Structure and diversity of a species-rich grassland community, treated with additional illumination, fertilization and mowing

We examined the changes of species-rich herbaceous community canopy structu re and species diversity in a five-year field experiment. Above- and below- ground resource addition (fertilization and additional illumination with th e help of mirrors) and cessation of annual mowing were applied as experimen tal treatments in 24 permanent plots of 40 x 40 cm. Canopy structure was st udied by point quadrat sampling using laser beam to randomly position point quadrats at two different angles of observation -60 degrees and 120 degree s from horizontal, north-south direction. We hypothezised that the studied plant community is not light-limited under the normal conditions (non-ferti lized and annually mown) but would become light-limited after nutrient addi tion and/or cessation of mowing. In this case the effect of fertilization a nd cessation of mowing could be mitigated by introducing additional light r esource (placing south-facing mirrors). Species richness in plots was not significantly altered by experimental tre atments, most probably because of a strong input of species from surroundin g areas by the means of lateral vegetative spread, compensating for the exp ected decrease of diversity with fertilization and cessation of mowing. Sur prisingly, plant total coverage in a plot was the only variable significant ly explaining plot richness, independent of experimental treatments and the year of observation. We therefore studied also species richness per unit c overage (standardized richness) which appeared to be significantly reduced by fertilization. Annual change of plant total coverage of a plot was good predictor of annual change of plot richness, except for the exceptionally d ry period (1994-1995). There was no indication of light being limiting for the plants in control plots, but light obviously became limiting after nutr ient addition-in fertilized plots additional illumination lead to a notably denser canopy. Generally, the average exposition of plant leaves surface w as towards the north, independent of experimental treatments. However, in t he exceptionally cloudy and cool year 1996 the situation was radically diff erent-leaves were exposed predominantly towards the south, indicating that multi-species canopy can plastically react to long-term changes in light av ailability.