Competition of wetland plants as affected by shade and nutrient supply

Many competition experiments have shown that plant species interactions are influenced by nutrient supply. In contrast, the effect of light supply on competitive interactions has received little attention. The question whether competitive interactions depend more on the absolute or on the relative supply of above- and below-ground resources has not been investigated for herbaceous vegetation. In this project we examine the influence of total light supply on interspecific competition between perennial wetland plants at different levels of nutrient supply. Total light supply is defined as the amount of light received by a vegetation stand as a whole, and can be varied independently of nutrient supply. Our main hypothesis is that light supply will influence interspecific competition in a way similar to nutrient supply, but that the two resources will interact. Five Carex species from wetlands are being grown with and without competitors at a range of light and nutrient levels in two competition experiments. The designs enable us to measure both competitive response and competitive effect, and to distinguish between above- and below-ground competition. A third experiment investigates the role of temporal variation in light availability. Mixtures of nine wetland graminoids have been established in an experimental field. All mixtures will receive a similar average light intensity during the main growing season, but periods of light and shade will differ. These experiments are relevant for nature conservation. Eutrophicated fen meadows are susceptible to invasion by tall plant species, e.g. Phragmites australis or Solidago serotina, which create two-layered vegetation by overtopping all other species. Our experiments will investigate whether or not the shade cast by such invaders is likely to affect species abundance and interspecific competition of the original fen vegetation.