Development of growth models for Pinus radiata in New Zealand — experience with management and process models

Abstract

The Forest Research Institute (FRI) has recently finished developing a series of management growth models for Pinus radiata within major bio-geo-climatic regions. These are stand-level models consisting of a series of stochastic differential equations, with the state of the stand at any one time being described by the major variables of top height, basal area and number of stems per hectare. Additional state variables and time-scale multipliers are used to account for regional differences in patterns of growth, loss of site occupancy, nitrogen and phosphate fertiliser effects, and increased growth due to improved breeds of trees. The research programme has been driven by the forest manager's need to accurately regulate yield in a period with a greater demand than supply, and to evaluate the profitability of new afforestation. It was necessary to predict the response to extremes of silvicultural regimes, ranging from dense pulpwood to widely spaced, high pruned stands. From 1986 onwards, an FRI/Industry Stand Growth Model Cooperative has facilitated data collection, research and technology transfer. Since the early 1980s, a research programme to construct process models for Pinus radiata has contributed to understanding physiological processes, particularly water balance and interception of solar radiation. However, initial expectations that such models would contribute to improvements in forest management, or to better management growth models have so far not been met. There is no evidence to suggest that either management or process models can be used with confidence outside the range of their experimental data.