Changes in fire regime break the legacy lock on successional trajectories in Alaskan boreal forest

TLDR

Predicting plant community responses to changing environmental conditions is essential for forecasting and mitigating global change, and disturbance-driven secondary succession allows long‑lived organisms to reorganize, but in interior Alaska short fire intervals limit relay succession so postfire seedling composition largely mirrors prefire stands with little net change. The study examined how environmental conditions, stand structure, and fire disturbance influence successional trajectories in black spruce‑dominated boreal forests. Using landscape‑scale data from an extreme fire year, the authors analyzed seedling recruitment across 90 burned stands to assess the effects of moisture, prefire spruce density, and fire severity on succession. Seedling recruitment of black spruce was strongest at moist sites with high prefire spruce density, whereas severely burned sites recruited abundant deciduous trees, indicating that increasing fire severity may shift the landscape toward deciduous‑dominated succession and alter ecosystem services.

Abstract

Abstract Predicting plant community responses to changing environmental conditions is a key element of forecasting and mitigating the effects of global change. Disturbance can play an important role in these dynamics, by initiating cycles of secondary succession and generating opportunities for communities of long‐lived organisms to reorganize in alternative configurations. This study used landscape‐scale variations in environmental conditions, stand structure, and disturbance from an extreme fire year in Alaska to examine how these factors affected successional trajectories in boreal forests dominated by black spruce. Because fire intervals in interior Alaska are typically too short to allow relay succession, the initial cohorts of seedlings that recruit after fire largely determine future canopy composition. Consequently, in a dynamically stable landscape, postfire tree seedling composition should resemble that of the prefire forest stands, with little net change in tree composition after fire. Seedling recruitment data from 90 burned stands indicated that postfire establishment of black spruce was strongly linked to environmental conditions and was highest at sites that were moist and had high densities of prefire spruce. Although deciduous broadleaf trees were absent from most prefire stands, deciduous trees recruited from seed at many sites and were most abundant at sites where the fires burned severely, consuming much of the surface organic layer. Comparison of pre‐ and postfire tree composition in the burned stands indicated that the expected trajectory of black spruce self‐replacement was typical only at moist sites that burned with low fire severity. At severely burned sites, deciduous trees dominated the postfire tree seedling community, suggesting these sites will follow alternative, deciduous‐dominated trajectories of succession. Increases in the severity of boreal fires with climate warming may catalyze shifts to an increasingly deciduous‐dominated landscape, substantially altering landscape dynamics and ecosystem services in this part of the boreal forest.