Arctic shrubs spreading slower despite climate change | Polarjournal
Large parts of the Arctic tundra are characterized by more or less dense shrub vegetation. Photo: Miko Fox Photography via Flickr (CC BY-NC-SA 2.0)

Vegetation in the Arctic has to withstand very harsh conditions, with all kinds of adaptations against cold, drought and strong winds. More favorable conditions, such as those brought by climate change, would drive the expansion of Arctic plants. However, that is limited by fires and seed dispersal, according to a new study by an Ohio State University research team.

Much of the tundra is now covered with shrubs, and based on previous studies, it was previously assumed that these dense shrubs would take over about 39 percent of the area in the Arctic not currently covered with shrubs as warming progressed. The new study, however, could not confirm this assumption. Analysis by Yanlan Liu, assistant professor of geosciences at Ohio State University and lead author of the study, and her team found that flora could spread to only 25 percent of the tundra by 2100.

Using historical growth patterns of shrubs in combination with various environmental variables such as precipitation, elevation, and days with temperatures above 5 degrees Celsius, the research team determined what might happen in the future. While shrub expansion was positively correlated with environmental suitability – the likelihood of a species surviving in a particular location – in past decades, this is no longer the case today.

With climate change in the Arctic, shrubs like these will spread less quickly than previously thought. Photo: Michael Wenger

So environmental conditions are now less crucial for the spread of shrubs and instead factors such as seed dispersal and fire have become more important. Gravity, animals, wind, or even ocean currents and ice floes play a role in seed dispersal, carrying seeds to areas where shrubs do not grow.

“Dispersal and fire are much more important than other environmental conditions, in that even if a given place is warm enough, shrubs won’t necessarily grow in that region,” Liu says. “Instead, it’s limited by whether seeds can arrive at that location, or whether the seed bed or the soil condition is nutritious enough to sustain shrub growth.”

Fires generally improve soil nutrient availability and seed germination, although fire can kill seeds and seedlings in the short term, Liu adds.

Over the past few decades, the researchers have noticed a significant change in vegetation distribution in the Arctic, which can be attributed to the rapid warming of the region. Unlike previous studies, which mostly relied on observations and field models, the team’s current work used high-resolution satellite imagery from NASA’s Arctic-Boreal Vulnerability Experiment (ABoVE) to monitor the spread of shrubs in Alaska and much of western Canada between 1984 and 2014.

In regions where no or hardly any shrubs grow today due to environmental conditions, denser vegetation could spread by 2100. Photo: Julia Hager

Using this data, they were able to estimate the spread of the shrubs for the years 2040, 2070, and 2100. They found that the future spread would not follow the expected warming patterns. Thus, previous studies had overestimated future shrub growth. According to Liu, this discrepancy could only be explained by taking into account factors such as seed dispersal and fire.

Accurately monitoring and predicting the spread of shrubs is particularly important, she says, because of their role in the global energy and carbon budget. Liu adds that her research opens up whole new avenues for studying how vegetation spreads elsewhere on Earth.

“This study is the very first step to look at large-scale shrub expansion in Arctic-Boreal regions at a high resolution,” Liu says. “Our data-driven analysis explains what happened, but the next step is to explain why.”

Next, Liu plans to use dynamic vegetation models to simulate what the distribution and structure of vegetation will look like in the future and then combine their observations to make the predictions more accurate. 

Julia Hager, PolarJournal

Link to the study: Yanlan Liu, William J. Riley, Trevor F. Keenan, et al. Dispersal and fire limit Arctic shrub expansion. Nature Communications, 2022; 13 (1) DOI: 10.1038/s41467-022-31597-6

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