Impact of human activity on species diversity, spatial genetic structure, and genetic diversity of P. alba in P. alba-communities

Species in nature do not live separated. They rather live in communities, which are tightly linked with the abiotic environment forming an ecosystem. Human activities such as exotic species introductions, river modifications, and clear-cuts are widespread and might influence the natural community/ecosystem composition, structure, function and thus their further evolution. Trees represent foundation species of forest communities, which "drive" the structure of communities by creating locally stable conditions for other species. They have developed two different reproductive strategies, sexual and asexual allowing them to resist to human and natural disturbances. Asexual reproduction allows safe colonisation of new habitats as well as fast regeneration after disturbances. Sexual reproduction compensates for the loss in genetic diversity brought upon by asexual reproduction and natural selection. Species hybridization followed by introgression is another phenomenon, which increases intraspecific genetic diversity. The plasticity of the reproductive strategy, the ability to hybridize and the tight connections between species within a community make some tree species highly dynamic in ecosystems. This project focuses on Populus alba L. (white poplar) and P. alba communities in the Danube river valley. The following characteristics make this species ideal to study interaction between human activity, the hybridization and the reproductive strategy: (1) P. alba is under strong selection of two forces; the river and human activity, (2) it possesses both reproductive strategies, (3) it is dioecious, (4) it hybridizes easily and its natural hybrid P. canescens lives in sympatry with P. alba, (5) it has favourable genetic characteristics, and (6) both managed and unmanaged types of P. alba communities have been identified in the Danube river valley. The primary aim of the proposed work is to study structure and composition (species diversity and genetic diversity of P. alba) of three managed and three unmanaged P. alba communities and look for (composition, structure and diversity)-trends within each community type. This will allow us to estimate the status of P. alba within the communities and to recognize in which way the human activity might affected the P. alba communities. The secondary aim is to study the relationship between genomic composition of P. alba and P. x canescens and environmental variables and thus estimate ecological amplitudes of both taxa. We believe that detailed investigations of P. alba as planned in this study scaled up from the population to the community level using molecular, and non-molecular methods will provide important insights into plant population- and community dynamics in general, in particular on the relationship of disturbance (management) to hybridization, and different reproductive strategies. In addition our work will allow us to address one extremely timely topic in plant evolutionary genetics: the role of hybridization in exploitation of new habitats.

Riparian forests are among the most productive and diverse of natural ecosystems of Central Europe. In Austria and Slovakia, the majority of these forests have been radically altered by river canalisation, construction of flood protection dikes and hydropower stations. These alternations have resulted not only in a significant drawdown of water levels but also in a complete cut-off of these forests from water dynamics (flooding). In addition, fast growing biomass- and high quality timber tree species have been planted here. At this point, it is important to highlight that apart from an economic commitment, there is also a social commitment to maintain and to conserve these riparian forests. Is it, however, possible to join both commitments in such altered forests? More specifically, which future does the white poplar (Populus alba) have in the existing riparian forests of the Danube? Did it already reach a climax in its distribution after which a decline in its dominance will follow? Shall these riparian forests be further managed or shall such activities be rather stopped and these areas declared to be under nature protection and conservation? Which reproductive strategy and attractive features does possess this forest tree in order to fulfill demands of current society and at the same time to withstand the destruction? To answer these questions, three managed and three unmanaged white poplar forests (stands) have been studied using different molecular and non-molecular methods. In addition, five other stands have been spot checked. White poplar is a native, pioneer tree species of softwood riparian forests, which reproduces both sexually and asexually by root sprouts. It plays a key role in early stages of succession, but later it is replaced by other hard-wood forest tree species. This was also confirmed by plant-sociological studies of this project, where white poplars were found at the upper tree level only, but not at the lower tree- and shrub-level. In addition, no seedlings were found (despite flowering female and male poplar trees) within the forests and root sprouts were identified at the herbal level only. Both findings point to a very low competitiveness of this tree species and its reliance on specific geomorphic sites for the seed germination. There was not a significant difference in plant species diversity between the managed and unmanaged white poplar forests. All white poplar trees corresponded to several genetic variants, so called clones. These have developed from root sprouts after clear-cuts (in managed forests), or other human activity and high water dynamics (floods) in unmanaged forests. It is apparent that white poplar which is often presented as a dynamic tree species, has reached the climax in the dominance of these forests. With missing high water dynamics, these stands can developed into new white poplar forests just by a targeted human activity using the natural regeneration of root sprouts. Although, white poplar is an economically unimportant forest tree species, mainly because of its bend growth and moderate growth rate, its persistence might be partly secured by its natural hybrid P. x canescens (grey poplar) representing its prolonged arm. They both prefer similar sites and possess identical reproduction strategies, nevertheless, the investigated grey poplar demonstrated a straight growth which might catch attention of forest industry and lead to an economic acceptance.