Forest and shrubland canopy carbon uptake in relation to foliage nitrogen concentration and leaf area index: A modelling analysis

Abstract

A multi-layer canopy model was used to simulate the effects of changing foliage nitrogen concentration and leaf area index on annual net carbon uptake in two contrasting indigenous forest ecosystems in New Zealand, to reveal the mechanisms regulating differences in light use efficiency. In the mature conifer-broadleaved forest dominated by Dacrydium cupressinum, canopy photosynthesis is limited principally by the rate of carboxylation associated with low nutrient availability. Photosynthesis in the secondary successional Leptospermum scoparium/Kunzea ericoides shrubland is limited by electron transport. Maximum carbon uptake occurred in spring at both sites. Annual increases in canopy photosynthesis with simulated increases up to 50% in leaf area index, L, or foliage nitrogen concentration per unit foliage area, N a, were largely offset by increases in night-time respiration. A realistic simulation where L was increased by 50% and Na by 20% together (equivalent to an increase in total canopy nitrogen of 80%) led to decreases in net annual carbon uptake because the increase in photosynthesis was offset by the increase in respiration. Given the environmental constraints, both canopies in their natural states appear to be operating at the optimum conditions of leaf area index and nitrogen concentration for maximum net carbon uptake. © INRA, EDP Sciences, 2005.