Trees out-forage understorey shrubs for nitrogen patches in a subarctic mountain birch forest

Nitrogen (N), acquired by roots and mycorrhizal fungi and supplied to plant foliage, is a growth-limiting nutrient at the subarctic treeline. Due to this limitation, interspecific competition and acquisition of N is an important control on plant community composition and distribution. The ability of trees and shrubs to access N shapes community dynamics at this ecotone undergoing species range shifts and changes in primary productivity driven by climate change. Using 15N soil labelling, we investigate the fate of soil inorganic N, and spatial distances over which trees and understorey shrubs access soil N, in a treeline forest. 15N was injected into soil rooting zones, and foliar samples were collected from trees between 1 and 50 m away and understorey shrubs between 0.5 and 11 m away from labelled soil. The 15N label was found in mountain birch trees up to 5 m and in understorey shrubs up to 2 m away from labelled soil. We estimate that 1.27% of pulse-derived N was found in foliage of ...