CONTRIBUTION OF PHYSIOLOGICAL AND MORPHOLOGICAL PLANT TRAITS TO A SPECIES COMPETITIVE ABILITY AT HIGH AND LOW NITROGEN SUPPLY - A HYPOTHESIS FOR INHERENTLY FAST-GROWING AND SLOW-GROWING MONOCOTYLEDONOUS SPECIES

Why do inherently fast-growing species from productive habitats genera lly have a higher rate of biomass production in short-term low-nitroge n experiments than slow-growing species from unproductive habitats, wh ereas the opposite is found in long-term experiments? Is this mainly d ue to inherent differences in biomass allocation, leaf characteristics or the plants' physiology? To analyse these questions we grew five mo nocotyledonous species from productive and unproductive habitats in a climate chamber at both high and low nitrogen supply. Nitrate was supp lied exponentially, enabling us to compare inherent differences in mor phological and physiological traits between the species, without any i nterference due to differences in the species' ability to take up nutr ients. At high nitrogen supply, we found major inherent differences in specific leaf area and nitrogen productivity, i.e. daily biomass incr ement per unit plant nitrogen, whereas there were only small differenc es in net assimilation rate, i.e. daily biomass increment per unit lea f area, and biomass partitioning. We propose that the higher specific leaf area and nitrogen productivity of inherently fast-growing species are the key factors explaining their high abundance in productive hab itats compared with inherently slow-growing ones. At low nitrogen supp ly, the net assimilation rate was decreased to a similar extent for al l species, compared with that at high nitrogen supply. The nitrogen pr oductivity of the inherently fast-growing species decreased with decre asing nitrogen supply, whereas that of the inherently slow-growing spe cies remained constant. There were no inherent differences in nitrogen productivity in this treatment. At this low nitrogen supply, the inhe rently fast-growing species invested relatively more biomass in their roots than the slow-growing ones did. The inherently fast-growing spec ies still had a higher specific leaf area at low nitrogen supply, but the difference between species was less than that at high nitrogen sup ply. Based on the present results and our optimization model for carbo n and nitrogen allocation ( Van der Werf et al. 1993a), we propose tha t the relatively large investment in root biomass of fast-growing spec ies is the key factor explaining their higher biomass production in sh ort-term experiments. We also propose that in the long run the competi tive ability of the slow-growing species will increase due to a lower turnover rate of biomass. It is concluded that the plant's physiology (net assimilation rate and nitrogen productivity), only plays a minor role in the species' competitive ability in low-nitrogen environments.