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

Maria Väisänen,Jeffrey M Welker,Hannah Bailey,Maria Tuomi

doi:10.1007/s00442-021-04957-0

Maria Väisänen, Jeffrey M Welker + Show 2 more

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https://doi.org/10.1007/s00442-021-04957-0

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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.

Highlights

Across boreal and tundra biomes, mosses contribute to plant biodiversity, biomass, and productivity (Gjerde et al 2005; Nilsson and Wardle 2005; Cornelissen et al 2007) and control ecosystem characteristics (Turetsky et al 2012)
Moss depth was twofold in the shaded in comparison to the sunlit habitats, and grazer exclusion deepened moss carpet 80% compared to the grazed area
The magnitude and direction of the impacts of habitat and grazing varied depending on the ecosystem component and measured variable: for instance, the depth of moss carpet responded to both habitat and grazing, soil inorganic N responded to grazer exclusion, while foliar δ15N showed species-dependent responses to habitat

Summary

Introduction

Across boreal and tundra biomes, mosses contribute to plant biodiversity, biomass, and productivity (Gjerde et al 2005; Nilsson and Wardle 2005; Cornelissen et al 2007) and control ecosystem characteristics (Turetsky et al 2012). Mosses trap N directly from deposition, support biological N fixation and N uptake from soil (DeLuca et al 2002; Rousk et al 2013), and supply soil with litter of varying decomposability that may all influence soil N availability, organic matter pools, and, vascular vegetation N availability (van der Wal et al 2001; Bengtsson et al 2018; Chiapusio et al 2018; Philben et al 2018). Moss effects on ecosystem functions may vary due to environmental conditions such as changes in air temperature and precipitation (De Long et al 2016a; Lett et al 2020). Mosses are one of the more susceptible life forms to the ongoing global environmental changes (Elmendorf et al 2012; Fraser et al 2014; Becker Scarpitta et al 2017). Since global changes—year-round warming, changes in snow depth and changes in land use—are pronounced and rapidly occurring in the North (IPCC 2019), unravelling the interactions

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