Abstract
AbstractClimate warming and drying are modifying the fire dynamics of many boreal forests, moving them towards a regime with a higher frequency of extreme fire years characterized by large burns of high severity. Plot‐scale studies indicate that increased burn severity favors the recruitment of deciduous trees in the initial years following fire. Consequently, a set of biophysical effects of burn severity on postfire boreal successional trajectories at decadal timescales have been hypothesized. Prominent among these are a greater cover of deciduous tree species in intermediately aged stands after more severe burning, with associated implications for carbon and energy balances. Here we investigate whether the current vegetation composition of interior Alaska supports this hypothesis. A chronosequence of six decades of vegetation regrowth following fire was created using a database of burn scars, an existing forest biomass map, and maps of albedo and the deciduous fraction of vegetation that we derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery. The deciduous fraction map depicted the proportion of aboveground biomass in deciduous vegetation, derived using a RandomForest algorithm trained with field data sets (n=69, 71% variance explained). Analysis of the difference Normalized Burn Ratio, a remotely sensed index commonly used as an indicator of burn severity, indicated that burn size and ignition date can provide a proxy of burn severity for historical fires. LIDAR remote sensing and a bioclimatic model of evergreen forest distribution were used to further refine the stratification of the current landscape by burn severity. Our results show that since the 1950s, more severely burned areas in interior Alaska have produced a vegetation cohort that is characterized by greater deciduous biomass. We discuss the importance of this shift in vegetation composition due to climate‐induced changes in fire severity for carbon sequestration in forest biomass and surface reflectance (albedo), among other feedbacks to climate.
Highlights
Fire is a primary driver of long-term boreal forest dynamics including successional changes in vegetation composition (Viereck, 1973; Foote, 1983; Wurtz et al, 2006) and carbon cycling (Bond-Lamberty et al, 2007)
Height of Medium Energy (HOME) was used because we found it was less sensitive to algorithms designed to estimate canopy height by first detecting an initial return signal and deconvolving a ground return
We first describe the deciduous cover map we derived from Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing observations, assess our approach used to estimate historical burn severity, and describe the chronosequences of biomass and albedo in response to burn severity
Summary
Fire is a primary driver of long-term boreal forest dynamics including successional changes in vegetation composition (Viereck, 1973; Foote, 1983; Wurtz et al, 2006) and carbon cycling (Bond-Lamberty et al, 2007). Historical estimates from Russia are sparse (Conard et al, 2002) but recent fires have been anomalously large (Kasischke et al, 2000; UNECE, FAO, GFMC, 2005; van der Werf et al, 2010), as have tundra fires recorded in Alaska (Jones et al, 2009). This intensification of the fire regime, with extreme fire years characterized by large burns becoming more frequent, has been attributed to climate warming (Gillett et al, 2004) and is expected to accelerate over the course of the 21st century. Estimates based on an empirical fire model for Alaska and Western Canada showed the average area burned per decade doubling by 2041–2050, relative to 1991–2000, and increasing on the order of 3.5–5.5 times by the end of the 21st century (Balshi et al, 2009)
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