Plant Species Composition and Diversity in Cliff and Mountain Ecosystems
Sammanfattning: Ecosystems today are under increasing pressure from a rapidly changing climate, changes in land-use, habitat fragmentation and degradation and increasing anthropogenic disturbance. Studying drivers of biodiversity patterns is of increasing importance for understanding the dynamics of communities and their reactions and adaptation to changes in the future. Climate change is predicted to be most dramatic in mountainous areas, influencing shifts in vegetation patterns, which have already been observed. Vegetation in cliff outcrops is highly distinct from surrounding vegetation, and commonly contains many rare species, including a high proportion of endemics. Despite this, cliffs are not well represented by ecological studies. This thesis is based on field studies investigating some aspects of processes shaping biodi-versity patterns in plant communities in terms of species distribution, composition and diversity. Patterns of diversity were related to environmental and spatial factors, human distur-bance and biotic interactions. The investigated plant communities were cliffs and surrounding matrix vegetation at a subarctic alpine tundra site and cliff ecosystems in south-west Sweden. Vegetation at the investigated sites were surveyed along with estimations of environmental variables and analyzed to identify the drivers of observed diversity patterns. At the subarctic alpine study site, results show that cliff habitats are highly important for landscape diversity. They contained many species specialized for this habitat, which cannot persist in the surrounding landscape. Bryophytes and lichens contained species more distinctly affiliated to the cliff habitats compared to vascular plants. At the same subarctic alpine site, the cushion plant Silene acaulis was shown to act as a nurse plant in cold-stressed environments, enhancing local biodiversity at higher elevation in the Scandes. At the investigated cliff sites in south-west Sweden, distribution of bryophytes specialized for cliff environments was indicated to be limited by dispersal. This implies that maintaining high connectivity of suitable habitat patches is of high importance for the conservation of bryophytes and probably also other organisms with small diaspores. In the same geographical region, it was found that recreational rock climbing had less influence on biodiversity patterns than environmental variables, in contrast to what has been reported in some previous studies from other regions. In summary, this thesis shows that cliffs are important for landscape diversity due to their contrasting composition of species compared to surrounding matrix habitat, especially in the investigated subarctic alpine study site. It also contributes to the documentation of dispersal limitation for bryophytes with affinities to patchy habitat types and a low frequency of spore production. In addition, it shows that including environmental factors when evaluating the impact of rock climbing on cliff vegetation is important for how patterns are interpreted. Furthermore, it shows the impact of a nurse plant effect in cold stressed environments, contributing to previous studies showing that biotic interactions shift from competition to facilitation in many environments, as stress increases.
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