For nearly two decades, scientists have noted dramatic changes in Arctic tundra habitat. Ankle-high grasses and sedges have given way to a sea of woody shrubs growing to waist- or neck-deep heights. This shrubification of the tundra challenges animals like caribou that are adapted to low-stature arctic vegetation. Like trees at lower latitudes, shrubs in the tundra form concentric rings around their stems each growing season. A research team used these annual rings to their advantage, collecting hundreds of shrub stems across a range of soil conditions. The team's research found that regardless of soil fertility or rainfall amounts, the single variable that was by far the strongest determinant of how much a shrub grew in a given year was the temperature in June. A warmer June means faster shrub growth.
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