COMPETITION IN A GLOBAL CHANGE ENVIRONMENT - THE IMPORTANCE OF DIFFERENT PLANT TRAITS FOR COMPETITIVE SUCCESS

Plant traits of both structural and physiological nature have been rep orted to play an important role in the outcome of a competitive intera ction. In this experiment a survey of the importance of characteristic s such as height, leaf area and vertical distribution, leaf inclinatio n, light transmission through a leaf and through the canopy together w ith leaf photosynthesis rates, was made for two grasses: Lolium perenn e L. and Festuca arundinacea Schreb. Both species were grown in a comp etitive set-up with minimal belowground interactions under four differ ent global change conditions of CO2 concentration and air temperature (ambient, elevated CO2 concentration 700 mu mol mol(-1), increased air temperature+4 degrees C, combined) in naturally sunlit air-conditione d half-open plastic greenhouses. Plants were grown for a whole growing season with regular (every 4 weeks) clippings at 3.5 cm height. Resul ts demonstrate that Lolium has a much greater competitive ability when compared to Festuca (aggressivity = 0.465). This may be a consequence of the slightly higher leaf photosynthesis rates (+15%) in combinatio n with a larger amount of foliage area, which could result in a higher canopy photosynthesis. The greater leaf area remaining after mowing m ay lead to an additional advantage for Lolium. Height, leaf inclinatio n, optical properties and the vertical distribution of leaf area seem to be of minor importance in this experiment. Global change treatments (elevated CO2 concentration and increased temperature) influence leaf photosynthesis rates predominantly and have little or no direct effec t on structural traits such as height, leaf angles and vertical leaf d istribution; nor do they exert an influence on light transmission thro ugh a leaf or through the canopy. However, treatment effects on leaf a rea can add to explain differences in canopy photosynthesis. In this c ompetition experiment, focused on above-ground interactions, both a st ructural (leaf area) and a physiological feature (leaf photosynthesis) contribute to the competitive success of a species.