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. 相似文献
The complexity inherent in variable, or mixed-severity fire regimes makes quantitative characterization of important fire
regime attributes (e.g., proportion of landscape burned at different severities, size and distribution of stand-replacing
patches) difficult. As a result, there is ambiguity associated with the term ‘mixed-severity’. We address this ambiguity through
spatial analysis of two recent wildland fires in upper elevation mixed-conifer forests that occurred in an area with over
30 years of relatively freely-burning natural fires. We take advantage of robust estimates of fire severity and detailed spatial
datasets to investigate patterns and controls on stand-replacing patches within these fires. Stand-replacing patches made
up 15% of the total burned area between the two fires, which consisted of many small patches (<4 ha) and few large patches
(>60 ha). Smaller stand-replacing patches were generally associated with shrub-dominated (Arctostaphylos spp. and Ceanothus spp.) and pine-dominated vegetation types, while larger stand-replacing patches tended to occur in more shade-tolerant, fir-dominated
types. Additionally, in shrub-dominated types stand-replacing patches were often constrained to the underlying patch of vegetation,
which for the shrub type were smaller across the two fire areas than vegetation patches for all other dominant vegetation
types. For white and red fir forest types we found little evidence of vegetation patch constraint on the extent of stand-replacing
patches. The patch dynamics we identified can be used to inform management strategies for landscapes in similar forest types. 相似文献
The characterization of the fire regime in the boreal forest rarely considers spatial attributes other than fire size. This study investigates the spatial attributes of fires using the physiography of the landscape as a spatial constraint at a regional scale. Using the Canadian National Fire Database, the size, shape, orientation and eccentricity were assessed for 1,136 fires between 1970 and 2010 in Quebec’s boreal forest and were summarized by ecodistrict. These spatial metrics were used to cluster 33 ecodistricts into homogeneous fire zones and then to determine which environmental variables (climate, topography, hydrography, and surficial deposits) influence the spatial attributes of fires. Analyses showed that 28 out of 33 ecodistricts belonging to a given fire zone were spatially contiguous, suggesting that factors driving the spatial attributes of fire are acting at a regional scale. Indeed, the orientation and size of fires vary significantly among the zones and are driven by the spatial orientation of the landscape and the seasonal regional climate. In some zones, prevailing winds during periods conducive to fire events parallel to the orientation of the landscape may favour the occurrence of very large fires (>100,000 ha). Conversely, an orientation of the landscape opposite to the prevailing winds may act as a natural firebreak and limit the fire size and orientation. This study highlights the need to consider the synergistic relationship between the landscape spatial patterns and the climate regime over the spatial attributes of fire at supra-regional scale. Further scale-dependant studies are needed to improve our understanding of the spatial factors controlling the spatial attributes of fire. 相似文献
Not all wildfire ignitions result in burned areas of a similar size. The aim of this study was to explore whether there was
a size-dependent pattern (in terms of resulting burned area) of fire ignitions in Portugal. For that purpose we characterised
71,618 fire ignitions occurring in the country in the period 2001–2003, in terms of population density in the local parish,
land cover type and distance to roads. We then assigned each ignition into subsets of five classes according to the resulting
burned area: >5 ha, >50 ha, >100 ha, >250 ha, >500 ha. The probability of an ignition resulting in different burned area classes
was modelled using binary logistic regression, and the relative importance, strength and signal (positive or negative) of
the three explanatory variables compared across the models obtained for the different classes. Finally, we explored the implications
of land cover and population density changes during the period 1990–2000 in Portugal for the likelihood of ignitions resulting
in wildfires >500 ha. Population density was the more important variable explaining the resulting burned area, with the probability
of an ignition resulting in a large burned area being inversely related to population density. In terms of land cover, ignitions
resulting in large burned areas were more likely to occur in shrubland and forest areas. Finally, ignitions farther away from
roads were more likely to result in large burns. The current land cover trends (decrease of agricultural land and increase
in shrublands) and population trends (decline in population densities except near the coast) are increasing the probability
that ignitions will result in large fires in vast regions of the country. 相似文献
The frequency and size of wildfires within the Mojave Desert are increasing, possibly due to climate and land cover changes and associated increases in non-native invasive plant biomass, as measured by normalized difference vegetation index (NDVI). These patterns are of particular concern to resource managers in regions where native plant communities are not well adapted to fire. We used an information-theoretic and mixed-model approach to quantify the importance of multiple environmental variables in predicting, separately, the probabilities of occurrence of all fires and the occurrence large (>20 ha) fires in five management units administered by the National Park Service in the Mojave Desert Network and based on fire ignition data obtained for the period 1992–2011. Fire occurrence was strongly associated with areas close to roads, high maximum NDVI values in the year preceding ignition, the desert montane ecological zone, and high topographic roughness. Large fire probability was strongly associated with lightning-caused ignition events, high maximum NDVI values in the spring preceding ignition, high topographic roughness, the middle-elevation shrubland ecological zone, and areas further from roads. Our probabilistic models and maps can be used to explore patterns of fire occurrence based upon variability in NDVI values and to assess the vulnerability of Mojave Desert protected areas to undesirable fire events. 相似文献
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. 相似文献
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.
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. 相似文献
A probabilistic spatial model was created based on empirical data to examine the influence of different fire regimes on stand
structure of lodgepole pine (Pinus contorta var. latifolia) forests across a >500,000-ha landscape in Yellowstone National Park, Wyoming, USA. We asked how variation in the frequency
of large fire events affects (1) the mean and annual variability of age and tree density (defined by postfire sapling density
and subsequent stand density) of lodgepole pine stands and (2) the spatial pattern of stand age and density across the landscape.
The model incorporates spatial and temporal variation in fire and serotiny in predicting postfire sapling densities of lodgepole
pine. Empirical self-thinning and in-filling curves alter initital postfire sapling densities over decades to centuries. In
response to a six-fold increase in the probability of large fires (0.003 to 0.018 year−1), mean stand age declined from 291 to 121 years. Mean stand density did not increase appreciably at high elevations (1,029
to 1,249 stems ha−1) where serotiny was low and postfire sapling density was relatively low (1,252 to 2,203 stems ha−1). At low elevations, where prefire serotiny and postfire lodgepole pine density are high, mean stand densities increased
from 2,807 to 7,664 stems ha−1. Spatially, the patterns of stand age became more simplified across the landscape, yet patterns of stand density became more
complex. In response to more frequent stand replacing fires, very high annual variability in postfire sapling density is expected,
with higher means and greater variation in stand density across lodgepole pine landscapes, especially in the few decades following
large fires. 相似文献
A measure of the historic range of variability (HRV) in landscape structure is essential for evaluating current landscape
patterns of Rocky Mountain coniferous forests that have been subjected to intensive timber harvest. We used a geographic information
system (GIS) and FRAGSTATS to calculate key landscape metrics on two ∼130,000-ha landscapes in the Greater Yellowstone Area,
USA: one in Yellowstone National Park (YNP), which has been primarily shaped by natural fires, and a second in the adjacent
Targhee National Forest (TNF), which has undergone intensive clearcutting for nearly 30 years. Digital maps of the current
and historical landscape in YNP were developed from earlier stand age maps developed by Romme and Despain. Maps of the TNF
landscape were adapted from United States Forest Service Resource Information System (RIS) data. Key landscape metrics were
calculated at 20-yr intervals for YNP for the period from 1705-1995. These metrics were used to first evaluate the relative
effects of small vs. large fire events on landscape structure and were then compared to similar metrics calculated for both
pre- and post-harvest landscapes of the TNF. Large fires, such as those that burned in 1988, produced a structurally different
landscape than did previous, smaller fires (1705-1985). The total number of patches of all types was higher after 1988 (694
vs. 340-404 before 1988), and mean patch size was reduced by almost half (186 ha vs. 319-379 ha). The amount of unburned forest
was less following the 1988 fires (63% vs. 72-90% prior to 1988), yet the number of unburned patches increased by nearly an
order of magnitude (230 vs. a maximum of 41 prior to 1988). Total core area and mean core area per patch decreased after 1988
relative to smaller fires (∼73,700 ha vs. 87,000-110,000 ha, and 320 ha vs. 2,123 ha, respectively). Notably, only edge density
was similar (17 m ha−1 after 1988) to earlier landscapes (9.8-14.2 m ha−1).Three decades of timber harvesting dramatically altered landscape structure in the TNF. Total number of patches increased
threefold (1,481 after harvest vs. 437 before harvest), and mean patch size decreased by ∼70% (91.3 ha vs. 309 ha). None of
the post-harvest landscape metrics calculated for the TNF fell within the HRV as defined in YNP, even when the post-1988 landscape
was considered. In contrast, pre-harvest TNF landscape metrics were all within, or very nearly within, the HRV for YNP. While
reference conditions such as those identified by this study are useful for local and regional landscape evaluation and planning,
additional research is necessary to understand the consequences of changes in landscape structure for population, community,
ecosystem, and landscape function.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
Crown fires create broad-scale patterns in vegetation by producing a patch mosaic of stand age classes, but the spread and behavior of crown fires also may be constrained by spatial patterns in terrain and fuels across the landscape. In this review, we address the implications of landscape heterogeneity for crown fire behavior and the ecological effects of crown fires over large areas. We suggest that fine-scale mechanisms of fire spread can be extrapolated to make broad-scale predictions of landscape pattern by coupling the knowledge obtained from mechanistic and empirical fire behavior models with spatially-explicit probabilistic models of fire spread. Climatic conditions exert a dominant control over crown fire behavior and spread, but topographic and physiographic features in the landscape and the spatial arrangement and types of fuels have a strong influence on fire spread, especially when burning conditions (e.g., fuel moisture and wind) are not extreme. General trends in crown fire regimes and stand age class distributions can be observed across continental, latitudinal, and elevational gradients. Crown fires are more frequent in regions having more frequent and/or severe droughts, and younger stands tend to dominate these landscapes. Landscapes dominated by crown fires appear to be nonequilibrium systems. This nonequilibrium condition presents a significant challenge to land managers, particularly when the implications of potential changes in the global climate are considered. Potential changes in the global climate may alter not only the frequency of crown fires but also their severity. Crown fires rarely consume the entire forest, and the spatial heterogeneity of burn severity patterns creates a wide range of local effects and is likely to influence plant reestablishment as well as many other ecological processes. Increased knowledge of ecological processes at regional scales and the effects of landscape pattern on fire dynamics should provide insight into our understanding of the behavior and consequences of crown fires. 相似文献
This study describes the results of the application of a single dormant season prescribed fire to two southern Ohio forest sites for the purposes of restoring the ecosystem functional properties that existed in these sites prior to major human intervention (clearcutting, fire suppression, and atmospheric deposition). Each forest site was composed of three contiguous watershed units, two of which were burned in April of 1996. The forest sites differed in soil pH and available litter mass prior to the fires, and in both sites pH and available inorganic N varied among landscape positions such that inorganic C increased with increasing longterm soil moisture potential (measured as the GIS-derived Integrated Moisture Index [IMI] developed for this region). The fire temperatures at 10 cm above the litter surface were generally 150–300°C, and 29–80% of the litter was consumed, depending on site and landscape position. Soil solution total inorganic N (TIN) present one month after the fires did not differ significantly from that present prior to the fires in either burned or unburned watersheds, but was consistently greater in mesic landscape positions than in more xeric ones. N mineralization potential and organic C content varied both among fires and landscape positions. At the site which burned at higher intensity, soil N mineralization and TIN were both decreased by fire. At the less intensely burned site, fire resulted in increased TIN in the soils from the more xeric landscape position, and greater soil organic C in soils from the intermediate soil moisture areas. Path analysis produced models for fire-induced changes in C and N dynamics capable of explaining 26–69% of the observed variation using combinations of landscape and fire behavior. Losses of N to volatilization from these single fires were generally < 1 kg N/ha, and thus could not be expected to ameliorate the effects of atmospheric N deposition in these sites. 相似文献
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.
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 large20thcentury 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. 相似文献
Understanding the driving forces behind the distribution of threatened species is critical to set priorities for conservation
measures and spatial planning. We examined the distribution of a globally threatened bird, the corncrake (Crex crex), in the lowland floodplains of the Rhine River, which provide an important breeding habitat for the species. We related
corncrake distribution to landscape characteristics (area, shape, texture, diversity) at three spatial scales: distinct floodplain
units (“floodplain scale”), circular zones around individual observations (“home range scale”), and individual patches (“patch
scale”) using logistic regression. Potential intrinsic spatial patterns in the corncrake data were accounted for by including
geographic coordinates and an autocovariate as predictors in the regression analysis. The autocovariate was the most important
predictor of corncrake occurrence, probably reflecting the strong conspecific attraction that is characteristic of the species.
Significant landscape predictors mainly pertained to area characteristics at the patch scale and the home range scale; the
probability of corncrake occurrence increased with potential habitat area, patch area, and nature reserve area. The median
potential habitat patch size associated with corncrake occurrence was 11.3 ha; 90% of the corncrake records were associated
with patches at least 2.2 ha in size. These results indicate that the corncrake is an area-sensitive species, possibly governed
by the males’ tendency to reside near other males while maintaining distinct territories. Our results imply that corncrake
habitat conservation schemes should focus on the preservation of sufficient potential habitat area and that existing management
measures, like delayed mowing, should be implemented in relatively large, preferably contiguous areas. 相似文献
In mountainous Mediterranean regions, land abandonment processes in past decades are hypothesized to trigger secondary vegetal
succession and homogenization, which in recent years has increased the size of burned areas. We conducted an analysis of temporal
changes in landscape vegetal spatial pattern over a 15-year period (1984–1998) in a rural area of 672.3 km2 in Eastern Spain to investigate the relationship between local landscape heterogeneity and wildfire occurrence. Heterogeneity
was analyzed from textural metrics derived from non-classified remote sensing data at several periods, and was related to
wildfire history in the study area. Several neural network models found significant relationships between local spatial pattern
and future fire occurrence. In this study, sensitivity analysis of the texture variables suggested that fire occurrence, estimated
as probability of burning in the near future, increased where local homogeneity was higher. 相似文献
Uncertainty in managing forested landscapes arises from many sources, including complexities inherent in forest ecosystems
and their disturbance processes. However, gaining knowledge about forested ecosystems at the landscape level is often impeded
by limitations in collecting comprehensive, representative, as well as accurate data sets. Historical reference data sets
about past disturbances are also mostly lacking. In the case of ground fires, however, records of past fires can be obtained
by analyzing fire scars using dendrochronology. While the temporal series of disturbance can be determined, there is still
uncertainty about the spatial limits of individual forest surface fires. Here, we investigate how a patch-based method (fuzzy
set membership) and a boundary-based uncertainty method (boundary membership) can help determine the spatial uncertainty related
to forest fire events and their boundary locations. We compare these methods using fire scar data from ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) sampled at 33 1-ha plots in a 1500-ha study area within the Stein River watershed (British Columbia). Patch-based fire maps,
using multiple constraints, were derived for years 1785–1937. We compared the resulting total fire event maps with the boundary-based
method, finding that depending on values chosen for the patch-based method, negative correlation was present (though very
modest: r = − 0.1, p ≤ 0.05) between some maps. However, significant positive correlation between maps (though again modest: r = 0.22, p ≤ 0.05) was found under the least constrained patch-based methods, suggesting that fire patches are counted more than once
in riparian zones. Our results suggest that these two methods provide complementary information about historical fire size
and spatial limits. Quantifying spatial uncertainty about fire size and fire boundary location using a boundary membership
method can contribute to not only understanding past fire regimes but also to providing better estimates of area burned. 相似文献
In fire-excluded forests across western North America, recent intense wildfire seasons starkly contrast with fire regimes of the past. The last 100 years mark a transition between pre-colonial and modern era fire regimes, providing crucial context for understanding future wildfire behavior.
Objectives
Using the greatest time depth of digitized fire events in Canada, we identify distinct phases of wildfire regimes from 1919 to 2019 by evaluating changes in mapped fire perimeters (>?20-ha) across the East Kootenay region (including the southern Rocky Mountain Trench), British Columbia.
Methods
We detect transitions in annual number of fires, burned area, and fire size; explore the role of lightning- and human-caused fires in driving these transitions; and quantify departures from historical fire frequency at the regional level.
Results
Relative to historical fire frequency, fire exclusion has created a significant fire deficit in active fire regimes, with a minimum of 1–10 fires missed across 46.4-percent of the landscape. Fire was active from 1919 to 1939 with frequent and large fire events, but the regime was already altered by a century of colonization. Fire activity decreased in 1940, coinciding with effective fire suppression influenced by a mild climatic period. In 2003, the combined effects of fire exclusion and accelerated climate change fueled a shift in fire regimes of various forest types, with increases in area burned and mean fire size driven by lightning.
Conclusions
The extent of fire regime disruption warrants significant management and policy attention to alter the current trajectory and facilitate better co-existence with wildfire throughout this century.
The relations between disturbance regime and landscape patterns have been developed from a theoretical perspective, but few studies have tested these relations when forces promoting opposing heterogeneity patterns are simultaneously operating on a landscape. This work provides quantitative evidence of these relations in areas dominated by human activity, showing that landscape heterogeneity decreases disturbance spread. In turn, disturbance introduces a source of landscape heterogeneity, but it is not enough to counterbalance the homogeneity trend due to agricultural abandonment. Land cover changes and wildfire occurrence (fires larger than 0.3 km2) have been monitored in the Tivissa municipality (208.4 km2) (Catalonia, NE Spain) from 1956 to 1993. Land cover maps were obtained from 1956, 1978 and 1993 and they were overlaid with fire occurrence maps obtained for the 1975–1995 period from 60 m resolution remote sensing images, which allow the identification of burned areas by sudden drops in Normalized Difference Vegetation Index (NDVI). Changes in landscape patterns in relation to fire regime have been analyzed considering several parameters: patch density, mean patch size, mean distance to the nearest neighbour of the same category, edge density, and the Shannon diversity index. In the 1956–1993 period there is a trend to increasing landscape homogenization due to the expansion of shrublands linked to a decrease in forest surface, and to the abandonment of agricultural lands. This trend, however, is not constant along all the period. Fires are more likely to occur in woody, homogenous areas, increasing landscape heterogeneity, as observed in the 1978–1993 period. This increase in heterogeneity does not counterbalance the general trend to landscape homogenization as a consequence of agricultural abandonment and the coalescence of natural vegetation patches.This revised version was published online in May 2005 with corrections to the Cover Date. 相似文献
