Fuel Shapes the Fire-Climate Relationship: Evidence from Mediterranean Ecosystems

Pausas et al. (2012) wish to understand how vegetation affects fire-climate dynamics. They predict that fuel and vegetation structure dictate ecosystem sensitivity to fire and will switch climatic conditions to high flammability. They observe 13 regions distributed along an aridity gradient on the Iberian Peninsula. They assessed the changes in the temporal fire-climate relationship across the regions by estimating various variables. The variables were then related to fuel structure indicators and regional aridity. Pausas et al. find that the aridity level switch to flammable conditions increased along the aridity gradient and that the differences in fire activity between regions was explained by the sensitivity of fire to Mediterranean conditions. They conclude that fuel structure is a more significant driver of fire activity and their results highlight the role of vegetation structure in shaping future and current fire-climate relationships at a regional scale. –Loren Stutts
Pausas, J.G., and Paula S. 2012. Fuel shapes the fire-climate relationship: evidence from Mediterranean ecosystems. Global Ecology & Biogeography, in press.

            Fire profoundly shapes ecosystems and biogeochemical cycles. Current changes in fire regimes are significantly impacting biodiversity and ecosystem functioning thus creating a growing interest for a deeper scientific understanding of the drivers of fire regimes. Climate influences fire regimes by affecting fuel structure and fuel moisture. Fuel moisture controls plant flammability while fuel structure determines the amount and connectivity of burnable resources. The roles of both fuel structure and fuel flammability in determining fire activity vary along the global productivity gradient. Specifically, in productive and moist regions, fire activity is controlled by the frequency with which flammable conditions are attained; however in unproductive and arid regions, fuel limitation restricts fire activity. In productive ecosystems, denser vegetation allows low intensity fire to spread more easily while sparse vegetation in arid ecosystem dry weather conditions propagates fire. This suggests that fuel or vegetation structure controls fire-climate relationships because it determines the climatic conditions needed to promote fires.
            Pausas et al. hypothesize that vegetation and landscape structure shape the fire climate relationship at a regional scale. The climatic conditions that increase flammability depend on fuel structure and thus change along the aridity/productivity gradient. The authors analyze whether the monthly aridity that dictates fire activity depends on regional climate and thus on fuel structure, along a climatic gradient on the Iberian Peninsula. Pausas et al. select the Iberian Peninsula because its high environmental variability provides a wide range of productivity conditions in a singular biogeographic unit.
            Pausas et al. use fire data (19682007) obtained from the Spanish Forest Service that include size, date, and location of each wildfire for all of Spain except the Basque Country and Navarra. They used a CORINE land cover map of Spain to differentiate wildland from non-forested areas and to analyze fuel cover statistics. They used the Forest Potential Productivity (FPP) map as an indicator of productivity. Monthly potential and actual evapotranspiration (AET, PET) for 19682007 was collected from layers produced by the Spanish government’s environmental bureau. AET layers were obtained by running the SIMPA hydrological model with PET and precipitation data. PET layers were produced from mean temperature data using the Thornthwaite method. To evaluate climatic variability within and between regions, Pausas et al. used temperature and precipitation records from the 19682007 period, and mean monthly wind velocity generated by the Spanish Meteorological Agency (AEMET). To define environmentally homogeneous region on the Iberian Peninsula, Pausas et al. combined available information and finally obtained 13 regions covering 82% of the Iberian Peninsula.
            In terms of data analyses, Pausas et al. considered the parameters of forest potential productivity (FPP), proportion of woodland area, distance between wildland patches, and proportion of wildland area to obtain of general characterization of the fuel structure in each region. They computed total woodland and wildland areas by adding up the corresponding patch areas obtained from the CORINE land cover map. The distance between forest patches was computed using FRAGSTATS. They used this measure since it directly related to fuel continuity across landscapes and thus to fire activity and spread. For climate analyses, Pausas et al.defined the Aridity Index as the difference between PET and AET. The difference integrates energy and water supplies, which are the climatic determinants of vegetation distribution and plant growth. This Aridity Index was computed monthly for each region for the whole study period (1968-2007) and for the average condition of each region. The mean annual Aridity Index was correlated with productivity indicators (AET and FPP) and with variables related to vegetation structure and landscape density. For fire season analyses, fire climate relationships were analyzed for the months of June to September. For thresholds, Pausas et al. sorted the monthly area burnt by the monthly Aridity Index and estimated the breakpoint with a test in their specific statistical analysis software. This breakpoint was considered to be the Aridity Index Threshold beyond which a switch to flammable conditions occurs. To determine patterns along the aridity gradient, Pausas et al. used the following variables: a) the Aridity Threshold, b) frequency of flammable conditions, and c) the anomaly in the area burnt under such conditions. They then analyzed the changes in these variables along the aridity gradient by testing their relation to the mean annual Aridity Index of each region. They used a linear regression analysis to test whether the aridity gradient explained the variability in the Aridity Threshold. They used a generalized mixed model (GLMM) to analyze changes in the frequency of flammable conditions along the aridity gradient. And finally to assess the variability of the standardized anomaly in the area burnt along the aridity gradient, Pausas et al. used a linear mixed model with the mean annual Aridity Index. For mixed models, model fit and estimation of dispersion was conducted using an analysis of deviance. They assessed the spatial autocorrelation in all studied parameters by using the Moran’s I Autocorrelation Index. They then estimated the Moran’s I of the residuals of each regression considered.
            Pausas et al. find that the relationship between monthly burned area and monthly Aridity Index exemplifies a threshold pattern in the 13 regions. Specifically they found that the drier the region, the higher the Aridity Threshold and similarly the Aridity Threshold was higher for less productive regions with lower fuel loads and connectivity. They also found that the required change in the Aridity Index to attain flammable conditions was negatively related to the mean annual Aridity Index meaning that productive (wet) regions need a greater reduction in moisture to become flammable. Yet fire activity was negatively related to the aridity of the region suggesting that productive regions burned more than arid regions. The Pausas et al. findings on the global aridity gradient imply that fuel structure is more relevant than the frequency of drought.
            Their findings provide evidence that flammability and fuel structure act simultaneously in driving fire regimes though not necessarily over the same temporal/spatial scale, and that the sensitivity of fire activity to dry conditions increases with productivity meaning the switch to flammable conditions has a greater effect on fire activity in productive system than in dry ones. In mesic or wet regions, fuel is less relevant and fire depends on the climatic conditions conducive to fire propagation and ignitability. In drier regions, area burned is low as a result of low fuel load and connectivity. In their study area, the more dry the region, the higher the dryness level needed for switching to flammable conditions thereby indicating that the Aridity Threshold is influenced by fuel.
            The essential role the Aridity Threshold plays in the ecosystems of the Iberian Peninsula reveals the importance of landscape structure in fire-climate relationships along the spatial scale. Specifically fuel structure climatically controls fire activity since fuels determine the climatic conditions that drive the switch to high flammability. Increased fire activity is predicted in highly productive regions, and Pausas et al.’s findings support this claim since they found that the fire-climate relationship changes along the productivity gradient and that wetter systems become flammable under wetter conditions in comparison to drier regions. Fuel structure plays a key role in shaping current fire regimes and will also dictate the direction of future fire regimes. Pausas et al. highlight that fuel structure does depend exclusively on environmental conditions. The relationship between fire and climate changes spatially with fuel along the aridity gradient but also temporally in response to different land use and management practices.

Forest Restoration in a Surface Fire-Dependent Ecosystem: An Example from a Mixed Conifer Forest, Southwestern Colorado, USA

Fire suppression in the warm and dry mixed conifer forests of southwestern Colorado has created changes that have disrupted ecosystem feedback interactions between vegetation composition and the region’s natural fire regime. This fire suppression has made the ecosystem more susceptible to high intensity fires that were previously absent in this forest type. Thus Korb et al. (2012) established four replicated treatments, each about 16-ha, of 1) thin/burn, 2) burn alone and 3) control to quantify the effects of restoration treatments on forest structure. Thinning trees meant removing them usually by chopping them down  while control meant no treatment. They sampled the pre-treatment plots in 2003 and post-treatment plots in 2009. There were no meaningful changes between pre and post-treatment in the control and burn alone treatments for canopy cover, basal area, tree density, and tree regeneration. There were significant changes in the thin/burn treatments with basal area declining, tree density declining, tree canopy cover decreasing, white fir regeneration decreasing and aspen tree regeneration increasing. Analysis of tree basal area by species in the thin/burn treatments in 2009 revealed a strong shift away from 2003 pre-treatment data towards the reconstructed historical (1870) forest structure, while burn alone treatments were distinct from controls after treatment in 2009. Thin/burn treatments moved warm/dry mixed conifer forests in southwestern Colorado toward historical reference conditions by altering forest composition and structure. Forest restoration will make forests more resilient to stand-replacing fires and subsequent transitions to novel ecosystems under a warmer drier climate. –Loren Stutts
Korb, J.E., Fulé, P.Z., and Stoddard M.T. 2012. Forest restoration in a surface fire-dependent ecosystem: An example from a mixed conifer forest, southwestern Colorado, USA. Forest Ecology and Management 269, 10-18, doi:10.1016/j.foreco.2012.01.002.

            Korb et al. claim there is an abundance of evidence that suggests 20thcentury fire suppression in ponderosa pine and low elevation mixed conifer forests in the southwestern U.S. has created changes in forest structure, composition, and ecological processes. Temperature and moisture are the crucial drivers that influence fire regimes and species composition for the mixed conifer forests in the San Juan Mountains of Southwest Colorado. Ponderosa pine and Douglas fir dominate the warm/dry mixed conifer forest while cool/moist mixed conifer is dominated by white fir, Douglas-fir, aspen, and blue spruce. More than a century of fire suppression in warm/dry mixed conifer forests has shifted species composition toward more shade tolerant species such as Douglas-fir and white fir, increased surface and aerial fuels as well as increased tree density.
            Under a warmer and drier climate, Korb et al. assert that the use of site-specific reference conditions is a scientifically sound target for forest-stand conditions in fire-dependent forest ecosystems because they increase tolerance to uncharacteristic fire behavior. Thus they created a controlled experiment in the warm/dry mixed conifer forest of the San Juan Mountains of Colorado to evaluate forest change and to test restoration alternatives. They established four replicated blocks of three treatments, each about 16 ha: 1) thin/burn, 2) burn alone, and 3) control. Burn alone treatment was included to determine if restoration goals could be attained without tree thinning. All treatments were tested against site-specific reconstructed reference conditions.
            They attempted to quantify post-treatment differences in forest composition and structure among treatments and compare post-treatment stands with site specific reference conditions and quantify changes in untreated controls over a six year period (2003-2009) to evaluate the stability of warm-dry mixed conifer stands.
            The study area is located in the San Juan Mountains in southwest Colorado within the San Juan National Forest. Forest vegetation includes ponderosa pine, white-fir, Douglas fir, and aspen. Fire suppression has been the management policy in the region since the early twentieth century. The thinning prescription retained all living trees established in 1870 or earlier as identified by canopy architecture, size, and bark color. The trees designated for thinning were mostly white fir and a bit of Douglas fir. Logs and limbs were lopped and scattered while old growth trees were not raked to remove fuels around tree boles.
            Korb et al. produced 20 permanent study plots on a 60-m grid per unit to characterize forest structure and vegetation. They collected pre-treatment data in the summer of 2003 and post treatment data in the summer of 2009. Species, crown base height, condition, diameter at breast height, total height, field classification of pre-settlement or post-settlement origin were recorded for each tree encountered in the plot. Tree regeneration, or trees below breast height, was measured on a nested circular plot; species, height class, and condition were recorded for each seedling or sprout. Tree canopy cover was recorded using a vertical projection densitometer every 3 meters along a permanently marked 50 m line transect upslope through the plot center. They measured dead woody biomass and forest floor on a permanently marked planar transect in a random direction from each plot center.
            For analysis the team compared tree density, canopy cover, mortality, basal area, and regeneration density among treatments with a Kruskal-Wallis test. They conducted post-hoc tests with pair wise Kruskal-Wallis two-sample tests following a statistically significant result for a total variable. They used Wilcoxon signed-ranks tests to quantify changes over time between 2003 and 2009 data to include the repeated measurements on the permanent plots. The team used nonmetric multidimensional scaling to examine changes in basal area of all tree species over time and among treatments. They compared stress value of the final solution to 50 random solutions using a Monte Carlo test. They examined differences between reconstructed 1870, 2003 pre-treatment, and 2009 post-treatment forest structure using multivariate analysis of variance to quantify differences in basal area and tree ha across time and among treatments. They used indicator-species analysis to identify species that were consistent indicators to the analysis dates of 2003 or 2009. They compared the maximum indicator value and random trials for occurrence of a given species to produce an approximate P-value.
            After running their experiment, Korb et al. found that in terms of forest structure, there were no differences in total tree density or basal area among treatment units prior to restoration in 2003. Following treatments, (2009) total density and basal area were lower in thin/burn units than the control and burn alone units. Thin/burn units had the only significant change in tree density and basal area with significant declines in both. After treatment diameter distributions in the controls were relatively unchanged. In terms of canopy cover, there were no differences among treatment units in tree canopy cover prior to restoration treatments in 2003. Tree canopy post treatment had the highest average in controls. In the thin/burn treatment, tree canopy cover decreased between 2003 and 2009. There were no differences in tree mortality for young trees post-treatment. The highest variability in tree mortality occurred in the thin/burn treatments. Total tree regeneration was no different before or after treatments. There were also no differences in forest floor depth prior to restoration treatments.
            Compared to reconstructed 1870 forest structure, there was a difference in tree basal area and tree density between reconstructed 1870, 2003 pre-treatment and 2009 post-treatment data. Tree basal area by species in 2009 in the thin/burn treatments showed a strong shift away from 2003 pre-treatment data toward the reconstructed 1870 forest structure. White-fir, aspen, and Douglas fir served as indicator species and dominated more highly in 2003 than in 2009.
            This experiment was the first to apply dendrochronologically reconstructed data on historical reference conditions to the design and testing of replicated ecological restoration treatments in a mixed conifer forest. The combination of thinning and burning in the study moved warm/dry mixed conifer forests in southwestern Colorado close to the historical reference condition, while burn alone treatments moved forests in the same direction towards the historical range. Fire had a thinning effect since younger (smaller) trees were most likely to be burned and die. Forest composition also shifted in the thin/burn treatments. For example ponderosa pine represented about 63% of the basal area in the historical reference condition, compared to 73% of the basal area in thin/burn treatments and only 44% in burn alone treatments. Old-growth trees represent a genetic and structural legacy that has largely vanished. Differential mortality amongst the treatments reveal that over time the reintroduction of the repeated surface fire regime will continue to shift composition away from less fire resistant species. Aspen regeneration in thin/burn treatments increased about fivefold over pre-treatment level while aspen regeneration in burn alone treatments did not double.
            The findings of this study support restoration studies in mixed conifer at other biogeographic locations where limited treatments did not restore stand composition and structure within historical reference conditions over the short-term. Creating forest conditions that increase forest heterogeneity at the landscape scale to replicate historical reference conditions will make forests more tolerant to altered fire regimes under a warmer drier climate. Since it has been revealed that widespread increases in mortality of old trees across the western U.S. was linked to regional warming, it is critical that restoration ecologists incorporate natural mortality into restoration treatment design because of the ensuing implication that background tree mortality has on forest stand structure and thus restoration goals. Fires are becoming easier to ignite and spread, the fire season is longer and extreme fire behavior is more common with warmer temperatures, longer growing seasons, and drier soils. Thus it is crucial to enact management actions to mitigate trajectories in species composition and ecological processes by restoring the self-regulating attributes of fire dependent forests.

Vulnerability of Landscape Carbon Fluxes to Future Climate and Fire in the Greater Yellowstone Ecosystem

Under climate change, an increase in fires would modify carbon stocks by decreasing the amount of carbon (C) stored in soil and biomass. Smithwick and team wish to explore under what conditions of future climate and what threshold of fire frequency would shift the Greater Yellowstone Ecosystem (GYE) from a C sink to a C source. They created downscaled climate projections for three general circulation models and used them in a dynamic ecosystem process model (CENTURY version 4.5). They also simulated C storage to 2100 for individual forest stands under three fire pathways ¾fires at every 30, 60, or 90 years¾and a control simulation (no fire) under the future and downscaled climate scenarios. Their results reveal that fire intervals less than about 90 years will cause lodgepole pine forest stands to move from a net C sink to a net C source (Smithwick et al. 2011). Under all future climate scenarios, their results show that a decreases in fire-return interval will likely reduce the ability of the GYE landscape to store C. –Loren Stutts
Smithwick, E.A.H., A. Westerling, M.Turner, W.H. Romme, and M.G. Ryan 2011. Vulnerability of Landscape Carbon Fluxes to Future Climate and Fire in the Greater Yellowstone Ecosystem. Proceedings of the 10th Biennial Scientific Conference on the Greater Yellowstone Ecosystem; October 11-13; Yellowstone National Park.

            Forests of the western United States are facing increasing fires and managers and scientists of these forests must anticipate any consequences of this trend. An increase in fire frequency will trigger significant ecological changes, in particular, carbon source-sink dynamics may be susceptible to changing fire regimes. Smithwick and team (2011) wish to further scientific understanding of the landscape-scale susceptibilities of key forest types as a result of climate change. Their goal was to identify the specific fire frequency at which conifer forests become sources of C to the atmosphere.
            To reach this goal, Smithwick and team utilized a dynamic ecosystem model to project future C stocks under different fire regimes and climate scenarios. They ran the ecosystem model (CENTURY version 4.5) aspatially for the dominant vegetation communities in the GYE and a range of estimated fire-return intervals and future and current climate conditions to identify how great a change in climate and fire regime would shift vegetation from C source to C sink. Their modeling focused on lodgepole pine, a representative forest type in the GYE, and to capture its variation in recovery, they modeled both a fast and slow recovery pathway. To estimate current and future climate conditions, Smithwick and team used historical climate data and a general circulation model (GCM) runs downscaled to the North American land Data Assimilation system. They used three GCMs (CCSM 3.0, CNRM CM 3.0, and GFDL CM 2.1) to generate a set of possible climate futures for the western US. They used climate data from the grid centered on the Yellowstone Lake climate station for simulations of lodgepole pine forest. Mortality, post-fire recovery and productivity were parameterized in CENTURY for lodgepole pine and warm-dry conifer trees based on empirical data. They assumed a C3 grass parameterization in CENTURY for all simulations. Fire return intervals used in CENTURY and landscape C modeling were based understanding of the canopy seed bank and its influence on post fire regeneration. To include the rapid and variable trend in development of a canopy seed bank, they used a 30-year fire interval. Forest C recovery time under both current and future climate scenarios was determined by comparing the time to recovery of pre-1988 C stocks of mature forest stands to that of future periods.
Smithwick and team’s simulation of the large 1988 fire resulted in 12 percent reduction of total C stocks from pre-fire levels. Assuming no fire in the post-1988 period, across the climate scenarios C stocks continued to increase through the end of the simulation.  For scenarios with fire return intervals less than 90 years, total C stocks did not recover and total ecosystem C storage declined through the future simulation period.  Yet for the 90-year fire return interval scenario, C stocks were within 5 percent of the pre-fire stocks. Their results suggest that fires would need to be separated by 90 years or longer for recovery of C stocks whereas closer more frequent fires lead to C losses.
Smithwick and team findings suggest that the threshold in which C stocks do not return to their pre fire levels for lodgepole pine forests of the GYE is at a fire return interval of about 90 years. If the fire return interval is below this threshold, forests re-burn before they re-accumulate the C lost in the previous fire. As a result, forests could become C sources in the global C cycle which may exacerbate climate change. The magnitude of the shift in C balance as a result of shorter fire intervals monitors the ability of the forests to store C. The extent of the shift in C balance will depend on future distribution of forest and nonforest ecosystems across the landscape. For example if future forests do not regenerate at all, then the balance between C sink to C source may prove to be more dramatic than Smithwick and team’s model predicts.
Overall Smithwick and team’s findings reveal that more frequent fire produces a shift in lodgepole forest from a C sink to a C source. Their results show that lodgepole forests are vulnerable to climate change and the associated increase in burning, specifically, the C storage of GYE forests is extremely sensitive to projected future fire regimes.

Invasion of Norway Spruce Diversifies the Fire Regime in Boreal European Forests

Since the Holocene ¾a geological epoch that began around 10,000 BC, global wildfire activity and biomass burning has varied greatly. At a regional to continental scale, it is generally accepted that macroclimate is the primary control that regulates wildfire. Forest and vegetation composition is usually treated as a secondary factor in studies assessing temporal variation in regional wildfire activity. Ohlson and team (2011) gather a spatially comprehensive data set of macroscopic charcoal records that illustrate forest landscapes and local burning spread over a large area of the European boreal forest. They demonstrate that the invasion of the Norway spruce Picea abies has greatly reduced wildfire activity, thereby changing forest disturbance dynamics at a subcontinental scale (Ohlson et al. 2011). Their findings reveal that independent of regional climate change, a biotic change in the forest ecosystem altered the region’s fire regime, demonstrating that forest composition is a significant factor that must be considered when modeling carbon dynamics and fire risk in boreal forests. –Loren Stutts
Ohlson, M., K. Brown, H.J.B. Birks, J. Grytnes, G. Hörnberg, M. Niklasson, H. Seppä, and R.H.W. Bradshaw 2011. Invasion of Norway spruce diversifies the fire regime in boreal European forests, Journal of Ecology, 99, 395-403, doi: 10.1111/j.1365-2745.2010.01780.

Changes in the amount of a single species can bring about significant alterations in the properties and behaviors of an ecosystem. Forest structure and biodiversity were profoundly affected with the invasion of Norway spruce Picea abies in northern Europe as the spruce emerged as a new boreal forest keystone species. Fire regime is a significant ecosystem process that was also affected by this ecosystem transformation. Fire regimes vary as a result of a complex interplay between human activities, climatic variability, sources of ignition, and vegetation and fuel characteristics. Yet at regional and continental scales, climatic factors are considered to be the primary controls that regulate fire regime. Forest tree composition is usually viewed as subsidiary factor in studies evaluating variation in wildfire activity. However Ohlson and team claim that new research on the interactions between forest tree species composition and fire in the boreal forests of Alaska have shown that vegetation composition can be an important driver of wildfire activity.
Ohlson and team assemble a network of peat, tree-ring, and humus records from forests landscapes down the longitudinal axis of Scandinavia to analyze fire disturbance and forest composition in the boreal forest of northern Europe. The records gained from their sampling reveal the history of local forest composition and a comparison of fire history before and after local spruce invasion at both local and regional spatial scales. Ohlson and team assert they can evaluate the influence of both climate and vegetation composition on fire regime through the comparison of charcoal and spruce pollen records. They sampled 75 spruce forest sites located in 24 forest landscapes in the boreal forest of northern Europe. The difference between pre and post-spruce charcoal concentrations were tested statistically while partial cross-sections were cut out of living trees to record fire scars and dated by counting tree rings.
The differences in charcoal values between pre- and post-spruce sections of the 75 soil profiles lead the team to make several conclusions. In terms of the general pattern of charcoal deposition across the soil sites, there was a significant decrease in abundance and deposition rate of charcoal following the local invasion of spruce. Concurrence between local spruce invasion and local cessation of wildfire is revealed by the permanent and immediate absence of historically substantial charcoal deposition following the emergence of spruce forest. Tree-ring analysis revealed that in spruce forests, no wood recorded past fires while charcoal records illustrated a complete lack of fire activity at various sites following the local invasion of spruce.
Ohlson and team draw several conclusions from their results.  First they assert that fire disturbance is not ubiquitous. Of the 75 forest sites analyzed in their study, 13 produced no macroscopic charcoal at all while seven produced records that showed a sporadic single peak occurrence of charcoal particles. They claim that such a large proportion of sites yielding little or no charcoal challenges the common notion that wildfire is a ubiquitous, important and frequent disturbance factor in the boreal European forest. Secondly, Ohlson and team claim that regional macroscale climate exerts a broad influence on fire regime. The marked variation of their charcoal records revealed a significant variability in the fire regime across forest sites.
 Next, Ohlson and team conclude that spruce invasion had a critical influence on fire regime. Specifically they claim their results reveal a close correspondence between the invasion of spruce and the decline in charcoal concentration, thereby suggesting that a change in the dominant tree species had a critical effect on the fire regime that exceeded the broad influence of climate change. Lastly they assert that the fire regime of northern Europe was diversified by the spruce invasion, which induced a more variable spatial occurrence of fire at the landscape scale. Specifically their records showed that the spread of spruce led to a marked reduction in local fire frequency and severity. The spruce invasion further diversified the fire regime by reducing wildfire activity significantly in mesic/moist forest types that typically occur in concave landscape forms. Ohlson and team also discuss how the reduction in burning as a result of the spread of spruce in northern Europe has most likely resulted in increased sequestration of carbon in forest ecosystems and thus initiated a major biological feedback to the climate system.

Ohlson and team do not claim their specific findings on the linkages between boreal forest composition and wildfire to be universal, but instead wish to raise awareness to the fact that tree species composition is an important factor capable of regulating the fire regime. They suggest replacing the concept of fire disturbance as a major determinant of boreal forest composition in favor of maintaining biological continuity

Impacts and Implication of an Intensifying Fire Regime on Alaskan Boreal Forest Composition and Albedo

Climate warming and climate drying are changing the fire dynamics of many boreal forests. This change is transforming the fire seasons of these forests and increasing the extremity and size of the fires as well increasing burn frequency. Studies reveal that the increase in burn severity allows for the growth of deciduous trees in the early years after fire. Pieter Beck and team (2011) wish to test if more deciduous trees are present in boreal forests followings severe burning along with the implications for energy and carbon balances in the forests. Beck and team use the vegetation composition of interior Alaska to test for their hypothesis. They create a set of forested sites of six decades of vegetation regrowth following fire using a database derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery. Using a RandomForestalgorithm with field data sets, the deciduous fraction map illustrated the aboveground biomass in deciduous vegetation. The team then analyzed the difference Normalized Burn Ratio, an index which indicates burn severity and ignition date that can produce a substitute for burn severity of historical fires. A bioclimatic model of evergreen forest distribution and LIDAR remote sensing were used to clarify stratification of the current landscape by burn severity. Their results reveal that since the 1950s, severely burned regions in interior Alaska have created vegetation with a heavily deciduous biomass. They discuss the implications of this climate-induced change in fire severity for carbon sequestration in the forests and surface reflectance (or albedo) with other potential feedbacks to climate. –Loren Stutts
Beck, P. S. A., Goetz, S. J., Mack, M. C., Alexander, H. D., Jin, Y., Randerson, J. T. and Loranty, M. M. 2011. The impacts and implications of an intensifying fire regime on Alaskan boreal forest composition and albedo. Global Change Biology17, 2853–2866, doi: 10.1111/j.1365-2486.2011.02412.

            Fire is a principal factor in boreal forest dynamics namely changes in vegetation composition and carbon cycling. Fire creates feedbacks between climate and vegetation. These feedbacks can modify the climate of the northern hemisphere because the biome is enormous and dense in carbon and fire impacts several agents including aerosols, surface albedo, and greenhouse gas fluxes. The area of forests burned in Alaska has increased over the last forty years and is predicted to increase as a result of drying and warming during the fire season. The agents causing greater areas to burn annually are claimed to also increase burn severity, or the proportion of organic matter consumed by fire. It is hypothesized that vegetation composition plays a central role in the expansiveness of productivity and albedo feedbacks to climate that affect the burn severity on boreal forests. In areas where burning is severe, the abundance of deciduous trees is projected to be higher following fire. The balance of agents crucial to climate forcing combined with the shift in vegetation composition will reveal the effect of the boreal fire regime on climate.
            The team tests their hypothesis using the following methods. Using remote sensing data, they map albedo and deciduous fraction of vegetation, defined as the percentage of aboveground biomass (AGB) stored in deciduous plants. They create chronosequences (over six decades) reflecting biomass regrowth and albedo post fire and they stratify burns in interior Alaska into high-and low-severity burns. Short wave albedo for spring and summer were calculated using MODIS satellite imagery Bidirectional Reflectance Distribution Function (BRDF) parameters at 500 resolution. Radiation values were derived from surface radiation budget data set supplied by the World Climate Research Program/Global Energy and Water-Cycle Experiments. The deciduous fraction (FD) of AGB was mapped from the monthly composites of MODIS reflectance and field measurements using a regression tree model. Plots were then spread across 69 MODIS pixels. The MODIS BRDF reflectance data were composited at 500 m spatial resolution to create monthly images of reflectance in spectral bands. To detect burned areas the team acquired the database of Alaskan fire perimeters created by the Alaska Fire Service and used it to define burns. Burns between 10 and 50 years old were used when constructing succession chronosequences on landscapes of interior Alaska. They used nonremote sensing or derived attributes of the burn scars to infer burn severity when stratifying historically burned areas. To team used the dNBR to test the validity of the burn severity stratification based on size and timing.
            The team then describes the deciduous cover map they generated from MODIS remote sensing observations, and the chronosequences of biomass and albedo in response to burn severity. The spatial patterns in the maps of FD reflect the distribution of burn scars and ecotones like alpine belts and the boreal forest-tundra transition. The deciduous fraction of vegetation in high severity burns was higher than in low-severity burns during the 10 to 50 years after burning. In low severity burns deciduous vegetation biomass steadily declined after 25 to 30 years while it increased in high severity burns. Figures illustrate deciduous fraction, total, and deciduous aboveground biomass in low and high severity burns. Total biomass in low severity burns and high severity burns were extracted from an existing map and partitioned into deciduous and evergreen biomass using the deciduous fraction mapped from field and MODIS data. Albedo was lower in more densely forested areas and decreased with stand age in low severity burns. Higher deciduous fraction in high severity burns resulted in higher summer albedo for the first 40 years of succession. In the 50 year regrowth period analyzed, higher burn severity promoted aboveground biomass (ABG) accumulation as a result of composition shifting toward deciduous tress that have higher growth rates.
The patterns of vegetation regrowth after fire in interior Alaska since 1950 reveal that severe burning induces the growth of deciduous species for several decades. The extent and severity of the fire and the consequences for post burn vegetation successional trajectories also support predictions of a shift toward more deciduous plants in Alaska. This has implications for climate feedbacks as well as land management and use of forest products in the region. Even when burn severity does not increase, the shortened fire return interval would limit the growth of mature spruce trees and thus increase the existence of deciduous vegetation. Climate warming is predicted to intensify the fire seasons and create a higher frequency of extreme fire years with large burns of high severity. Negative feedback from newly deciduous land cover may modify and facilitate further changes. An increase in the area burned annually will reduce the conifer forest cover throughout interior Alaska. Deciduous forests that replace the conifer-dominated regions are less likely to burn because of their lower flammability architecture and limited accumulation of fuels. This increase in deciduous cover may limit the intensification of fire regime and warmer temperatures may promote the long existence of deciduous trees. Alaska may be approaching a tipping point where its boreal forests move from evergreen dominance to more deciduous species in which a biome shift could be accelerated by ongoing effects of climate change. Beck and team then indicate that further research is needed to observe the direction and magnitude of these newly created feedbacks to climate and their potential for climate disturbance.

Impacts of Climate Change on Fire Regimes and Carbon Stocks of the U.S. Pacific Northwest

The vegetation types and carbon groupings present in the U.S. Pacific Northwest (PNW) are closely tied to fire seasons that rely on fire suppression and climate change. To evaluate the effects of current climate change on PNW carbon and fire relationships, the team created a new fire suppression rule for the MC1 general vegetation model and ran simulations under three climate change scenarios (Rogers et al. 2011). Moderately moist forests may be vulnerable to future fires and emit large amounts of carbon while drier forests displayed carbon sequestration despite projections of increased fire frequencies under climate change. The simulations showed substantial increases in area burned and burn severity suggesting that fire suppression will become less effective leaving ecosystems vulnerable to larger fires in the future. –Loren Stutts

Rogers, B. M., R. P. Neilson, R. Drapek, J. M. Lenihan, J. R. Wells, D. Bachelet, and B. E. Law 2011. Impacts of limate change on fire regimes and carbon stocks of the U.S. Pacific Northwest, J. Geophys. Res., 116, G03037, oi:10.1029/2011JG001695.

Given the diverse vegetation of the PNW, Rogers and his team use a dynamic general vegetation model (DGVM) with specific suppression rules on regional domains to understand the future impact of fire in the PNW. To assess which processes may control the PNW’s carbon budget (or the balance between spring precipitation, CO2 fertilization, summer drought and intensity of fire season) and fire regimes, the team used the MAPSS-CENTURY 1 model (MC1), a DGVM that captures the feedbacks and interactions between major ecosystem processes. They ran the MC1 with their created fire suppression rule over the PNW on a fine scale grid a grid that represents geological variation on very fine scales, in this case using 30 arc-seconds resolution. They ran this MC1 over the PNW under historical climates and three projected future climates, and performed sensitivity analyses to emphasize potential changes.

For the historical vegetation types and climates, the team used three regions in western Oregon and Washington: Western Forests, Eastern Forests, and the Columbia Plateau. The Western Forests experience high rainfall with long intervals of no fire, the Eastern Forests are drier and burn more often, while the Columbia Plateau is the driest region. Historical climate data were obtained from these regions and modeled at a 30 arc- second resolution. Three general circulation models (GCMs) were used to obtain the three future climate projections: CSIRO Mk3, MIROC 3.2 medres, and Hadley CM3 (CSIRO, MIROC, and Hadley) chosen for their range of temperature changes.

The team then used the MC1 to simulate the most common vegetation types with full fire for the historical and future periods under the three climate projections. In terms of historical comparisons, MCI results were favorable compared to historical observations although there was some disagreement. In terms of the three climate projections and their precipitation, the CSIRO projection is cool and wet, MIROC is hot and wet, while Hadley is hot and dry. With MIROC and Hadley, the growing season was lengthened and exaggerated the already strong seasonal cycles which increased net primary productivity (NPP) during the rainy season and decreased summer NPP by amplifying summer drought. Under the milder conditions of CSIRO, NPP increases. Under all climate projections simulated fire increased across the domain. Under CSIRO and MIROC these increases surface late in the twenty first century while with Hadley’s conditions create large fires in the early to mid-twenty-first century. Due to larger and more intense fires in the Western and Eastern Forests and the woodiness of the Columbia Plateau, under all three scenarios burn severities increase across the domain through the twenty first century. The simulated twenty first century PNW carbon budget balances biomass losses from summer drought and fire and biomass gains from higher rainy season NPP. The domain gains carbon emissions under CSIRO and gains less under MIROC but loses emissions under Hadley.

Sensitivity analyses were run to assess the influence of fire and fire suppression on the carbon balance. MC1 was first run with fire suppression off (full fire) and with all fires off (no fire). Fire suppression creates less burn area and biomass consumed than full fire yet when compared to results for historical periods with the identical fire rules, fire suppression results in greater increases in burn area and biomass consumption than does full fire under all scenarios. Current fire suppression efforts may not be as effective against future fires. Simulated, observed, and historical fire suppression causes elevated fuel loads which suggests intensification of future PNW fire regimes yet because biomass consumption is less and carbon is gained after the initiation of simulated fire suppression, suppression creates smaller losses than full fire simulations (except for under Hadley).

Research suggests that fire regimes will be amplified during the twenty first century. The MC1 illustrates an increase in fire intensity and severity, worsened by the history of fire suppression.

Pathways for Climate Change Effects on Fire: Models, Data, and Uncertainties

As the climate continues to warm, fire activity has gained importance because of the way in which it affects the biosphere and the atmosphere. Current research has focused on measuring the changing patterns in wildland fire activity, mainly area burned and fire frequency, with less emphasis on understanding the factors responsible for these changing patterns. To understand these factors, research must observe fire history records but incorporate changes in vegetation and changes in human activities alongside history records (Hessl et al. 2011). –Loren Stutts

Hessl, A. E., 2011. Pathways for Climate Change Effects on Fire: Model, Data, and Uncertainties. Progress in Physical Geography vol. 35, 393-407, doi: 10.1177/0309133311407654.

The Intergovernmental Panel on Climate Change predicts that in areas where drought is persistent, fire intensity and frequency will increase. These predictions are supported by many modeling studies but few empirical studies have attempted to document changes in fire activity. To better understand the causes responsible for changes in fire activity and better project future changes in fire seasons, researchers must study a combination of fire history data, model-based studies, and empirical studies.

Because the relationship between fire and climate respond to vegetation and fuel, new models for predicting fire activity should include potential changes in vegetation and fuel structure as a result of climate change. Using empirical evidence to support climate change impact on fire season is often difficult because of the long term, consistent records of fire occurrence needed to detect change. Tree rings, sedimentary charcoal, and soil charcoal are the three fire history methods used to predict fire regimes and to understand the relationship between fire and climate.

After reviewing empirical, model, and fire history studies, the author suggests three pathways in which fire seasons respond to climate change: changes in fuel condition, fuel volume, and ignitions. Fuel condition refers to the moisture or aridity of grasslands, woodlands, or shrubland, while fuel volume refers to changes in density of plants, trees, and shrubs as a result of drier or wetter conditions, while ignitions refer to the ability of fuels to be ignited by lightening or other natural causes and/or by humans.

Because empirical and model-based studies should better define human impact on fuel volume, ignitions and density, the author proposes that research use hindcasting or models of vegetation and fire during past climates to help justify model projections of fire activity. This would improve fire climate models through its applicability across land-use histories and types of vegetation. To further understand the interactions between climate change and human activities, research must include fire history records from areas with varied land use. Projections of fire activity should address the complexity of changing human activities, vegetation, and climate.