The relationship between the relative growth rate and nitrogen economy of alpine and lowland Poa species

This study investigates the nitrogen economy of six altitudinally contrasting Poa species which differ in their relative growth rate (R). Two alpine (Poa fawcettiae and P. costiniana), one sub-alpine (P. alpina) and three temperate lowland species (P. pratensis, P. compressa and P. trivialis) were grown hydroponically under identical conditions in a growth room. The low R exhibited by the alpine species mas associated with lower plant organic nitrogen concentration (n(P)) and lower nitrogen productivity (Pi(P), amount of biomass accumulation per mol organic nitrogen and time). The differences in Pi(P) between the alpine and lowland species did not appear to be due to differences in the carbon concentration or the proportion of total plant organic nitrogen allocated to the leaves, stems or roots. Variations in Pi(P) were also not due to variations in photosynthetic nitrogen use efficiency (Phi(N), the rate of photosynthesis per unit organic leaf nitrogen) or shoot or root respiration rates per unit organic nitrogen (Gamma(SH) and Gamma(R), respectively) per se. Rather, the lower Pi(P) in the alpine species was probably due to a combination of small variations in several of the parameters (e.g. slightly lower Phi(N), slightly higher Gamma(SH), and Gamma(R), and slightly higher proportions of total plant organic nitrogen allocated to the roots). The alpine species exhibited lower organic acid and mineral concentrations. However, no differences in whole-plant construction costs (grams of glucose needed to synthesize one gram of biomass) were observed between the alpine and lowland Poa species. The lack of substantial differences in Phi(N) between the alpine and lowland species contracts with the large differences in Phi(N) between slow- and fast-growing lowland species that have been reported in the literature. The reasons for the unusually high Phi(N) values exhibited by the alpine Poa species are discussed.