Alaska's Rainforests Stayed Surprisingly Stable Through Extreme Temperature Swings

When researchers studied trees growing next to a glacier in Alaska, they expected to find that wild temperature swings had thrown the forest into chaos. Instead, they found the opposite — and the discovery may change how scientists think about the future of temperate rainforests worldwide.

Sea lions on a rock near a living forest in Glacier Bay National Park and Preserve. Image credit: NPS

Scientific projections suggest that if current trends continue, Glacier Bay National Park in Alaska could feel as temperate as coastal Washington State does today — a climate shift of 900 miles in roughly a century. Glaciers would retreat, fires might become common, and new wildlife would arrive. For long-lived, stationary organisms like trees, that kind of change raises urgent questions. A team of researchers led by Ben Gaglioti found a way to look for answers without waiting a century to find them.

A Glacier as a Natural Thermostat

About 27 percent of Glacier Bay National Park is covered by more than 1,000 glaciers, many of them sitting alongside old-growth, temperate rainforest. Cold winds flowing off large masses of ice cool the forests at their edges. That cooling effect gave Gaglioti and his colleagues an idea: those forests had already lived through dramatic temperature changes as glaciers advanced and retreated, and the trees had recorded those changes in their rings.

The idea took shape during a June 2018 backpacking trip to the remote Outer Coast region of Glacier Bay. Camping on the strip of gravel separating La Perouse Glacier from the nearby old-growth forest, the team felt the glacier's chilling effect constantly. On a clear night, without the usual cloud cover of Southeast Alaska, temperatures dropped sharply — enough that Gaglioti regretted packing only a summer sleeping bag.

Geologist Dan Mann was on the trip, and over breakfast the two began discussing whether the advance and retreat of La Perouse Glacier had measurably altered air temperatures in the nearby forest — and whether the trees had recorded those changes in their rings.

Six Years of Data Collection

Turning the idea into science took six years. The team installed a network of sensors recording hourly air temperatures along a transect one kilometer long, running perpendicular to the glacier's ice margin. The data showed that the glacier cooled the surrounding area within about 600 yards of the ice edge, most strongly in summer. Sites within 100 yards of the margin experienced summers around 3–4°C (7–9°F) colder than sites 1,000 yards away.

To reconstruct the glacier's position over the past several centuries, the team used multiple sources. Historical accounts helped — including a description by John Muir, who capsized his dory near La Perouse Glacier during the Harriman Expedition in summer 1899 and hiked around the ice margin to observe the forest. They also studied a "ghost forest": trees that the advancing glacier had buried under gravel during the Little Ice Age, the cold period between 1300 and 1900. As the glacier later retreated, the gravel eroded away, exposing the preserved trees. By matching the ring-width patterns of those dead trees against living trees of known age — a technique called cross-dating — the team built a precise timeline of glacial advance and retreat since 1830.

John Muir as a young man. Muir overturned his boat near La Perouse Glacier while on the Harriman Expedition in 1899. He hiked around the ice margin and wrote a description of the La Perouse forest. Image credit: NPS

The reconstruction showed that after sitting well away from the living forest before 1830, the glacier began advancing. Its margin came within 600 meters of the living forest around 1850, and within about 50 meters a few years before Muir's 1899 visit. After minor retreats and advances, the glacier began retreating in earnest around 1950, with retreat accelerating over the last 20 years.

Combining the sensor data with the glacial timeline, the team estimated that the rates and magnitudes of summer temperature change in the La Perouse forest were three to four times greater than those experienced by other Alaskan forests. Over the last 50 years, the La Perouse forest likely experienced one of the fastest rates of warming on the planet — comparable to what scientists predict will occur in other forests during the coming century.

The Portfolio Effect in the Trees

The team then measured tree rings across the five conifer species growing in the La Perouse forest to see how the forest as a whole had responded to those temperature extremes. The results were unexpected.

When climate changes were relatively small — before the glacier was close enough to amplify them — the trees of different species grew in near-synchrony, and the forest's overall growth rate swung widely with regional temperatures. But when the glacier was close enough to accentuate temperature trends, the growth rates of different species diverged. Their boom-and-bust responses canceled each other out, stabilizing forest-wide growth rates.

This is what researchers call the "portfolio effect" — the same principle behind diversifying a financial portfolio to reduce risk. A colleague, Sean Brennan, had documented the same phenomenon in salmon populations in Southwest Alaska: when some creek populations had boom years, others went bust, keeping the watershed's overall salmon numbers stable from year to year. In the La Perouse forest, tree species diversity appeared to play the same stabilizing role, but only when climate swings were large and fast.

As Markus Stoffel, a global change scientist at the University of Geneva, put it: "These types of natural experiments provide insight into how forest ecosystems respond to the kind of rapid and radical temperature changes we can expect in the future."

Martin Hutten, a terrestrial ecologist at Glacier Bay National Park, noted that the research "observed resilience to rapid, long-term temperature change in a pristine forest," and that studying these climate gradients "can help us predict how diversity and ecosystem services in parks may respond to climate change in the future."

What Comes Next

The findings are not a guarantee of safety for Alaska's forests. The portfolio effect appeared only under conditions of large, fast climate change — and its limits are not yet known. The team plans to replicate the study in other glacier-adjacent forests, including those in Wrangell–St. Elias and Olympic National Parks, as well as in protected areas in Switzerland and New Zealand. One key question: do forests with fewer tree species lose the portfolio effect, making them more vulnerable to accelerated climate change? Another: can individual trees with diverse growth responses produce the same stabilizing effect even in a monoculture forest?

Chugach National Forest in Alaska is one of several other temperate rainforests that lie adjacent to glaciers. Image credit: U.S. Forest Service / Marion Glaser

The goal is to identify which national park forests are likely to be resilient as climate extremes intensify — and which may not be.

Sources

This article is based on "One Way to See the Future of Alaska's Unparalleled Forests: Look at Their Past," published by the National Park Service in Park Science magazine, Volume 39, Number 1, Winter 2024–25; it is a work of the United States government and is in the public domain.