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.

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