Little Sign of a Boreal Forest Retreat in Interior Alaska, a Two-Area Study Finds

Researchers comparing Denali National Park and Preserve with Yukon-Charley Rivers National Preserve found no evidence that conifers are thinning out in Alaska's warmer, more fire-affected forests, as recent literature had suggested. The study, by Carl Roland, Joshua Schmidt, Samantha Winder, Sarah Stehn and Fleur Nicklen (2019), is described on a National Park Service page.

Transect tapes laid out in a boreal forest monitoring plot. Image from NPS's page.

A natural experiment

High-latitude regions are warming rapidly, with major ecological effects. To understand the likely changes in Alaska's boreal forests, the team compared forest survey data from the two areas. They differ in summer warmth and in their history of fire, so together they form a useful "natural experiment".

What they expected, and what they found

Some results fit the hypotheses. Broadleaved species were more common in the warmer, more fire-affected area, and trees were more common at high elevations there.

Others did not. The team found no evidence of fewer conifers, or of more tree death unrelated to fire, as recent literature had suggested. Instead, the number of trees, their density and their variety were considerably higher in the warmer, more fire-influenced area, except in the warmest and driest places: steep, south-facing slopes at low elevation in Yukon-Charley. Spruce occurred widely even in warmer, fire-influenced areas, except under those most extreme conditions.

What it means

The researchers expect changes in tree distribution and forest structure in interior Alaska to proceed unevenly, depending on local terrain conditions such as slope, aspect and elevation, and on species, which will shift according to differences in their surrounding landscape, including fire disturbance and soil temperature. They found little support for an ongoing biome shift that would shrink boreal forest cover in the short to medium term. Cooler areas of interior Alaska may even support modest increases in forest cover as warming continues, until warming and fire become more extreme than anything the study captured.

The landscape behind the data

The paper's photographs show why terrain matters so much.

Young birch trees (Betula neoalaskana) recruited after a fire that killed an open black spruce canopy in Yukon-Charley Rivers National Preserve. Birch and other broadleaved trees were considerably more abundant in the more fire-affected study area than in Denali. NPS/Gwen Schneider.

The subalpine landscape of Godge Creek in the Charley River watershed. Permafrost under the sedge and ericaceous plants in the foreground strongly controls where trees grow; shallow thaw layers, cold and poorly drained soil support few trees, while white spruce stands in the background grow in less solidly frozen ground. A warming climate is thawing permafrost in subarctic Alaska. NPS/Rachel King.

From near the top of Kathul Mountain, warm south-facing lower slopes rise from a terrace of the Yukon River. Tan open slopes support subarctic steppe vegetation, too warm and dry for trees, which grades into open aspen forest, while deeper green mixed white spruce and birch forest grows on deeply thawed, well-drained terraces. NPS/Haig Diradourian.

Limestone barrens in the upper Funnel Creek watershed in the Ogilvie Mountains, with widely scattered white spruce near the upper elevation limit for trees. Trees grew at much higher elevations in Yukon-Charley than in Denali, even though it lies about 2 degrees of latitude farther north, probably in part because of warmer growing seasons. NPS/Benjamin Hart.

A mosaic of forest types in the upper Kandik River drainage near Johnson Gorge. Fire history, terrain and river disturbance shape the patterns, and permafrost beneath the open black spruce woodland interacts with them. Thawing with rapid warming will profoundly affect subarctic ecosystems in the coming decades, the authors say. NPS/Erin Corrigan.

Sources

• National Park Service, "Fire, Topography, and Climate Drive Variation in Alaska's Boreal Forests", summarizing Carl A. Roland, Joshua H. Schmidt, Samantha G. Winder, Sarah E. Stehn and E. Fleur Nicklen (2019), "Regional variation in interior Alaskan boreal forests is driven by fire disturbance, topography, and climate"; rewritten in hubnx's own words.