Oligotrophic boreal forests with lichen dominated field layer are important winter pastures for reindeer (Rangifer tarandus). In addition to the lichen dominated, drier sunlit habitats, these forests may also have moister shaded habitats with varying moss abundance. Mosses, in turn, control vascular plant-soil interactions, yet they all can also be altered by reindeer grazing. We determined how two decades of reindeer exclusion affects feather moss (Pleurozium schreberi) depth, and the accompanying soil N dynamics, plant foliar N and stable isotopes of N and C in two contrasting habitats of an oligotrophic Scots pine forest.

The study species were pine seedling (Pinus sylvestris), bilberry (Vaccinium myrtillus), lingonberry (V. vitis-idaea) and feather moss. We found that moss carpet was 80% deeper after excluding reindeer both in shaded and sunlit habitats. In the humus horizon, the proportion of heavy N isotope increased due to exclusion in the sunlit habitats and, in the mineral soil, exclusion also increased inorganic N in both habitats. These soil responses were correlated with moss depth. Foliar chemistry did not respond to reindeer exclusion and varied solely due to habitat but depending on species identity.

We conclude that despite strong reindeer grazing-induced shifts in mosses and subtler shifts in soil N, the N dynamics of vascular vegetation remain unchanged. These indicate that plant-soil interactions are resistant to shifts in grazing intensity, a pattern that appears to be common across boreal oligotrophic forests.

Reference: Väisänen M, Tuomi M, Bailey H, Welker JM. Plant and soil nitrogen in oligotrophic boreal forest habitats with varying moss depths: does exclusion of large grazers matter? Oecologia https://doi.org/10.1007/s00442-021-04957-0