The effects of management and environmental variation on population stage structure in three river-corridor violets

摘要

Population stage structure of plants, i.e., the density and frequency of individuals in different stages of the life cycle, is a crucial aspect of population viability that depends on a variety of factors. In this paper, we evaluated the effects of (i) management and year, (ii) location (population) and time (year) and (iii) of local habitat quality and population factors on population stage structure of three morphologically similar, closely related violets from floodplains, Viola elatior V pumila and V stagnina. We hypothesised that owing to similar life cycles there should be no significant differences in population stage structure among species. We analysed population stage structure in managed vs. abandoned populations to test whether a proposed effect of management acts through the creation of regeneration niches. We further tried to identify which habitat factors are responsible for possible management effects. We established permanent plots (0.25 m(2)) in 27 populations of the species in two different regions (Rhine floodplains, Germany; Dyje River floodplains, Czech Republic) and recorded frequency and density of seedlings, small and large vegetative plants and small and large flowering plants during 2 years. There were significant differences among species, indicating that the species have different life histories. Furthermore, there was a significant effect of management on population stage structure in two of the species. Management significantly increased the proportion of seedlings, over and above possible differences between regions. In our data set, the effects of spatial variation among populations were generally larger than the effects of temporal variation. The only factor that affected the density of life-cycle stages was the cover of bryophytes, while the cover of higher plants, litter or soil (local habitat quality), or isolation and population size (population factors) had no effects. (C) 2003 Elsevier SAS. All rights reserved.