How Tree Density and Wildfire Impact Ecosystems on the Forest-Tundra Edge

Map of the study area within the state of Alaska showing locations of burned (red points) and control (blue) sampling sites.

Black line represents the Denali National Park and Preserve boundary (left side is within the Park boundary), orange lines represent major roads, and the red line represents the perimeter of the 2013 Toklat River Fire.

Pre-fire vegetation influences post-fire dynamics in permafrost-influenced treeline ecotone

The Central Alaska Network vegetation monitoring program started fieldwork in 2001 by visiting three “mini-grid” study areas and installing 25 permanent plots laid out int 5 rows of 5 plots each spaced 500 m apart (see map image). One of those study areas was located in the Wigand Creek drainage between the East Fork and main stem of the Toklat River in Denali National Park and Preserve, including areas of rolling boreal forest and tussock tundra located in the heavily permafrost-influenced Toklat basin ecoregion of the park. The following year, the program installed an additional six mini-grid study areas, including the “East Toklat” mini-grid, located 10 km south of Wigand Creek study area.

More than a decade later, during the very warm summer of 2013, the East Toklat fire was ignited by a lightning strike and burned about 34,216 acres of the Toklat Basin, including the two study areas mentioned above. The fire consumed much of the surface vegetation and changed the face of the area, instigating thawing of permafrost and other important ecological changes. The severity of the fire was variable across the area depending on weather and site conditions. The intersection of a random fire event and the network of measured permanent plots created a unique opportunity to evaluate a "natural experiment" that scientists and researchers could use to learn about the role of fire in an area experiencing rapid changes in climatic conditions.

In 2016, two crews from the NPS and Northern Arizona University revisited these burned networks of permanent plots spending almost two months all told, remeasuring the plots and initiating seedling experiments in an adjacent area to address a suite of hypotheses regarding the role of fire in Alaska’s ecosystems in an era of climate change. This work was possible because markers were placed at the center of the permanent plots and the initial crews and acquired GPS locations and extensive photographic documentation of these sites during the original measurement.

The insights gained from our work help to quantify the impacts of wildfire in a forest-tundra ecotone, which in turn inform our understanding of the rate and magnitude of possible changes in boreal-tundra landcover, its future flammability, and associated feedbacks to the global carbon cycle (and climate). We found that sites located in warm, dry landscape positions receiving more solar radiation supported a higher density of trees before the fire and then experienced greater fire severity and soil carbon emissions during the fire compared to sites in cooler and wetter portions that supported very few spruce trees in our study areas. Our findings suggest that densely forested areas within the forest-tundra ecotone are thus more vulnerable to wildfire induced losses of long-term carbon storage in the soil than are tundra areas. Forested sites also experienced the largest fire-induced changes in understory vegetation communities (as measured three years post-fire). We conclude that if forest infilling (i.e., increased tree densities) occurs throughout the forest-tundra ecotone, increases in wildfire severity and carbon emissions, and changes in understory vegetation communities are also likely to occur. However, we observed very low natural seed availability and recruitment of spruce seedlings, which indicate that infilling of tundra by tree seedlings did not occur in response to the recent fire and is unlikely to occur in the future without substantial increases in the availability of viable seeds.

Impacts of pre-fire conifer density and wildfire severity on ecosystem structure and function at the forest-tundra ecotone

Abstract

Wildfire frequency and extent is increasing throughout the boreal forest-tundra ecotone as climate warms. Understanding the impacts of wildfire throughout this ecotone is required to make predictions of the rate and magnitude of changes in boreal-tundra landcover, its future flammability, and associated feedbacks to the global carbon (C) cycle and climate. We studied 48 sites spanning a gradient from tundra to low-density spruce stands that were burned in an extensive 2013 wildfire on the north slope of the Alaska Range in Denali National Park and Preserve, central Alaska. We assessed wildfire severity and C emissions, and determined the impacts of severity on understory vegetation composition, conifer tree recruitment, and active layer thickness (ALT). We also assessed conifer seed rain and used a seeding experiment to determine factors controlling post-fire tree regeneration. We found that an average of 2.18 ± 1.13 Kg C m-2 was emitted from this fire, almost 95% of which came from burning of the organic soil. On average, burn depth of the organic soil was 10.6 ± 4.5 cm and both burn depth and total C combusted increased with pre-fire conifer density. Sites with higher pre-fire conifer density were also located at warmer and drier landscape positions and associated with increased ALT post-fire, greater changes in pre- and post-fire understory vegetation communities, and higher post-fire boreal tree recruitment. Our seed rain observations and seeding experiment indicate that the recruitment potential of conifer trees is limited by seed availability in this forest-tundra ecotone. We conclude that the expected climate-induced forest infilling (i.e. increased density) at the forest-tundra ecotone could increase fire severity, but this infilling is unlikely to occur without increases in the availability of viable seed.

Walker, X. J., B. K. Howard, M. Jean, J. F. Johnstone, C. A. Roland, B. M. Rogers, E. A. G. Schuur, K. K. Solvik, and M. C. Mack. 2021. Impacts of pre-fire conifer density and wildfire severity on ecosystem structure and function at the forest-tundra ecotone. PLOS ONE 16(10): e0258558.