The change from grasses and sedges to larger, woody-stemmed shrubs, called shrubification, has already had an impact on caribou herds which normally feed on grass, herbs, and lichens.
There have been studies on the socio-economic impacts of shrubification on northern Indigenous cultures, for whom the caribou hunt is important, including a study published in April 2017 that found the changes to vegetation put the animal’s food sources at risk.
University of Minnesota study
With all that scientists do know about the changing Arctic tundra, pinpointing a cause for the shrubification has been difficult. However, new UMN research published in Environmental Research Letters last week found that regardless of soil fertility or rainfall amounts, there was only one variable that determined shrub growth – the temperature in June.
“It was a surprising result,” said Daniel Ackerman, a Ph.D. candidate in the Department of Ecology, Evolution, and Behavior in the College of Biological Sciences. Ackerman and his research team traveled to the tundra of northern Alaska to investigate why the habitat was changing.
“Other variables, including temperatures during the rest of the growing season in July and August, barely had an impact on shrub growth,” Ackerman added.
The science of dendroecology, the study of annual growth increments in woody plants, played a role in the research. Just like trees at lower altitudes, shrubs in the tundra also form concentric rings around their stems each growing season, hence the familiar term, “tree rings.”
Ackerman’s team collected hundreds of shrub stems across a broad range of soil conditions. They measured the size of the annual rings in each sample, a formidable task seeing as the shrubs, Salix pulchra, can contain more than 50 rings in a stem with the diameter of a pencil.
Salix pulchra, commonly known as the diamondleaf willow or tealeaf willow, is a member of the willow family and is a dominant species on the tundra in many areas and is expanding rapidly, say the authors. Samples of the shrub were collected at six sites throughout the North Slope of Alaska.
Sites spanned four landscapes that varied in time since glaciation and hence in soil properties, such as nutrient availability. The team painstakingly analyzed 19,624 annual growth ring measurements through a microscope, creating a record of historic shrub growth across northern Alaska.
The researchers then compared their growth records to recorded climate observations, particularly precipitation, temperature, and solar radiation data. What they found is very interesting. Ring growth was remarkably coherent among sites and responded positively to the mean June temperature.
Additionally, shrubs at all sites exhibited diminishing marginal growth gains in response to increasing temperatures, indicative of alternative growth-limiting mechanisms in particularly warm years, such as temperature-induced moisture limitation.
“Our new understanding of the link between June temperature and shrub growth means that we can expect shrubification to continue throughout northern Alaska,” said Ackerman.
“With this study and others like it, we’re beginning to understand the causes of shrubification. However, we still have a ways to go in predicting its effects. It seems like larger shrubs will benefit some animals, like moose and ptarmigan, while other animals, like caribou, could be harmed.”
