Kris Verheyen

Permeability of ancient forest edges for weedy plant species invasion

Abstract Within highly fragmented forest landscapes, the flux of matter, energy and species from the landscape matrix into the forest fragment interior is an important ecological phenomenon. We studied the invasion of ancient forest edges by weedy plant species, which are normally confined to the agricultural landscape matrix. We related this to edge orientation and to the occurrence of natural and anthropogenical environmental edge gradients like microclimate and soil chemistry. The plant community composition of the forest edge zone differed significantly from the community composition of the forest core area, and is characterized by a relatively high number of competitive and light demanding species. The maximal width of this edge zone is more extensive at south facing (maximum ca. 20–23xa0m) than at north facing edges (ca. 0–3xa0m). However, most of the weedy plant species were unable to penetrate the forest and were confined to the boundary between forest and arable land. We found no negative edge effects on the more stress tolerant ‘true’ forest species going further than 0–3xa0m (only at south facing edges). The invasion pattern generally fitted the edge gradient for photosynthetically active radiation (PAR). The negative effects of fertilizer misplacement from adjacent arable land on forest soil chemistry (and community composition) are also restricted to the boundary of the forest edge zone. Based on the derived penetration distances of the edge effects and on the shape index of a forest patch it is possible to derive the minimal required forest patch area for forest plant species conservation.

Ecological perspectives for the restoration of plant communities in European temperate forests

Simultaneously with increasing afforestation efforts in western Europe, among conservationists the consciousness is growing that protecting areas to conserve biodiversity will not be sufficient in the long term, and that also the ecological restoration of more or less severely altered areas will be necessary. The probability that recently established forest stands develop towards their ecological reference (i.e. ancient forest) depends largely on the possibility of the target species to colonize them. We focused on the colonization ability of forest plant species and particularly on so-called ancient forest plant species. Major constraints for ecological forest restoration are the spatial characteristics of the target site (isolation, shape and area), imposing dispersal limitations, and in the duration and intensity of the historical land use, leading to changes in habitat characteristics influencing recruitment probability. We reviewed the ecological literature with respect to these constraints and conclude that it takes at least a century to restore the understorey layer of recent forests, even when the target stand is adjacent to a well-developed ancient forest. Both recruitment and dispersal limitation of the target species are responsible for this. Newly established forests should therefore be situated at a minimal distance of the ancient forest source. In other cases, forest plant species will not be able to colonize the newly established forest on a measurable time scale and artificial introduction of forest plant species can be taken into consideration. The negative effects of habitat characteristics, and mainly high soil nutrient values in the recent forest stand can be mitigated by soil nutrient lowering measures. Disturbances in the recent forest should be minimized to maintain a high canopy closure level, preventing light demanding, high competitive species from establishing a stable population. An additional negative consequence of soil disturbances is that it stimulates germination of species from the soil seed bank, which is mainly composed of highly competitive or ruderal species.

METAPOPULATION DYNAMICS IN CHANGING LANDSCAPES: A NEW SPATIALLY REALISTIC MODEL FOR FOREST PLANTS

In fragmented landscapes, habitat patches are often destroyed and created through time, though most metapopulation models treat patch networks as static. Here we present a generally applicable, modified version of Hanskis Incidence Function Model (IFM) that incorporates landscape dynamics (i.e., habitat patch turnover), and we param- eterize the model with data on patch occupancy patterns for forest plants in central Lin- colnshire, UK. The modified IFM provided a better, or equally good, fit to species patch occupancy patterns than logistic regression. Estimated colonization and extinction rates, and the results of logistic regression analyses, varied significantly among species with different life history traits. For example, species with low seed production and predomi- nantly short-distance seed dispersal showed lower rates of colonization and extinction and were more likely to show effects of patch age and connectivity on patch-level presence than species with the opposite set of traits. Model simulations demonstrate a profound negative influence of habitat turnover rate on metapopulation dynamics and persistence, particularly for slow-colonizing species. The incorporation of temporal habitat dynamics into the metapopulation paradigm will permit its application to organisms in a much wider range of real landscapes.

Large herbivores as mobile links between isolated nature reserves through adhesive seed dispersal

Abstract Question: Do large herbivores contribute to the dispersal of plant seeds between isolated habitats by epizoochory? Location: Nature reserves in Flanders, Belgium. Methods: Epizoochory was studied by brushing plant seeds from the fur of 201 domesticated large herbivores (Galloway cattle, donkeys and horses), grazing in 27 Flemish nature reserves. Several herbivores were examined after transport between different nature reserves as part of the seasonal grazing system in Flanders, allowing detection of seed dispersal both within and between reserves. The seedling emergence method was used to identify the dispersed plant species. Results: In total, 6385 epizoochorous seeds from 75 species germinated, yet the real seed quantity was underestimated by the seedling emergence method. A wide variety of seed morphology, seed weights and plant heights was represented among the dispersed species, many of which had a transient seed bank. There was a gradual turnover in epizoochorous species composition in the course of the vegetation season, and seed dispersal occurred both within and between different nature reserves. Conclusions: Domesticated large herbivores, as models for wild mammals in the present and the past, are important dispersers of many plant species. Through seasonal grazing, the herbivores function as ‘mobile link organisms’, connecting isolated nature reserves through seed dispersal, possibly influencing vegetation development and long-term survival of plant populations. As such, large herbivores are important instruments in ecological restoration, especially in fragmented ecosystems. Nomenclature: Lambinon et al. (1998).

Forest biodiversity : lessons from history for conservation

This book focuses on the diverse impact of forest history in general, and of forest continuity, fragmentation and past management in particular, on the diversity and distribution of species. The implications for the conservation of biodiversity in forests are also addressed. Chapters have been developed from papers presented at a conference held in Leuven in January 2003. The emphasis is on temperate forests in Europe and North America, but the information may also be applicable to other regions or biomes. The book will be of significant interest to researchers working within the areas of forestry, ecology, conservation and environmental history.

Meta-Analysis of Standing Crop Reduction by Rhinanthus spp. and Its Effect on Vegetation Structure

We performed a quantitative literature review on the effect of the root hemiparasiteRhinanthus on vegetation standing crop.(1) Across all available experimental studies in mixed vegetation and in pots, above-ground biomass of co-occurring species is generally reduced, with on average 40% and 60% of the value in the controls respectively. Total above-ground biomass, as the sum of parasite biomass and biomass of co-occurring species, decreases in most cases. For field experiments this reduction amounts, on average, to 26% of the control value. This implies that there is no compensation by the parasites’ biomass for the loss of biomass of co-occurring species due to parasite infection. This can be attributed to the low resource-use efficiency of hemiparasites. Meta-analysis confirmed these trends.(2) In pot experiments, the negative effect of the parasite on the above-ground biomass of the host increases with the number ofRhinanthus plants. In field experiments, we found no relationship between biomass reduction andRhinanthus density.(3) Total above-ground biomass reduction in field experiments increases with standing crop of the vegetation. However, reduction in above-ground biomass of co-occurring species seems to decrease with standing crop. Functional and species diversity buffer the community against negative effects ofRhinanthus.(4) In field experiments, functional groups are affected differently byRhinanthus spp. Grasses and legumes are mostly strongly reduced by the hemiparasites. Non-leguminous dicots mostly benefit from the presence ofRhinanthus.(5) In one out of four weeding experiments,Rhinanthus spp. has a significant (positive) effect on species number. However, the response of plant diversity to invasion of parasitic plants requires further research.

An integrated analysis of the spatio-temporal colonization patterns of forest plant species

An integrated analysis of the colonization patterns of forest plant species was carried out in a 34-ha, mixed deciduous forest in northern Belgium. First, we sought to describe the relationships between land use history and envi- ronmental conditions. Land use history and soil type were related and negative correlations between pH and secondary forest age were found. The density of the shrub layer increases with secondary forest age. Litter quantity and cover of Urtica dioica were mainly indirectly influenced by land use history. Litter starts accumulating at low pH values and high shrub density and Urtica dioica grows vigorously on nutrient en- riched soils where much light can reach the ground. Next, the importance of these human-altered environmental conditions for the colonization of forest plant species was assessed rela- tive to the importance of dispersal limitation. Therefore, the distribution of 16 forest species was mapped and species- specific spatio-temporal isolation measures were calculated. The analysis revealed that the colonization patterns of the slowly colonizing species (i.e. ancient forest plant species) are best explained by a combination of spatio-temporal isola- tion, soil type, pH and the (non-)cover of Urtica dioica. By contrast, spatio-temporal isolation was never a limiting factor for good colonizing forest species. Our results suggest that colonization of ancient forest plant species is hampered by a combination of dispersal and recruitment limitation and that the relative importance of both factors is species-specific.An integrated analysis of the colonization patterns of forest plant species was carried out in a 34-ha, mixed deciduous forest in northern Belgium. First, we sought to describe the relationships between land use history and envi- ronmental conditions. Land use history and soil type were related and negative correlations between pH and secondary forest age were found. The density of the shrub layer increases with secondary forest age. Litter quantity and cover of Urtica dioica were mainly indirectly influenced by land use history. Litter starts accumulating at low pH values and high shrub density and Urtica dioica grows vigorously on nutrient en- riched soils where much light can reach the ground. Next, the importance of these human-altered environmental conditions for the colonization of forest plant species was assessed rela- tive to the importance of dispersal limitation. Therefore, the distribution of 16 forest species was mapped and species- specific spatio-temporal isolation measures were calculated. The analysis revealed that the colonization patterns of the slowly colonizing species (i.e. ancient forest plant species) are best explained by a combination of spatio-temporal isola- tion, soil type, pH and the (non-)cover of Urtica dioica. By contrast, spatio-temporal isolation was never a limiting factor for good colonizing forest species. Our results suggest that colonization of ancient forest plant species is hampered by a combination of dispersal and recruitment limitation and that the relative importance of both factors is species-specific.

High-resolution continuous soil classification using morphological soil profile descriptions

Soil grid data were gathered from 156 points in the 30-ha Muizen forest (Ranst, Belgium). At each grid point, soil profiles were examined morphologically by augering to 120-cm depth. In the laboratory, pH(KCl) was determined on samples from every horizon. To allow numerical analyses, all the morphological attributes were given ordinal scores. The analysis consisted of two parts. First, the master horizons were split up into subtypes using Principal Components Analysis and a non-hierarchical clustering technique. This was necessary to overcome the problem of the anisotropy of the soil profiles, which makes it impossible to pool the data of all the horizons and analyse them together. Next, the distinguished horizon subtypes were used as input for the continuous soil profile classification with the ‘fuzzy k-means with extragrades’ algorithm. n nFive different soil classes plus an extragrade class were distinguished. The distinguished soil classes exhibited a fair degree of spatial autocorrelation and correlated well with the Belgian Soil Map. n nThe technique developed ensures the compatibility with national or global soil classification systems based on diagnostic horizons and properties on the one hand and the production of high-resolution soil classes for local use on the other. Furthermore, the developed technique allows reanalysis and optimisation of data from previous surveys.

Plant species loss in an urban area (Turnhout, Belgium) from 1880 to 1999 and its environmental determinants

Summary Information from four archival literature sources from the late 19 th century was matched to present-day plant species distribution data for the region of Turnhout (Belgium) and for 15 smaller sub-regions within this region. In the entire study area 25% of th e species recorded in the late 19 th century went extinct during the 20 th century and the extinction rate doubled at the more detailed sub-region level. Binary survival-extinction data and continuous residuals from a linear regression between historical and present-day abundance categories were used to investigate underlying ecological factors of change including habitat preference, ecological amplitude and life strategy. Species increasing relative to the overall trend were generally correlated with nutrien trich habitats while declining species were more associated with nutrient-poor situations. Generalist species have become relatively more common whilst habitat specialists have strongly declined, resulting in a flora with many ‘losers’ and a few tolerant ‘winners’. The winners are often competitive species while the losers are mainly stress-tolerating species and species with combined life strategies (e.g. SC, SR). Correlations between the decline of historically present habitats and extinction rates of related habitat specialist species show clear trends. We suggest the most important factors involved in changes in flora divers ity and vegetation composition are habitat loss due to urbanization and habitat deterioration, mainly due to agricultural intensification.

Herbaceous plant community structure of ancient and recent forests in two contrasting forest types

It has already been extensively documented that major floristic differences between ancient and recent temperate broad-leaved forests exist. Hence, the question raises whether the herb layer community structure and organization of ancient and recent forests also differs and whether these differences are the same in productive vs. unproductive forest types? Therefore, we selected 127 releves situated in productive Alno-Padion forests and 69 releves situated in less productive Quercion forests out of a larger data-set containing 640 releves from northern Belgium. The plots differed with respect to land use history but it was assured that no covariation between land use history on the one hand, and soil texture, soil drainage and canopy composition on the other occurred. In both forest types, about 30% of the studied species exhibited an association with either ancient or recent forests. Persistent differences between ancient and recent forests in life-form spectra in general and in the number and abundance of geophytes in particular were found as well. Few changes in the community structure of productive Alno-Padion forests were observed after 70 years of recovery. Only in the youngest Alno-Padion forests (i.e. < 70 years) species numbers and total cover of the herb layer were lower and co-occurrence patterns did not differ from random. In general, community recovery appeared to be slower in unproductive Quercion forests. Species numbers, species abundance distributions, species co-occurrence patterns and plot dissimilarity still changed after 120 years. The persistent differences in both communities can be explained by the strong dispersal limitation of many of the involved species, but in the Quercion forests recovery is probably also severely hampered by establishment limitation. Es wurde bereits ausfuhrlich dokumentiert, dass grosere floristische Unterschiede zwischen sehr alten und jungeren gemasigten Laubwaldern existieren. Daher stellt sich die Frage, ob sich die Lebensgemeinschaftsstruktur und -organisation der Krautschicht ebenfalls unterscheidet und ob diese Unterschiede die gleichen bei produktiven vs. unproduktiven Waldtypen sind? Deshalb wahlten wir aus einem groseren Datensatz, der 640 Aufnahmen aus dem nordlichen Belgien enthalt, 127 Aufnahmen, die in produktiven Alno-Padion-Waldern lagen, und 69 Aufnahmen, die in weniger produktiven Quercion-Waldern lagen. Die Probeflachen unterschieden sich bezuglich der Landnutzungsgeschichte, aber es wurde sichergestellt, dass keine Kovarianz zwischen der Landnutzungsgeschichte auf der einen Seite und der Bodenstruktur, der Bodenentwasserung und der Blatterdachstruktur auf der anderen Seite bestand. In beiden Waldtypen zeigten ungefahr 30% der untersuchten Arten eine Assoziation entweder mit sehr alten oder jungeren Waldern. Nachhaltige Unterschiede zwischen sehr alten und jungeren Waldern wurden ebenfalls bei den Lebensform