J.M. van Groenendael

GERMINATION STRATEGIES DURING GRASSLAND SUCCESSION

1. We analysed the germination of 91 herbaceous species in response to a temperature gradient, and to alternating temperatures and stratification (chilling). A principal-components analysis revealed that the species were distributed along two statistically independent axes, the first of which primary represented the optimal temperature for germination and the second the rate of germination. 2 These results were further related to data on species replacement during 25 years of succession in grassland after the cessation of fertilizer application but with continued hay making. Separate successional sequences were distinguished for the dry parts and the wet parts of the fields. Changes in canopy structure were studied by counting gaps and mapping light at the soil surface. 3 Changes in species composition of species present at the different stages were related to the changes in the germination attributes during the succession towards nutrient-poor grassland. The productive grassland was characterized by rapidly germinating species and in the wetter parts, by those germinating at low temperatures. This might enable these species to escape from light competition by germinating in the autumn or winter, i.e. soon after seed set. The species from the less productive, more open stages germinated more slowly, and responded more clearly to stratification and alternating temperatures. It is therefore likely that germination of most seeds of these species is delayed until the following summer. 4 The importance of the observed interspecific differences in germination characteristics in relation to the observed changes in vegetation structure as an explanation for species replacement during succession is discussed.

The trade‐off between dispersability and longevity ‐ an important aspect of plant species diversity

Abstract. Local presence of plant species is determined by population colonizations and extinctions. All traits that influence the capacity of individuals to colonize patches and survive within patches, are therefore important for community diversity. Spatial models can explain the coexistence of species provided that the inferior competitor has a greater spatial mobility and thereby can avoid competition. We searched the literature for empirical evidence for such trade-offs and included all available information on correlations between traits associated with the capacity to colonize and traits promoting the ability to survive. A lower reproductive effort of a species is associated with a longer life span and a higher competitive ability. Morphological adaptations for dispersal are less common in species which better tolerate stress, that are better competitors or possess seed dormancy. Such patterns suggest that species that are good survivors may have a limited ability to colonize new patches and vice versa. A negative correlation between dispersability and longevity has important effects on the regional dynamics of single species as well as on the coexistence of species. From a conservation perspective differences in the colonization capacity among species imply that restoration of plant biodiversity must not only focus on conditions within patches, but also consider the spatial arrangement of patches in order to enable plants to bridge gaps in time and space.

The contrasting dynamics of two populations of Plantago lanceolata L. classified by age and size.

(1) The dynamics of two populations of Plantago lanceolata from contrasting habitats are simulated with the use of matrix projection models. The fecundity and survival parameters that together form the matrix are based on demographic fieldwork and evaluated for both size and age categories at the same time. (2) After verification of the model, elasticity analysis is applied to determine the model parameters that are most important in defining the population growth rate. The effect of more complex changes in the life histories on the population growth rate is evaluated by means of simulation. (3) The results are in general agreement with expectations based on field observations: seed production is more important in an unpredictable environment, whereas adult survival is more important in a stable environment. In both populations seedling establishment is the most important phase in the life history, especially in the stable habitat. (4) The formation of side rosettes is not so much a mode of vegetative reproduction as a way to increase the current years seed production in an unpredictable environment. (5) Timing of germination in spring or in autumn is important, but the effect of delay by means of a seedbank is unexpectedly small. (6) The particular form of this matrix projection model is time-invariant. Because fecundity and survival are rarely constant over time, however, random sequences of bad, normal and good years are generated and incorporated in the model. The impact of this variation over time is evaluated using time-to-extinction as a measure.

Reproductive strategy, clonal structure and genetic diversity in populations of the aquatic macrophyte Sparganium emersum in river systems

Many aquatic and riparian plant species are characterized by the ability to reproduce both sexually and asexually. Yet, little is known about how spatial variation in sexual and asexual reproduction affects the genotypic diversity within populations of aquatic and riparian plants. We used six polymorphic microsatellites to examine the genetic diversity within and differentiation among 17 populations (606 individuals) of Sparganium emersum, in two Dutch‐German rivers. Our study revealed a striking difference between rivers in the mode of reproduction (sexual vs. asexual) within S. emersum populations. The mode of reproduction was strongly related to locally reigning hydrodynamic conditions. Sexually reproducing populations exhibited a greater number of multilocus genotypes compared to asexual populations. The regional population structure suggested higher levels of gene flow among sexually reproducing populations compared to clonal populations. Gene flow was mainly mediated via hydrochoric dispersal of generative propagules (seeds), impeding genetic differentiation among populations even over river distances up to 50 km. Although evidence for hydrochoric dispersal of vegetative propagules (clonal plant fragments) was found, this mechanism appeared to be relatively less important. Bayesian‐based assignment procedures revealed a number of immigrants, originating from outside our study area, suggesting intercatchment plant dispersal, possibly the result of waterfowl‐mediated seed dispersal. This study demonstrates how variation in local environmental conditions in river systems, resulting in shifting balances of sexual vs. asexual reproduction within populations, will affect the genotypic diversity within populations. This study furthermore cautions against generalizations about dispersal of riparian plant species in river systems.

Nutrient limitation and nutrient-driven shifts in plant species composition in a species-rich fen meadow

Question: We studied the development and persistence of the effects of nutrient pulses on biomass production and species composition in a fen meadow. Location: Nature reserve, central Netherlands, 5 m a.s.l. Methods: Single pulse fertilization with N and P in a factorial design on an undrained central and a drained margin site in a species-rich fen meadow (Cirsio dissecti-Molinietum). Biomass production and species composition were monitored during four years. Results: At the central site, N addition boosted biomass production, but only during one year. The species composition was not changed. P fertilization increased the biomass production and changed the species composition from a vegetation dominated by Carex panicea to a grassland community with abundant Holcus lanatus, but not before the second year. At the margin site, P fertilization changed the species composition in a similar way, but biomass production was not increased. N fertilization had no effect. At both sites the P induced shift in species composition persisted for four years although the P effect declined during the experiment. Conclusions: The biomass responses show that N was limiting in the central site. Another nutrient, besides N and P (probably K) must have been limiting in the marginal site. The fast decline of the N effect on biomass is ascribed to increased denitrification and biomass removal. The delay in the P effect on biomass and species composition and the persistence of the P effect on species composition are ascribed to fast immobilisation and subsequent slow release of fertilizer P in the peat soil. Recurrence of the P pulses is expected to cause permanent changes in species composition.Abstract Question: We studied the development and persistence of the effects of nutrient pulses on biomass production and species composition in a fen meadow. Location: Nature reserve, central Netherlands, 5 m a.s.l. Methods: Single pulse fertilization with N and P in a factorial design on an undrained central and a drained margin site in a species-rich fen meadow (Cirsio dissecti-Molinietum). Biomass production and species composition were monitored during four years. Results: At the central site, N addition boosted biomass production, but only during one year. The species composition was not changed. P fertilization increased the biomass production and changed the species composition from a vegetation dominated by Carex panicea to a grassland community with abundant Holcus lanatus, but not before the second year. At the margin site, P fertilization changed the species composition in a similar way, but biomass production was not increased. N fertilization had no effect. At both sites the P induced shift in species composition persisted for four years although the P effect declined during the experiment. Conclusions: The biomass responses show that N was limiting in the central site. Another nutrient, besides N and P (probably K) must have been limiting in the marginal site. The fast decline of the N effect on biomass is ascribed to increased denitrification and biomass removal. The delay in the P effect on biomass and species composition and the persistence of the P effect on species composition are ascribed to fast immobilisation and subsequent slow release of fertilizer P in the peat soil. Recurrence of the P pulses is expected to cause permanent changes in species composition. Nomenclature: van der Meijden (1990) for phanerogams and Schaminée et al. (1996) for syntaxa.

Density dependent simulation of the population dynamics of a perennial grassland species, Hypochaeris radicata

Hypochaeris radicata L. is a rosette-forming herb that often attains dominance after a number of years in newly established grassland like roadverges, but decreases afterwards. To evaluate the possible causes of its success and eventual decline, a transition matrix model was constructed simulating different management regimes, with parameter values estimated from preliminary field data. Mowing twice a year results in the largest initial population growth rate and the largest equilibrium density compared with the other simulated mowing frequencies (mowing once a year, uncut verge). Elasticity analysis indicated that the parameters that together form the pathway of sexual reproduction and those that represent vegetative ramification, are balanced with respect to their impact on population growth. This balance shifts with decreasing mowing frequency towards vegetative ramification. This is largely the result of variation in transitions of the pathway of sexual reproduction. Large rosette densities are maintained only when these variable transitions, such as germination and establishment of seedlings are optimal. This characterizes H. radicata as a fugitive species, rather than as a competitor, and helps to understand its replacement in a succession.

Assessing the relative importance of dispersal in plant communities using an ecoinformatics approach

Increased insight into the factors that determine the importance of dispersal limitation on species richness and species composition is of paramount importance for conservation and restoration ecology. One way to explore the importance of dispersal limitation is to use seed-sowing experiments, but these do not enable the screening of large sets of species and habitats. In the present paper we present a complementary approach based on comparing small plots with larger regions with regard to species composition and distribution of functional traits. We developed a GIS tool based on ecological and geographical criteria to quantify species pools at various spatial scales. In this GIS tool, containing floristic, large databases, phytosociological and functional information are exploited. Our premise is that differences in the nature of the species in local and regional species pools with regard to functional traits can give important clues to the processes at work in the assembly of communities.We illustrate the approach with a case study for mesotrophic hay meadows (Calthion palustris). We tested the effects of differences in frequency in the local Habitat Species Pool and differences in dispersal and persistence traits of species on local species composition. Our results show that both species pool effects and functional traits affect the probability of occurrence in small plots. Species with a high propagule weight have, given the frequency in the Local Habitat Species Pool, a lower probability of occurrence in small plots. The probability of local occurrence, however, is increased by the ability to form a persistent soil seed bank and by adult longevity. This provides support for the view that the degree of dispersal limitation is dependent on the degree of spatial isolation of the focal site relative to source populations and moreover that species inherently differ in the degree to which dispersal limitation is a limiting factor for local occurrence.

Regional gene flow and population structure of the wind-dispersed plant species Hypochaeris radicata (Asteraceae) in an agricultural landscape

Using microsatellites, we investigated population structure and gene flow of the short‐lived, wind‐dispersed plant species Hypochaeris radicata in a fragmented agricultural landscape where more than 99% of the nutrient‐poor grasslands have disappeared over the last century. We sampled populations in the few remaining high density populations in conservation areas, as well as individuals that occurred, with lower densities, in linear landscape elements, at two spatial scales. In a re‐inventory of the landscape, after 3 years, both extinctions and colonizations of populations were observed. Contrary to expectations, no differences in genetic diversity between high and low density populations were observed. Both types of populations had relatively high levels of diversity. Overall genetic differentiation (θ) was 0.04 and significantly different from zero (P < 0.01). A significant isolation‐by‐distance pattern was found when all populations were simultaneously analysed (r = 0.24, P = 0.013). Isolation by distance was (marginally) significant at the small scale (r = 0.32, P = 0.06), whereas nonsignificant at the large spatial scale (r = –0.05, P = 0.66). A maximization‐of‐explained‐variance procedure resulted in a threshold distance of 3.5 km above which populations were effectively genetically isolated. An additional partial exclusion Bayesian‐based assignment test showed that overall 32.3% of the individuals were assigned to their population of origin, 48% were assigned to another population in the area and 19.7% were not assigned. Together, these results suggest high levels of gene flow. Seed dispersal contributes to the observed gene flow up to several hundred metres, which is higher than previously modelled using aerodynamic models on seed dispersal of H. radicata. We discuss the consequences of these results for an evaluation of the probability of persistence of this species in the fragmented landscape.