Ecological role of reindeer summer browsing in the mountain birch (<Emphasis Type="Italic">Betula pubescens</Emphasis> ssp. <Emphasis Type="Italic">czerepanovii</Emphasis>) forests: effects on plant defense,litter decomposition,and soil nutrient cycling |
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Authors: | Sari Stark Riitta Julkunen-Tiitto Jouko Kumpula |
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Institution: | (1) Rovaniemi Research Unit, Finnish Forest Research Institute, P.O. Box 16, Rovaniemi, 96301, Finland;(2) Natural Product Research Laboratory, Department of Biology, University of Joensuu, Joensuu, 80101, Finland;(3) Finnish Game and Fisheries Research Institute, Reindeer Research Station, Kaamanen, 99910, Finland |
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Abstract: | Mammalian herbivores commonly alter the concentrations of secondary compounds in plants and, by this mechanism, have indirect
effects on litter decomposition and soil carbon and nutrient cycling. In northernmost Fennoscandia, the subarctic mountain
birch (Betula pubescens ssp. czerepanovii) forests are important pasture for the semidomestic reindeer (Rangifer tarandus). In the summer ranges, mountain birches are intensively browsed, whereas in the winter ranges, reindeer feed on ground lichens,
and the mountain birches remain intact. We analyzed the effect of summer browsing on the concentrations of secondary substances,
litter decomposition, and soil nutrient pools in areas that had been separated as summer or winter ranges for at least 20 years,
and we predicted that summer browsing may reduce levels of secondary compounds in the mountain birch and, by this mechanism,
have an indirect effect on the decomposition of mountain birch leaf litter and soil nutrient cycling. The effect of browsing
on the concentration of secondary substances in the mountain birch leaves varied between different years and management districts,
but in some cases, the concentration of condensed tannins was lower in the summer than in the winter ranges. In a reciprocal
litter decomposition trial, both litter origin and emplacement significantly affected the litter decomposition rate. Decomposition
rates were faster for the litter originating from and placed into the summer range. Soil inorganic nitrogen (N) concentrations
were higher in the summer than in the winter ranges, which indicates that reindeer summer browsing may enhance the soil nutrient
cycling. There was a tight inverse relationship between soil N and foliar tannin concentrations in the winter range but not
in the summer range. This suggests that in these strongly nutrient-limited ecosystems, soil N availability regulates the patterns
of resource allocation to condensed tannins in the absence but not in the presence of browsing. |
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Keywords: | Antiherbivore defense Litter decomposition Mountain birch Reindeer |
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