Browsing by Author "Mod, Heidi"

Plant-plant interactions, i.e. biotic interactions, shape plant communities and the vegetation's succession along abiotic environmental factors. Positive interactions (e.g. facilitation) may expand species niches and enhance growth and reproduction. Negative interactions (e.g. competition, allelopathy) can interfere with growth and reproduction, even out competing some species from their niches. Negative and positive interactions co-occur, but research has shown that positive interactions are generally more common and important than negative ones in harsh environments. The theory of change of net-interaction from negative to positive along an environmental gradient is called the stress gradient hypothesis (=SGH). This work examines nordic crowberry's (Empetrum nigrum ssp. hermaphroditum) effect on arctic-alpine species' sexual reproduction under different environmental stress levels. Crowberry is a dominant species in low-nutrient, acidic arctic-alpine ecosystems. Its competitive ability is based on allelopathic characteristics and a forming of dense mats. It is also unpalatable for herbivores. The species facilitative characteristics include providing shelter from the wind and maintaining an ericoidmycorrhiza community. In this research environmental stress is represented by geomorphological disturbance and soil moisture, with the interactions between crowberry and other species are examined as the relationship between crowberry cover and the fitness measures (e.g. abundance of flowers or fruits) of study species. Explanation for the variation in the effects of crowberry is tried to find from the traits of the study species. Data was collected in May 2011 from Kilpisjärvi, northernmost Finland. The study area comprised 960 1m2 cells. In each cell the cover of each species (including crowberry), the abundance of each species flowers or fruits, the cover of geomorfological disturbance and soil moisture were recorded. Generalized linear models (=GLM) were run for all species to identify the best model for predicting fitness (as selected by the AIC-criterion). Spatial autocorrelation was accounted for by repeating analyses using generalized estimation equation models (GEE), which explicitly account for the spatial structure of data. 17 species were included to the research based on the abundance of their flowers and berries in the research area. Crowberry is included in the best fit model for 14 out of 17 species. The effect of crowberry was positive for four species and negative for ten species based on the modeling results. Interactions of the crowberry and one of the environmental variables are included to the models 19 times. In ten of these cases the interactions agreed with the predictions of the SGH (i.e. effect of crowberry became less negative with increasing abiotic stress). No species traits were consistently related to the outcome of interaction between crowberry and environmental variable, although crowberry effects on dwarf shrub species appeared to be more commonly positive than on other growthforms. According to these results, crowberry has dominant role in arctic-alpine plant communities. The species effect on sexual reproduction of other plant species is commonly negative, but the effect can change to positive along environmental stress gradients, supporting the SGH. Dwarf shrubs may interact positively with crowberry because of sharing the same mycorrhiza type, while more generally species may benefit from crowberry due to its provisioning of shelter from the wind and increased soil moisture. The negative effect of crowberry might be related to its production of allelopathic compounds or its dense growth. The reason for crowberry having a facilitative affect under disturbed conditions might be an indirect effect of disturbance decreasing crowberry's allelopathic effects. These results show that the roles of crowberry and biotic interactions in arctic-alpine vegetation are important. Therefore understanding their effects and mechanisms is important in predicting how this vegetation will respond to changing climate.