Habitat distribution and population genetics of riparian Salix species in space and time – a restoration framework for softwood forests along the Elbe River
Riparian softwood forests belong to the most endangered vegetation types in Central Europe due to diverse river management measures (e.g. dyking, river training, etc.). As unmodified hydro-geomorphic processes along rivers are widely lacking, which are necessary to create competition-free establishm...
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|Summary:||Riparian softwood forests belong to the most endangered vegetation types in Central Europe due to diverse river management measures (e.g. dyking, river training, etc.). As unmodified hydro-geomorphic processes along rivers are widely lacking, which are necessary to create competition-free establishment sites, natural regeneration of the respective species is almost absent. Hence, restoration measures are required but are a challenging task given species’ ecological requirements as well as the different interests of economy, society, and nature conservation.
My thesis was conducted within the framework of a project to develop a concept for the establishment of riparian softwood forests combining ecological and flood protection issues along the Middle Elbe River. Main aims were to investigate ecological as well as population genetic aspects of riparian softwood forest species to deliver a basis for improving the success of restoration measures such as plantings. Five studies were conducted dealing with different aspects of riparian softwood forest restoration.
In the first study the ecological relationships of distribution patterns of riparian softwood forests and hydrological variables were examined on the basis of habitat distribution models, which were developed for two softwood vegetation types as well as two age classes. They revealed that hydrological variables are significantly related to and substantially determine species distribution. Based on the observed ecological patterns advices are given for the selection of suitable planting sites.
In the second study, an experimental approach, potential effects of competition and biomass loss on the regeneration capability of vegetative propagules, i.e. cuttings, of diverse riparian Salix species were tested. It revealed that cuttings, similarly as seedlings, are prone to competition mainly due to demands for light, indicating that competition should be eliminated during establishment phases to increase the success of planting measures. The additionally tested resprouting ability of the species disclosed vigorous growth capacities of all investigated floodplain species making them particularly suitable for restoration purposes.
Since much debate exists on softwood forest restoration plantings from a flood protections point of view the third study was conducted to assess the effect of riparian softwood forest plantings on flooding risk combining habitat distribution models as well as a two-dimensional hydraulic-numerical model. With this approach we could demonstrate that it is possible to identify sites which meet the criteria of both ecological as well as hydraulic suitability. Hence, substantial potential for “safe” plantings can be expected in managed river-floodplain systems.
The fourth study dealt with possible effects of climate change on habitat distribution of riparian softwood forest species. While current conditions still disclose large potential for softwood forest restoration, habitat projections based on climate change induced hydrological alterations point to a further loss of suitable habitat in the future. Great uncertainty in habitat availability for restoration measures due to substantial variability in hydrological projections exists. However, this uncertainty should be implemented into restoration management to preserve riparian softwood forests in the long term.
While current natural population structures and dynamics of riparian softwood forest species warn about an extremely critical condition regarding the long-term persistence of these vegetation types, results of the fifth study, a genetic population analysis, are less alarming. Clonal patterns and genetic diversity of Salix viminalis as a model species were investigated as basis for the selection of suitable planting material. Though clonal structures were prevalent no dominance of single clones was detected. Levels of genetic diversity were generally high and no negative effects of floodplain or stand fragmentation on diversity levels were found. From the observed genetic patterns, advices for the selection of planting material are deduced.
Overall, the results presented in this thesis deliver a sound basis for the restoration of riparian softwood forests based on reforestation. Given the restricted sustainability of these measures additionally other restoration approaches should be considered enabling the “re-dynamisation” of the floodplain and hence the self-preservation of riparian softwood forests.|