Expansion of deciduous tall shrubs but not evergreen dwarf shrubs inhibited by reindeer in Scandes mountain range.

Tage Vowles,Ulf Molau,Thomas Hickler,Bengt Gunnarsson,Leif Klemedtsson,Robert G Björk,Hans Cornelissen

doi:10.1111/1365-2745.12753

Abstract

Summary One of the most palpable effects of warming in Arctic ecosystems is shrub expansion above the tree line. However, previous studies have found that reindeer can influence plant community responses to warming and inhibit shrubification of the tundra.We revisited grazed (ambient) and ungrazed study plots (exclosures), at the southern as well as the northern limits of the Swedish alpine region, to study long‐term grazing effects and vegetation changes in response to increasing temperatures between 1995 and 2011, in two vegetation types (shrub heath and mountain birch forest).In the field layer at the shrub heath sites, evergreen dwarf shrubs had increased in cover from 26% to 49% but were unaffected by grazing. Deciduous dwarf and tall shrubs also showed significant, though smaller, increases over time. At the birch forest sites, the increase was similar for evergreen dwarf shrubs (20–48%) but deciduous tall shrubs did not show the same consistent increase over time as in the shrub heath.The cover and height of the shrub layer were significantly greater in exclosures at the shrub heath sites, but no significant treatment effects were found on species richness or diversity.July soil temperatures and growing season thawing degree days (TDD) were higher in exclosures at all but one site, and there was a significant negative correlation between mean shrub layer height and soil TDD at the shrub heath sites. Synthesis. This study shows that shrub expansion is occurring rapidly in the Scandes mountain range, both above and below the tree line. Tall, deciduous shrubs had benefitted significantly from grazing exclosure, both in terms of cover and height, which in turn lowered summer soil temperatures. However, the overriding vegetation shift across our sites was the striking increase in evergreen dwarf shrubs, which were not influenced by grazing. As the effects of an increase in evergreen dwarf shrubs and more recalcitrant plant litter may to some degree counteract some of the effects of an increase in deciduous tall shrubs, herbivore influence on shrub interactions is potentially of great importance for shaping arctic shrub expansion and its associated ecosystem effects.

Highlights

In recent years, several studies have noted a shift in plant abundance and community structure in alpine and arctic areas, many of them linking these shifts to increasing temperatures (ACIA 2005; Elmendorf et al 2012b; Gottfried et al 2012)
In the field layer at the shrub heath sites, evergreen dwarf shrubs had increased in cover from 26% to 49% but were unaffected by grazing
At the birch forest sites, the increase was similar for evergreen dwarf shrubs (20–48%) but deciduous tall shrubs did not show the same consistent increase over time as in the shrub heath

Summary

Introduction

Several studies have noted a shift in plant abundance and community structure in alpine and arctic areas, many of them linking these shifts to increasing temperatures (ACIA 2005; Elmendorf et al 2012b; Gottfried et al 2012). Tall shrubs are usually the largest plant life-form and can influence biodiversity, albedo, snow cover, nutrient availability and soil temperature (Myers-Smith et al 2011). On the other hand, shading from canopies can decrease soil temperatures and active layer depths (Blok et al 2010) These changes in soil temperature, along with increases in litter input, may in turn have implications for nutrient cycling. Larger nitrogen pools and faster mineralization rates during the summer have been found in tall compared to low dwarf birch vegetation due to input of higher quality litter (Buckeridge et al 2010) and the increased winter temperatures beneath shrubs have been hypothesized to increase annual mineralization rates by 25% (Chapin et al 2005), possibly leading to positive feedback effects on primary production. An increase in shrub biomass could have fundamental effects on tundra ecosystems, by dramatically changing living conditions for a range of species as well as potentially influencing carbon and nitrogen cycles

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