodiversity-rich Amazon forest is a region of growing concern because many global climate model (GCM) scenarios of climate change forecast reduced precipitation and, in some cases, coupled vegetation models predict dieback of the forest. To date, fires have generally been spatially co-located with road networks and associated human land use because almost all fires in this region are anthropogenic in origin. Climate change, if severe enough, could alter this situation, potentially changing the fire regime to one of increased fire frequency and severity for vast portions of the Amazon forest. High moisture contents and dense canopies have historically made Amazonian forests extremely resistant to fire spread. Climate will affect the fire situation in the Amazon directly, through changes in temperature and precipitation, and indirectly, through climate-forced changes in vegetation composition and structure. The frequency of drought will be a prime determinant of both how often forest fires occur and how extensive they become. Fire risk management needs to take into account landscape configuration, land cover types and forest disturbance history as well as climate and weather. Maintaining large blocks of unsettled forest is critical for managing landscape level fire in the Amazon. The Amazon has resisted previous climate changes and should adapt to future climates as well if landscapes can be managed to maintain natural fire regimes in the majority of forest remnants.     

Gradient Analysis of Fire Regimes in Montane Forests of the Southern Cascade Range,Thousand Lakes Wilderness,California, USA

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.