P. Rotach

The influence of population size and spatial isolation on contemporary gene flow by pollen and mating patterns in temperate forest trees are not well documented, although they are crucial factors in the life history of plant species. We analysed a small, isolated population and a large, continuous population of the insect-pollinated tree species Sorbus torminalis in two consecutive years. The species recently experienced increased habitat fragmentation due to altered forest management leading to forests with closed canopies. We estimated individual plant size, percentage of flowering trees, intensity of flowering, degree of fruiting and seed set per fruit, and we determined mating patterns, pollen flow distances and external gene flow in a genetic paternity analysis based on microsatellite markers. We found clear effects of small population size and spatial isolation in S. torminalis. Compared with the large, continuous population, the small and isolated population harboured a lower...

Distinct spatial genetic structure, as the result of various evolutionary and ecological processes, is a common feature of tree populations. The rare pioneer forest tree Sorbus torminalis occurs in scattered populations of low density and exhibits both clonal propagation and gametophytic self-incompatibility. Clonal reproduction can promote considerable spatial genetic structure and, together with a self-incompatibility system, may substantially reduce mating opportunities within S. torminalis populations, i.e. an Allee-effect owing to mate limitation. All 10 S. torminalis stands mapped in northern Switzerland and analysed with allozymes showed a considerable degree of clonal reproduction, but they were also characterised by large numbers of genotypes that occurred only once. However, spatial autocorrelation analysis revealed significant spatial genetic structure at distances between 15 and 30m as the result of clonal reproduction. Once the effect of clonal propagation was removed from the analysis, the stands no longer exhibited significant spatial autocorrelation. This implies that seed dispersal was not locally restricted. The degree of clonal reproduction was neither correlated with population size, nor did smaller populations exhibit less genetic diversity. Because clonal patches were rather small and interspersed with other genetically unique and unrelated individuals, clonal reproduction seemed to have no negative impact on the species’ sexual reproduction. It is thus likely that the combination of an effective self-incompatibility system and high interstand gene flow helps to maintain genetic diversity in S. torminalis stands, while clonal propagation preserves the genetic diversity over time even if environmental conditions become less favourable during the course of succession.

ABSTRACT Conservation of genetic resources of forest trees has become a major objective for the management of forests. Much theoretical work has been devoted to the subject, and implementation has already started at the local, national, or international scales. Poplars are probably the most representative and threatened forest tree species of old natural floodplain forests in the temperate zone. Gene conservation needs to be integrated with intensive breeding activities, habitat conservation and restoration. For Populus nigra, while research in genetics and ecology is reinforced, a combined conservation strategy is applied at the European scale; simultaneously, the conservation of riparian ecosystems is also a priority. Research and application benefit from each other. The question now is the evaluation of such an integrated strategy. Criteria and indicators for the follow-up of gene resource management are progressively developed, but still need to be tested on the operational scale.