WILLOW-CHARACTERISED SHRUB VEGETATION IN TUNDRA AND ITS RELATION TO ABIOTIC, BIOTIC AND ANTHROPOGENIC FACTORS, ACTA UNIVERSITATIS OULUENSIS A Scientiae Rerum Naturalium 546
|Kustantaja:||Oulun yliopisto|| |
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|Tekijät:||PAJUNEN ANU|| |
Deciduous shrubs form the tallest type of vegetation in arctic-alpine areas and are important forecosystem function. In the southern part of the Eurasian tundra zone, willows (Salix spp.) are themost common species in the shrub layer. In the alpine areas of Northern Fennoscandia, willowshrubs are characteristic to areas between tree line and treeless tundra heaths. Vertical structureand composition of willow-characterized tundra vegetation is affected by a variety of ecologicalfactors including climate and herbivory. In turn, the abundance of the willow canopy affectsunderstory species in several ways that still remain inadequately understood.
In this PhD work I describe compositional differentiation of willow-characterized vegetationby using a large data set spanning from north-western Fennoscandia to the Yamal Peninsula innorth-western Siberia. I studied environmental factors affecting willow-characterized vegetationand willow growth by using correlative analyses. The factors under investigation were latitude,distance from the sea, depth of thaw, position in the slope, industrial disturbance and reindeergrazing. In addition, I examined the relationships between the shrub biomass estimate andcomposition and species richness of understory vegetation. The effects of reindeer grazing onvegetation in an alpine forest-tundra ecotone were studied experimentally using reindeer-proofexclosures.
I found that willow-characterized vegetation is floristically variable and comprises at leasteight vegetation types. The most abundant willow thickets typically have a forb-rich understory.The growth of willow increased along with increasing summer temperatures. However the heightof willow was more determined by distance from the sea, thaw depth and slope position. Reindeergrazing decreased the abundance of willow and changed the composition of understory vegetation.In addition, industrial activities were detected to have destructed shrub vegetation and turned itinto graminoid-dominated vegetation. Shrub canopies facilitated forbs but decreased the cover ofall the other groups including dwarf shrubs, bryophytes and lichens. The species richness ofvegetation decreased along with increasing shrub abundance.
My study shows that arctic-alpAine willow vegetation is more diverse than previously thought.There is a predictable relationship between summer temperatures and willow growth. However,the results also show that there are many factors, both physical and anthropogenic, that are likelyto complicate this pattern. Most important of these counteracting effects are industrial activitiesand reindeer grazing. In the areas where shrubs grow in abundance, the species richness ofunderstory vegetation is likely to decrease and forbs are likely to replace other tundra species.