Modeling Annual Carbon Uptake for the Indigenous Forests of New Zealand

Abstract

Abstract A regional estimate of annual gross carbon uptake for the indigenous forests of New Zealand is presented. Carbon uptake for forest canopies occupying 59,311 sites on a 1 km grid (25% of the total land area) is estimated daily using a leaf-level photosynthesis–stomatal conductance model with appropriate scaling procedures (Leuning et al. 1995), and a model of site water balance. For each site, estimates of monthly climate data, soil physical properties, root-zone depth, and nitrogen availability were matched from maps. Forest type was distinguished into 18 classes and values for the maximum rate of carboxylation, Vcmax0, were assigned according to the ratio of conifer to broadleaved trees and nitrogen availability. Mean annual carbon uptake across all sites was 1.5 kg m-2. Variability between sites was attributable principally to the value for Vcmax0, followed by temperature. The effects of water stress integral, air saturation deficit, and solar radiation on carbon uptake were much less. Mean radiation conversion efficiency (ratio of carbon uptake to intercepted solar radiation) was low at 0.79 kg GJ-1, consistent with low nitrogen availability. The analysis highlights the sensitivity of carbon uptake to physiological characteristics and nitrogen status and thus the need to determine values for photosynthesis parameters for trees growing in field conditions. FOR. Sci. 47(1):9–20.