Olivier Honnay

An ecological comparison between ancient and other forest plant species of Europe, and the implications for forest conservation

An analysis is presented of the ecological characteristics of ancient forest plant species in deciduous forests of Europe. Twenty-two literature sources were used to generate a list of 132 ancient forest plant species, described from at least eight countries in Europe. The affinity for ancient forests of these species differs considerably from country to country, but they have a definite ecological profile. There is a significant difference in the response of the ancient forest plant species compared with other forest plant species for a variety of ecological characteristics, based on Ellenberg indicators, plant strategies and phytosociological associations. Ancient forest plant species tend to be more shade-tolerant than the other forest plant species; dry and wet sites are avoided. They are typical of forest sites with an intermediate pH and nitrogen availability. Geophytes and hemicryptophytes are more frequent amongst ancient forest plant species. The stress-tolerant plant strategy type is significantly more abundant under the ancient forest species than expected when compared with other forest plant species and vice versa for the competitive plant strategy. This distinct ecological profile suggests that ancient forest plant species may be considered as a guild. The poor ability of these species to colonize new forest sites may be attributed to a complex of interacting variables: limited dispersal abilities (many have a short-distance dispersal strategy), low diaspore production and recruitment problems (e.g. low competitive ability). The regional variation in ancient forest plant species suggests that regional lists are more appropriate for assessing the nature conservation value of forests than one global European list. Due to their distinct ecological profile and low colonizing abilities, ancient forest plant species may be considered as important biodiversity indicators for forests.

Effects of area, age and diversity of forest patches in Belgium on plant species richness, and implications for conservation and reforestation

Abstract The distribution of 203 forest plant species over 234 isolated forest patches in the western part of Belgium and the most northern part of France was studied. An analysis that considered species richness in the context of the SLOSS (single large or several small reserves) debate gave no evidence of habitat subdivision reducing total plant species richness in the forests. The presence of some functional ecological plant species groups was correlated with habitat features and patch area. Habitat diversity was found to be important in explaining the presence of species groups of high conservation value, but patch age (as an indicator for habitat quality) also played a major role. Habitat diversity was not a surrogate for patch age. For most of the species groups, patch area sensu stricto is a redundant variable in explaining species richness relative to habitat diversity and patch age; area-dependent stochastic extinctions of forest plant species are of minor importance, at least at the present level of forest fragmentation. In contrast we suggest that extinction of forest plants occurred and still occurs mainly in a deterministic way. Finally, we conclude that even small forest fragments can be very important for maintaining plant species diversity, at least if they are of high habitat quality and if the forest management is appropriate.

Can the seed bank be used for ecological restoration? An overview of seed bank characteristics in European communities

Abstract Question: Can seeds in the seed bank be considered as a potential source of material for the restoration of European plant communities including forest, marsh, grassland and heathland? Methods: This study reviews seed bank studies (1990–2006) to determine if they provide useful and reliable results to predict restoration success. We formally selected 102 seed bank studies and analyzed differences between four plant community types in several seed bank characteristics, such as seed density, species richness and similarity between seed bank and vegetation. We also assessed the dominant genera present in the seed bank in each plant community. Results: We observed remarkably consistent trends when comparing seed bank characteristics among community types. Seed density was lowest for grassland and forest communities and highest in marshes, whereas species richness, diversity and evenness of the seed bank community was lowest in heathland and highest in grassland. Similarity between seed bank and vegetation was low in forest, and high in grassland. There was a lot of overlap of the dominant genera of seed bank communities in all studies. Conclusions: The absence of target species and the high dominance of early successional species, in particular Juncus spp., indicate that restoration of target plant communities relying only on seed germination from the seed bank is in most cases not feasible. The exceptions are heathland and early successional plant communities occurring after temporally recurring disturbances. Restoration of plant communities composed of late successional species, such as woody species or herbaceous species typical of woodland or forest rely mainly on seed dispersal and not on in situ germination.

Impact of habitat quality on forest plant species colonization

The impact of habitat quality and site history on the recolonization potential of ancient-forest plant species on abandoned farmland was studied in the forest of Ename, Flanders, Belgium. With the exception of a network of fringe relics (linear elements mainly along exploitation roads), our study area was cleared and converted to arable land ca. 1850. From 1869 onward, most fields were gradually abandoned, resulting in a progressive, partly spontaneous reforestation. Each of the 42 actual forest parcels (amounting to 62 ha) was surveyed and a total of 466 plant species were inventoried. Twenty seven of these were identified as ancient-forest plant species and cataloged in a separate subset. Additionally, the spatial distribution of six ancient-forest plant species (Anemone nemorosa, Corylus avellana, Hyacinthoides non-scripta, Paris quadrifolia, Mercurialis perennis, Vinca minor) was systematically surveyed and digitized in a GIS environment. Habitat quality was assessed on the parcel level using intrinsic soil variables on the one side, and historically related variables (length of the agricultural-occupation period, length of woody fringe relics, and total length of fringe relics s.l.) on the other. Soil texture had a major impact on the duration of agricultural land use after deforestation. Soil phosphate content and pH are positively correlated with the duration of the agricultural land use. The number of ancient-forest plant species was negatively affected by the length of the agricultural-occupation period and soil phosphate content, and positively by the total length of the surrounding fringe relics. The same trends are observed studying the systematically surveyed ancient-forest species. We propose that soil phosphate content affected ancient-forest plant species distribution, because it stimulates vigorous vegetation development and as such has a definite effect on evolving competitive plant relationships. Using raster-G.I.S analysis tools, mean and maximum colonization distances and approximate mean and maximum colonization rates per century were calculated for each systematically surveyed species. We conclude that not only seed dispersal capabilities, but also site quality variables play an important role in the colonization process of ancient-forest plant species. In the short term, afforestation of previously heavily fertilized farmland will not result in floristically diverse and, thus, valuable forest habitats. The relatively immobile soil phosphate represents a major barrier.

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.

Satellite based land use and landscape complexity indices as predictors for regional plant species diversity

It is known that habitats composed of spatially heterogeneous abiotic conditions provide a great diversity of potentially suitable niches for plant species. The scientific premises of landscape ecology suggest that, at a higher spatial level, also the composition and structure of the landscape mosaic, influences biotic processes and hence species richness. In this exploratory study we investigated if plant species diversity could be correlated with landscape structure and complexity indices which were based on Landsat Thematic Mapper satellite imagery. Plant species data were derived from the 4 km × 4 km resolution Flora Database of Flanders (i.e. northern Belgium). Plant species number within the 4 km × 4 km grid cells was positively correlated with most of the landscape diversity indices whereas landscape fragmentation indices only affected the group of the threatened species. We found a gradient of increasing species richness beginning from the rural areas of Flanders over the suburban towards the urban areas. This gradient was mostly due to the higher number of alien plant species, warmth indicators and threatened species in urbanised areas. We conclude that, at least in the studied region, the effects of landscape changes on plant species diversity can be monitored and predicted on a large scale and over long periods of time using land cover data. Bottleneck in this kind of analyses remains the reliability of the land cover data and the availability and reliability of the biological data.

Meta-Analysis of Susceptibility of Woody Plants to Loss of Genetic Diversity through Habitat Fragmentation

Shrubs and trees are assumed less likely to lose genetic variation in response to habitat fragmentation because they have certain life-history characteristics such as long lifespans and extensive pollen flow. To test this assumption, we conducted a meta-analysis with data on 97 woody plant species derived from 98 studies of habitat fragmentation. We measured the weighted response of four different measures of population-level genetic diversity to habitat fragmentation with Hedges d and Spearman rank correlation. We tested whether the genetic response to habitat fragmentation was mediated by life-history traits (longevity, pollination mode, and seed dispersal vector) and study characteristics (genetic marker and plant material used). For both tests of effect size habitat fragmentation was associated with a substantial decrease in expected heterozygosity, number of alleles, and percentage of polymorphic loci, whereas the population inbreeding coefficient was not associated with these measures. The largest proportion of variation among effect sizes was explained by pollination mechanism and by the age of the tissue (progeny or adult) that was genotyped. Our primary finding was that wind-pollinated trees and shrubs appeared to be as likely to lose genetic variation as insect-pollinated species, indicating that severe habitat fragmentation may lead to pollen limitation and limited gene flow. In comparison with results of previous meta-analyses on mainly herbaceous species, we found trees and shrubs were as likely to have negative genetic responses to habitat fragmentation as herbaceous species. We also found that the genetic variation in offspring was generally less than that of adult trees, which is evidence of a genetic extinction debt and probably reflects the genetic diversity of the historical, less-fragmented landscape.

Nested plant communities in deciduous forest fragments : species relaxation or nested habitats?

The presence or absence of 75 forest-core plant species in 156 forest fragments in Western Belgium was surveyed. We performed a nestedness analysis both at the total species pool level and at the individual species level. Community composition of these plant species seemed to exhibit a highly significant nested subset pattern. We tested the classic hypotheses concerning the ecological mechanisms generating nestedness, i.e. stochastic species extinction and differential colonization. Neither patch area nor patch isolation, however, adequately explained the species nestedness pattern, hence our conclusion that these variables are of little use in predicting the ultimate plant species composition of a forest fragment. We then derived a complex synthetic habitat diversity measure for each forest fragment and ranked the patches accordingly. The resulting plant species occurrence patterns approximated the nestedness pattern very closely. Hence we suggest that nested plant species assemblages result in the first place from nested habitats, and that species diversity in forest fragments is determined primarily by habitat diversity and is predictable on the basis thereof. Nested species subsets are not generated by differences in minimal sustainable population sizes between species but by differences in habitat specialization. We conclude that forest plant species extinctions in Western Belgium were probably not caused by patch-area-dependent stochastic extinction processes resulting from shrinking forest size (i.e. species relaxation), but in contrast must have their origin in deterministic processes caused by overall habitat deterioration. We also learned from our dataset that a significantly positive species-area relationship and significant species nestedness do not necessary imply that one large forest fragment supports more species than several small patches of the same total size. This result is also associated with habitat diversity and the intrinsic high sensitiveness of forest-core plant species.

Forest restoration, biodiversity and ecosystem functioning.

Globally, forests cover nearly one third of the land area and they contain over 80% of terrestrial biodiversity. Both the extent and quality of forest habitat continue to decrease and the associated loss of biodiversity jeopardizes forest ecosystem functioning and the ability of forests to provide ecosystem services. In the light of the increasing population pressure, it is of major importance not only to conserve, but also to restore forest ecosystems.Ecological restoration has recently started to adopt insights from the biodiversity-ecosystem functioning (BEF) perspective. Central is the focus on restoring the relation between biodiversity and ecosystem functioning. Here we provide an overview of important considerations related to forest restoration that can be inferred from this BEF-perspective.Restoring multiple forest functions requires multiple species. It is highly unlikely that species-poor plantations, which may be optimal for above-ground biomass production, will outperform species diverse assemblages for a combination of functions, including overall carbon storage and control over water and nutrient flows. Restoring stable forest functions also requires multiple species. In particular in the light of global climatic change scenarios, which predict more frequent extreme disturbances and climatic events, it is important to incorporate insights from the relation between biodiversity and stability of ecosystem functioning into forest restoration projects. Rather than focussing on species per se, focussing on functional diversity of tree species assemblages seems appropriate when selecting tree species for restoration. Finally, also plant genetic diversity and above - below-ground linkages should be considered during the restoration process, as these likely have prominent but until now poorly understood effects at the level of the ecosystem.The BEF-approach provides a useful framework to evaluate forest restoration in an ecosystem functioning context, but it also highlights that much remains to be understood, especially regarding the relation between forest functioning on the one side and genetic diversity and above-ground-below-ground species associations on the other. The strong emphasis of the BEF-approach on functional rather than taxonomic diversity may also be the beginning of a paradigm shift in restoration ecology, increasing the tolerance towards allochthonous species.

Genetic structure of the forest herb Primula elatior in a changing landscape

To investigate whether changes in land use and associated forest patch turnover affected genetic diversity and structure of the forest herb Primula elatior, historical data on landscape changes were combined with a population genetic analysis using dominant amplified fragment length polymorphism markers. Based on nine topographic maps, landscape history was reconstructed and forest patches were assigned to two age classes: young (less than 35 years) and old (more than 35 years). The level of differentiation among Primula populations in recently established patches was compared with the level of differentiation among populations in older patches. Genetic diversity was independent of population size (P > 0.05). Most genetic variation was present within populations. Within‐population diversity levels tended to be higher for populations located in older forests compared with those for populations located in young forests (Hj = 0.297 and 0.285, respectively). Total gene diversity was also higher for old than for young populations (Ht = 0.2987 and 0.2828, respectively). The global fixation index FST averaged over loci was low, but significant. Populations in older patches were significantly more differentiated from each other than were populations in recently established patches and they showed significant isolation by distance. In contrast, no significant correlations between pairwise geographical distance and FST were found for populations in recently established patches. The location of young and old populations in the studied system and altered gene flow because of increased population density and decreased inter‐patch distances between extant populations may explain the observed lower genetic differentiation in the younger populations. This study exemplifies the importance of incorporating data on historical landscape changes in population genetic research at the landscape scale.