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
There is considerable concern over the occurrence of stand-replacing fire in forest types historically associated with low- to moderate-severity fire. The concern is largely over whether contemporary levels of stand-replacing fire are outside the historical range of variability, and what natural forest recovery is in these forest types following stand-replacing fire. In this study we quantified shrub characteristics and tree regeneration patterns in stand-replacing patches for five fires in the northern Sierra Nevada. These fires occurred between 1999 and 2008, and our field measurements were conducted in 2010. We analyzed tree regeneration patterns at two scales: patch level, in which field observations and spatial data were aggregated for a given stand-replacing patch, and plot level. Although tree regeneration densities varied considerably across sampled fires, over 50 % of the patches and approximately 80 % all plots had no tree regeneration. The percentage of patches, and to a greater extent plots, without pine regeneration was even higher, 72 and 87 %, respectively. Hardwood regeneration was present on a higher proportion of plots than either the pine or non-pine conifer groups. Shrub cover was generally high, with approximately 60 % of both patches and individual plots exceeding 60 % cover. Patch characteristics (size, perimeter-to-area ratio, distance-to-edge) appeared to have little effect on observed tree regeneration patterns. Conifer regeneration was higher in areas with post-fire management activities (salvage harvesting, planting). Our results indicate that the natural return of pine/mixed-conifer forests is uncertain in many areas affected by stand-replacing fire. 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.
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.
Fire regime characteristics of high-elevation forests on the North Rim of the Grand Canyon, Arizona, were reconstructed from fire scar analysis, remote sensing, tree age, and forest structure measurements, a first attempt at detailed reconstruction of the transition from surface to stand-replacing fire patterns in the Southwest. Tree densities and fire-/non-fire-initiated groups were highly mixed over the landscape, so distinct fire-created stands could not be delineated from satellite imagery or the oldest available aerial photos. Surface fires were common from 1700 to 1879 in the 4,400 ha site, especially on S and W aspects. Fire dates frequently coincided with fire dates measured at study sites at lower elevation, suggesting that pre-1880 fire sizes may have been very large. Large fires, those scarring 25% or more of the sample trees, were relatively infrequent, averaging 31 years between burns. Four of the five major regional fire years occurred in the 1700s, followed by a 94-year gap until 1879. Fires typically occurred in significantly dry years (Palmer Drought Stress Index), with severe drought in major regional fire years. Currently the forest is predominantly spruce-fir, mixed conifer, and aspen. In contrast, dendroecological reconstruction of past forest structure showed that the forest in 1880 was very open, corresponding closely with historical (1910) accounts of severe fires leaving partially denuded landscapes. Age structure and species composition were used to classify sampling points into fire-initiated and non-fire-initiated groups. Tree groups on nearly 60% of the plots were fire-initiated; the oldest such groups appeared to have originated after severe fires in 1782 or 1785. In 1880, all fire-initiated groups were less than 100 years old and nearly 25% of the groups were less than 20 years old. Non-fire-initiated groups were significantly older (oldest 262 years in 1880), dominated by ponderosa pine, Douglas-fir, or white fir, and occurred preferentially on S and W slopes. The mixed-severity fire regime, transitioning from lower-elevation surface fires to mixed surface and stand-replacing fire at higher elevations, appeared not to have been stable over the temporal and spatial scales of this study. Information about historical fire regime and forest structure is valuable for managers but the information is probably less specific and stable for high-elevation forests than for low-elevation ponderosa pine forests.This revised version was published online in May 2005 with corrections to the Cover Date. 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
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. 相似文献
Climate and land-use change have led to disturbance regimes in many ecosystems without a historical analog, leading to uncertainty about how species adapted to past conditions will respond to novel post-disturbance landscapes.
Objectives
We examined habitat selection by spotted owls in a post-fire landscape. We tested whether selection or avoidance of severely burned areas could be explained by patch size or configuration, and whether variation in selection among individuals could be explained by differences in habitat availability.
Methods
We applied mixed-effects models to GPS data from 20 spotted owls in the Sierra Nevada, California, USA, with individual owls occupying home ranges spanning a broad range of post-fire conditions after the 2014 King Fire.
Results
Individual spotted owls whose home ranges experienced less severe fire (<?5% of home range severely burned) tended to select severely burned forest, but owls avoided severely burned forest when more of their home range was affected (~ 5–40%). Owls also tended to select severe fire patches that were smaller in size and more complex in shape, and rarely traveled?>?100-m into severe fire patches. Spotted owls avoided areas that had experienced post-fire salvage logging but the interpretation of this effect was nuanced. Owls also avoided areas that were classified as open and/or young forest prior to the fire.
Conclusions
Our results support the hypothesis that spotted owls are adapted to historical fire regimes characterized by small severe fire patches in this region. Shifts in disturbance regimes that produce novel landscape patterns characterized by large, homogeneous patches of high-severity fire may negatively affect this species.
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
In the southwestern U.S., wildland fire frequency and area burned have steadily increased in recent decades, a pattern attributable
to multiple ignition sources. To examine contributing landscape factors and patterns related to the occurrence of large (⩾20 ha
in extent) fires in the forested region of northern Arizona, we assembled a database of lightning- and human-caused fires
for the period 1 April to 30 September, 1986–2000. At the landscape scale, we used a weights-of-evidence approach to model
and map the probability of occurrence based on all fire types (n = 203), and lightning-caused fires alone (n = 136). In total, large fires burned 101,571 ha on our study area. Fires due to lightning were more frequent and extensive
than those caused by humans, although human-caused fires burned large areas during the period of our analysis. For all fires,
probability of occurrence was greatest in areas of high topographic roughness and lower road density. Ponderosa pine (Pinus ponderosa)-dominated forest vegetation and mean annual precipitation were less important predictors. Our modeling results indicate
that seasonal large fire events are a consequence of non-random patterns of occurrence, and that patterns generated by these
events may affect the regional fire regime more extensively than previously thought. Identifying the factors that influence
large fires will improve our ability to target resource protection efforts and manage fire risk at the landscape scale. 相似文献
Wildfire spatial patterns drive ecological processes including vegetation succession and wildlife community dynamics. Such patterns may be changing due to fire suppression policies and climate change, making characterization of trends in post-fire mosaics important for understanding and managing fire-prone ecosystems.
Methods
For wildfires in California’s yellow pine and mixed-conifer forests, spatial pattern trends of two components of the post-fire severity matrix were assessed for 1984–2015: (1) unchanged or very low-severity and (2) high-severity, which represent remnant forest and stand-replacing fire, respectively. Trends were evaluated for metrics of total and proportional burned area, shape complexity, aggregation, and core area. Additionally, comparisons were made between management units where fire suppression is commonly practiced and those with a history of managing wildfire for ecological/resource benefits.
Results
Unchanged or very low-severity area per fire decreased proportionally through time, and became increasingly fragmented. High-severity area and core area increased on average across most of California, with the high-severity component also becoming simpler in shape in the Sierra Nevada. Compared to suppression units, managed wildfire units lack an increase in high-severity area, have less aggregated post-fire mosaics, and more high-severity spatial complexity.
Conclusions
Documented changes in severity patterns have cascading ecological effects including increased vegetation type conversion risk, habitat availability shifts, and remnant forest fragmentation. These changes likely benefit early-seral-associated species at the expense of mature closed-canopy forest-associated species. Managed wildfire appears to moderate some effects of fire suppression, and may help buy time for ecosystems and managers to respond to a changing climate.
Recovery from disturbances is a prominent measure of forest ecosystem resilience, with swift recovery indicating resilient systems. The forest ecosystems of Central Europe have recently been affected by unprecedented levels of natural disturbance, yet our understanding of their ability to recover from disturbances is still limited.
Objectives
We here integrated satellite and airborne Lidar data to (i) quantify multi-decadal post-disturbance recovery of two indicators of forest structure, and (ii) compare the recovery trajectories of forest structure among managed and un-managed forests.
Methods
We developed satellite-based models predicting Lidar-derived estimates of tree cover and stand height at 30 m grain across a 3100 km2 landscape in the Bohemian Forest Ecosystem (Central Europe). We summarized the percentage of disturbed area that recovered to >?40% tree cover and >?5 m stand height and quantified the variability in both indicators over a 30-year period. The analyses were stratified by three management regimes (managed, protected, strictly protected) and two forest types (beech-dominated, spruce-dominated).
Results
We found that on average 84% of the disturbed area met our recovery threshold 30 years post-disturbance. The rate of recovery was slower in un-managed compared to managed forests. Variability in tree cover was more persistent over time in un-managed forests, while managed forests strongly converged after a few decades post-disturbance.
Conclusion
We conclude that current management facilitates the recovery of forest structure in Central European forest ecosystems. However, our results underline that forests recovered well from disturbances also in the absence of human intervention. Our analysis highlights the high resilience of Central European forest ecosystems to recent disturbances.
Varying altitudes and aspects within small distances are typically found in mountainous areas. Such a complex topography complicates the accurate quantification of forest C dynamics at larger scales.
Objectives
We determined the effects of altitude and aspect on forest C cycling in a typical, mountainous catchment in the Northern Limestone Alps.
Methods
Forest C pools and fluxes were measured along two altitudinal gradients (650–900 m a.s.l.) at south-west (SW) and north-east (NE) facing slopes. Net ecosystem production (NEP) was estimated using a biometric approach combining field measurements of aboveground biomass and soil CO2 efflux (SR) with allometric functions, root:shoot ratios and empirical SR modeling.
Results
NEP was higher at the SW facing slope (6.60?±?3.01 t C ha?1 year?1), when compared to the NE facing slope (4.36?±?2.61 t C ha?1 year?1). SR was higher at the SW facing slope too, balancing out any difference in NEP between aspects (NE: 1.30?±?3.23 t C ha?1 year?1, SW: 1.65?±?3.34 t C ha?1 year?1). Soil organic C stocks significantly decreased with altitude. Forest NPP and NEP did not show clear altitudinal trends within the catchment.
Conclusions
Under current climate conditions, altitude and aspect adversely affect C sequestering and releasing processes, resulting in a relatively uniform forest NEP in the catchment. Hence, including detailed climatic and soil conditions, which are driven by altitude and aspect, will unlikely improve forest NEP estimates at the scale of the studied catchment. In a future climate, however, shifts in temperature and precipitation may disproportionally affect forest C cycling at the southward slopes through increased water limitation.
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.
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
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
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
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
The open and free access to Landsat and MODIS products have greatly promoted scientific investigations on spatiotemporal change in land mosaics and ecosystem functions at landscape to regional scales. Unfortunately, there is a major mismatch in spatial resolution between MODIS products at coarser resolution (≥?250 m) and landscape structure based on classified Landsat scenes at finer resolution (30 m).
Objectives
Based on practical needs for downscaling popular MODIS products at 500 m resolution to match classified land cover at Landsat 30 m resolution, we proposed an innovative modelling approach so that landscape structure and ecosystem functions can be directly studied for their interconnections. As a proof-of-concept of our downscaling approach, we selected the watershed of the Kalamazoo River in southwestern Michigan, USA as the testbed.
Methods
MODIS products for three fundamental variables of ecosystem function are downscaled to ensure the approach can be extrapolated to multiple functional measurements. They are blue-sky albedo (0–1), evapotranspiration (ET, mm), and gross primary production (GPP, Mg C ha?1 year?1). An object-oriented classification of Landsat images in 2011 was processed to generate a land cover map for landscape structure. The downscaling model was tested for the five Level IV ecoregions within the watershed.
Results
We achieved satisfactory downscaling models for albedo, ET, and GPP for all five ecoregions. The adjusted R2 was?>?0.995 for albedo, 0.915–0.997 for ET, and 0.902–0.962 for GPP. The estimated albedo, ET, and GPP values appear different in the region. The estimated albedo was the lowest for water (0.076–0.107) and the highest for cropland (0.166–0.172). Estimated ET was the highest for the built-up cover type (525.6–687.1 mm) and the lowest for forest (209.7–459.7 mm). The estimated GPP was the highest for the build-up cover type (8.65–9.85 Mg C ha?1 year?1) and the lowest for forest.
Conclusions
Estimated values for albedo, ET, and GPP appear reasonable for their ranges in the Kalamazoo River region and are consistent with values reported in the literature. Despite these promising results, the downscaling approach relies on strong assumptions and can carry substantial uncertainty. It is only valid at a spatial scale where similar climate, soil, and landforms exist (i.e., values in isolated patches of the same cover type are similar). Plausibly, the uncertainties associated with each estimation, as well as the model residuals, can be explored for other pattern-process relationships within the landscape.
