The floodplain meadows of Soomaa National Park, Estonia. Vegetation - Dispersal - Regeneration

In the years 1998-2000 various aspects of the vegetation ecology and plant population dynamics of the floodplain meadows of the Soomaa National Park in Estonia were studied. Based on phytosociological methods the floodplain meadows and related communities were surveyed and habitat parameters like groundwater dynamics and soil properties were recorded. The survey of 171 releves resulted in the description of 15 meadow, reed and sedge communities. These ranged from dry communities on sandy ridges, mesic communities in lower but well drained sites, moist communities on poorly drained sites of the central floodplains to wet communities on the lowest and frequently inundated sites. Most of these communities can conveniently be placed in the established syntaxonomical system, at least at the level of alliances. True meadow communities of the Arrhenatheretalia and Molinietalia were easily recognizable but showed distinct properties setting them apart from their Central European counterparts. Communities with Sesleria caerulea were found to be unique to the Baltic area but could be placed inside the Molinion. In a set of three experimental studies various aspects of the regeneration dynamics of floodplain plants were assessed. Seed traps were used to study the composition of the local seed rain, in a seed sowing study the germination of ten species in different gap types was investigated and a third study examined the role of seed bank and seed rain in the colonization of artificial gaps. Data were also analyzed in view of the role of plant traits and functional groups. 140 seed traps were placed at 28 sample locations along three transects. 34423 seeds of ca. 76 species were collected during one year of sampling; the distribution of species was highly skewed with only five species accounting for 84 % of the total seed count, median seed density was 6579 seeds/sqm. Approximately half of the seed rain originated from the direct neighborhood of the seed traps but only very few species came from outside the studied transects; those were either wind-dispersed or water-dispersed. Temporal variation of seed rain was considerable with three distinct peaks in the course of one year. The distribution of plant traits showed substantial variation between seed rain and vegetation. Generally, wind- and water-dispersed species were more important in the seed rain whereas animal-dispersed species and those without apparent dispersal agent were more prominent in the vegetation. Multivariate analysis revealed that established vegetation, seed rain and different vegetation types were each characterized by a distinct combination of vegetative and generative traits. Ten species typical of the Soomaa floodplain meadows were sown into experimental plots with different disturbance treatments and into the undisturbed vegetation. Most of the studied species were able to germinate and establish in the experimental plots; emergence rate in the first year generally declined in the order bare soil > bryophytes > litter removed > intact vegetation. No significant differences between treatments were found for establishment after three years and for survival; however, both values tended to be lowest in intact vegetation. In some species, however, survival tended to be higher in the semi-open conditions, which is discussed in terms of sheltering effects. Implications of seed sowing studies for habitat restoration are discussed and it is suggested that deliberate (re-)introduction of species by seed may be a valuable method to overcome seed limitation. 20 plots with three treatments were set up to study the role of seed rain and seed bank in the re-colonization of gaps in the meadow vegetation. Soil inversion was used to measure the seed rain; exclusion of the seed rain by nylon mesh allowed the assessment of the seed bank. A disturbed but uncovered plot and an undisturbed control received input from both seed bank and seed rain. 4363 seedlings of 61 species were identified in the plots. The seed bank generally was four to five times richer in both species and seeds than the seed rain. Similarity between the established vegetation and the seedling assemblages was very low. Analysis of life history traits suggested that both established vegetation and seedling assemblages were characterized by unique sets of plant traits. The data collected for the functional analysis of the experimental results was also used for a general assessment of trait distribution and functional group differentiation of the grassland vegetation as a whole. Correlation analysis showed that a number of traits like clonal growth, reproductive mode, guild membership, seed number and weight etc. were inter-correlated. These correlations are explained in terms of general bio-physical constraints and trade-offs. A cluster analysis of a trait matrix of 135 floodplain species resulted in nine morphologically and functionally defined groups of species.