Regional variation in interior Alaskan boreal forests is driven by fire disturbance, topography, and climate

Abstract

High latitude regions are warming rapidly with important ecological and societal consequences. Utilizing two landscape‐scale data sets from interior Alaska, we compared patterns in forest structure in two regions with differing fire disturbance, topography, and summer climate norms. Our goal was to evaluate a set of hypotheses concerning possible warming‐driven changes in forest structure suggested by recent literature. We found essentially consistent habitat associations for the tree flora across two disparate study areas concomitant with considerable differences in observed patterns of forest structure and composition. Our results confirmed expected increases in broadleaved species occupancy and abundance in the warmer, more fire‐affected study region along with considerably higher tree occupancy and abundance in high elevation areas there. However, contrary to our predictions, we found no evidence of expected reductions in conifer occupancy or increases in non‐fire related tree mortality. Instead, both individual and combined tree species occupancy, density, abundance, and richness were considerably higher in the warmer, more fire‐influenced region, except in the warmest, driest areas (steep and south‐facing slopes at low elevation). Our comparison of two landscape‐scale data sets suggests that changes in tree distribution and forest structure in interior Alaska will proceed unevenly, governed by a mosaic of site‐dependent influences wherein forest community composition and species dominance will shift along different trajectories and at different rates according to variation in underlying landscape attributes. Although there were clear differences in forest structure between the two areas that were likely attributable to differences in growing season warmth and fire disturbance, we found scant support for the concept of an incipient, ongoing biome shift in interior Alaska resulting from impending diminution of boreal forest cover over the short to medium term. Indeed, we suggest that (depending on severity of disturbance dynamics and the rapidity of future warming) cooler areas of interior Alaska's forest may reasonably be expected to sustain marginal increases in forest cover with additional warming, at least in certain topographic positions (such as poorly drained basins and cool treeline sites) and/or geographic regions, prior to any landscape‐scale diminution of forest cover due to warming.

Effects of short‐interval disturbances continue to accumulate, overwhelming variability in local resilience

“…However, the bulk of evidence provided by Johnstone and Chapin (2006 ) and others indicate that early patterns of regeneration in the boreal tend to be highly prescriptive of multidecadal successional trajectories. Furthermore, the spatial extent of larger ecosystem transition in the boreal remains unknown: Emerging deciduous communities appear to be spatially constrained within fire or reburn perimeters (Roland et al 2019). The results of this study are similarly limited in scale.…”

“…Aspen ( Populus tremuloides; Greene and Johnson 1999 ) and birch ( Betula neoalaskana; Zasada 1971) produce large quantities of small wind‐borne seeds (Greene et al 2007, Johnstone et al 2009, Roland et al 2013), which may benefit from the interaction of limited black spruce seed availability and greater available mineral soil surface, depending on the amount of substrate consumed during fire (Hesketh et al 2009). Models suggest that the interaction of seedbank and soil consumption driven by short‐interval fires will lead to a shift in local community composition from conifer‐dominated stands to deciduous forest (Rupp et al 2002, Mann et al 2012, Roland et al 2019) or grassland (Brooks et al 2004, Roland et al 2013).…”

Borealization and its discontents: drivers of regional variation in plant diversity across scales in interior Alaska

“…Ecological dynamics associated with warmer and wetter conditions during the Holocene stimulated the widespread formation of poorly drained soils with thick, organic horizons coincident with the establishment of coniferous forests underlain by deep feather moss and Sphagnum mats in their understory (Edwards et al 2000, Anderson et al 2003, Wang et al 2016). These conditions, widespread in Alaska's interior lowlands, inhibit the establishment of most herbaceous species (Roland et al 2017(Roland et al , 2019b. Because herbaceous species groups represent the largest proportion of the regional vascular plant species pool, the exclusion of these taxa from muskeg and boreal forest situations has created diversity coldspots encompassing much of the landscape of interior Alaska (Roland et al 2017).…”

“…The implementation of a landscape-scale vegetation monitoring program in interior Alaska over the last 20 yr has amassed sufficient data to begin to assess the drivers of patterns in species richness and occurrence at multiple spatial scales. As part of this program, two intensive and extensive landscape-level datasets have been collected at different positions along the continentality gradient in interior Alaska: (1) DNPP astride the Alaska Range in the central interior with a continental climate, and (2) the hyper-continental Yukon-Charley Rivers National Preserve (YCNP) located deep in the eastern interior along the Canadian border (see Roland et al 2013Roland et al , 2019b; Fig. 1).…”

“…3. Due to warmer growing season conditions in YCNP, species distributions will be shifted higher in elevation relative to DNPP (Roland et al 2019b), and species with boreal forest and scrub community affiliations will thus have significantly greater occupancy probabilities in YCNP. Correspondingly, tundra species and species associated with open vegetation types will have significantly higher occupancy probabilities in DNPP, and tundra species will occur at considerably lower elevations in DNPP as compared to YCNP.…”