Spatial dynamics of plant species in an agricultural landscape in the Netherlands

This study examined the changes in distribution patterns of 13 herbaceous plant species from 1998 to 2000 in ditch banks along the edges of arable fields in the Netherlands. The objective was to test if spatial dynamics could be related to spatial isolation and disturbance of habitat and to the dispersal and seed bank characteristics of the species. Knowledge of these relations should be used to increase the effectivity of agri-environmental schemes aiming at an increase of botanical diversity. All species frequently colonized empty patches and populations in occupied patches frequently went extinct. Most colonization events occurred within 50 m of conspecific source patches in the preceding year, but colonization events in patches at distances more than 200 m from conspecific source patches were also observed. The colonization probabilities decreased with isolation distance. For nine species this relation was statistically significant, after correction for year and habitat. The extinction probabilities increased with isolation. For only four species this relation was statistically significant. Both colonization and extinction probabilities were more often statistically significant related to isolation for species with transient seed banks than species with persistent seed banks. Implications for management options aiming at survival of plant species in fragmented landscapes are discussed.     

Numerical assessment of plant species as indicators of the groundwater regime

Abstract. The relation between the occurrence of plant species in environments varying in moisture status and groundwater regime was tested using numerical methods. The groundwater regime during the vegetation period was expressed by means of four parameters, the average (AVG), mean highest (HIGH), mean lowest (LOW) groundwater level and the maximum fluctuation (AMP). 67 records of five vegetation types were selected from hydrologically stable sites in brook valleys in the northern part of The Netherlands. Response curves were calculated for 30 representative species. Calculated optima for AVG, HIGH and LOW are strongly correlated to each other. The vegetation reacts independently from overall wetness to the amount of fluctuation of the groundwater level (AMP). Response curves of single species as well as combinations of both present and absent species were used to find the best set of indicators for each parameter. The use of combinations of species clearly improves the indicating value of vegetation records. The vegetation appears to be the most sensitive to the parameter HIGH, which can thus be considered to be a key factor in controlling vegetation composition. The four parameters can be predicted satisfactorily only in the middle part of the investigated gradient. This is not only due to arithmetic artifacts, inherent to the applied method, but also to the fact that at average groundwater levels below - 60 cm or above 0 cm other factors become predominant.     

The species pool concept applied to forests in a fragmented landscape: dispersal limitation versus habitat limitation

Abstract. The species pool concept has been used as a theoretical framework for understanding local community richness. A significant problem in putting the concept into practice is the lack of methods for determining the size of the species pool. We tested the hypothesis that species composition of recent forests is primarily determined by the species composition of neighbouring older forests against the null‐hypothesis that species are a random sample of the species occurring in the study area. Forest plant species composition of recently established fragments was significantly correlated with species composition in neighbouring older forests (i.e. the local species pool). When older forest within a neighbourhood of 1000m radius is considered, seed dispersal sources can be found for 91% of the flora in the recent forests. For an individual fragment, dispersal is a much more important determinant of species presence than the environment, with an average of 46% of the total pool excluded from local pools by dispersal limitation and only 8% excluded by environmental limitations. The species richness of recent forests is on average 23% of the local species pool. Several hypotheses are proposed for this low percentage, such as asymmetric competition due to the early successional state or the limited colonization period.     

Hydrology of Dutch Cirsio‐Molinietum meadows: Prospects for restoration

Abstract. Fen meadows (Cirsio dissecti‐Molinietum) are seriously threatened by desiccation, acidification and eutro‐phication. In The Netherlands several projects were launched to restore damaged fen meadows. This review describes how successes and failures of these restoration projects depend on hydrological systems. Six hydrological systems have been distinguished, which all provide the site conditions required by this community. Nowadays, the best developed fen meadows are found in the higher Pleistocene landscape of The Netherlands, where they depend on base‐rich groundwater discharging from local or large groundwater systems. Fen meadows of the lower Holocene landscape usually occur in man‐made surface water systems. Almost all stands have been severely deteriorated. Restoration of fen meadows in the Pleistocene landscape is promising when the hydrology is only slightly disturbed or when hydrological measures are taken in combination with sod cutting. Restoration prospects of fen meadows in the Holocene landscape are low. Until now a complete regeneration of Cirsio‐Molinietum meadows has not been realized. Restoration measures failed to restore high pH values in the top soil. It is hypothesized that viable seeds of many target species lack in the soil seed bank. In addition, the dispersal capacities of these species seem to be limited.     

Patterns of species diversity on an oceanic versus a continental island mountain: a hypothesis on species diversification1

Abstract. The vegetation on a wet mountain slope on Haleakala (an oceanic island) is compared with that on Kinabalu (a continental island) to examine relationships between regional floristic richness and α- and β-diversities. The two mountains are similar in their constant tropical climate, generic and family-level floristic elements and geological age of the summit regions, but different in regional floristic richness (rich on Kinabalu vs. poor on Haleakala). α-diversity of canopy and subcanopy tree species was much higher on Kinabalu than in comparable zones on Haleakala. Average turnover rate of species (as logarithmic community similarity) on the slope was one order of magnitude greater on Kinabalu than on Haleakala (0.127 vs. 0.017 per 100 m alt.). While there were genera with wide altitudinal ranges on both mountains, a large proportion of the genera was differentiated into parapatric altitudinal congeners on Kinabalu. By contrast, most genera are altitudinally monotypic on Haleakala. The number of sympatric congeners per genus, and the frequency of multi-specific genera per plot were high on lower slopes but decreased with increasing altitude on Kinabalu, whereas the values were low across all altitudes on Haleakala. These patterns suggest that sympatric and parapatric species radiation was less on Haleakala than on Kinabalu. This may be related to Haleakala's initially poor and disharmonic flora.     

Vegetation development in dune slacks: the role of persistent seed banks

Abstract. The soil seed bank composition was determined at four sites in the dune slack ‘Koegelwieck’ on the Dutch Wadden Sea island of Terschelling. At three different sites in the slack, where sod-cutting experiments down to the mineral sand had been carried out, the established vegetation and seed bank were assessed after 5, 9 and 39 yr of undisturbed development, respectively. In addition, a fourth site in the slack was investigated, where vegetation development had proceeded for 80 yr since plant colonization of bare soil and where nowadays a vegetation dominated by Calamagrostis epigejos and Salix repens occurs. Together these four sites can be regarded as a chronosequence of dune slack formation. Clear time sequences were detected in the seed bank data. Many late successional species showed a significant increase in the number of seeds during the succession. Some of the early successional basiphilous pioneer species such as Anagallis minima, Centaurium littorale, Littorella uniflora, Radiola linoides and Samolus valerandi, showed either a decrease during the time of succession or an optimum in the vegetation while remaining present in the seed bank in low but detectable numbers. They could, therefore, play a role in re-establishment of the vegetation after sod-cutting. One of the target species, Schoenus nigricans, established within a few years after removal of the sod. However, no seeds of this species have been detected in the soil below either of the successional stages. Based on the species disappearance from the established vegetation and based on the independent data of Thompson et al. (1997) an estimation of seed longevity could be made for several Red List species of wet dune slacks.     

Alternative stable states in a wet calcareous dune slack in The Netherlands

Abstract. Evidence is presented for the occurrence of alternative stable states in a wet calcareous dune slack on the Frisian island of Texel, The Netherlands. An early pioneer stage (0.5 kgm^?2 total standing crop) and a more productive later successional stage (2.9 kg m^?2) occur side by side, with sharp boundaries between them. The pioneer vegetation has been recorded at the site for more than 62 yr. These features indicate the occurrence of a positive‐feedback mechanism that has led to alternative stable states. Analyses of ground and surface water composition, and decalcification depths, indicated that hydrologically the study site can be characterized as a flow‐through slack, with exfiltration of calcareous groundwater on one side and infiltration of surface water on the other side of the slack. These differences in hydrological conditions have led to distinct differences in environmental conditions within the dune slack. The occurrence of the two successional stages can, however, not be explained by differences in hydrological conditions since both stages occur side by side in the centre of the dune slack. It is, therefore, more likely that biotic interactions are the cause of the vegetation pattern. Three possible mechanisms for feedback processes are discussed: (1) enhanced nitrogen loss; (2) sulfide toxicity and (3) nutrient accumulation in internal cycle.     

Impact of different‐sized herbivores on recruitment opportunities for subordinate herbs in grasslands

Abstract. Potential effects of herbivores on plant species diversity depend on herbivore size, species and density. In this study we examine the effect of different‐sized herbivores (cattle and rabbits) on recruitment of subordinate herbs in grasslands. We show that in a grazed floodplain, grassland plant species richness is mainly determined by the presence of many species of subordinate herbs. These herbs experience high colonization and extinction rates. We conclude that the creation of colonization opportunities for subordinate herbs plays a crucial role in maintaining plant species richness in productive grasslands. We found that cattle disperse large amounts of seeds via their dung, over ten times more than rabbits. Rabbits create more and on average larger bare soil patches than cattle. In a field experiment artificial disturbances improved germination success tremendously for four tested herb species. We found that bare soil is the best regeneration site, while cattle dung gave a too strong nutrient stimulus, resulting in tall vegetation and therefore light limitation. These results can be confirmed with results from field monitoring plots where plant species richness was positively related to the occurrence of bare soil patches. Therefore both large and small herbivores have a major impact on dispersal and colonization, but for different reasons. Cattle are identified as most important for seed dispersal whereas rabbits have a main effect as creators of disturbances. These results emphasize the importance of distinguishing between herbivore species in assessing their (potential) effects.     

Nutrient limitation and vegetation changes in a coastal dune slack

Abstract. Basiphilous pioneer species are among the most endangered plant species in The Netherlands. They find most of their refuges in young coastal dune slacks, especially on the Wadden Sea islands. For the purpose of nature management it is important to know which processes control the presence of basiphilous pioneer communities, and to learn about the nature of slacks harbouring the concerning successional sequences. In a large dune slack on the Island of Terschelling, we assessed soil nutrient status and tested for nutrient limitation in four chronosequential stages: 2, 6, 37 and ca. 80 yr of age. Stage 2 harboured a basiphilous pioneer vegetation; in the stages 3 and 4 a dense vegetation of dwarf shrubs and grasses occurred. Soil organic matter and nutrient concentrations in each stage were measured in 1991. In 1992 and 1993 fertilizers were applied to all stages to detect nutrient limitation. Rates of accumulation of organic matter, nutrients and above-ground biomass were estimated. When interpreted as successional stages, the different stages represent a sequence as expected on the basis of general successional theory. There was a peak in yearly nutrient accumulation between the 6- and 37-yr old stage and a steady state after ca. 80 yr. Between the first two and the latter two stages a shift occurred from allogenic to autogenic succession which correlated with a shift in emphasis from available nutrients to light availability as limiting resources. Basiphilous pioneer species suffered only deficiency of nitrogen, probably because of their low phosphorus requirements. It is concluded that in dune slack habitats, in addition to a low nutrient availability in general, a very low phosphorus availability favours basiphilous pioneer species to species showing co-limitation of nitrogen and phosphorus as found in some grasses and dwarf shrubs. A comparison between the effects of lime addition and the effects of nitrogen and phosphorus additions suggests that, in the early stages, soil buffering increases the availability of nitrogen and inhibits the availability of phosphorus. Sod cutting is an effective technique for restoring basiphilous pioneer vegetation, when slacks are acidified only superficially and buffering-mechanisms can be reactivated. Yearly mowing and removing of standing crop may prolong the life-span of basiphilous pioneer vegetation, when soil acidification has not yet dropped below pH 6.     

Two decades of change in the ground vegetation of a mixed deciduous forest in an agricultural landscape

Abstract. A vegetation survey of semi‐permanent plots was performed in 1977–1980 and in 1997–1998 to study the floristic changes in a mixed deciduous forest situated in a region of intensive livestock breeding. Our aim was to test the hypothesis of allogenic succession in the forest ground vegetation caused by the emissions of the surrounding agricultural activities. During the last 20 years, a significant increase of nitrogen indicators and a decline of moisture indicators were observed. Results were consistent at the community, the plot and the species level. Nitrogen deposition and the lowering of the groundwater table are suggested as the driving forces behind the changes. The results are discussed in the context of current ecological theory concerning resource competition and community diversity.     

Wind dispersal in freshwater wetlands: Knowledge for conservation and restoration

Questions: For wetland plants, dispersal by wind is often overlooked because dispersal by water is generally assumed to be the key dispersal process. This literature review addresses the role of seed dispersal by wind in wetlands. Why is wind dispersal relevant in wetlands? Which seeds are dispersed by wind and how far? And how can our understanding of wind dispersal be applied to wetland conservation and restoration? Methods: Literature review. Results and conclusions: Wind is a widely available seed dispersal vector in wetlands and can transport many seeds over long distances. Unlike water, wind can transport seeds in all directions and is therefore important for dispersal to upstream wetlands and to wetlands not connected by surface water flows. Wind dispersal transports seeds to a wider range of sites than water, and therefore reaches more sites but with lower seed densities. Many wetland plant species have adaptations to facilitate wind dispersal. Dispersal distances increase with decreasing falling velocity of seeds, increasing seed release height and selective release mechanisms. Depending on the adaptations, seeds may be dispersed by wind over many km or only a few m. The frequency of long‐distance wind dispersal events depends on these adaptations, the number of produced seeds, the structure of the surrounding vegetation, and the frequency of occurrence of suitable weather conditions. Humans reduce the frequency of successful long‐distance wind dispersal events in wetlands through wetland loss and fragmentation (which reduce the number and quality of seeds) and eutrophication (which changes the structure of the vegetation so that seed release into the wind flow becomes more difficult). This is yet another reason to focus on wetland conservation and restoration measures at increased population sizes, prevention of eutrophication, and the restoration of sites at short distances from seed sources.     

Effects of age and distance on the composition of mixed deciduous forest fragments in an agricultural landscape

Abstract. Forest patches in central Belgium were inventoried twice for the presence or absence of forest plant species to study the effects of age and distance on species composition. All forests in the study area were subdivided based on their land use history. To avoid effects of autocorrelated environmental characteristics on species composition, habitat homogeneity was indirectly investigated using a TWINSPAN classification of the vegetation data. Two major habitats (alluvial and non‐alluvial forests) were distinguished and analysed separately. Patterns of species composition were investigated at the landscape level using Mantel tests. Species composition similarity measures were calculated between all pairs of fragments based on the floristic data. A highly significant correlation was found between species composition similarity and inter‐patch distance. Difference in age, which we used as a measure for habitat quality, was less important in explaining species composition patterns. The effects of spatial configuration became significant when difference in age was accounted for, and a partial correlation was calculated between inter‐patch distance and species composition similarity. Different results were found for alluvial and non‐alluvial forest types. Alluvial forests were more influenced by the spatial configuration than the non‐alluvial. For the non‐alluvial forest type effects measured with the difference in age between forest fragments obscured the effects of inter‐patch distance. Based on our findings we suggest that species composition is not only internally generated, but external processes such as differential colonization caused by varying degrees of isolation may be of overriding importance.     

Degeneration of species-rich Calthion palustris hay meadows; some considerations on the community concept

Abstract. An overview of the vegetation history of Calthion palustris meadows is presented with special emphasis on the natural habitats of the character species. The response of meadow species under the influence of drainage was calculated using species-environment response curve techniques. The changes were monitored in permanent plots situated on sites with various intensities of drainage. The similarity with an undisturbed local reference type of the hay meadow community was calculated for all plots and years. On this basis a half-life time of the Calthion palustris stands could be assessed. The half-life time was small in the plots which are most affected by drainage. The response to drainage of character species of the Calthion palustris was very different. This imposes problems for some definitions of the community concept.     

Vegetation succession in lakes of West Connemara,Ireland: comparing predicted and actual changes

Abstract. Changes in the vegetation of lakes and wetlands were investigated over a period of 18 years. It was assumed that changes in vegetation were related to changes in agricultural land use resulting in increased phosphate levels in surface waters. Data were collected in 1975, 1988 and 1993. Multivariate techniques were used to relate changes in vegetation to changes in environmental factors. With the use of a Markovian chain model, vegetation development was projected into the future. Projections based on vegetation dynamics between 1975 and 1988 were compared with actual changes in the vegetation. The vegetation dynamics appeared stable on a regional scale but quite dynamic on a local scale. A continuous decline in species diversity was noted as well as an overall increase of phosphate level. However, only minor changes in vegetation could be attributed to this increase of phosphate. Major changes were a result of fluctuations in water level. These changes coincide with periods of drier and wetter climate. Because of the fluctuating nature of these changes, predicted vegetation change did deviate from the observed change.     

Ecosystem development on Hawaiian lava flows: biomass and species composition

Abstract. The strong environmental gradients and ‘natural experimental design’ of Mauna Loa volcano, Hawaii, provide an outstanding opportunity to study controls on ecosystem development. We measured above-ground vascular plant biomass and species composition on 42 sites on which precipitation, temperature, substrate texture, and substrate age varied substantially and largely independently. Biomass and species richness of live plants were strongly correlated with precipitation and lava flow age, but not with temperature or lava flow texture. Species composition, as measured by correspondence analysis, was likewise correlated with precipitation and flow age, but composition was also strongly influenced by temperature. Lava texture had a complex effect on vegetation, with ‘a’ a lava favoring vegetation development on wet sites and pāhoehoe favoring development on dry sites. Many locations remain virtually free of invasion by alien species; aliens appear where disturbance has facilitated invasion, either from stand-level dieback in rainforest or a grass-fire cycle on the dry, leeward side of the mountain. All four of the environmental factors studied here (precipitation, temperature, substrate texture, and substrate age) exert significant and independent control over vegetation biomass and/or species composition on Mauna Loa.     

Effects of woodlot isolation on the dispersion of plants with fleshy fruits

By relating species presence, number and density to the perimeter and isolation of small (0.1–2.75 ha) homogeneous woodlots (n = 43), indications were obtained that some fleshy-fruited species are negatively affected by woodlot isolation. The number of fruit-bearing herbaceous perennial species increased with woodlot area, probably because of the increasing heterogeneity of the herb layer. The density of individuals of the investigated species were negatively correlated to woodlot area. There were also some indications thatthe presence/absence of species was related to the length of the forest edge (e.g. Hederá helix). Because woodlot area and shape index were strongly correlated, the effects of increased forest area and forest edge were hard to separate. The probability of occurrence of Lonicera periclymenum and the density oí Ilex aquifolium decreased with an increasing degree of isolation of present forest islands. The chance of occurrence of L. periclymenum decreased also with the degree of isolation in the past, measured from topographical maps of about 60 yr ago. The total area of broad-leaved forest within a radius of 500 m, as well as the distance to the nearest connecting landscape element, seem to be good indicators of woodlot isolation.     

Distribution of plant functional types along gradients of disturbance intensity and resource supply in an agricultural landscape

Abstract. In this study, plant functional types are understood as groups of plants with similar biological traits displaying significant optima or maxima on a gradient plane of resource supply and disturbance intensity. The biological traits refer to expansion, vegetative regeneration, generative reproduction, dispersal and seed bank longevity. 129 vegetation samples were taken in an agricultural landscape in southwestern Germany, covering a wide range of terrestrial vegetation types – but with the exception of forests and wetlands. For each site, also soil data were recorded. Mean daily soil moisture was estimated with a simple model. Soil moisture, balanced nitrogen supply and available phosphorus were combined into a factor ‘resource supply’. In addition, disturbance intensity was estimated for each site. This factor was based on (1) frequency of disturbance, (2) disturbance depth below or above the soil surface, and (3) proportion of the area affected by a discrete disturbance event. 30 plant groups with similar biological characteristics resulted from a cluster analysis, based on a compilation of 19 biological traits for a regional species pool. Logistic regression on a gradient plane of disturbance intensity and resource supply yielded response curves for 28 groups. The dependent variable was defined as the probability of encountering all members of a group in a sample. 17 groups display a significant response curve on the gradient plane. Plants with a potential for long- range dispersal are concentrated on sites with low or high disturbance intensities (e.g. fallow land, fields, lawns). On sites with medium disturbance intensity (e.g. meadows) and low to medium resource supply, small-range dispersal predominates. There are no distinct trends concerning seed bank longevity. The potential for vertical and lateral expansion increases with decreasing disturbance intensity. Only at medium disturbance intensities does vertical expansion correlate positively with resource supply. Rapid detachment of daughter individuals occurs more often on productive sites than on less productive sites. Diversity of groups with similar biological traits is highest on sites with medium disturbance intensities.     

Recruitment in Dentaria bulbifera; the roles of dispersal,habitat quality and mollusc herbivory

Abstract. We investigated the effects of dispersal limitation, diaspore density, soil chemistry, disturbance and mollusc herbivory on the abundance and distribution of the perennial forest herb Dentaria bulbifera (Brassicaceae). After experimental sowing of bulbils in originally empty patches, juveniles of Dentaria emerged in 83% of plots. The seventh year after sowing, plants still persisted and were reproducing in several plots, demonstrating that the species is dispersal‐limited. However, long‐term survival of transplants was higher at sites where Dentaria occurred naturally, and this difference increased with time after sowing. Thus, the importance of local factors may only be expressed fully several years after colonisation. Differences in soil chemistry and disturbance treatment were not associated with differences in patch suitability. Removal of molluscs, however, significantly increased recruitment, and the effects of a single molluscicide treatment persisted after four years. We conclude that dispersal is not the only limitation to the distribution of Dentaria, and that local factors, such as mollusc herbivory, are also crucial.