Population structure of Salvia pratensis in relation to vegetation and management of Dutch dry floodplain grasslands.
Abstract
We need to understand how threatened plant species respond to natural or management-induced habitat changes to conserve them successfully. Because long-term demographic studies are not feasible for large numbers of species, there is a clear need for simple short-term methods to assess demographic responses. The population structure of 23 populations of the endangered perennial Salvia pratensis was studied in relation to the vegetation and management of dry floodplain grasslands along Dutch (Netherlands) rivers. The aims of the study were to: (i) evaluate the suitability of single censuses of population stage structures as a tool for quick assessments of the viability of plant populations; (ii) test whether viable populations of S. pratensis indicate sites of higher conservation value; (iii) obtain information on the viability of the remaining populations; and (iv) provide advice for optimal habitat management. Using cluster analysis, we distinguished three different types of populations: (i) 'dynamic' populations, characterized by a large proportion of young individuals (seedlings, juveniles and immatures); (ii) 'normal' populations, with a relatively higher proportion of adults but still a considerable number of young individuals; and (iii) 'regressive' populations, in which adult stages, especially large flowering individuals, dominated and rejuvenation hardly occurred. The three population types differed with respect to population size and total plant density, which were highest in dynamic, intermediate in normal and lowest in regressive populations. Both the structure and composition of the surrounding vegetation were associated with the type of population found. The percentage of bare soil surface (indicating an open vegetation structure) was positively related to recruitment of S. pratensis. The dynamic populations occurred in a species-rich vegetation, comprising species of nutrient-poor soils and characteristic of floodplain grasslands. The regressive populations occurred more often in species-poor vegetation, comprising mainly species of nutrient-rich conditions. Salvia population structure was correlated with management. We observed that those populations with a late mowing regime had higher proportions of young stages and larger population sizes. Although we expected that recently established populations on the young river dunes formed during ecological restoration projects would be dynamic, most sampled plots were of the regressive type, which suggests that either site conditions were not yet optimal or Allee effects limited further expansion of the small founder populations. We conclude that conservation of the remaining populations of S. pratensis in Dutch dry floodplain grasslands will be best achieved by late mowing with hay removal.