Author: Hannah Hoag / Source: Science News
The Chugach people of southern Alaska’s Kenai Peninsula have picked berries for generations.
Tart blueberries and sweet, raspberry-like salmonberries — an Alaska favorite — are baked into pies and boiled into jams. But in the summer of 2009, the bushes stayed brown and the berries never came.For three more years, harvests failed. “It hit the communities very hard,” says Nathan Lojewski, the forestry manager for Chugachmiut, a nonprofit tribal consortium for seven villages in the Chugach region.
The berry bushes had been ravaged by caterpillars of geometrid moths — the Bruce spanworm (Operophtera bruceata) and the autumnal moth (Epirrita autumnata). The insects had laid their eggs in the fall, and as soon as the leaf buds began growing in the spring, the eggs hatched and the inchworms nibbled the stalks bare.
Chugach elders had no traditional knowledge of an outbreak on this scale in the region, even though the insects were known in Alaska. “These berries were incredibly important. There would have been a story, something in the oral history,” Lojewski says. “As far as the tribe was concerned, this had not happened before.”
At the peak of the multiyear outbreak, the caterpillars climbed from the berry bushes into trees. The pests munched through foliage from Port Graham, at the tip of the Kenai Peninsula, to Wasilla, north of Anchorage, about 300 kilometers away.
In summer, thick brown-gray layers of denuded willows, alders and birches lined the mountainsides above stretches of Sitka spruce.
Two summers ago, almost a decade after the first infestation, the moths returned. “We got a few berries, but not as many as we used to,” says Chugach elder Ephim Moonin Sr., whose house in the village of Nanwalek is flanked by tall salmonberry bushes. “Last year, again, there were hardly any berries.”
For more than 35 years, satellites circling the Arctic have detected a “greening” trend in Earth’s northernmost landscapes. Scientists have attributed this verdant flush to more vigorous plant growth and a longer growing season, propelled by higher temperatures that come with climate change. But recently, satellites have been picking up a decline in tundra greenness in some parts of the Arctic. Those areas appear to be “browning.”
Like the salmonberry harvesters on the Kenai Peninsula, ecologists working on the ground have witnessed browning up close at field sites across the circumpolar Arctic, from Alaska to Greenland to northern Norway and Sweden. Yet the bushes bereft of berries and the tinder-dry heaths (low-growing shrubland) haven’t always been picked up by the satellites. The low-resolution sensors may have averaged out the mix of dead and living vegetation and failed to detect the browning.
Scientists are left to wonder what is and isn’t being detected, and they’re concerned about the potential impact of not knowing the extent of the browning. If it becomes widespread, Arctic browning could have far-reaching consequences for people and wildlife, affecting habitat and atmospheric carbon uptake and boosting wildfire risk.
Growing greenbelt
The Arctic is warming two to three times as fast as the rest of the planet, with most of the temperature increase occurring in the winter. Alaska, for example, has warmed 2 degrees Celsius since 1949, and winters in some parts of the state, including southcentral Alaska and the Arctic interior, are on average 5 degrees C warmer.
An early effect of the warmer climate was a greener Arctic. More than 20 years ago, researchers used data from the National Oceanic and Atmospheric Administration’s weather satellites to assess a decade of northern plant growth after a century of warming. The team compared different wavelengths of light — red and near-infrared — reflecting off vegetation to calculate the NDVI, the normalized difference vegetation index. Higher NDVI values indicate a greener, more productive landscape. In a single decade — from 1981, when the first satellite was launched, to 1991 — the northern high latitudes had become about 8 percent greener, the researchers reported in 1997 in Nature.
The Arctic ecosystem, once constrained by cool conditions, was stretching beyond its limits. In 1999 and 2000, researchers cataloged the extent and types of vegetation change in parts of northern Alaska using archival photographs taken during oil exploration flyovers between 1948 and 1950. In new images of the same locations, such as the Kugururok River in the Noatak National Preserve, low-lying tundra plants that once grew along the riverside terraces had been replaced by stands of white spruce and green alder shrubs. At some of the study’s 66 locations, shrub-dominated vegetation had doubled its coverage from 10 to 20 percent. Not all areas showed a rise in shrub abundance, but none showed any decrease.
In 2003, Howard Epstein, a terrestrial ecologist at the University of Virginia in Charlottesville, and colleagues looked to the satellite record, which now held another decade of data. Focusing on Alaska’s North Slope, which lies just beyond the crown of the Brooks Range and extends to the Beaufort Sea, the researchers found that the highest NDVI values, or “peak greenness,” during the growing season had increased nearly 17 percent between 1981 and 2001, in line with the warming trend.
As the Arctic warms up, it’s getting greener, but some pockets have been going brown instead. Satellite imagery and ecologists on the ground have observed browning in the circled areas on this map.
Earth-observing satellites have been monitoring the Arctic tundra for almost four decades. In that time, the North Slope, the Canadian low Arctic tundra and eastern Siberia have become especially green, with thicker and taller tundra vegetation and shrubs expanding northward. “If you look at the North Slope of Alaska, if you look at the overall trend, it’s greening like nobody’s business,” says Uma Bhatt, an atmospheric scientist at the University of Alaska Fairbanks.
Yet parts of the Arctic, including the Yukon-Kuskokwim Delta of western Alaska, the Canadian Arctic Archipelago (the islands north of the mainland that give Canada its pointed tip) and the northwestern Siberian tundra, show extensive browning over the length of the satellite record, from the early 1980s to 2016. “It could just be a reduction in green vegetation. It doesn’t necessarily mean the widespread death of plants,” Epstein says. Scientists don’t yet know why plant growth there has slowed or reversed — or whether the satellite signal is in some way misleading.
“All the models indicated for a long time that we would expect greening with warmer temperatures and higher productivity in the tundra, so long as it wasn’t limited in some other way, like [by lower] moisture,” says Scott Goetz, an ecologist and remote-sensing specialist at Northern Arizona University in Flagstaff. He is also the science team lead for ABoVE, NASA’s…
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