"We really have very little direct evidence of what has been happening in northern Siberia," Gerald Frost of the University of Virginia, US, told environmentalresearchweb. "We exploited a little-used archive of declassified, Cold War-era satellite photography from the 1960s to quantify recent changes in vegetation in Siberian tundra."

Frost and colleagues from the University of Alaska, US, Lomonosov Moscow State University, Russia, and the Siberian Branch of the Russian Academy of Sciences compared the satellite images with modern photos to track changes in shrub abundance as well as conducting field studies. They found that the recruitment of tall shrubs is facilitated by geomorphic processes in patterned-ground, "a widespread and visually striking" feature of permafrost landscapes.

"One common form of patterned ground consists of small, barren patches separated from one another by a metre or two in very regular, geometric patterns," said Frost. "In the landscapes that we studied, alder shrubs – a tall shrub that is more common in the boreal forest – colonize the bare patches and quickly overtop the surrounding tundra."

The researchers discovered that alders have increased substantially in patterned-ground areas since the 1960s, despite a lack of summer warming in recent decades, but not in other parts of the landscape. They say it is critical to understand these differential responses, in order to predict what tundra landscapes will look like in coming decades.

"Shrub recruitment in patterned ground also initiates the development of shrublands in which individual alders are regularly spaced from one another – a phenomenon that is commonly seen in other parts of the Arctic," said Frost. "This phenomenon has previously been explained by intense competition for nutrients. In the areas we studied, though, regular spacing of shrubs is a legacy of initial patterns of recruitment on patterned-ground features."

Since patterned-ground landscapes are widespread in northwest Siberia, the researchers say the area will likely be a "hotspot" of shrub-induced biome shifts. "When tall shrubs invade tundra areas, what's left behind is not really tundra anymore – the spread of tall shrubs not only changes the structure of vegetation, it also impacts permafrost temperatures, carbon balance, and energy exchange between the land surface and the atmosphere," said Frost.

Now the team is focusing on seasonal changes to permafrost temperatures following shrub expansion, as well as using satellites such as Landsat to look at changes in tundra vegetation elsewhere in Siberia. "The Siberian forest-tundra transition is one of the largest 'ecotones' on Earth, but we know relatively little about recent changes across much of the ecotone, which is spread across a very large geographic area," said Frost.

Frost and colleagues reported their results in Environmental Research Letters (ERL) as part of the ERL Focus on Northern Eurasia in the Global Earth System: Changes and Interactions.

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