Site classification and growth models for Sitka spruce plantations in Ireland

Site classification has been identified as an important tool in forestry for the quantification of a forest site's productivity potential. It is useful in forest management and silvicultural decision making for example when deciding on when to start thinning and or when to clearfell a forest of a particular species, and for estimating stands development patterns across sites. This paper presents top height-age site classification and growth models for Sitka spruce (Picea sitchensis (Bong.) Carr.) forest plantations in Ireland using nonlinear quantile regression (NLQR) methodology. Repeated top height - age measurements were recorded from over 700 Coillte Teoranta (Irish Forestry Board) silvicultural permanent sample plots (PSPs) in seven Irish forest regions over the past six decades. The conditional quantiles (0 <= tau <= 1) of the top height distribution at a given age were used as a sorrugate to classify Irish Sitka spruce plantations into five polymorphic site classes of I, II, III, IV and V in order of decreasing productivity, and for fitting the site class growth models using the Chapman-Richard nonlinear growth function. The NLQR methodology performed better than the traditional guide curve methodology for constructing top height-age site index curves when compared to the NLQR median (tau = 0.5) predictive growth model. The fit statistics deviance of 10639.6 for the NLQR median (tau = 0.5) predictive growth model was considerably smaller than the fit statistics deviance of 71736.3 for the comparable Nonlinear least square (NLS) mean top height-age growth model. The model predicted top heights of 27.6 m +/- 2.4%, 23.3 m +/- 2.9%, 20.4 m +/- 2.8%, 17.4 m +/- 2.9% and 16.0 m +/- 2.1% are to be expected in Sitka spruce stands at age 30 years on site classes I, II, III, IV and V respectively. (c) 2012 Elsevier B.V. All rights reserved.