Bottom-up control of a northern Arizona ponderosa pine forest fire regime in a fragmented landscape

Fire regimes often vary at fine spatial scales in response to factors such as topography or fuels while climate usually synchronizes fires across broader scales. We investigated the relative influence of top-down and bottom-up controls on fire occurrence in ponderosa pine (Pinus ponderosa) forests in a highly fragmented landscape at Mount Dellenbaugh, in northwestern Arizona. Our study area of 4,000?ha was characterized by patches of ponderosa pine forest in drainages that were separated by a matrix of pinyon?Cjuniper woodlands, sagebrush shrublands, and perennial grasslands. We reconstructed fire histories from 135 fire-scarred trees in sixteen 25-ha sample sites placed in patches of mature ponderosa forest. We found that, among patches of ponderosa forest, fires were similar in terms of frequency but highly asynchronous in terms of individual years. Climate synchronized fire but only across broader spatial scales. Fires occurring at broader scales were associated with dry years that were preceded by several wet years. The remarkable level of asynchrony at finer scales suggests that bottom-up factors, such as site productivity and fuel continuity, were important in regulating fire at Mount Dellenbaugh. Understanding where bottom-up controls were historically influential is important for prioritizing areas that may best respond to fuel treatment under a warming climate.     

Use of artificial landscapes to isolate controls on burn probability

Techniques for modeling burn probability (BP) combine the stochastic components of fire regimes (ignitions and weather) with sophisticated fire growth algorithms to produce high-resolution spatial estimates of the relative likelihood of burning. Despite the numerous investigations of fire patterns from either observed or simulated sources, the specific influence of environmental factors on BP patterns is not well understood. This study examined the relative effects of ignitions, fuels, and weather on mean BP and spatial patterns in BP (i.e., BP variability) using highly simplified artificial landscapes and wildfire simulation methods. Our results showed that a limited set of inputs yielded a wide range of responses in the mean and spatial patterning of BP. The input factors contributed unequally to mean BP and to BP variability: so-called top-down controls (weather) primarily influenced mean BP, whereas bottom-up influences (ignitions and fuels) were mainly responsible for the spatial patterns of BP. However, confounding effects and interactions among factors suggest that fully separating top-down and bottom-up controls may be impossible. Furthermore, interactions among input variables produced unanticipated but explainable BP patterns, hinting at complex topological dependencies among the main determinants of fire spread and the resulting BP. The results will improve our understanding of the spatial ecology of fire regimes and help in the interpretation of patterns of fire likelihood on real landscapes as part of future wildfire risk assessments.     

Drivers and trends in landscape patterns of stand-replacing fire in forests of the US Northern Rocky Mountains (1984–2010)

Context

Resilience in fire-prone forests is strongly affected by landscape burn-severity patterns, in part by governing propagule availability around stand-replacing patches in which all or most vegetation is killed. However, little is known about drivers of landscape patterns of stand-replacing fire, or whether such patterns are changing during an era of increased wildfire activity.

Objectives

(a) Identify key direct/indirect drivers of landscape patterns of stand-replacing fire (e.g., size, shape of patches), (b) test for temporal trends in these patterns, and (c) anticipate thresholds beyond which landscape patterns of burn severity may change fundamentally.

Methods

We applied structural equation modeling to satellite burn-severity maps of fires in the US Northern Rocky Mountains (1984–2010) to test for direct and indirect (via influence on fire size and proportion stand-replacing) effects of climate/weather, vegetation, and topography on landscape patterns of stand-replacing fire. We also tested for temporal trends in landscape patterns.

Results

Landscape patterns of stand-replacing fire were strongly controlled by fire size and proportion stand-replacing, which were, in turn, controlled by climate/weather and vegetation/topography, respectively. From 1984 to 2010, the proportion of stand-replacing fire within burn perimeters increased from 0.22 to 0.27. Trends for other landscape metrics were not significant, but may respond to further increases proportion stand-replacing fire.

Conclusions

Fires from 1984 to 2010 exhibited tremendous heterogeneity in landscape patterns of stand-replacing fire, likely promoting resilience in burned areas. If trends continue on the current trajectory, however, fires may produce larger and simpler shaped patches of stand-replacing fire with more burned area far from seed sources.

     

Influence of landscape structure,topography, and forest type on spatial variation in historical fire regimes,Central Oregon,USA

Context

In the interior Northwest, debate over restoring mixed-conifer forests after a century of fire exclusion is hampered by poor understanding of the pattern and causes of spatial variation in historical fire regimes.

Objectives

To identify the roles of topography, landscape structure, and forest type in driving spatial variation in historical fire regimes in mixed-conifer forests of central Oregon.

Methods

We used tree rings to reconstruct multicentury fire and forest histories at 105 plots over 10,393 ha. We classified fire regimes into four types and assessed whether they varied with topography, the location of fuel-limited pumice basins that inhibit fire spread, and an updated classification of forest type.

Results

We identified four fire-regime types and six forest types. Although surface fires were frequent and often extensive, severe fires were rare in all four types. Fire regimes varied with some aspects of topography (elevation), but not others (slope or aspect) and with the distribution of pumice basins. Fire regimes did not strictly co-vary with mixed-conifer forest types.

Conclusions

Our work reveals the persistent influence of landscape structure on spatial variation in historical fire regimes and can help inform discussions about appropriate restoration of fire-excluded forests in the interior Northwest. Where the goal is to restore historical fire regimes at landscape scales, managers may want to consider the influence of topoedaphic and vegetation patch types that could affect fire spread and ignition frequency.

     

Comparing fire spread algorithms using equivalence testing and neutral landscape models

We demonstrate a method to evaluate the degree to which a meta-model approximates spatial disturbance processes represented by a more detailed model across a range of landscape conditions, using neutral landscapes and equivalence testing. We illustrate this approach by comparing burn patterns produced by a relatively simple fire spread algorithm with those generated by a more detailed fire behavior model from which the simpler algorithm was derived. Equivalence testing allows objective comparisons of the output of simple and complex models, to determine if the results are significantly similar. Neutral landscape models represent a range of landscape conditions that the model may encounter, allowing evaluation of the sensitivity and behavior of the model to different landscape compositions and configurations. We first tested the model for universal applicability, then narrowed the testing to assess the practical domain of applicability. As a whole, the calibrated simple model passed the test for significant equivalence using the 25% threshold. When applied to a range of landscape conditions different from the calibration scenarios, the model failed the tests for equivalence. Although our particular model failed the tests, the neutral landscape models were helpful in determining an appropriate domain of applicability and in assessing the model sensitivity to landscape changes. Equivalence testing provides an effective method for model comparison, and coupled with neutral landscapes, our approach provides an objective way to assess the domain of applicability of a spatial model.     

Spatial patterns of large natural fires in Sierra Nevada wilderness areas

The effects of fire on vegetation vary based on the properties and amount of existing biomass (or fuel) in a forest stand, weather conditions, and topography. Identifying controls over the spatial patterning of fire-induced vegetation change, or fire severity, is critical in understanding fire as a landscape scale process. We use gridded estimates of fire severity, derived from Landsat ETM+ imagery, to identify the biotic and abiotic factors contributing to the observed spatial patterns of fire severity in two large natural fires. Regression tree analysis indicates the importance of weather, topography, and vegetation variables in explaining fire severity patterns between the two fires. Relative humidity explained the highest proportion of total sum of squares throughout the Hoover fire (Yosemite National Park, 2001). The lowest fire severity corresponded with increased relative humidity. For the Williams fire (Sequoia/Kings Canyon National Parks, 2003) dominant vegetation type explains the highest proportion of sum of squares. Dominant vegetation was also important in determining fire severity throughout the Hoover fire. In both fires, forest stands that were dominated by lodgepole pine (Pinus contorta) burned at highest severity, while red fir (Abies magnifica) stands corresponded with the lowest fire severities. There was evidence in both fires that lower wind speed corresponded with higher fire severity, although the highest fire severity in the Williams fire occurred during increased wind speed. Additionally, in the vegetation types that were associated with lower severity, burn severity was lowest when the time since last fire was fewer than 11 and 17 years for the Williams and Hoover fires, respectively. Based on the factors and patterns identified, managers can anticipate the effects of management ignited and naturally ignited fires at the forest stand and the landscape levels.     

Landscape-scale controls over 20th century fire occurrence in two large Rocky Mountain (USA) wilderness areas

Topography, vegetation, and climate act together to determine thespatial patterns of fires at landscape scales. Knowledge oflandscape-fire-climate relations at these broad scales (1,000s hato 100,000s ha) is limited and is largely based on inferences andextrapolations from fire histories reconstructed from finer scales. In thisstudy, we used long time series of fire perimeter data (fire atlases) and datafor topography, vegetation, and climate to evaluate relationships between large20^thcentury fires and landscape characteristics in two contrastingareas: the 486,673-ha Gila/Aldo Leopold Wilderness Complex (GALWC)in New Mexico, USA, and the 785,090-ha Selway-BitterrootWilderness Complex (SBWC) in Idaho and Montana, USA. There were importantsimilarities and differences in gradients of topography, vegetation, andclimatefor areas with different fire frequencies, both within and between study areas.These unique and general relationships, when compared between study areas,highlight important characteristics of fire regimes in the Northern andSouthernRocky Mountains of the Western United States.Results suggest that amount and horizontal continuity of herbaceous fuels limitthe frequency and spread of surface fires in the GALWC, while the moisturestatus of large fuels and crown fuels limits the frequency of moderate-to-highseverity fires in the SBWC. These empirically described spatial and temporalrelationships between fire, landscape attributes, and climate increaseunderstanding of interactions among broad-scale ecosystem processes. Resultsalso provide a historical baseline for fire management planning over broadspatial and temporal scales in each wilderness complex.This revised version was published online in May 2005 with corrections to the Cover Date.     

The influence of topography and fire in controlling landscape composition and structure in Sierra de Gredos (Central Spain)

Mediterranean landscapes are dynamic systems that undergo temporal changes in composition and structure in response to disturbances, such as fire. Neither landscape patterns nor driving factors that affect them are evenly distributed in space. Accordingly, disturbances and biophysical factors interact in space through time. The aim of this paper is to assess the relative influence of topography and fire on the landscape patterns of a large forested area located in Sierra de Gredos (Central Spain) through time. A series of Landsat MSS images from 1975 to 1990, and a digital elevation model (DEM) were used to map fires, assess topographical complexity and evaluate changes in landscape composition and structure. Functional regions across the entire landscape were identified using different classification criteria (i.e., percentage burned area and topographic properties) to model topographic and fire impacts at regional scales. A canonical variance partition method, with a time series split-plot design, quantified the relative influence and co-variation of topography and fire on land cover patterns through time. Main results indicated that analyzing portions of the landscape under similar environmental conditions and fire histories, the effects of different fire regimes on the spatio-temporal dynamics of main land covers can be highlighted. However, the impact of fire on landscape patterns was high variable among regions due to the different regeneration abilities of main land covers, the topographic constraints and the fire histories of each region. Hence, broad patterns of fire related variance and co-variation with topography emerged across the entire area due to the different conditions of each landscape portion in which this large Mediterranean landscape was divided. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Long-term,landscape patterns of past fire events in a montane ponderosa pine forest of central Colorado

Parameters of fire regimes, including fire frequency, spatial extent of burned areas, fire severity, and season of fire occurrence, influence vegetation patterns over multiple scales. In this study, centuries-long patterns of fire events in a montane ponderosa pine – Douglas-fir forest landscape surrounding Cheesman Lake in central Colorado were reconstructed from fire-scarred trees and inferences from forest stand ages. We crossdated 153 fire-scarred trees from an approximately 4000 ha study area that recorded 77 total fire years from 1197 to the present. Spatial extent of burned areas during fire years varied from the scale of single trees or small clusters of trees to fires that burned across the entire landscape. Intervals between fire years varied from 1 to 29 years across the entire landscape to 3 to 58 years in one stand, to over 100 years in other stands. Large portions of the landscape did not record any fire for a 128 year-long period from 1723 to 1851. Fire severity varied from low-intensity surface fires to large-scale, stand-destroying fires, especially during the 1851 fire year but also possibly during other years. Fires occurred throughout tree growing seasons and both before and after growing seasons. These results suggest that the fire regime has varied considerably across the study area during the past several centuries. Since fires influence plant establishment and mortality on the landscape, these results further suggest that vegetation patterns changed at multiple scales during this period. The fire history from Cheesman Lake documents a greater range in fire behavior in ponderosa pine forests than generally has been found in previous studies.     

Landscape heterogeneity and fire behavior: scale-dependent feedback between fire and grazing processes

Fire and grazing are ecological processes that frequently interact to modify landscape patterns of vegetation. There is empirical and theoretical evidence that response of herbivores to heterogeneity is scale-dependent however the relationship between fire and scale of heterogeneity is not well defined. We examined the relationship between fire behavior and spatial scale (i.e., patch grain) of fuel heterogeneity. We created four heterogeneous landscapes modeled after those created by a fire–grazing interaction that differed in grain size of fuel patches. Fire spread was simulated through each model landscape from 80 independent, randomly located ignition points. Burn area, burn shape complexity and the proportion of area burnt by different fire types (headfire, backfire and flankfire) were all affected by the grain of fuel patch. The area fires burned in heterogeneous landscapes interacted with the fuel load present in the patch where ignition occurred. Burn complexity was greater in landscapes with small patch grain than in landscapes with large patch grain. The proportion of each fire type (backfire, flankfire and headfire) was similar among all landscapes regardless of patch grain but the variance of burned area within each of the three fire types differed among treatments of patch grain. Our landscape fire simulation supports the supposition that feedbacks between landscape patterns and ecological processes are scale-dependent, in this case spatial scale of fuel loading altering fire spread through the landscape.     

Historical patterns of fire severity and forest structure and composition in a landscape structured by frequent large fires: Pumice Plateau ecoregion,Oregon, USA

Context

Lack of quantitative observations of extent, frequency, and severity of large historical fires constrains awareness of departure of contemporary conditions from those that demonstrated resistance and resilience to frequent fire and recurring drought.

Objectives

Compare historical and contemporary fire and forest conditions for a dry forest landscape with few barriers to fire spread.

Methods

Quantify differences in (1) historical (1700–1918) and contemporary (1985–2015) fire extent, fire rotation, and stand-replacing fire and (2) historical (1914–1924) and contemporary (2012) forest structure and composition. Data include 85,750-ha tree-ring reconstruction of fire frequency and extent; >?375,000-ha timber inventory following >?78,900-ha fires in 1918; and remotely-sensed maps of contemporary fire effects and forest conditions.

Results

Historically, fires?>?20,000 ha occurred every 9.5 years; fire rotation was 14.9 years; seven fires?>?40,469 ha occurred during extreme drought (PDSI <?? 4.0); and stand-replacing fire occurred primarily in lodgepole (Pinus contorta var. murrayana). In contemporary fires, only 5% of the ecoregion burned in 30 years, and stand-replacing fire occurred primarily in ponderosa (Pinus ponderosa) and mixed-conifer. Historically, density of conifers?>?15 cm dbh exceeded 120 trees/ha on?<?5% of the area compared to 95% currently.

Conclusions

Frequent, large, low-severity fires historically maintained open-canopy ponderosa and mixed-conifer forests in which large fire- and drought-tolerant trees were prevalent. Stand-replacing patches in ponderosa and mixed-conifer were rare, even in fires >?40,469 ha (minimum size of contemporary “megafires”) during extreme drought. In this frequent-fire landscape, mixed-severity fire historically influenced lodgepole and adjacent forests. Lack of large, frequent, low-severity fires degrades contemporary forest ecosystems.

     

Using Neutral Models to Identify Constraints on Low-severity Fire Regimes

Climate, topography, fuel loadings, and human activities all affect spatial and temporal patterns of fire occurrence. Because fire is modeled as a stochastic process, for which each fire history is only one realization, a simulation approach is necessary to understand baseline variability, thereby identifying constraints, or forcing functions, that affect fire regimes. With a suitable neutral model, characteristics of natural fire regimes estimated from fire history data can be compared to a “null hypothesis”. We generated random landscapes of fire-scarred trees via a point process with sequential spatial inhibition. Random ignition points, fire sizes, and fire years were drawn from uniform and exponential family probability distributions. We compared two characteristics of neutral fire regimes to those from five watersheds in eastern Washington that have experienced low-severity fire. Composite fire intervals (CFIs) at multiple spatial scales displayed similar monotonic decreases with increasing sample area in neutral vs. real landscapes, although patterns of residuals from statistical models differed. In contrast, parameters of the Weibull distribution associated with temporal trends in fire hazard exhibited different forms of scale dependence in real vs. simulated data. Clear patterns in neutral landscapes suggest that deviations from them in empirical data represent real constraints on fire regimes (e.g., topography, fuels). As with any null model, however, neutral fire-regime models need to be carefully tuned to avoid confounding these constraints with artifacts of modeling. Neutral models show promise for investigating low-severity fire regimes to separate intrinsic properties of stochastic processes from the effects of climate, fuel loadings, topography, and management.     

Climatic and topographic controls on patterns of fire in the southern and central Appalachian Mountains,USA

Climate and topography are two important controls on spatial patterns of fire disturbance in forests globally, via their influence on fuel moisture and fuel production. To assess the influences of climate and topography on fire disturbance patterns in a temperate forest region, we analyzed the mapped perimeters of fires that burned during 1930–2003 in two national parks in the eastern United States. These were Great Smoky Mountains National Park (GSMNP) in the southern Appalachian Mountains and Shenandoah National Park (SNP) in the central Appalachian Mountains. We conducted GIS analyses to assess trends in area burned under differing climatic conditions and across topographic gradients (elevation, slope position, and aspect). We developed a Classification and Regression Tree model in order to further explore the interactions between topography, climate, and fire. The results demonstrate that climate is a strong driver of both spatial and temporal patterns of wildfire. Fire was most prevalent in the drier SNP than the wetter GSMNP, and during drought years in both parks. Topography also influenced fire occurrence, with relatively dry south-facing aspects, ridges, and lower elevations burning most frequently. However, the strength of topographic trends varied according to the climatic context. Weaker topographic trends emerged in the drier SNP than GSMNP, and during low-PDSI (dry) years than high-PDSI (wet) years in both parks. The apparent influence of climate on the spatial patterning of fire suggests a more general concept, that disturbance-prone landscapes exhibit weaker fine-scale spatial patterning of disturbance than do less disturbance-prone landscapes.     

From pattern to process: estimating expansion rates of a forest tree species in a protected palm savanna

We assessed the possible influences of dominant tree density (Butia yatay palm trees) and fire on the expansion of a riparian tree population (Myrcianthes cisplatensis) over El Palmar National Park, a protected savanna in Argentina. Our approach is based on Skellam’s model of population expansion, which predicts that populations with density-independent reproduction and random dispersal will exhibit Gaussian-shaped expansion fronts. Using Poisson regression, we fitted Gaussian curves to Myrcianthes density data collected at varying distances from a riparian forest, within four environmental conditions resulting from combinations of palm density (dense and sparse) and fire history (burned and unburned). Based on the estimated parameters, we derived statistics appropriate to compare attained expansion velocity, mean squared effective dispersal distance, and density-independent population growth among environmental conditions. We also analyzed the effects of palm density, fire history, and distance from the riparian forest on local maximum size of Myrcianthes individuals. Gaussian curves fitted the data reasonably well and slightly better than two alternative front models. Palm density and fire history interacted to control Myrcianthes spread, making unburned dense palm savannas the preferential avenue for Myrcianthes population expansion across the landscape. Limitation of Myrcianthes expansion by fire appeared to result from low survival of small individuals to fire, whereas facilitation of Myrcianthes expansion by palm trees may have resulted from increased population growth. Our results stress the interactive role of fire regime and local biotic influences in determining propagule pressure and tree establishment at the forefront, and the overall vulnerability of savannas to colonization by forest species.     

Temporal patterns of ecosystem processes on simulated landscapes in Glacier National Park,Montana, USA

The mechanistic, spatially-explicit fire succession model, Fire-BGC (a Fire BioGeoChemical succession model) was used to investigate long-term trends in landscape pattern under historical and future fire regimes and present and future climate regimes for two 46000 ha landscapes in Glacier National Park, Montana, USA. Fire-BGC has two spatial and temporal resolutions in the simulation architecture where ecological processes that act at a landscape level, such as fire, are simulated annually from information contained in spatial data layers, while stand-level processes such as photosynthesis, transpiration, and decomposition are simulated both daily and annually. Fire is spread across the landscape using the FARSITE fire growth model and subsequent fire effects are simulated at the stand-level. Fire-BGC was used to simulate changes in landscape pattern over 250 years under four scenarios: (1) complete fire exclusion under current climate, (2) historical wildfire occurrence and current climate, (3) complete fire exclusion under a possible future climate, (4) future wildfire occurrence and future climate. Simulated maps of dominant tree species, aboveground standing crop, leaf area index, and net primary productivity (NPP) were contrasted across scenarios using the metrics of patch density, edge density, evenness, contagion, and interspersion. Simulation results indicate that fire influences landscape pattern metrics more that climate alone by creating more diverse, fragmented, and disconnected landscapes. Fires were more frequent, larger, and more intense under a future climate regime. Landscape metrics showed different trends for the process-based NPP map when compared to the cover type map. It may be important to augment landscape analyses with process-based layers as well as structural and compositional layers.     

Landscape characteristics of disturbed shrubsteppe habitats in southwestern Idaho (U.S.A.)

We compared 5 zones in shrubsteppe habitats of southwestern Idaho to determine the effect of differing disturbance combinations on landscapes that once shared historically similar disturbance regimes. The primary consequence of agriculture, wildfires, and extensive fires ignited by the military during training activities was loss of native shrubs from the landscape. Agriculture created large square blocks on the landscape, and the landscape contained fewer small patches and more large shrub patches than non-agricultural areas. In contrast, fires left a more fragmented landscape. Repeated fires did not change the distribution of patch sizes, but decreased the total area of remaining shrublands and increased the distance between remaining shrub patches that provide seed sources. Military training with tracked vehicles was associated with a landscape characterized by small, closely spaced, shrub patches.Our results support the general model hypothesized for conversion of shrublands to annual grasslands by disturbance. Larger shrub patches in our region, historically resistant to fire spread and large-scale fires because of a perennial bunchgrass understory, were more fragmented than small patches. Presence of cheatgrass (Bromus tectorum), an exotic annual, was positively related to landscape patchiness and negatively related to number of shrub cells. Thus, cheatgrass dominance can contribute to further fragmentation and loss of the shrub patch by facilitating spread of subsequent fires, carried by continuous fuels, through the patch. The synergistic processes of fragmentation of shrub patches by disturbance, invasion and subsequent dominance by exotic annuals, and fire are converting shrubsteppe in southwestern Idaho to a new state dominated by exotic annual grasslands and high fire frequencies.     

Changes in mixed deciduous forests of boreal Sweden 1866–1999 based on interpretation of historical records

Current knowledge of patterns and abundance of deciduous trees in thepre-industrial landscape of boreal Sweden is limited. This is due to a dramatictransformation of the forest landscape during the last 100 years and the lackofrepresentative forest reserves. We used historical records to study change infive mixed deciduous forests between 1866 and 1999. The results show that largechanges occurred due to complex interactions between fire disturbance, firesuppression, logging and silviculture. Before fire suppression, the presence ofdeciduous trees was mainly determined by earlier fire influence. Laterselectivelogging disturbed natural succession and favoured regeneration of deciduoustrees. During the 20^th century deciduous trees were removed bygirdling, thinning and herbicide spraying. Much of the mixed deciduous standschanged to coniferous stands between 1906–15 and 1969–70, and thendeciduous trees were totally removed from these stands between late 1960s and1999. Today mixed deciduous forest occurs mainly in young stands and on othersites than previously. Our results also show that large coniferous trees andmulti-aged forest occurred in all sites in the early 1900s. Most sites weredominated by coniferous species and forest dominated by deciduous treesoccurredonly in smaller areas. These results are not consistent with the current viewthat deciduous-dominated forest occupied substantial areas in boreal Swedenbefore fire suppression. Appropriate changes in forest management arediscussed,as is the value of historical data in interpreting changes in forestlandscapes.This revised version was published online in May 2005 with corrections to the Cover Date.     

Postcards from the past: charting the landscape-scale conversion of tropical Australian savanna to closed forest during the 20th century

Repeated sequences of digitised and geo-referenced historical aerial photography provide a powerful means of understanding landscape change. We use this method to demonstrate a landscape wide expansion of closed forest (42% increase in total coverage) in the Australian monsoon tropics over the past five decades. Retrospective habitat suitability models (HSI) of closed forest derived using four landscape measures (drainage distance, slope angle, aspect and elevation) for imagery taken in 1947 correctly forecast the subsequent spatial distribution of the expansion, with topographic fire protection primarily determining the closed-forest distribution. The dynamics of the closed forest-savanna boundary were predicted accurately by generalised linear models, with closed-forest expansion in fire-protected sites along forest edges and regression in the more fire-prone areas. Two factors may plausibly explain the expansion of closed forests. First, eco-ethnographic records stress the skilful use of fire by Aboriginal people in protecting isolated and locally resource-rich closed-forest patches. Second, the recent global increase in atmospheric CO₂ may be changing the competitive balance between savanna and forest by enabling C₃ trees to grow fast enough to escape the fire trap presented by flammable C₄ grasses.     

Effects of landscape patterns of fire severity on regenerating ponderosa pine forests (<Emphasis Type="Italic">Pinus ponderosa</Emphasis>) in New Mexico and Arizona,USA

Much of the current effort to restore southwestern ponderosa pine forests to historical conditions is predicated upon assumptions regarding the catastrophic effects of large fires that are now defining a new fire regime. To determine how spatial characteristics influence the process of ponderosa pine regeneration under this new regime, we mapped the spatial patterns of severity at areas that burned in 1960 (Saddle Mountain, AZ) and (La Mesa, NM) 1977 using pre- and post-fire aerial photography, and quantified characteristics of pine regeneration at sample plots in areas where all trees were killed by the fire event. We used generalized linear models to determine the relationship of ponderosa pine stem density to three spatial burn pattern metrics: (1) distance to nearest edge of lower severity; (2) neighborhood severity, measured at varying spatial scales, and (3) scaled seed dispersal kernel surfaces. Pine regeneration corresponded most closely with particular scales of measurement in both seed dispersal kernel and neighborhood severity. Spatial patterns of burning remained important to understanding regeneration even after consideration of subsequent disturbance and other environmental variables, with the exception of a few cases in which simpler models were equally well-supported by the data. Analysis of tree ages revealed slow progress in early post-fire years. Our observations suggest that populations spread in a moving front, as well as by remotely dispersed individuals. Based on our results, recent large fires cannot be summarily dismissed as catastrophic. We conclude that management should focus on the value and natural recovery of post-fire landscapes. Further, process centered restoration efforts could utilize our findings in formulating reference dynamics under a changing fire regime.     

Spatial models for inferring topographic controls on historical low-severity fire in the eastern Cascade Range of Washington,USA

Fire regimes are complex systems that represent an aggregate of spatial and temporal events whose statistical properties are scale dependent. Despite the breadth of research regarding the spatial controls on fire regime variability, few datasets are available with sufficient resolution to test spatially explicit hypotheses. We used a spatially distributed network of georeferenced fire-scarred trees to investigate the spatial structure of fire occurrence at multiple scales. Mantel’s tests and geostatistical analysis of fire-occurrence time series led to inferences about the mechanisms that generated spatial patterns of historical fire synchrony (multiple trees recording fire in a single year) in eastern Washington, USA. The spatial autocorrelation structure of historical fire regimes varied within and among sites, with clearer patterns in the complex rugged terrain of the Cascade Range than in more open and rolling terrain further north and east. Results illustrate that the statistical spatial characteristics of fire regimes change with landform characteristics within a forest type, suggesting that simple relationships between fire frequency, fire synchrony, and forest type do not exist. Quantifying the spatial structures in fire occurrence associated with topographic variation showed that fire regime variability depends on both landscape structure and the scale of measurement. Spatially explicit fire-scar data open new possibilities for analysis and interpretation, potentially informing the design and application of fire management on landscapes, including hazardous fuel treatments and the use of fire for ecosystem restoration.