Abstract
In northernmost Fennoscandia, grazing by reindeer
(Rangifer tarandus L.) has a substantial impact on
the vegetation of boreal forests and arctic-alpine tundra heaths, which
are reflected in below-ground processes, such as nutrient mineralization
and soil organic matter decomposition. In the present thesis, the effects
of reindeer grazing on soil nutrient cycling were studied by comparing
grazed situation with an ungrazed control area in ten boreal forests and
six arctic-alpine tundra heaths.
In boreal forests, reindeer grazing reduced microbial respiration in
both the oligotrophic and mesotrophic study areas, indicating a deficiency
of labile substrates for the soil microbes due to reindeer grazing.
Simultaneously, there was heterogeneity in the impact on nitrogen
mineralization rates as at some sites, mineralization was enhanced by
grazing. The fertilization effect of urine and faeces can therefore be
strong enough a factor to outweigh a reduction in quality of soil organic
matter. In the oligotrophic forests, low soil moisture content in the
grazed areas could sometimes limit the mineralization rates even when the
potential for mineralization was enhanced by grazing.
In the tundra ecosystems, there was spatial variation in the impact
of grazing on microbial respiration and nitrogen mineralization. Low
grazing intensity occurring outside the growing season had a retarding
impact on nutrient cycling in both unfertilized, nutrient-poor and
fertilized, nutrient-rich conditions. In contrast, a relatively high
grazing intensity enhanced the mineralization rates in two nutrient-poor
and two nutrient-rich tundra heaths. When three different grazing
intensities were compared in one oceanic, nutrient-rich and one
continental, nutrient-poor tundra heath, the strongest positive effect of
grazing on soil nutrient cycling occurred in the heavily grazed areas. The
data do not support the assumption that soil nutrient availability
regulates whether herbivores enhance or retard nutrient cycling in the
soil. Instead, the net effect of grazing is determined by the balance
between the underlying mechanisms that may work at opposite directions.
The most important of these mechanisms are the grazer-mediated impact on
the decomposability of the dominant vegetation and fertilization by urine
and faeces.
The duration, intensity and seasonal timing of the grazing seem to
be important factors that regulate whether reindeer grazing enhances or
retards soil nutrient cycling in each specific area. Due to the high
spatial and temporal variation in the effects of grazing observed in this
study, it is not possible to generalize the overall impact of grazing.
Further study is required in order to determine the exact conditions under
which grazing enhances or it retards soil nutrient cycling.
| Identifer | oai:union.ndltd.org:oulo.fi/oai:oulu.fi:isbn951-42-6692-7 |
| Date | 18 May 2002 |
| Creators | Stark, S. (Sari) |
| Publisher | University of Oulu |
| Source Sets | University of Oulu |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/doctoralThesis, info:eu-repo/semantics/publishedVersion |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess, © University of Oulu, 2002 |
| Relation | info:eu-repo/semantics/altIdentifier/pissn/0355-3191, info:eu-repo/semantics/altIdentifier/eissn/1796-220X |
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