Tree species effects on herb layer development in post-agricultural forests

摘要

The forest cover in Flanders and many similar lowland regions has been subjected to many land use changes in previous centuries. Especially on fertile loam and sand loam soils, the forest cover has been declining rather continuously. Only in the last decades, fertile agricultural land has been afforested for biodiversity conservation and other objectives. These so called post-agricultural forests have, however, strongly altered soil characteristics, especially elevated phosphorus (P) levels. The elevated P concentrations cause a strong increase in growth of competitive species, like Urtica dioica L.. While, stress tolerant species like many forest plant species, show only a limited response in growth. Many forest plant species, so called ancient forest species (AFS), are predominantly present in sites with a long and continuous forest land use due to their limited colonisation capacity of recent forest. It concerns a large group of species and consequently they are important for the functional and taxonomic biodiversity of the herb layer of forests. The colonisation of AFS in post-agricultural forests can thus be considered as a bottleneck for forest restoration. In this thesis, tree species effects on the colonisation of AFS are studied in post-agricultural forests. Tree species are known to have a strong impact on soil characteristics, light transmittance and understory vegetation. It is hypothesized that increased shade levels will suppress competitors and facilitate the colonisation of AFS while acidifying tree species will strongly limit the survival of many AFS. The thesis comprises an observational vegetation study and four different experiments focusing on tree species effects on AFS. The vegetation study showed a diverging vegetation development in a chronosequence with two contrasting tree species, i.e. poplar and oak. This is explained by the quick soil acidification under oak compared to stable near neutral pH under poplar. No difference was found in the cover of forest plants, but oak had mainly acid tolerant forest plants while poplars had more acid intolerant forest plants. For the first experiment six AFS were introduced under eight different tree species in a common garden. The survival of the introduced AFS was limited by soil acidifying tree species while growth of these AFS was higher under tree species with low light levels. Temporary canopy gaps promoted the establishment of AFS. In a sowing experiment, germination and recruitment of both acid tolerant and intolerant species was higher under tree species with higher soil pH. However, competitive species from the seed bank also germinated more numerously by higher pH. In another sowing experiment, recruitment showed to be independent for early and late leafing out shrub species. However, four out of seven species showed higher recruitment in shaded versus non shaded treatment. In a final experiment, tree species induced soil acidification is linked to decreasing plant Ca and P concentrations and increasing Mg and Al shoot/root ratios, likely explained by Al antagonism. A potential interaction between light availability and soil acidification effects was not found in the various experiments. The results lead to the conclusion that tree species are indeed important drivers of the understory development in post-agricultural forests. This tree species effect can be explained by their impact on soil acidification and understory light availability. Dark stands suppress the competitors and facilitate the vegetative colonisation of many AFS. Temporary canopy gaps seem to facilitate recruitment of AFS. However, it is important that gaps close quickly enough to prevent competitors from establishing. The recruitment and survival of acid intolerant AFS is strongly limited to moderately acidifying tree species. In the concluding chapter, a simple framework is proposed to evaluate tree species effects based on light transmittance and soil acidity. It is concluded that topsoil is best kept clearly above pH-H2O 4.2 and light transmittance below 8% of the open field for the optimal recovery of AFS.