Population structure of Salvia pratensis in relation to vegetation and management of Dutch dry floodplain grasslands

Summary

In the Netherlands, meadow clary Salvia pratensis is a rare plant. It is red-listed and legally protected, and the Dutch Ministry of Agriculture, Nature Management and Fisheries list it as a 'target' species of dry floodplain grasslands and dykes, which means that specific efforts should be made to maintain this species and its habitat.

The population structure of 23 meadow clary populations was studied in relation to the vegetation and management of dry floodplain grasslands along Dutch rivers. The aims of the study were to (i) evaluate the suitability of single censuses as a tool for quick population viability assessments; (ii) test whether viable Salvia populations indicate sites of higher conservation value; (iii) obtain information on the viability of the remaining populations; and (iv) provide advice for optimal habitat management. This case study focuses on information on the management regime of each meadow clary population with a view to providing advice for optimal habitat management.

Study species and study areas: In the Netherlands, meadow clary Salvia pratensis is characteristic of semi-natural grassland communities. It is a diagnostic species of the Medicagini–Avenetum pubescentis association, characteristic of dry grasslands on nutrient-poor sandy soils. It is occasionally found in calcareous grasslands in the south of the Netherlands, but its main habitat is dry floodplain grasslands situated mostly along the Rhine and the Meuse river system. It has been estimated that 89% of such grasslands have been lost between 1968 and 1992. As a result, meadow clary has a highly fragmented habitat, consisting generally of very small nature reserves of a few hectares one to several kilometres apart.

Management: Information on the management regime of each meadow clary population (mostly within nature reserves) was obtained through personal observation and interviews with site managers. Some areas were managed traditionally as hay meadows by means of mowing and haymaking once or twice each year. Others were seasonally grazed by domestic cattle, and another group was managed by year-round low-intensity grazing by Galloway cattle and/or Konik horses. None of the sites was artificially fertilized. The management regimes observed were classified as follows:

1. Early mowing, between 15 June and 15 July, usually early in this period, and some sites were cut a second time later in the season

2. Late mowing, after 15 July

3. Seasonal grazing from spring to autumn by domestic cattle or horses

4. Year-round low-intensity grazing

Population types: Three groups of meadow clary population types emerged from cluster analysis (K-means clustering):

Population Group 1 - Populations of group 1 had a relatively high proportion of young life stages, primarily juveniles. Consequently, the proportion of adult plants was low and most of the generative plants were small. On average, the proportion of young stages was 0.60. This population was named 'dynamic'.

Population Group 2 - The second population group was characterised by a lower proportions of seedlings and juveniles than group 1, and relatively higher proportions of immatures, vegetatives and small generatives. Large generative plants were scarce. The average proportion of young life stages was 0.36. This population was termed the 'normal' type.

Population Group 3 – The third group consisted primarily of later life stages, such as adult vegetatives and particularly large generative plants. Only two juveniles were found in populations of this type. The mean proportion of young life stages was only 0.1. This population was named 'regressive'.

Effects of management: The different management methods (mowing or grazing) had a significant effect on the vegetation. Vegetation differed in composition among the meadow clary population types, and this was not merely a result of differences in management. The percentage of bare soil surface (indicating an open vegetation structure) was positively related to recruitment of meadow clary. Dynamic populations occurred in a species-rich vegetation, comprising species characteristic of nutrient-poor floodplain grasslands. Regressive populations occurred more often in species-poor vegetation, comprising mainly species of nutrient-rich conditions.

There was a significant association between population type and the four different management regimes:

Meadow clary populations that were mown late had significantly more dynamic populations than expected; sites with year-round grazing had more regressive populations than expected (although there were too few sites with this management type to draw useful conclusions). No differences in the frequencies of specific population types were found between mowing or grazing.

There were significant differences between the proportions of several life stages dependant on the management: Seedlings were mainly found in late-mown grasslands. Of the seven plots in which seedlings were observed, six were in mown areas and only one in a grazed site. Four of those six plots were situated in sites that were mown late. However, there was no difference in the proportion of juvenile plants between management types. Immature and adult vegetative plants were relatively more frequent in the seasonally grazed sites and less frequent on those grazed year-round.

The proportion of large flowering plants was highest when grazed year-round or mown early, and very low under the two other management types. Importantly, the late mowing regime was associated with a significantly larger population size. This was partly caused by the fact that the two largest populations, Bijland (3,800 individuals) and Koekoeksche waard (3,500), were both mown late.

Conclusions: In the Dutch meadow clary populations investigated in this study, population structure was correlated with management. Those populations with a late mowing regime had higher proportions of young stages and larger population sizes. It was expected that recently established populations on young river dunes formed during ecological restoration projects would be 'dynamic', but in fact most sampled were 'regressive', which suggests that either site conditions were not yet optimal or Allee effects limited expansion of the small founder populations. It is considered in terms of habitat management therefore, that conservation of remaining Salvia pratensis populations in Dutch dry floodplain grasslands will be best achieved by late mowing with hay removal.


Note: If using or referring to this published study, please read and quote the original paper. The original paper can be viewed at:http://www.blackwell-synergy.com/doi/full/10.1046/j.0021-8901.2001.00679.x