Description and validation of C_change: A model for simulating carbon content in managed Pinus radiata stands

A model has been developed that predicts the amount of carbon contained in the stem, crown, roots, forest floor, and understorey of managed Pinus radiata D. Don stands at any age over a rotation. A key concept underlying the C_change model is that, with current knowledge of growth partitioning, mortality, and decay of tree components, stem volume production can be used to predict carbon content of forest biomass components. The advantage of taking this approach is that data input requirements for predicting forest carbon are minimised, given a system for determining stem volume growth and mortality over time. The Stand Growth module of STANDPAK predicts P. radiata stem volume for each of the major forest-growing regions in New Zealand, based on an extensive network of permanent sample plots (PSP). By linking the Growth Partitioning module with Stand Growth, a minimum set of data inputs is required to calibrate C_change to the region. The utility of this approach was tested by running C_change to make predictions of stem volume and carbon at several sites where stand biomass measurements had been made. These sites covered a range in nitrogen fertility, stocking, stand ages, and climate. Across all studies, actual above-ground stand carbon content (i.e., excluding understorey and forest floor) was highly correlated with that predicted by C_change (r2 = 0.97, n = 25, p < 0.01). Assuming that suitable regional growth models are available for predicting stem volume and that growth relationships are constant across regions, these results give confidence in the use of C_change for prediction of carbon on a stand and regional scale in New Zealand.