Abstract
Empirical growth models are widely used to predict the growth and yield of plantation tree species, and the precise estimation of site quality is an important component of these models. The most commonly used proxy for site quality in growth models is Site Index (SI), which describes the mean height of dominant trees at a specified base age. Although SI is widely used, considerable research shows significant site-dependent variation in height for a given volume, with this latter variable more closely reflecting actual site productivity. Using a national dataset, this study develops and describes a stand-level growth and yield model for even-aged New Zealand-grown coast redwood (Sequoia sempervirens). We used a novel modelling approach that quantifies site quality using SI and a volume-based index termed the 300 Index, defined as the volume mean annual increment at age 30 years for a reference regime of 300 stems ha−1. The growth model includes a number of interrelated components. Mean top height is modelled from age and SI using a polymorphic Korf function. A modified anamorphic Korf function is used to describe tree quadratic mean diameter (Dq) as a function of age, stand density, SI and a diameter site index. As the Dq model includes stand density in its formulation, it can predict tree growth for different stand densities and thinning regimes. The mortality model is based on a simple attritional equation improved through incorporation of the Reineke stand density index to account for competition-induced mortality. Using these components, the model precisely estimates stand-level volume. The developed model will be of considerable value to growers for yield projection and regime evaluation. By more robustly describing the site effect, the growth model provides researchers with an improved framework for quantifying and understanding the causes of spatial and temporal variation in plantation productivity.
Highlights
A central element of forest planning is prediction of plantation growth and yield.Growth models can be broadly grouped as being empirical, process-based or hybrid, model categorisation is often quite arbitrary as they all lie along a spectrum of empiricism [1]
Site quality was more robustly described using the 300 Index as this metric is a direct measure of site productivity, which is typically defined for forests as above-ground wood volume
Site Index has the advantage of not being greatly influenced by stand density, but its use as an index of site quality depends on the assumption that height and diameter growth are closely related at a constant stand density
Summary
A central element of forest planning is prediction of plantation growth and yield.Growth models can be broadly grouped as being empirical, process-based or hybrid, model categorisation is often quite arbitrary as they all lie along a spectrum of empiricism [1]. Through incorporation of the key physiological processes that influence growth, process-based models can characterise tree and stand development [4–6] and are often used for understanding and exploring system behaviour [2,7–9]. Hybrid models combine useful features from both statistical and process-based approaches and aim to utilise physiological knowledge while simultaneously incorporating inputs and outputs applicable to forest management [6]. These models encompass many variations in structure, they are often more sensitive to the environment than empirical models as they incorporate links to climatic and edaphic information [10–13] and do not require the parameterisation of process-based models [6]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
