Why do inherently fast-growing species from productive habitats generally have a higher rate of biomass production in short-term low-nitrogen experiments than slow-growing species from unproductive habitats, whereas the opposite is found in long-term experiments? Is this mainly due to inherent differences in biomass allocation, leaf characteristics or the plants' physiology? To analyse these questions we grew five monocotyledonous species from productive and unproductive habitats in a climate chamber at both high and low nitrogen supply. Nitrate was supplied exponentially, enabling us to compare inherent differences in morphological and physiological traits between the species, without any interference due to differences in the species' ability to take up nutrients. At high nitrogen supply, we found major inherent differences in specific leaf area and nitrogen productivity, i.e. daily biomass increment per unit plant nitrogen, where-as there were only small differences in net assimilation rate, i.e. daily biomass increment per unit leaf 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 habitats compared with inherently slow-growing ones. At low nitrogen supply, the net assimilation rate was decreased to a similar extent for all species, compared with that at high nitrogen supply. The nitrogen productivity of the inherentlyfast-growing species decreased with decreasing nitrogen supply, whereas that of the inherently slow-growing species remained constant. There were no inherent differences in nitrogen productivity in this treatment. At this low nitrogen supply, the inherently fast-growing species invested relatively more biomass in their roots that the slow-growing ones did. The inherently fast-growing species still had a higher specific leaf area at low nitrogen supply, but the difference between species was less than that at high nitrogen supply. Based on the present results and our optimization model for carbon and nitrogen allocation (Van der Werf et al. 1993a), we propose that the relatively large investment in root biomass of fast-growing species is the key factor explaining their higher biomass production in short-term experiments. We also propose that in the long run the competitive 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.

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ABSTRACToften described in tbe literature migbt be explained by tbe absence of solar UV-B radiation in tbese cbambers.

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Van der Werf, A., Van Nuenen, M., Visser, A.J. and Lambers, H. 1993. Effects of N-supply on the rates of photosynthesis and shoot and root respiration of inherently fast-and slow-growing monocotyledonous species. -Physiol. Plant. 89: 563-569.Are there intrinsic differences in the rates of photosynthesis, shoot-and root-respiration between inherently fast-and slow-growing monocotyledons at high and low nitrogen supply? To analyze this question we grew 5 monocotyledons, widely differing in their inherent relative growth rate at high and low nitrogen supply in a growth room. Nitrate was exponentially added to the plants, enabling us to compare inherent differences in plant characteristics, without any effect of species differences in the ability to take up nutrients. At high nitrogen supply, the fast-growing species from productive habitats had a higher photosynthetic nitrogen use efficiency and rate of root respiration than the slowgrowing ones from unproductive habitats. Only minor differences were observed in their rates of photosynthesis and shoot respiration per unit leaf area. At low nitrogen supply, the rates of photosynthesis and shoot-and root respiration decreased for all species, even though there were no longer any differences in these processes between inherently fast-and slow-growing species. The photosynthetic nitrogen use efficiency increased for all species, and no differences were found among species. Differences in the photosynthetic nitrogen use efficiency among species and nitrogen treatments are discussed in terms of the utilization of the photosynthetic apparatus, whereas differences in respiration rate are discussed in terms of the energy demand for growth, maintenance and ion uptake and their related specific respiratory energy costs. It is concluded that the relatively high abundance of slow-growing species compared to fast-growing ones in unproductive habitats is unlikely to be explained by differences in rates of photosynthesis and respiration or in photosynthetic nitrogen use efficiency.

Van der Werf, A., Van Nuenen, M., Visser, A.J. and Lambers, H. 1993. Effects of N-supply on the rates of photosynthesis and shoot and root respiration of inherently fast-and slow-growing monocotyledonous species. -Physiol. Plant. 89: 563-569.Are there intrinsic differences in the rates of photosynthesis, shoot-and root-respiration between inherently fast-and slow-growing monocotyledons at high and low nitrogen supply? To analyze this question we grew 5 monocotyledons, widely differing in their inherent relative growth rate at high and low nitrogen supply in a growth room. Nitrate was exponentially added to the plants, enabling us to compare inherent differences in plant characteristics, without any effect of species differences in the ability to take up nutrients. At high nitrogen supply, the fast-growing species from productive habitats had a higher photosynthetic nitrogen use efficiency and rate of root respiration than the slowgrowing ones from unproductive habitats. Only minor differences were observed in their rates of photosynthesis and shoot respiration per unit leaf area. At low nitrogen supply, the rates of photosynthesis and shoot-and root respiration decreased for all species, even though there were no longer any differences in these processes between inherently fast-and slow-growing species. The photosynthetic nitrogen use efficiency increased for all species, and no differences were found among species. Differences in the photosynthetic nitrogen use efficiency among species and nitrogen treatments are discussed in terms of the utilization of the photosynthetic apparatus, whereas differences in respiration rate are discussed in terms of the energy demand for growth, maintenance and ion uptake and their related specific respiratory energy costs. It is concluded that the relatively high abundance of slow-growing species compared to fast-growing ones in unproductive habitats is unlikely to be explained by differences in rates of photosynthesis and respiration or in photosynthetic nitrogen use efficiency.