The role of regeneration in plant niche differentiation and habitat specialization
Sammanfattning: To predict the effects of environmental change and nature management on the distribution of plant species, it is crucial to understand the mechanisms of plant niche differentiation and habitat specialization. The importance to habitat specialization of particular plant traits and requirement of the regenerative phase of the plants life has received scanty interest. Our experiments on seed germination and seedling establishment have shown that plant strategies in the regenerative phase contribute strongly to the habitat specialization of species. A conceptual model explains how germination strategies and seasonal environmental conditions interact to allow certain species to establish in forests, whereas other species are effectively excluded from establishing under the prevailing adverse conditions that characterize forest floor environments throughout the year. Also the specific adaptations in the seedling stage contribute to the specialization of species to their habitat. Specialization to shaded habitats was associated with more conservative growth strategy, i.e. showing a more modest growth response to increasing light. Species from open habitats showed a stronger shade avoidance response than forest species, which is more adaptive in herbaceous vegetation than in a forest floor herb layer.
Seed size also appeared to be important for the recruitment success of plants in different habitats. Seed size is a compromise between a numeric advantage of producing many seeds against a recruitment advantage, e.g. higher resistance to environmental adverse conditions, of larger seeds. In our study, the recruitment advantage of larger seeded species relative to smaller seeded species appeared to increase with from meadows and open forests to closed canopy-forest. Besides the responses of individual plant species to the environment, community assembly processes and the resulting species and trait composition of local assemblages has also been studied in this thesis. Within-community trait variation appeared to be larger than between-community traits variation for seed mass, plant height, lateral spread and species indices of competition, stress and ruderality in rocky habitat vegetation. This would indicate that limiting similarity plays a role in structuring plant communities in rocky habitats if the underlying environmental gradient was mainly varying in productivity. However, due to the shallow soils, rocky habitats are drought-prone by nature and chemical stress from aluminium increases towards low soil pH parallel to phosphorus.
In the last paper, we studied the effect of deterministic processes on the reproducibility of species assemblages, when the stochastic factor of seed dispersal is controlled for. In synthetically assembled plant communities, patterns in species composition were highly repeatable at each level of soil fertility, indicating strong deterministic processes in community assembly. A severe drought resulted in disproportionately high mortality in the high soil fertility microcosms, where a dense sward of fast-grown and hence drought-susceptible plant individuals was formed.
The findings in this thesis reveal an important role of regeneration in niche differentiation between species and the resulting habitat specialization. Plants strategies in the regenerative phase contribute strongly to the habitat specialization of species and their adaptation to specific habitats. The abiotic environment causes species to separate along productivity gradients, and simultaneously affect local biotic interactions by governing species’ relative abundances.
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