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

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.

Resistance of the boreal forest to high burn rates

“…Conversely, in some parts of the boreal forest, the abundance of aspen (Populus tremuloides Michx. ), a broadleaved taxon less flammable than conifers, has been shown to increase after severe fires (14). As large fires are generally more severe than small ones (19), models predict than aspen abundance will increase in a future with more fires, thus potentially strengthening the negative feedback on burn rates (15,41,42).…”

“…Models suggest this biome will experience rapid temperature increases during the 21st century, with a potential 30-500% increase in burn rates (6)(7)(8)(9). Both fire size and the frequency of large fire years are expected to increase (8,10,11), with a cascading effect on ecosystem dynamics (12)(13)(14)(15)(16) and carbon storage (17)(18)(19)(20)(21).…”

“…Large and severe fires may alter seed sources and seedbeds for tree recruitment, which may trigger shifts to novel forest ecosystems (14,22). In addition, shortened fire-free intervals (FI) may shift young, sexually immature tree populations to faster-maturing species (12,(23)(24)(25).…”

Recent burning of boreal forests exceeds fire regime limits of the past 10,000 years

“…Our fire reconstructions from the YF reveal that the boreal forest can sustain a high-severity fire regime for centuries under warm and arid conditions, but they also suggest that vegetation feedbacks attenuated the climate-fire relationship over these timescales in the past. Such dynamics have potentially dramatic ecological impacts (9,48) that could interact with or override the direct effects of shifting climate and fire regime. Estimating the longterm trajectory of boreal forest ecosystems thus requires understanding if and how these feedbacks will progress under scenarios of future change.…”

Quantifying ecological drivers of ecosystem productivity of the early‐successional boreal Larix gmelinii forest

“…As a result, young forests are increasing throughout the boreal forest in response to greater fire activity. Because patterns of ecosystem structure and function in these young forest can strongly affect the future successional trajectory and biogeochemistry (Johnstone et al 2010, Donato et al 2012, better understanding of the ecological drivers of post-fire ecosystem structure and function can help us understand how forest succession and landscape patterns will change in future climates.…”