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- Materials and methods
The evaluation of changes in habitat quality or ecological conditions often occurs on the basis of vegetation monitoring. In addition, changes in frequencies or abundances of rare species with a narrow ecological tolerance are frequently used as indicators for local ecological change. Nevertheless, observed changes in abundance are only rarely linked to demographic data. In the case of plant conservation, it is frequently assumed that rare species occurring in specific plant communities will benefit from management measures that maintain the community. However, because the demographic responses of endangered or target species to this community-based management are often largely unknown, the outcome is generally unpredictable. Counts of flowering plants yield very little information on population viability and its relation to different management regimes. Particularly when there is no time or money for detailed demographic monitoring of individual plants over a period of years (Menges & Gordon 1996), information on population structure can reveal important information (Oostermeijer, van’t Veer & den Nijs 1994).
The main aim of the present study was to analyse the population stage structure of the rare perennial plant Salvia pratensis L. in order to (i) learn more about the interactions between different life stages and habitat and identify factors that are correlated with demographic viability; (ii) test whether viable populations of S. pratensis are also indicative of sites with higher conservation value in terms of vegetation composition and species richness; and (iii) evaluate further the suitability of using the population stage structure as an indicator of the demographic viability of a rare plant species.
We hypothesized that populations of S. pratensis can be classified into different types that vary in the relative proportions of young life stages and that can be related to vegetation structure. Furthermore, we expected that viable (i.e. growing or stable) populations of S. pratensis occur in more species-rich vegetation types of higher conservation value. Finally, we hoped to identify management regimes that positively affect the demographic viability of S. pratensis in order to provide advice on the best short-term conservation management.
- Top of page
- Materials and methods
The observed division of S. pratensis populations into three types was based on the relative proportions of the different life stages. However, besides the stage structure, there were certain other features that distinguished the population types. These included differences in population size, total density of S. pratensis individuals, and vegetation structure (mainly the percentage of bare soil cover) and composition (species characteristic of nutrient-poor vs. nutrient-rich soils). The type of habitat management also seemed to have an effect on the population structure of S. pratensis.
The data support our hypothesis that it was possible to distinguish different types of population structure for S. pratensis. Comparing our findings with the literature, we named the three population clusters dynamic (invasive; sensuRabotnov 1985; Oostermeijer, van’t Veer & den Nijs 1994), normal and regressive (Rabotnov 1985; Oostermeijer, van’t Veer & den Nijs 1994). With long-term demographic studies and matrix projection models, Oostermeijer et al. (1996) could relate the three population types found in the wet heathland species Gentiana pneumonanthe to the actual population growth rates (measured as the finite rate of increase, λ). Dynamic (invasive) populations increased (λ > 1), normal populations were stable (λ≈ 1) and regressive populations declined (λ < 1). It is possible that the three population types of S. pratensis show similar growth rates, although this remains to be confirmed.
In contrast to the predominance of regressive populations in Gentiana pneumonanthe (Oostermeijer, van’t Veer & den Nijs 1994), most of the Salvia populations were either of the dynamic or the normal type. This suggests that the management of the remaining sites was generally favourable. However, the small populations under an early mowing regime currently depend on the survival of a few large, and probably rather old, flowering plants, and therefore risk local extinction if the current conditions do not change. In those reserves, it is necessary to either change the mowing time to a later date, after seed shedding of Salvia and most other species has occurred, or change to seasonal grazing. Nevertheless, in such small populations, the response to improved management in the form of seedling recruitment could be impaired by inbreeding depression in combination with low seed production resulting from the Allee effect (Oostermeijer 2000).
Bare soil was the most important vegetation structure parameter, suggesting that the presence of open ground is important for the performance of the population. The proportions of seedlings and juveniles and the S + J/G ratio were positively correlated with the percentage of bare soil. Open patches in the vegetation are thus apparently important as ‘safe sites’ for germination and seedling survival. This is consistent with field observations on recruitment of S. pratensis in British populations (Rich 1999) and with studies on the recruitment of various other plant species in regularly disturbed areas (Holderegger 1996) and grasslands (Johnson & Thomas 1978; Fowler 1988; Rusch 1992; Krenová & Leps 1996). According to Spackova, Kotorova & Leps (1998), removal of the vegetation and/or bryophyte layer is important for the seedling recruitment of many wet meadow species. In general, it has been suggested (Tilman 1993) that lack of seedling recruitment may be one of the major causes of declines in species diversity in grasslands.
It is interesting that the percentage of generative individuals showed the same positive correlation with bare soil surface. Given that there are also relatively high percentages of young plants, this indicates that a higher proportion of adults is flowering in open patches. Hence, the vegetation structure seems to affect the most important phases in the demography of this species: germination, seedling establishment and flowering.
We hypothesized that viable populations would be found in vegetation with a higher conservation value, which would be reflected in a higher species diversity or the occurrence of other rare species. To some degree, the species composition of the surrounding vegetation was associated with the viability of S. pratensis populations. Consistent with our hypothesis, species diversity was higher in dynamic and normal populations, although the differences were small and only marginally significant. However, other (rare) species of the Medicagini–Avenetum pubescentis, the characteristic plant community of dry river grasslands (Schaminée et al. 1996), were indeed restricted to sites with dynamic Salvia populations, whereas species typical of nutrient-rich conditions were associated with regressive populations. Under the latter conditions, population size may still be rather large, and the remaining flowering Salvia individuals are often tall and bear many flowering spikes and hence appear quite vital. Both types of information clearly give a false impression of population viability. By assessing the population stage structure, more useful information can be obtained in a simple way.
Vegetation management is one of the most important and interesting aspects of applied vegetation science. In this study, the most striking results were the differences in structure between populations of S. pratensis with a late mowing regime and the early mown or the grazed populations. Late mowing was performed in three populations, which together had more plots with a dynamic structure than were expected by chance. Two of those populations were the largest in the Netherlands and the majority of the few seedlings observed was found here. In contrast, many of the regressive populations were mown early, although there were several early mown populations with a normal or dynamic structure. This result suggests that late mowing is and has been the most favourable management for S. pratensis. Other authors also propose late mowing as a method for recovering typical dry floodplain grassland vegetations from nutrient-rich situations (van Eck, van Zuijen & Sykora 1997). However, early mowing of river dykes (in June) has been recommended more recently (Liebrand 1999).
The difference in recruitment between the late and early mown populations is due to the timing of seed shedding. In the early mown populations, seeds are generally not ripe at the time of mowing. The seeds produced after regrowth later in the season might be less viable, for instance because of reduced pollinator visitation, higher selfing rates and subsequent inbreeding depression (van Treuren et al. 1993; Ouborg & van Treuren 1994). One population where mowing was performed relatively early, but after the seed was shed, was classified as dynamic. A less rigid mowing schedule, which takes the temporal and spatial variation among populations into account, seems the best way to maintain or promote the remaining populations of S. pratensis. Another argument in favour of late mowing is that it opens up the vegetation canopy in the autumn period (Olff et al. 1994), which is when S. pratensis germinates.
Many ecological restoration projects are in progress in the Dutch floodplains. This will eventually result in new habitats and will enlarge the remaining habitats for many characteristic plants and animals. The dominant management regime in these areas, year-round grazing by free-ranging animals, will definitely require larger areas to avoid too high a grazing pressure on the remaining rare plant species. Disturbance by year-round grazing is more unpredictable and dynamic than the traditional mowing and haymaking (Bokdam & Gleichman 2000). On the basis of the few populations included in this study, it is difficult to draw conclusions concerning the effects of year-round grazing on the population structure of S. pratensis, the more so because this type of management is relatively recent and populations have only recently established. Three populations were situated in ecological restoration areas with year-round grazing, and of the nine plots analysed in those sites, one was dynamic, two were normal and six were regressive. At the same time, all populations were relatively small. Hence, contrary to our expectations, very few of the colonization events seem to have yielded dynamic (rapidly) expanding populations. Although the small population sizes may merely be a consequence of rather recent colonization, it could also be that the regressive structure of these small founder populations is caused by problems with reproduction (seed quantity) and inbreeding depression (seed quality) as a result of the Allee effect (Ouborg & van Treuren 1994, 1995; Groom 1998; Oostermeijer 2000). Additional studies are needed to provide evidence for this hypothesis.
Bakker (1989) doubts whether species-rich grasslands can be restored with the management regimes associated with ecological restoration of intensively used agricultural areas. This management generally involves removal of the nutrient-rich topsoil of pastures or fields, followed by year-round grazing by hardy cattle breeds or horses. Nowadays, most populations of S. pratensis occur in the few semi-natural grasslands that are still under traditional management, such as mowing and haymaking or seasonal grazing by domestic cattle. Our findings suggest that year-round grazing of ecologically restored areas has not yielded populations of sufficient vitality to compensate for any local extinctions that would be caused by unfavourable management of remaining sites. Hence, maintaining the quality of the remaining reserves will be an important management task until any restoration projects have recreated sufficiently large new habitat patches in the immediate locality.