Analysis of the growth of rimu (Dacrydium cupressinum) in South Westland, New Zealand, using process-based simulation models

Abstract

Two process-based models were used to identify the environmental variables limiting productivity in a pristine, mature forest dominated by rimu (Dacrydium cupressinum Sol. ex Lamb.) trees in South Westland, New Zealand. A model of canopy net carbon uptake, incorporating routines for radiation interception, photosynthesis and water balance was used to determine a value for quantum efficiency when climate variables were not limiting. The annual net carbon uptake by the canopy was estimated to be 1.1 kg C m(-2) and the quantum efficiency 22.6 mmol mol quanta(-1). This value of quantum efficiency, combined with other parameters obtainable from the literature, was then used in a model of forest productivity (3-PG), to simulate changes in net productivity and the allocation of carbon to tree components. The model was adjusted to match a measured stem increment of 10.6 Mg ha(-1) over a period of 13 years. To achieve this while maintaining a low, but stable value for leaf area index, it was necessary to set the site fertility rating very low and select high values for the parameters describing the proportional allocation of total carbon to roots. This approach highlighted nutrient availability as the principal constraint on productivity for the ecosystem and identified critical measurements that will be necessary for using the model to predict the effects of climate change on carbon sequestration. The low rates of carbon uptake and productivity are consistent with the low nutrient supply available from the highly leached, acid soils, most likely attributable to frequent saturation and a very shallow aerobic zone.

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

“…However, evidence from modelling approaches shows that productivity in mature forests [57,58] and shrublands [56] in New Zealand is limited principally by low nutrient availability.…”

“…The soils are extremely acid (pH 3.8-4.4) with medium levels of nitrogen (2.1 mol kg -1 ) in the upper 150 mm, falling to very low values (0.14 mol kg -1 ) at a depth of 150 mm, and low values of acidextractable phosphorus and low phosphorus retention [37]. The mean annual biomass increment for the site was estimated to be 0.05 kg C m -2 [58] and the effective leaf area index (half-total surface area basis) was 3.5. Average foliage nitrogen concentration was 128 mmol m -2 [50].…”

“…Mean annual temperature is 11.3 ºC with a small range between winter and summer of 8.6 ºC and annual rainfall is approximately 3400 mm. Further details of the site can be found in Whitehead et al [58].…”

“…The model has been described fully elsewhere [58][59][60][61], so only brief details will be provided here. The canopy was divided into 20 layers based on the vertical distribution of cumulative canopy leaf area index.…”

“…Values for the parameters required for the model were taken from [58] for the Dacrydium site and [59] for the Leptospermum/Kunzea site and are listed in Table III. Daily weather data were used for 1 year with the model to estimate annual net canopy photosynthesis, A, and annual night-time respiration, R d , for the actual conditions at both sites.…”