Alaska Is Likely Becoming Woodier as Warming Reshapes Its Plant Life

Looking north to Kankone Peak, a shrubline-tundra ecotone, in Denali National Park and Preserve. NPS/Carl Roland.

Alaska's boreal forests are relative newcomers. During the glacial advances of the ice ages, forests mostly vanished from the state, replaced by open steppe-tundra, tundra, meadows and shrublands, and the forests we know were mostly absent from these landscapes before the current interglacial period, the Holocene, began about 12,000 years ago. Carl Roland, a National Park Service plant ecologist for Denali National Park and Preserve and the Central Alaska Network, uses that long history as the backdrop for a review of how today's warming is changing Alaska's plants.

Stable patterns on the land

Roland starts with a paradox. Alaska's vegetation is always in flux, with individual plants establishing, growing, reproducing and decaying, yet it also shows clear, repeating patterns that reflect temperature, precipitation, solar energy, drainage, soils and disturbance. Beside a stream, open gravelly soils with scattered colonizing plants give way to willow thickets, then stands of poplar, then spruce forest where flowing water has not disturbed the ground for many decades. Climbing a mountainside leads from forest to a shrubby treeline, then to low tundra of cushion plants, mosses, lichens and dwarfed plants, and finally to rocky high alpine ground. These patterns, Roland says, reflect the strong control that environment has over plants at high latitudes.

A long history of change

During cold phases of the Pleistocene, snow that failed to melt each summer built ice sheets that isolated Alaska from the rest of North America. With so much water locked in ice, sea levels fell and the shallow shelf between Alaska and northeastern Asia was exposed as the Bering Land Bridge, creating Beringia. In ice-age eastern Beringia, which is now Alaska, deep blankets of silt were laid down by running water and fierce winds across open, treeless ground. When the climate warmed again, trees and shrubs that had been confined south of the ice sheets moved in and displaced the low, open plant communities.

Beringia during the last glacial maximum, when Asia and North America were connected by the Bering Land Bridge. NPS map.

Roland draws two conclusions. Climate is an overarching driver of the character, structure and distribution of plant communities in the Far North, and periodic, massive change is woven into Alaska's ecosystems.

Two kinds of change

Roland writes that Alaska's climate is changing very rapidly, with warming driven primarily by heat-trapping gases from industrialized human activities, and that this will likely have profound consequences for all life there, though some species, especially those whose ranges extend farther south, may thrive. To make sense of the consequences, Roland separates two kinds of change:

The two are closely linked, because process changes that are pronounced and long-lasting eventually change structure. Roland's example is wetland conversion. As a wetland dries, its plants suffer drought stress and grow and reproduce more slowly, wetland seedlings cannot establish in periodically dry ground, and drought-tolerant species take over, so a sedge-dominated wetland can become a community of shrubs or trees. That would probably change the animals too, since insects, birds and other animals that nest in open wetlands or eat herbaceous wetland plants likely do not also use shady, well-drained forest or eat woody shrubs. Major disturbances can force structural change at once: a crown fire, floodwaters that strip a stand, or the catastrophic draining of a lake after permafrost thaw.

As wetlands dry, they gradually fill with non-wetland plants. This area was once a shallow lake, was colonized by a sedge wetland as it dried, and is now rapidly being colonized by trees and shrubs as it dries further. NPS/Carl Roland.

What researchers have observed

Roland's survey of recent studies finds Alaska's vegetation already responding in many ways. Among process-related and disturbance-related changes:

• Summer greenness trends differ across the state: some colder, moister regions are getting greener, which suggests greater productivity with warming, while some warmer, drier regions show a "browning" trend tied to heat and drought stress.
• Growing seasons are longer over very large areas, following earlier snowmelt and warmer early-season temperatures.
• Plants respond to warming soils and thawing permafrost, including local "wetting" as ice wedges degrade and changes in tree growth tied to the depth of the soil's active layer and to air temperature.
• Insect outbreaks have been widespread or severe in many areas, likely enhanced by warming, and have damaged and killed plants.

Widespread and severe insect outbreaks have recently occurred across southcentral Alaska, enhanced by climate warming. Insect damage in forests and shrublands is likely to accelerate with further warming. NPS/Sarah Stehn.

And among changes in structure and composition:

• White spruce is advancing northward into tundra in northwestern Alaska, described in the journal Nature in 2022, and warming has driven a wave of spruce seedlings on the southern coast.
• Woody plants are spreading on floodplains and other early-successional ground, including recently deglaciated sites, making vegetation taller and denser.
• Satellite imagery shows net increases in deciduous and evergreen shrubs, broadleaf trees and conifers, and decreases in moss, lichen and graminoid cover.
• Lakes and ponds are shrinking in parts of the state, shrubs are invading drying wetlands, and in the Chugach and Kenai Mountains tall shrubs are moving upslope into tundra faster than the treeline.
• Researchers predict that more fire, at shorter intervals, could convert areas long dominated by conifers into birch, aspen and balsam poplar woodland, whose huge, easily dispersed seed crops thrive in recently burned terrain.
• Botanists forecast the gradual displacement of many endemic plants and wildflowers unique to eastern Beringia, which now persist in cold, open, high-elevation sites, by larger boreal species that will likely outcompete them.

Taken together, Roland writes, Alaska has likely become a much woodier place as the climate has warmed.

Resilience and ripple effects

The changes are uneven, and some areas, species and plant communities are more sensitive than others. The dominant boreal trees are widespread and tolerate a wide range of conditions, and well-insulated, undisturbed permafrost can resist structural change, but Roland notes that such refuges are not permanent given projections that permafrost will disappear from large areas in coming decades.

Because plants are the base of food webs and define habitat for most animals, Roland expects cascading effects, some hard to predict. Tundra and neighboring spruce forest have markedly different insects and birds, and moose are common in the boreal lowlands while caribou are rarely found in dense forest in any numbers. One example comes from far northwestern Alaska, where aerial images showed a proliferation of beaver ponds between 1949 and 2019, consistent with Indigenous knowledge and local observations. Researchers estimated it likely that 10,000 beaver ponds were built in tundra areas in recent decades, in step with warming. The shallow water absorbs heat and likely thaws permafrost, releasing carbon as peat decomposes, and a likely contributing factor is the spread of woody forage as shrubs expand. A warming world prompts shrub growth, which may draw in beavers, whose engineering may worsen permafrost thaw.

Roland calls these some of the first results of a huge experiment and says the changes described would likely have been unfathomable even to experienced ecologists working in Alaska 50 years ago. The review ends with a question: what further changes will Alaska see in the next fifty years that cannot be imagined now?

Sources

• Carl Roland (National Park Service, Denali National Park and Preserve and Central Alaska Network), Alaska's Changing Vegetation Processes and Patterns: Plant Responses to Unprecedented Levels of Warming in the Far North, Alaska Park Science 22(1), 2023, pp. 34-47, National Park Service; rewritten in hubnx's own words.