On the tundra of northern Alaska, spring is coming earlier and plants are sprouting earlier, but migrating caribou are not missing out on the high-quality food plants they need in spring and early summer, new research shows.

A study led by the U.S. Geological Survey found that the many plants that caribou eat still have high nutritional levels when the animals arrive at their summer grounds, even though the thaws and plant growth start much earlier than they did three decades ago.

A bull caribou in velvet. (Bob Hallinen / Alaska Dispatch News)
A bull caribou in velvet. (Bob Hallinen / Alaska Dispatch News)

"We observed long-term changes in temperatures, timing, and the length of the growing seasons, but found little support for a mismatch between caribou and the plants they consume," Dave Gustine, lead author of the study and a former USGS wildlife biologist who is now with the National Park Service, said in a statement.

The study is published in the journal PLoS One.

Unlike some other migratory animals, like certain birds that fly to the Arctic from long distances, caribou are not suffering from what is known as a "phenological mismatch," a disruption in timing of migration and food supplies, the study found. Some animals are dependent on the plants' first growth, which can be the most nutritious, or on the season's first emergence of insects. Migratory species hurt by a phenological mismatch arrive at the summer grounds too late to eat the habitat's highest-quality food. Past USGS research shows that some Arctic-breeding birds are migrating earlier as the climate warms, possibly avoiding such mismatches.

For caribou, the plants used for food do not have a loss in nutritional quality, even if they start to grow earlier, before the migratory herd arrives, the study found.

It compared plant conditions, including the important quality of nitrogen content, measured in the years for 2011 to 2013 and compared those to qualities recorded in detailed studies done in the 1970s by the Alaska Department of Fish and Game. The researchers found no dropoff in the nitrogen in the forage plants used by caribou in their calving and nursing season, a life stage that has not changed in its timing.

The study evaluated caribou-eaten plants along a 125-mile section of the Dalton Highway on Alaska's North Slope, an area important to the Central Arctic Caribou Herd. The study area spanned three different types of Arctic ecosystems — the coastal plain, the Brooks Range foothills and the rugged mountains within the northern Brooks Range. The three ecosystems have similar winter temperatures, but summer temperatures in the foothills and mountains are higher than those on the coastal plain. The coastal plain is warming the fastest, the study said.

The growing season has expanded in all three eco-regions. The season starts earlier and ends later, making it 15 to 21 days longer than it was in 1970, the study says.

Future conditions might be different as the Arctic climate continues to warm, the study cautions. And future research might consider plants' nutritional quality at a different time of year — late summer and autumn, when female caribou are storing energy to carry them through pregnancy and future calving, the study says.

Correction: An earlier version of this story incorrectly said the caribou study was conducted along a 200-mile section of the Dalton Highway. It was actually a 200-kilometer, or 125-mile, section of the highway.