Fire severity as a key determinant of aboveground and belowground biological community recovery in managed even-aged boreal forests

Abstract

Changes in fire regime of boreal forests in response to climate warming are expected to impact postfire recovery. However, quantitative data on how managed forests sustain and recover from recent fire disturbance are limited.Two years after a large wildfire in managed even-aged boreal forests in Sweden, we investigated how recovery of aboveground and belowground communities, that is, understory vegetation and soil microbial and faunal communities, responded to variation in the severity of soil (i.e., consumption of soil organic matter) and canopy fires (i.e., tree mortality).While fire overall enhanced diversity of understory vegetation through colonization of fire adapted plant species, it reduced the abundance and diversity of soil biota. We observed contrasting effects of tree- and soil-related fire severity on survival and recovery of understory vegetation and soil biological communities. Severe fires that killed overstory Pinus sylvestris promoted a successional stage dominated by the mosses Ceratodon purpureus and Polytrichum juniperinum, but reduced regeneration of tree seedlings and disfavored the ericaceous dwarf-shrub Vaccinium vitis-idaea and the grass Deschampsia flexuosa. Moreover, high tree mortality from fire reduced fungal biomass and changed fungal community composition, in particular that of ectomycorrhizal fungi, and reduced the fungivorous soil Oribatida. In contrast, soil-related fire severity had little impact on vegetation composition, fungal communities, and soil animals. Bacterial communities responded to both tree- and soil-related fire severity. Synthesis: Our results 2 years postfire suggest that a change in fire regime from a historically low-severity ground fire regime, with fires that mainly burns into the soil organic layer, to a stand-replacing fire regime with a high degree of tree mortality, as may be expected with climate change, is likely to impact the short-term recovery of stand structure and above- and belowground species composition of even-aged P. sylvestris boreal forests.

Keywords: Boreal forest; Pinus sylvestris; climate change; ecosystem recovery; even‐aged forestry; fire severity; ground fire; soil biota; stand‐replacing fire.

FIGURE 1

Conceptual diagram showing how we hypothesize that two separate fire severity gradients will affect biological communities: one aboveground gradient that is mainly attributed to damage of the trees and one gradient attributed to damage on the forest floor.

FIGURE 2

Map of the Västmanland burn that occurred in Sweden in 2014, and of the physical location of each of the 32 individual forest stands sampled in this study. Red contour line represents the fire boundary. Black outer line dots represent 25 individual burned stands and blue outer line dots represent the seven unburned reference stands. Circles corresponding to burned plots are colored according to tree‐related fire severity (from yellow [low] to brown [high]) and sized according to soil‐related fire severity. GPS coordinates for each stand are given in Table S1.

FIGURE 3

Principal component analysis (PCA) of aboveground and belowground variables that are indicative of tree‐related fire severity and soil‐related fire severity, respectively. The first two axes explaining the largest proportions of total variation (in parenthesis) are shown. Numbers represent forest stand IDs (Table S1). Mean values of each variable can be found in Table S4.

FIGURE 4

Mean frequency of species of (a) bryophytes, (b) ericaceous dwarf shrubs, (c) grasses and sedges, (d) herbs, (e) newly established tree seedlings, and (f) others (lichen and ferns) that are present in more than one stand in the understory of the burned and unburned forest stands. Asterisks indicate significant differences for individual species between burned and unburned forests (results of the statistical tests in Table S5).

FIGURE 5

Ordination analysis of response variables displayed by NMDS for community composition of (a) understory vegetation, (b) soil fungi, (c) soil bacterial PLFAs, and (d) soil Nematoda. Vectors indicate the direction and magnitude of correlations of community assemblage with tree‐related fire severity and soil‐related fire severity indexes. Asterisks denote level of significant correlations (*p < 0.05, **p < 0.01, and ***p < 0.001). Stress values shown at the bottom of each graph correspond to two dimensions (k = 2). Numbers represent forest stand IDs (Table S1).