Plant and soil nitrogen in oligotrophic boreal forest habitats with varying moss depths: does exclusion of large grazers matter?

Abstract

The boreal forest consists of drier sunlit and moister-shaded habitats with varying moss abundance. Mosses control vascular plant–soil interactions, yet they all can also be altered by grazers. We determined how 2 decades of reindeer (Rangifer tarandus) exclusion affect feather moss (Pleurozium schreberi) depth, and the accompanying soil N dynamics (total and dissolvable inorganic N, δ15N), plant foliar N, and stable isotopes (δ15N, δ13C) in two contrasting habitats of an oligotrophic Scots pine forest. The study species were pine seedling (Pinus sylvestris L.), bilberry (Vaccinium myrtillus L.), lingonberry (V. vitis-idaea L.), and feather moss. Moss carpet was deeper in shaded than sunlit habitats and increased with grazer exclusion. Humus N content increased in the shade as did humus δ15N, which also increased due to exclusion in the sunlit habitats. Exclusion increased inorganic N concentration in the mineral soil. These soil responses were correlated with moss depth. Foliar chemistry varied due to habitat depending on species identity. Pine seedlings showed higher foliar N content and lower foliar δ15N in the shaded than in the sunlit habitats, while bilberry had both higher foliar N and δ15N in the shade. Thus, foliar δ15N values of co-existing species diverged in the shade indicating enhanced N partitioning. We conclude that despite strong 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.

The Nitrogen Removal Ability of Salt Marsh Improved After Grazing Prohibition

“…Most of the world's research on grazing prohibition has focused on grassland and forest ecosystems and has concentrated on studies related to plant community composition, primary production, soil properties and carbon cycling (Chen et al, 2012;Chen et al, 2016;Hu et al, 2016;Väisänen et al, 2021). For example, Hu et al (2016) found that grazing prohibition increased vegetation biomass and soil carbon content by synthesizing the results of 51 sites in grasslands in China; Olofsson et al (2006) revealed that herbivore exclusion increased primary production in the long term; Moradi et al (2021) showed that soil physicochemical properties had a significant improvement after livestock exclusion.…”

Contrasting impacts of short‐ and long‐term large herbivore exclusion on understory net CO2 exchange in a boreal forest

“…Short-term exclusion of (reindeer) grazing may alleviate trampling-induced soil compaction and disturbances that are known to reduce microbial activity in forest soils (Ponder andTadros 2002, Tuomi et al 2021) and limit plant fine root growth (Väre et al 1996, Ohtonen and Väre 1998, Morneau and Payette 2000. The organic matter content is higher in the shaded habitats at our study site (Väisänen et al 2021). Therefore, we propose that the alleviation of trampling-induced disturbances by reindeer may rapidly increase microbial biomass, decomposition, fine root growth, and root respiration and, thus, C release in the shaded habitats with more resources (Väisänen et al 2021) and not in the drier sunlit habitats.…”

“…The organic matter content is higher in the shaded habitats at our study site (Väisänen et al 2021). Therefore, we propose that the alleviation of trampling-induced disturbances by reindeer may rapidly increase microbial biomass, decomposition, fine root growth, and root respiration and, thus, C release in the shaded habitats with more resources (Väisänen et al 2021) and not in the drier sunlit habitats. In addition, in the shaded habitats, the short-term grazer exclusion increased moss depth that significantly explained the magnitude of ecosystem respiration, similar to earlier studies (Goulden and Crill 1997).…”

“…that grow on top of a shallower carpet of feather mosses. Soil is classified as haplic podzol and the organic horizon is 0.5-5 cm deep, comprising litter and humus (Väisänen et al 2021). In Finland, the herding of semi-domesticated reindeer is organized into reindeer herding districts, and our study area belongs to Alakitka reindeer herding district, which had a reindeer density of 1.1-1.2 individuals per km 2 during the time of this study.…”

“…close to trees). In our study site, the shaded habitats are more fertile than the sunlit habitats as indicated by higher foliar nitrogen contents (Väisänen et al 2021). For manual CO 2 measurements, we established in total 30 experimental plots (0.5 × 0.5 m) as follows.…”