Hedgerows in agricultural landscapes as a habitat for forest plant species

Abstract

In many countries of the world the landscape today is dominated by agri- and silvicultural land, whereas the former natural habitats, such as forests, have largely been destroyed or reduced in size and quality. As a consequence, many herbaceous forest plant species are endangered, especially in the intensively used farming landscapes of Central Europe. Hedgerows, common elements of rural landscapes, partly offer a similar environment as forests. They have been proposed as habitat and conservation corridor for woodland herbs, but their importance for the survival of these species is still not clear. In this study the value of hedgerows for the conservation of forest plant species was examined with special attention paid to the influencing factors mainly responsible for the occurrence of these herbs. Selected for study was an agricultural used region with a highly fragmented forest cover and a relatively dense hedgerow network in the hinterland of Bremen, a city in north-western Germany.The first objective of this study was to survey the hedgerows for the occurrence of forest specialists and to determine which factors are influencing their distribution. In the study region almost a quarter of all found herbaceous species (43 of 173) could be classified as typical woodland plants. Endangered species were found more often in forests than in the adjacent hedgerows. Averaged across species there was a predominant effect of environmental factors on the occurrence of forest species in hedges. Forest herbs were favoured by low nutrient and light availability and a close distance to nearby woodland. A further objective was to test the suitability of the habitat hedgerow for the survival of five selected forest species (Adoxa moschatellina, Anemone nemorosa, Circaea lutetiana, Polygonatum multiflorum and Stellaria holostea) with common occurrence in hedges. The comparison of several life-history attributes (as a measurement of plant fitness) between the habitat types forest and hedgerow resulted only in few differences, which show no consistent patterns across species. Adoxa, Anemone and Polygonatum performed equally well in hedgerows and forests, whereas Stellaria appeared to have a higher fitness in hedgerows. In contrast, Circaea showed a higher reproduction under forest conditions. However, performance of these species was mainly influenced by light and soil water availability.The survey of forest-hedgerow transects for changes in forest species richness and frequency revealed a strong negative relationship with species number and distance from the forest edge. This was assumed to be an indication for the conservation corridor function of hedgerows for these species. However, almost a quarter of the forest species of the surrounding forests were not able to spread within hedgerows. Hedgerow age was not influencing the number of species, but older hedgerows showed a smaller variation in species composition. Furthermore, woodland species with high frequencies in forest-hedgerow transects had comparatively high Ellenberg values for light, and were mostly associated with seed dispersal by wind or animals.The last objective was to examine the suitability of a set of ecological and life-history variables, reported to have some effect on the occurrence of woodland species, predicting the occurrence of forest herbs in hedgerow networks throughout Europe. The single factor light availability had a stronger relationship to the frequency of forest herbs of a tested hedgerow network than the chosen set of variables. It is likely that the regional pool of woodland species is more influential for the species composition of the tested hedgerow network than other variables. The investigation of changes in environmental conditions and species composition along a climate gradient across Europe resulted in some differences, which are mainly explained by changes in climate. Hedgerows in more continental areas showed species compositions indicating drier and less acid soils and had lower proportions of chamaephytes and wintergreen species. In sum, climatic differences together with variations in the regional forest species pool have an important effect on the woodland species composition of hedgerows.In conclusion, hedgerows of north-western Germany provide for the majority of woodland herbs a suitable habitat and they help them to survive in an otherwise hostile environment most probably by increased connectivity between habitat patches. Endangered forest species seemed, in contrast, to be restricted to (ancient) forests and not to thrive in hedgerows. Further analyses have to consider the regional pool of forest species, which was found to be most decisive for the species composition of hedgerow networks.