EVIDENCE FOR OPTIMAL PARTITIONING OF BIOMASS AND NITROGEN AT A RANGE OF NITROGEN AVAILABILITIES FOR A FAST-GROWING AND SLOW-GROWING SPECIES

1. Do species of nutrient-poor environments growing at low nitrogen availability partition their biomass and nitrogen closer to their optimum with respect to relative growth rate and nitrogen productivity than species of nutrient-rich environments? And does the opposite occur at high nitrogen availability? To answer these questions we examined Briza media, a species characteristic of nutrient-poor soils, and Dactylis glomerata, a species characteristic of nutrient-rich soils, at a range of nitrogen availabilities. Based on the experimentally determined rates of physiological processes and morphological characteristics (photosynthesis, shoot and root respiration, allocation of fixed carbon and nitrogen, and specific leaf area), a dynamic vegetative growth model was constructed. The model describes the relative growth rate (biomass increment per unit of biomass present per day) and nitrogen productivity (biomass increment per unit of organic nitrogen present in the plant per day) as a function of nitrogen availability. 2. Model simulations indicate that at low availability of nitrogen, B. media and D. glomerata partitioned their biomass and nitrogen in such a way as to maximize both their relative growth rate and nitrogen productivity. At high availability of nitrogen, the two species partitioned their biomass and nitrogen close to their optimum with respect to relative growth rate, but not with respect to nitrogen productivity. 3. The model simulation shows that at a low rate of nitrogen uptake per unit root weight, i.e. reflecting a low availability of nitrogen in a given environment, species attain an equal relative growth rate and nitrogen productivity, whereas at high availability of nitrogen D. glomerata achieves a higher relative growth rate and nitrogen productivity than B. media does. It is concluded that optimal partitioning of biomass and nitrogen with respect to relative growth rate and nitrogen productivity cannot explain the success of an inherently slow-growing species in a nutrient-poor environment.