Growth Models for Even-Aged Stands of Hesperocyparis macrocarpa and Hesperocyparis lusitanica.

The cypress species Hesperocyparis lusitanica (Mill.) Bartel and H. macrocarpa (Hartw.) Bartel (formerly known as Cupressus lusitanica and C. macrocarpa) are widely grown in New Zealand where they are noted for their high-value timber. Preliminary cypress growth models were developed in New Zealand in 2004 based on limited growth data. Here, we describe new stand-level growth models developed for these two species using measurements from 521 permanent sample plots. Each model consists of several component sub-models including a height/age model, a diameter/age model, a mortality function and a stand-level volume function, with different parameter estimates used for each species. The mean top height is predicted from age and site index using common-asymptote Richards models. Modified common-asymptote Korf functions are used to predict the mean diameter as a function of age, stand density and a diameter index. The volume-based 300 Index, defined as the mean annual volume increment at age 30 years for a reference regime of 300 stems ha⁻¹, can be calculated directly from the site index and diameter index using the volume function. These models will be of considerable value to forest managers for regime evaluation and yield prediction. By more robustly describing site productivity using the 300 Index, the models provide an improved framework for understanding and quantifying site productivity. [ABSTRACT FROM AUTHOR]