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.

¹⁵N enrichment of leaves from Betula pubescens, Vaccinium vitis-idaea, Vaccinium myrtillus and Empetrum nigrum after addition of ¹⁵N injected directly into treeline forest soils in Abisko Sweden. Stable isotope analysis was conducted on milled foliar samples using EA-IRMS (Elemental Analysis 161- Isotope Ratio Mass Spectrometry).

The data has been analysed using R Version 4.0.2 with packages including drm, SSasymp and aomisc. 

Data files are provided as Excel .csv or .xlsx and code script files are provided as .R files which can be opened in R Studio.