Sari Stark

Links between plant community composition, soil organic matter quality and microbial communities in contrasting tundra habitats.

Plant communities, soil organic matter and microbial communities are predicted to be interlinked and to exhibit concordant patterns along major environmental gradients. We investigated the relationships between plant functional type composition, soil organic matter quality and decomposer community composition, and how these are related to major environmental variation in non-acid and acid soils derived from calcareous versus siliceous bedrocks, respectively. We analysed vegetation, organic matter and microbial community compositions from five non-acidic and five acidic heath sites in alpine tundra in northern Europe. Sequential organic matter fractionation was used to characterize organic matter quality and phospholipid fatty acid analysis to detect major variation in decomposer communities. Non-acidic and acidic heaths differed substantially in vegetation composition, and these disparities were associated with congruent shifts in soil organic matter and microbial communities. A high proportion of forbs in the vegetation was positively associated with low C:N and high soluble N:phenolics ratios in soil organic matter, and a high proportion of bacteria in the microbial community. On the contrary, dwarf shrub-rich vegetation was associated with high C:N and low soluble N:phenolics ratios, and a high proportion of fungi in the microbial community. Our study demonstrates a strong link between the plant community composition, soil organic matter quality, and microbial community composition, and that differences in one compartment are paralleled by changes in others. Variation in the forb-shrub gradient of vegetation may largely dictate variations in the chemical quality of organic matter and decomposer communities in tundra ecosystems. Soil pH, through its direct and indirect effects on plant and microbial communities, seems to function as an ultimate environmental driver that gives rise to and amplifies the interactions between above- and belowground systems.

Phenolic Composition and Antioxidant Capacity of Bilberry (Vaccinium myrtillus) Leaves in Northern Europe Following Foliar Development and Along Environmental Gradients

Bilberry is a characteristic field layer species in the boreal forests and is an important forage plant for herbivores of the North European ecosystem. Bilberry leaves contain high levels of phenolic compounds, especially hydroxycinnamic acids, flavonols, catechins, and proanthocyanidins. We investigated the phenolic composition of bilberry leaves in two studies, one following foliar development in forest and open areas, and the other along a wide geographical gradient from south to north boreal forests in Finland. An analysis of bilberry leaves collected in open and forest areas showed that major phenolic changes appeared in the first stages of leaf development, but, most importantly, synthesis and accumulation of flavonoids was delayed in the forest compared to the high light sites. Sampling along a geographical gradient in the boreal zone indicated that leaves from higher latitudes and higher altitudes had greater soluble phenolic and flavonol levels, higher antioxidant capacity, and lower contents of chlorogenic acid derivatives. The ecological significance of the results is discussed.

Ecological role of reindeer summer browsing in the mountain birch (Betula pubescens ssp. czerepanovii) forests: effects on plant defense, litter decomposition, and soil nutrient cycling

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 20xa0years, 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.

Concentrations of Foliar Quercetin in Natural Populations of White Birch (Betula pubescens) Increase with Latitude

We investigated latitudinal and regional variations in the composition and concentrations of foliar flavonoids and condensed tannins in wild populations of white birch (Betula pubescens EHRH) in a large climatic transect in Finland. Concentrations of quercetin derivatives were correlated positively with latitude. By contrast, the concentrations of apigenin and naringenin derivatives were correlated negatively with latitude. These compound-specific latitudinal gradients compensated each other, resulting in no changes in the concentration of total flavonoids. Our results thus demonstrate a qualitative, but not quantitative, latitude-associated gradient in the foliar flavonoids in white birch. Due to higher antioxidant capacity of the quercetin derivatives in relation to other flavonoids, the qualitative change can reflect higher adaptation to light in the north than south. An investigation on a regional scale in the northern boreal zone showed that the temperature sum was correlated positively and soil P concentration was correlated negatively with the concentrations of foliar flavonoid, while the concentration of condensed tannins was correlated with slope. The variation in concentrations of flavonoids at large-scale geographical patterns is in line with the conjecture that foliar flavonoids are synthesized for protection against photooxidative stress.

Seasonal grazing effects by semi-domesticated reindeer on subarctic mountain birch forests

In northern Fennoscandia, the spatial and temporal grazing practices of semi-domesticated reindeer (Rangifer tarandus tarandus) vary, which implies different grazing effects dependent on natural conditions as well as management regime (i.e., timing and intensity of grazing). We compared density and biomass of main plant groups in semi-dry mountain birch forests exposed to either long-term summer or winter grazing in three reindeer herding districts in the northernmost Finland. Percent plant cover, height, and biomass of reindeer lichens (Cladonia spp.) and dwarf shrubs (Vaccinium uliginosum, Calluna vulgaris, and Betula nana) were lower on summer ranges compared with winter ranges. The biomass of other dwarf shrubs (Vaccinium myrtillus and V. vitis-idaea), and graminoids and herbs, and the % cover of non-vegetated bare soil and litter were, however, higher on summer ranges than on winter ranges. Young mountain birch shoots (Betula pubescens ssp. czerepanovii) were less frequent on summer ranges than on winter ranges. The total leaf biomass under the browsing height of reindeer (<1.5xa0m) was also lower on summer ranges compared with winter ranges. Especially in drier and nutrient poor mountain birch forests, intensive summer grazing reduces the quantity of lichens and total plant biomass which affects the ecological state and productivity of these forests and also reduces especially their winter grazing value for reindeer. Therefore, in addition to regulating the maximum sustained numbers of reindeer, pasture rotation systems that effectively protect dry and nutrient poor vegetation from summer grazing and trampling should be encouraged.

Nutrient Cycling in the Tundra

The term tundra is generally applied to treeless areas that are situated beyond the climatic limit for tree growth. This definition can denote both regions of high latitude north or south of the tree line (i.e. arctic/Antarctic or polar tundra), or high altitude belts above the natural tree line in all climatic zones (i.e. alpine tundra). Arctic and alpine tundras have many similarities, but also great differences. This chapter describes the processes of carbon (C) and nutrient cycling in arctic or arctic-alpine tundra, the latter referring to high elevation belts in the boreal region above 60 °N. The majority of this area is located in northern parts of Russia, North America and Scandinavia. For a comprehensive description of tundra systems, see Wielgolaski (1997).