Vowles Tage, Gunnarsson Bengt, Molau Ulf, Hickler Thomas, Klemedtsson Leif, Björk Robert G
Department of Biological and Environmental Sciences University of Gothenburg Box 46140530 Göteborg Sweden.
Senckenberg Biodiversity & Climate Research Centre Bik F Senckenberganalge 25D-60325 Frankfurt Germany.
J Ecol. 2017 Nov;105(6):1547-1561. doi: 10.1111/1365-2745.12753. Epub 2017 Mar 16.
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. . 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.
北极生态系统变暖最显著的影响之一是树线以上灌木的扩张。然而，先前的研究发现，驯鹿会影响植物群落对变暖的反应，并抑制苔原的灌木化。我们重新考察了瑞典高山地区南部和北部边界的放牧（环境）和未放牧研究地块（围栏），以研究1995年至2011年间，在两种植被类型（灌木石南和山地桦树林）中，长期放牧效应以及植被对气温升高的变化。在灌木石南地的地被层，常绿矮灌木的覆盖度从26%增加到了49%，但不受放牧影响。落叶矮灌木和高灌木随着时间的推移也有显著增加，尽管增幅较小。在桦树林地，常绿矮灌木的增加情况类似（20% - 48%），但落叶高灌木并没有像在灌木石南地那样随着时间呈现出一致的增加。灌木石南地围栏内的灌木层覆盖度和高度显著更高，但在物种丰富度或多样性方面未发现显著的处理效应。除一个地点外，所有围栏内7月土壤温度和生长季解冻度日（TDD）都更高，并且在灌木石南地，灌木层平均高度与土壤TDD之间存在显著的负相关。这项研究表明，在斯堪的纳维亚山脉，树线上下的灌木都在迅速扩张。高大的落叶灌木在覆盖度和高度方面都从放牧围栏中显著受益，这反过来又降低了夏季土壤温度。然而，我们研究地点植被的总体变化是常绿矮灌木显著增加，且不受放牧影响。由于常绿矮灌木增加以及更难分解的植物凋落物的影响可能在一定程度上抵消落叶高灌木增加带来的一些影响，食草动物对灌木相互作用的影响对于塑造北极灌木扩张及其相关生态系统效应可能具有至关重要的意义。
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