With climate change, shrubs and trees expand northwards in the Subarctic

Mosses in subarctic tundra are colonized by bacteria that fix atmospheric nitrogen (N₂), and together, they can contribute 50% to total ecosystem N input. Despite this key role, the effects of climate warming and increased litter input as a result of shrub expansion on N₂ fixation in mosses are ambiguous.

Assistant professor Kathrin Rousk, Department of Biology, explains, "to aid in predicting the role of moss-associated N₂ fixation in a warmer, future climate, we quantified N₂ fixation throughout the snow-free period in subarctic tundra. We used data from a field experiment in Northern Sweden, in which climate change was simulated with open top chambers to increase the air and soil temperature, and with addition of plant litter from willow and birch-shrubs.

The results show that N₂ fixation was highest in the warmed and in the birch litter addition plots (around 3 kg N ha-1 yr-1), while the willow litter additions lead to decreased N₂ fixation rates (less than 2 kg N ha-1 yr-1)

Field experiment in tundra vegetation in Northern Sweden. Open top chambers simulate future warming of soil and plants caused by climate change, and leaf litter is added to simulate enhanced litterfall caused by expansion of birch trees and willow shrubs (Photo: Anders Michelsen)

Kathrin Rousk concludes, 'Warming will lead to increased N₂ fixation rates in mosses, while the consequences of further shrub expansion will depend on the dominant shrub invading: the expansion of willow will likely limit the N input via N₂ fixation, whereas a predominance of birch shrubs will increase N₂ fixation and with that, N supply to the ecosystem."

The amount of N₂ that is fixed is important because N is an essential nutrient for plant growth and is available only in limited supply in arctic ecosystems. Nitrogen availability influences how much CO2 plants are able to acquire through photosynthesis, and hence it impacts the carbon balance. Changes in N₂ fixation due to global warming will alter N input to arctic ecosystems with significant consequences for plant growth.

Micrograph of moss leaves with insert micrograph of a chain of blue-green algae (cyanobacteria) (red) on a c. 0.1 mm long fragment of a moss leaf (green). The micrographs of the cyanobacteria, which fix free N₂ the air, are taken in a fluorescence microscope with a green filter (200x magnification). Micrographs by Kathrin Rousk.