Spatial prediction of optimal final stand density for even-aged plantation forests using productivity indices

Abstract

Surfaces that describe spatial variation in optimal stand density following final thining (Sopt) are likely to be of considerable use to forest managers. Using a comprehensive series of growth model simulations, the aim of this research was to (i) develop a model of Sopt that maximises volume of large-diameter, small-branched sawlogs (S27) for unpruned New Zealand radiata pine (Pinus radiata D. Don) stands, (ii) use this model to examine how site productivity and tree morphology influence Sopt, and (iii) generate a map of Sopt for New Zealand. A model predicting Sopt from clearfell age and two productivity indices, Site Index (SI) and 300 Index (I300), was found to predict optimal stand density with a high degree of accuracy. Optimal stand density was found to increase with I300 and clearfell age but decrease with SI. Within New Zealand plantations, the mean predicted Sopt for clearfell age 28 years was 614 stems·ha−1. The proportion of plantations predicted to have Sopt greater than 400, 500, and 600 stems·ha−1 was 0.99, 0.88, and 0.61, respectively. The predicted Sopt was found to exceed the actual mean final crop stand density in stands managed under unpruned sawlog regimes of ca. 500 stems·ha−1 within most plantation areas in New Zealand. This disparity highlights the potential of this approach for increasing crop value in New Zealand P. radiata plantations.