Devotees of Wolverine Peak, an Anchorage mountain with a well-used hiking trail, might feel like it takes them longer than in the past to climb to the areas of open alpine tundra.
The suspicion isn’t unfounded, according to new research from scientists at Alaska Pacific University and partners at the University of Alaska Anchorage and Kenai National Wildlife Refuge.
As temperatures have inched up over the past decades, woody shrubs like alders and willows have spread steadily higher in elevation in the Chugach Mountains and Kenai Mountains, according to the scientists’ calculations and analysis.
Anchorage’s Wolverine Peak is a classic example, said APU biology and mathematics professor Roman Dial, leader of the study and the lead author of a newly published paper in the journal Global Change Biology.
“I’ve lived here for 20 years and I’ve watched that area get shrubbier and shrubbier,” Dial said.
Shrubs creeping up
Dial and his research partners found that shrubs — multistem plants like alders and willows — have advanced at a rate of 1.3 meters per year in the Chugach Mountains. In the Kenai Mountains farther south, where maritime influences may be greater and where warming has been faster, the shrubs have advanced at rate of 2.8 meters per year.
The study uses analysis of local temperature trends and site photographs that date back to the middle of the 20th century. For the Chugach Mountains, the study period was 1972 to 2012; for the Kenai Mountains the study period was 1950 to 1996.
While much scientific attention has focused on the spread of plants into higher-latitude regions of the far north, the Chugach-Kenai study focuses instead of the spread into higher altitudes.
The shrub spread is not exactly linear, Dial said. Rather, it’s a patchy spread, with clumps of alders and willows growing at ever-higher points on the mountains, he said.
“It’s not like a piece of toast and you’re dipping it in milk. It’s more like the way that mold spreads on your piece of toast,” he said.
Different effect on trees
That means the alpine tundra is getting squeezed into smaller spaces at higher elevations on mountains, a sign of trouble for Dall sheep, animals dependent on such open alpine terrain, Dial and his research partners said in their study.
Not spreading as much in elevation were trees, which differ from shrubs because they have single main trunks, or occasionally dual trunks. The study found that trees advanced in elevation at rates ranging between 0.1 meters and 1.1 meters per year, depending on location.
Shrubs grow faster and spread their seeds farther, so their quicker colonization of higher-altitude areas could be crowding out trees, Dial said. Alternatively, “there might be something else besides temperature that affects trees,” such as soil quality.
A similar trend is found at Denali National Park, where a long photographic record showing before-and-after images documents the spread of shrubs to higher elevations.
Impact on birds
A National Park Service study published in the journal Ecosphere describes shifts in bird distribution from 1995 to 2013 that reflect shifts in shrubs and trees. For birds associated with shrubs, especially those that use terrain where shrubs are scattered right at the edge of the alpine tundra, an “upward distributional shift was pervasive,” the study said.
Unlike the APU-led study, the National Park Service study did not quantify the spread of shrubs and trees. Instead, it focused on passerines — perching birds, like sparrows and songbirds — that dwell in varying types of habitat.
Species with the biggest changes in distribution were Arctic warblers, Savannah sparrows and golden-crowned sparrows, that study found.
Movement of those birds up in elevation could be part of a “novel assemblages of species,” with yet-to-be-determined effects on wildlife, said Jeremy Mizel, a Park Service biologist and the study’s lead author.
The Park Service study makes use of an extensive photographic record dating back to the early 20th century. Those photographs show a long-term evolution in the plant life at Denali.
In some places, the changes are obvious to longtime park denizens, even if they are not scientists, said Carl Roland, a Park Service botanist and co-author of the study. The plant transformation can be seen from the windows of tour buses that take visitors through the park, for example, he said. “Bus drivers certainly talk about the degree to which it has changed over the years,” he said.
But in some parts of the park, little vegetation change has occurred over the past decades, he said.
“It is really site dependent,” he said.
As with the Chugach and Kenai mountain sites, shrubs are outpacing trees in their expansion. But the relationship is not quite the same at Denali as at the more southern sites, said Roland, who pointed out the climate north of the Alaska Range is drier than that south of the range.
‘Lush, dense shrub cover’
Compared to Denali, conditions in the Chugach and Kenai mountain areas encourage faster growth, resulting in “lush, dense shrub cover,” he said. That leaves less sunlight available at the higher-elevation points for new trees to grow, he said.
Dial said shrub expansion into higher elevations is evident at other subarctic sites in Alaska. One example is the Lake Clark National Park region, where he has noticed big changes, he said. “Those alders are just growing like gangbusters,” he said. “In some places, they’ve grown right up to the bare rock.”