The mean density of stem wood growth sheaths laid down annually in Pinus radiata
D. Don stands was modelled as a function of site mean annual temperature, soil nitrogen fertility, ring age, and stocking. The model was based on measurements of mean outerwood basic density at breast height of 30 trees per stand from a single seedlot established at 17 sites located throughout New Zealand. Soil and climate data were obtained from each site, and stem wood disks were sampled at 5-m intervals along the stem from 10 trees per stand from 15 of these sites. Previously reported breast-height basic density data from a comprehensive national survey were used to examine ring age trends from pith to bark, supplemented with data from four trials to determine the influence of tree stocking on outerwood density. The model was tested using data from an independent study of mean outerwood basic density at breast height (30-120 trees per stand) undertaken at 21 stands selected to cover a wide range in site fertility, temperature, tree stocking, and stand age.
Site mean breast-height outerwood basic density ranged betwe en 356 and 494 kg/m3 in the model development dataset, and 316 and 482 kg/m3 in the validation dataset, and increased significantly with site mean annual temperature (T), mineral soil adjusted carbon /nitrogen (C/N) ratio, and stocking. Breast-height density of annual growth rings from pith to bark increased with ring age, and this pattern was consistent at all heights within the stem. The ratio of sheath density to breast-height ring density varied with ring age and increased with increasing nitrogen fertility. To predict the density of annual stem wood growth sheaths, the model firstly estimates the effects of site mean annual temperature, soil nitrogen fertility, and stocking on mean outerwood density at breast height. Secondly, the effect of ring age on annual ring density at breast height from pith to bark is taken into account. Finally, the ratio of sheath density to breast-height ring density for each ring is estimated as a function of stand age and outerwood density.
The national wood density model explained 93% of the variation in outerwood density at breast height for the model development dataset, with model predictions within 0.2% of the measured values. The model explained 86% of the variation in breast-height ring density in the model validation dataset, with predictions from the model averaging 3.2% higher than the measured values. Seedlot differences in breast-height outerwood density contributed in part to the greater variability evident in the validation dataset. The modelled ratios used to predict the density of annual growth increments were not directly tested. However, analogous ratios of whole stem wood density/breast-height outerwood density were derived by site density class for stands across a range of age classes using an independent national dataset, and these were consistent with those predicted using the model. Areas for further model testing and development were identified.
The model can be applied to predict stem biomass and carbon sequestration in P. radiata stands from the increment in stem wood volume. The wood density model has been incorporated in a carbon modelling system (C_Change) to facilitate the prediction of carbon stocks and changes in New Zealand's exotic plantation forest estate.