Jo H. Willems

Challenges for the conservation of calcareous grasslands in northwestern Europe : integrating the requirements of flora and fauna

The conservation of biological diversity requires an integrated approach covering the ecological demands of a multitude of species. Integration may be achieved by focusing on a careful selection of target species, which is rare in practice. Calcareous grasslands offer a case in point. Although they harbour a high diversity of both plant and insect species, in management the emphasis is placed on the flora. This results in an underestimation of, notably, the importance of structural heterogeneity in the vegetation. As an apparent consequence, conservation management in the Netherlands has been much more successful for the flora than for butterflies. In contrast, Germany shows promising efforts to integrate both plant and animal species in conservation management and landscape planning. The main constraints for a successfully integrated conservation management presently consist of a limited availability and exchange of information, and an insufficient organisation of research and management at an international level.

Why do we need permanent plots in the study of long-term vegetation dynamics?

This paper presents a survey of vegetation studies on permanent plots, with an emphasis on the long-term char- acter of these studies. It makes remarks on the external and internal causes of succession, pays attention to the pathways of succession, links up permanent plots studies and chrono- sequences, discusses internal causes and mechanisms of suc- cession, and finally mentions the significance of long-term vegetation dynamics for nature conservation.

Increasing dominance of Brachypodium pinnatum (L.) beauv. in chalk grasslands: A threat to a species-rich ecosystem

Chalk grassland is a semi-natural, species-rich vegetation which was once widespread in West Europe. Due to the high species diversity a number of remnants of this vegetation have been set aside as nature reserves and managed by man to prevent natural succession into woodland. During the last decade an increase of dominance of the grass Brachypodium pinnatum has been observed in chalk grassland within the greater part of the distribution area. Increasing dominance is strongly negatively correlated with the species diversity. Increase of this species is observed in grassland with different management regimes, e.g. sheep grazing, mowing or burning. Brachypodium can gain 80% or more of the relative above-ground phytomass. In such vegetation the species number decreases to less than 50% of the original number within a few years—the low growing forbs especially disappear almost completely. It is argued that the impact of nutrients by air pollution, especially of nitrogen, causes the increasing dominance of Brachypodium. Further research will focus on the effects of this extra nutrient supply. The aim of the present research is to formulate management strategies in order to prevent degradation of the species diversity in chalk grasslands.

Changes in chalk‐grassland structure and species richness resulting from selective nutrient additions

A series of fertilization experiments was carried out over a 5-yr period in a chalk grassland in Limburg (The Netherlands) as part of a study of the maintenance of species richness in species-rich grasslands. Phosphorus and nitrogen were shown to be the most limiting nutrients. Addition of both elements doubled above-ground production, and species rich- ness dropped ca. 50 % in 0.01-m 2 subplots, relative to controls. However, neither the above-ground production nor plant growth-forms were sufficient to explain the observed changes in species richness. Small-scale structural heterogeneity of the vegetation is probably critical for maintaining high levels of richness. Historically, high nitrogen, low phosphorus condi- tions were rarely encountered in the Dutch landscape and few species appear adapted to these conditions. Among the chalk grassland species, Brachypodium pinnatum seems well adapted to these conditions, where it dominates and excludes most other species. A detailed understanding of the small-scale processes responsible for maintenance of species richness is critically important in efforts to maintain the biodiversity of natural ecosystems.

High species mobility in species-rich plant communities: An intercontinental comparison

Using results from a long-term study of fine-scale dynamics in grasslands in four widely separated study areas from two continents, we provide further evidence to support the idea of the carousel model as an aid to describe the high fine-scale temporal and spatial species mobility found in grassland communities. Cumulative species numbers on small subplots in plots situated in stable plant communities, determined as the sum of species appearing in these subplots in one or more years over a period of time, are very high. In floristically different species-rich grasslands, varying from moist pine savannas in North Carolina and Mississippi, to humid chalk grassland in the Netherlands and seasonally dry limestone grassland in Sweden, average species numbers on subplots of 0.01 m2 in plots of 2.5 m2 over the period 1985–1989 were similar, most plots falling in the range 10.8–13.2. The total cumulative species numbers were similar as well, most plots falling in the range 17.4 and 20.9. Yearly average species numbers remained relatively constant. Considerable species turnover is occurring in all these communities; on average three species appear and three disappear each year in each 0.01 m2 subplot. Total species accumulation on 0.01 m2 subplots over the period 1985–1989 varied considerably, from 4.1 to 11.6, and is correlated with the cumulative species total on the plot, the latter figure being considered as correlated with the size of the species pool.

Integration of local and regional species-area relationships from space-time species accumulation

A long‐standing observation in community ecology is that the scaling of species richness, as exemplified by species‐area curves, differs on local and regional scales. This decoupling of scales may be largely due to sampling processes (the increasing constraint imposed by sampling fewer individuals at fine scales), as distinct from ecological processes, such as environmental heterogeneity, that operate across scales. Removal of the sampling constraint from fine‐scale richness estimates should yield species‐area curves that behave like those of the regions in which they are embedded, but an effective method for this removal has not been available. We suggest an approach that incorporates the manner in which small areas accumulate species over time as a way to remove the signature of sampling processes from fine‐scale species‐area curves. We report for three species‐rich grasslands from two continents how local plant species richness is distributed through time at multiple, nested spatial scales, and we ask whether sampling‐corrected curves reflect the spatial scaling of richness of each larger floristic province. Our analysis suggests that fine‐scale values of richness are highly constrained by sampling processes, but once these constraints are removed, the spatial scaling of species richness is consistent from the scale of individuals to that of an entire province.

The impoverishment of the bryophyte and lichen flora of the Dutch chalk grasslands in the thirty years 1953-1983

Abstract The Dutch chalk grasslands were formerly very rich in bryophyte and lichen species. A comparison of data from Barkman (1953) and more recent inventories yielded the following results: (a) Lichens have disappeared nearly completely from the Dutch chalk grasslands; (b) Many characteristic acrocarps (e.g., Tortella spp., Trichostomum spp., Aloina spp., Pleurochaete squarrosa ) and some pleurocarps ( Abietinella abietina, Camptothecium lutescens ) have decreased drastically; (c) Some common litter-indicator species ( Brachythecium rutabulum and others) are now increasing. We hypothesise that this change is mainly a result of combined effects of the abandonment of the old grazing regime, which is in only a few places replaced by mowing, and of air pollution. We are strongly in favour of current attempts to reintroduce grazing by the old race of ‘Mergelland’ sheep.

Soil seed bank, seedling recruitment and actual species composition in an old and isolated chalk grassland site

A soil seed bank study was conducted in an old and isolated chalk grassland site (area ca. 0.05 ha) with a high species richness in The Netherlands, over a two-year period. Each soil seed bank sample was divided into two halves: one part was sown immediately in trays with a layer of sterilized sand and put in a greenhouse, whilst the other half received a chilling period before being sown. Plant recruitment in the field was studied during the same period in permanently marked plots.Seeds of species which had become extinct during the period 1944–1970 were not found in the soil seed bank in the late eighties. One group of species still present in the actual vegetation neither showed any signs of recruitment from seed nor had a soil seed bank. Species in this group that do not recruit vegetatively can be considered as directly threatened by extinction. A group in the actual vegetation did not shown any recruitment, but nevertheless had germinable seed present in the soil. Among this group, there are some annuals; however, the majority of the species are characterized by the potential to recruit vegetatively. The majority of the species present in the actual vegetation (45%) demonstrate seedling recruitment and are also present as seeds in the soil, although there is some quantitative discrepancy between these categories. The seed bank did not reflect former stages of the actual vegetation.Based on the results, predictions can be made on the future development of the vegetation and on the fate of a number of species. Adjustment of the management regime is aimed at maintenance of the species richness of the site. This is important for nature preservation purposes since a high number (ca. 27%) of the species in the chalk grassland of this small site are on the Dutch Red List.

Flood events overrule fertiliser effects on biomass production and species richness in riverine grasslands

Abstract Question: Do severe winter flood events lift the nutrient limitation of biomass production in a river floodplain? How does this affect plant species richness? How long do the effects last? Location: Floodplain grassland on calcareous sandy loam near river Rhine in The Netherlands. Methods: Plots were fertilised with four treatments (control, N, P, N+P) for 21 years; plant species composition, vegetation biomass and tissue nutrient concentrations were determined every year between 1985 and 2005. Results: Fertilisation with N generally increased biomass production and reduced species richness, but these effects varied over time. During the first four years of the experiment, biomass production appeared to be co-limited by N and P, while N fertilisation dramatically reduced plant species richness; these effects became weaker subsequently. Following two extreme winter floods in 1993–94 and 1994–95 and a drought in spring 1996, the effects of fertilisation disappeared between 1998 and 2001 and then appeared again. Flooding caused an overall reduction in species richness (from c. 24 to 15 species m−2) and an increase in biomass production, which were only partly reversed after ten years. Conclusions: Long time series are necessary to understand vegetation dynamics and nutrient limitation in riverfloodplains, since they are influenced by occasional flood and drought events, whose effects may persist for more than ten years. A future increase in flooding frequency might be detrimental to species richness in floodplain grasslands. Nomenclature: van der Meijden (1996).

Effects of selective clipping and mowing time on species diversity in chalk grassland

Phytomass distribution and species diversity were followed in a chalk grassland with different selective clipping treatments during 4 years. Removal of the dominantBrachypodium resulted in a sharp increase in forb phytomass and in species number. The dominance-diversity curve changed in shape from geometric to log-normal. Obviously,Brachypodium restricted the growth of most other species. Removal of all species except the dominant showed, in contrast to the niche pre-emption theory, a marked increase in phytomass and reproductive effort ofBrachypodium. It seems that other mechanisms can also lead to a geometric distribution. Removal in a type of vegetation without obvious dominants and with a log-normal species distribution revealed that interspecific competition is not an important factor in this vegetation. This suggested a low intensity of species interaction and a large influence of restricting density-independent factors. Finally, to reduce the dominance ofBrachypodium we implemented a 4-year mowing experiment. Mowing earlier in the season resulted in a reduced exclusion of species byBrachypodium. This alternative mowing time may be sufficient to restore species diversity in chalk grassland. Experiments are under way to further substantiate this conclusion.