Vertical structure of a species-rich grassland canopy, treated with additional illumination, fertilization and mowing

The canopy architecture of herbaceous communities has received little atten tion despite its obvious relevance for how plants interact. The objective o f this study was to determine how the position of species within the canopy of a species-rich grassland depends on nutrient availability, light availa bility, and mowing regime. To enhance the generality of the results, specie s were classified into four growth-form groups according to two important m orphological traits - narrow vs. wide leaves, leafy vs. leafless stem - gra sses, sedges, upright forbs and rosette forbs. Vertical structure of the ca nopy was examined by point quadrat sampling of 40 x 40 cm permanent plots, treated by fertilization, additional illumination and mowing. As expected, fertilization and cessation of mowing allowed plants to grow taller, an eff ect of additional illumination was not detected. The relative height of spe cies in the canopy was related to their leaf and stem morphology - species with narrow leaves and/or with a leafy stem were significantly taller. In a ddition, each growth-form group had a characteristic relative height: grass es > sedges > upright forbs > rosette forbs. All the experimental treatment s led to a smaller number of more clearly distinguishable canopy strata wit h certain growth-form group(s) dominating in each. In (non-fertilized, non- illuminated and mown) control plots the dominance of grasses in the upper c anopy layers was the most obvious - the remaining plants tended to improve their vertical position with all manipulations. The two contrasting growth- form groups - grasses and rosette forbs - responded similarily to illuminat ion, becoming relatively shorter with added light. Sedges and upright forbs used additional light to 'improve' their position in the canopy.