B14C-01
The vulnerability of carbon storage in boreal North America during the 21st Century to increases in wildfire activity
The boreal forest contains large reserves of carbon. Across this region, wildfires influence the temporal and spatial dynamics of carbon storage, which has the potential to be altered under a changing climate. The temporal and spatial dynamics of fire are also likely to be altered as the climate continues to change. In this study, we develop temporally and spatially explicit relationships between air temperature and fuel moisture codes derived from the Canadian Fire Weather Index System to estimate annual area burned at 2.5° resolution using a Multivariate Adaptive Regression Splines (MARS) approach across boreal North America. At the boreal North American scale, the empirical fire models explain on the order of 80 % of the variation in annual area burned for the period 1960-2002. To understand how the temporal and spatial dynamics of fire might be altered by future climate change, the empirical fire models were driven by output from the A2 and B2 scenarios from the Canadian Climate Center CGCM2 global climate model to predict annual area burned through year 2100. Historical and future area burned estimates are then coupled to the process-based Terrestrial Ecosystem Model (TEM) to simulate fire emissions and changes in carbon storage for boreal North America in the context of changing atmospheric CO2 concentration and climate from the start of the historically recorded fire records in the 20th century through the end of the 21st century. Relative to the last decade of the 20th century, decadal total carbon emissions from fire increase on the order of 2.5 to 4.4 times by 2091-2100, depending on the climate scenario and assumptions about CO2 fertilization. The effect of CO2 fertilization is a major uncertainty in this analysis. For the 21st century, our simulations indicate that boreal North America is a carbon sink in response to CO2 fertilization, climate variability, and fire, but an increase in fire results in a decrease in the sink strength. While this study highlights the importance of future atmospheric CO2, climate, and fire on the carbon dynamics of boreal North America, several limitations and uncertainties exist and should be addressed in future process- based analyses. Future studies should incorporate the role of dynamic vegetation to more accurately represent post-fire successional processes, incorporate fire severity parameters that change in time and space, and integrate the role of other disturbances and their interactions with future fire regime.
B14C-02 INVITED
Landscape and climate controls on fire severity in Alaskan black spruce forests
Black spruce (Picea mariana) forests are the dominant forest cover found in the Interior (boreal) region of Alaska and throughout most of Canada, with most of the forests established through secondary succession following fires. These forests have deep surface organic layers (10 to > 40 cm thick) and are frequently underlain by permafrost. An important measure of fire severity in these forests is the depth of burning of the surface organic layer. Variations in this depth of burning controls many post-fire ecosystem characteristics and processes, including ground temperature and moisture, permafrost dynamics, seasonal rates of soil thawing, vegetative reproduction, and seedling establishment and growth. Over the past several years, several different research groups have focused on collecting data on the depth of the surface organic layers in burned and unburned black spruce forests in Interior Alaska. Combined, these data were collected in some 800 burned and unburned stands associated with 37 fire events that occurred between 1983 and 2006. The data set consists of over 18,000 individual organic layer depth measurements, nearly equally divided between burned and unburned stands. Our analyses of this data set show that variations in the depths of both unburned and burned organic layers is strongly controlled by the topographic position of the black spruce forests, with deeper organic layers being found on flat sites compared to sites on slopes, and deeper organic layers being found on north-facing slopes compared to south-facing slopes. In addition, we found that sites that burned later in the growing season (after July 20) had shallower organic layers compared to sites that burned early in the growing season (prior to 20 July). This difference is attributed to the effects that seasonal thawing of the ground layer (in sites with permafrost) has on site drainage. Finally, sites that burned during the ultra-large fire years of 2005 and 2006 (where greater than 3 percent of the total land surface burned during each year) had shallower organic layers than sites that burned during small fire years (where less than one percent of the land surface burned during a year). We attribute this difference to the overall drought conditions that occurred during the ultra-large fire years, which led to lower duff moisture and deeper burning fires. These results show that critical differences in fire severity in black spruce forests are linked to climate driven-variations in the fire regime, specifically the frequency of large fire years at a regional scale and the seasonality of burning.
B14C-03
Effects of Fire on Boreal Bogs and Implications of Climate Change
Peatland ecosystems, which are predominantly found in northern boreal regions of Canada and Russia, accumulate carbon because photosynthetic production of the mosses dominating the ground layer exceeds their decomposition, thereby generating peat. Production and decomposition rates, and therefore peat accumulation, are species-specific and climatically controlled. While primary production of these systems is relatively low, the cold, wet, nutrient poor conditions found therein result in slow rates of decomposition. Therefore, changes in climate or vegetation composition will have an affect on boreal peatland function. Fire is the most prevalent disturbance for boreal peatlands of western Canada. Ombrotrophic, forested bog peatlands are most affected by fire due to a drier peat surface relative to other peatland landforms and an extensive Picea mariana canopy. In addition to direct C losses during peat combustion, fire has indirect affects on bog C cycling through removal of the ground layer vegetation and alteration of the surface environment. Because peat accumulation varies among species, functional recovery post-fire is linked to ground layer succession, which varies with combustion severity. To assess the post-fire compositional and functional recovery trajectories of western Canadian bogs, we monitored the ground layer community structure, production, and decomposition from 2003 to 2006 along a chronosequence of historically burned bogs (1-105 years since fire). Ground layer succession was tri-phasic, grading from pioneer true mosses early post-fire (1-10 ysf) to a Sphagnum-dominated community (20-80 ysf), followed by feathermoss encroachment at the longest recovery times (>90 ysf). However, the ground layer biomass production trajectory was asymptotic, stabilizing at ca. 20 years post-fire coinciding with Sphagnum dominance of the ground layer community. Decomposition in the upper peat column (top 40-cm) did not vary along the chronosequence, although surface (<2-cm) decomposition was not assessed. From our results, we developed models to assess the impact of an altered fire regime on peatland C storage. Increases in annual extent of wildfire and combustion severity under a 2xCO2 scenario substantially extend the peatland C pool recovery time. Furthermore, other models suggest a substantial reduction of the fire return interval (< 70 yrs) will cause peatlands to become sources, rather than sinks, of atmospheric C. Warming will enhance this effect, requiring less of a reduction in fire interval to trigger the functional switch.
B14C-04 INVITED
Climate drivers of fire extent and severity in US Rocky Mountains
Years of extensive forest fires, 1900-2003, in the US Northern Rockies in Idaho and western Montana had warm springs followed by warm, dry summers and positive Pacific Decadal Oscillation (PDO). We identified these years as those exceeding the 90th percentile in annual fire extent derived from an existing fire atlas that includes 5,038 fire polygons recorded from 12,070,086 ha, or 71% of the forested land in Idaho and Montana west of the Continental Divide for 1900-2003. The long mid-20th century period lacking regional-fire years (1935-1987) had generally cool springs, generally negative PDO, a lack of extremely dry summers, and was a period of active fire suppression. In years of extensive fires, the climate drivers did not differ among dry forests, mesic forests and cold forests. Similarly, in the 32 years in which fires occurred on at least 5 of 21 widely dispersed sites sampled for fire scars 1650-1900, spring-summers were significantly warm and summers were significantly warm/dry compared to the 99 years when no fires were recorded at any site. Given projections for warmer springs and continued warm, dry summers, forests of the US Northern Rockies are likely to experience synchronous, widespread fires in the future. We also analyzed burn severity interpreted from a time series of Landsat satellite images (114 fires burned 195,600 ha, 1986-2004, on the 1.4 million-ha Gila National Forest, New Mexico), daily weather and SNOTEL data. Snowpack and precipitation influenced fire extent and severity in upper elevation forests. Fire extent and severity in mid-and low elevation forest types were strongly correlated with fire season (April-July) precipitation variability. Recent cross-continental and cross-regional analyses highlight the importance of climate in driving fire occurrence. Less is known about why, where and when large, severe fires are most likely and the influence of climate, land use, vegetation, fuels management, topography, and other disturbances.
B14C-05
Could managed burning of peatlands lead to carbon storage?
Peatlands are the UK's largest single terrestrial carbon store with carbon stored in UK peatlands than in forests of Britain and France combined. Unlike most northern peatlands in the peat soils of the UK are heavily managed for recreation and agriculture and due to their proximity to major centres of population are under more anthropogenic pressure than most peatlands. A typical management strategy on UK upland peats is the use of managed fire to restrict vegetation. Fires are used upon a 10-25 year rotation and are described as "cool" as they remove the crown of the vegetation without scorching the litter layer or the underlying soil. In this case the fire destroys primary productivity and limits litter production but produces char. Char is a low volume, highly refractory, high carbon content product while litter is a high volume, decomposable, lower carbon content product. Therefore, the question is if there are fire conditions underwhich the production of char causes more carbon to be stored in the peat than would have been stored if no fire management had been employed. This study uses detailed vegetation studies from a long term monitoring site in order to assess litter and biomass production; in laboratory experimental burns were undertaken in order to assess the amount and controls upon char production and the carbon content of that char. Results of field and laboratory observations are used to model carbon accumulation under s aseries of fire management scenarios and the modelling shows that cools burns at long rotations could lead to higher carbon storage than if no fire had occurred, further than in several cases more carbon accumulation occurred even if less depth of peat was generated.
B14C-06
Variability in fuel consumption across fire-effected boreal and western North American forest regions
In estimating total carbon emissions from wildland fire, three inputs are needed: area burned, carbon density (in the form of biomass and soil organic carbon) within the burn site, and the fraction of the biomass consumed during the fire. The last of these, biomass consumption, is one of the most difficult to measure and the most variable. We present results of a study to quantify and characterize the variability of biomass (fuel) consumption by wildland fires in forest regions of western and northern North America. The results can be used to improve model inputs and to better define the uncertainty in model-based estimates of fire emissions. Remote sensing- based maps of severity are used to partition fires into severity classes. Field measures of consumption and model outputs relating fuel moisture to consumption are used to quantify fuel consumption as a function of severity class and fuel type. Demonstration of empirically-driven fuel consumption models to derive consumption levels is presented as well as methods to use remote sensing to map fire severity in temperate and boreal regions. The final results of this study to quantify fuel consumption and consumption variability in North American forested regions will be presented and discussed. Results show severely burned areas with high fuel consumption represent the smallest amount of the landscape in all ecoregions, while some ecoregions often have light levels of fuel consumption across large areas. Despite smaller incidence of severe fires (high fuel consumption), these events contribute a considerable proportion of the total pyrogenic carbon emissions to the atmosphere from forest fires. http://fireconsumption.mtri.org/