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 structure and species diversity in a five-year field experiment. Above- and below-ground resource addition (fertilization and additional illumination with the help of mirrors) and cessation of annual mowing were applied as experimental treatments in 24 permanent plots of 40 x 40 cm. Canopy structure was studied by point quadrat sampling using laser beam to randomly position point quadrats at two different angles of observation -60 degrees and 120 degrees from horizontal, north-south direction. We hypothezised that the studied plant community is not light-limited under the normal conditions (non-fertilized and annually mown) but would become light-limited after nutrient addition and/or cessation of mowing. In this case the effect of fertilization and cessation of mowing could be mitigated by introducing additional light resource (placing south-facing mirrors). Species richness in plots was not significantly altered by experimental treatments, most probably because of a strong input of species from surrounding areas by the means of lateral vegetative spread, compensating for the expected decrease of diversity with fertilization and cessation of mowing. Surprisingly, plant total coverage in a plot was the only variable significantly explaining plot richness, independent of experimental treatments and the year of observation. We therefore studied also species richness per unit coverage (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 dry period (1994-1995). There was no indication of light being limiting for the plants in control plots, but light obviously became limiting after nutrient addition-in fertilized plots additional illumination lead to a notably denser canopy. Generally, the average exposition of plant leaves surface was towards the north, independent of experimental treatments. However, in the exceptionally cloudy and cool year 1996 the situation was radically different-leaves were exposed predominantly towards the south, indicating that multi-species canopy can plastically react to long-term changes in light availability.