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 birch trees, compared to 1.16% in the understorey. However, mountain birch trees contributed only 31% of ecosystem leaf area index (LAI), thus there was a disproportionate allocation of added label to the birch canopy compared with its contribution to ecosystem LAI. The difference in root and mycorrhizal exploration distances and community N partitioning between mountain birch trees and understorey shrubs may confer competitive advantage to trees, which may alter plant community structures within these forests. This is particularly important considering predicted climate-driven tree and tall shrub expansion in subarctic regions, with likely consequences for ecosystem N and carbon (C) cycling, as well as for community composition and biodiversity.