Moss Mediates the Influence of Shrub Species on Soil Properties and Processes in Alpine Tundra

<div><p>In tundra ecosystems, bryophytes influence soil processes directly and indirectly through interactions with overstory shrub species. We experimentally manipulated moss cover and measured seasonal soil properties and processes under two species of deciduous shrubs with contrasting canopy structures, <i>Salix planifolia pulchra</i> and <i>Betula glandulosa-nana</i> complex. Soil properties (seasonal temperature, moisture and C:N ratios) and processes (seasonal litter decomposition and soil respiration) were measured over twelve months. Shrub species identity had the largest influence on summer soil temperatures and soil respiration rates, which were higher under <i>Salix</i> canopies. Mosses were associated with lower soil moisture irrespective of shrub identity, but modulated the effects of shrubs on winter soil temperatures and soil C:N ratios so that moss cover reduced differences in soil winter temperatures between shrub species and reduced C:N ratios under <i>Betula</i> but not under <i>Salix</i> canopies. Our results suggest a central role of mosses in mediating soil properties and processes, with their influence depending on shrub species identity. Such species-dependent effects need to be accounted for when forecasting vegetation dynamics under ongoing environmental changes.</p></div

Measurements of soil processes in the experimental plots.

<p>Soil processes (winter litter decomposition (mg day^-1; <b>a</b>), summer litter decomposition (mg day^-1; <b>b</b>) and soil respiration (μmol CO₂ m^-2s^-1; <b>c</b>), in plots with an intact moss layer (control, light grey bars) or with moss experimentally removed (removal, dark grey bars) under <i>Betula</i> and <i>Salix</i> shrubs. Means and SE are shown.</p

Snow measurements in the experimental plots during winter 2012.

<p>Snow depth (cm; <b>a</b>) and snow cover duration (days; <b>b</b>) values (mean ± SE), during winter 2012 under the two focal shrub species (<i>Betula</i> and <i>Salix</i>).</p

Predicted responses of soil parameters under two dominant expanding shrubs in North American alpine tundra (<i>Betula</i> and <i>Salix</i>), when moss cover is intact (A) and artificially removed (B).

<p>Because of the insulating properties of moss, we hypothesized that when moss cover is intact (A) the effects of mosses will dominate (orange arrows), leading to similar responses in soils irrespective of shrub identity. However, when the active layer of mosses is removed (B), we expected differences in shrub canopy properties to drive soil responses: taller and sparser canopies (<i>Salix</i>) will ameliorate conditions for soil decomposition processes, while dense canopies (<i>Betula</i>) will maintain cooler soils in summer, slowing down soil processes.</p

Measurements of soil properties in the experimental plots.

<p>Soil properties (winter soil temperature (°C; <b>a</b>), summer soil temperature (°C; <b>b</b>), soil moisture (volumetric water content; <b>c</b>), total soil C (%; <b>d</b>), total soil N (%; <b>e</b>), and soil C:N ratio (<b>f</b>)) of plots with an intact moss layer (control, light grey bars) or with moss experimentally removed (removal, dark grey bars) under <i>Betula</i> and <i>Salix</i> shrubs. Means and SE are shown; small-case letters indicate statistically significant differences between treatments (shrub identity and moss removal combinations).</p

Effects of moss treatment (control or moss removal) and the identity of the overstory shrub species (<i>Betula</i> or <i>Salix</i>) on soil properties and soil processes derived from Linear Mixed Models (LMM).

<p>Effects of moss treatment (control or moss removal) and the identity of the overstory shrub species (<i>Betula</i> or <i>Salix</i>) on soil properties and soil processes derived from Linear Mixed Models (LMM).</p

Characteristics of the plots in relation to the two shrub species, <i>Betula</i> and <i>Salix</i>.

<p>Characteristics of the plots in relation to the two shrub species, <i>Betula</i> and <i>Salix</i>.</p

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Individual Based Model Simulations

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Data on Dummy Caterpillars

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