Abstract
This study develops some components to complete a management-oriented dynamic model system for simulating current and future volume and biomass of even-aged stands of managed downy birch (Betula pubescens Ehrh.) in Galicia (NW Spain). The specific components developed are a mortality equation, a basal area growth equation, a diameter distribution prediction system and equations for total and component biomass prediction. Functions to predict decreases in numberof trees per hectare and basal area growth were simultaneously fitted using Nonlinear Seemingly Unrelated Regression (NSUR) and a base-age invariant dummy variable method; critical errors of 19 and 16 per cent were obtained, respectively, when projections were made. The Weibull function modelled successfully all but four of the 198 diameter distributions examined, using the parameter recovery method through moments. To predict components and total aboveground biomass, a system of additive equations was simultaneously fitted using the generalized method of moments (GMM), which takes into account heteroscedasticity and inherent correlations among the biomass components. A two-step fitting procedure was used because of the different number of observations for the components considered. The biomass equations explained more than 85 per cent of the observed variability. The Galician birch stands which were surveyed were found to have grown by 3–10 m ha year in volume and by 2–8 Mg ha year in aboveground total biomass, with rotations between 30 and 60 years. The model system now forms an accessible decision support system for forest management and land development in the region.
Highlights
This study develops some components to complete a management-oriented dynamic model system for simulating current and future volume and biomass of even-aged stands of managed downy birch (Betula pubescens Ehrh.) in Galicia (NW Spain)
To predict components and total aboveground biomass, a system of additive equations was simultaneously fitted using the generalized method of moments (GMM), which takes into account heteroscedasticity and inherent correlations among the biomass components
The Betula genus is distributed throughout most of Europe, where it is represented by four species, two of which are trees: downy birch (Betula pubescens Ehrh.) and silver birch (B. pendula Roth.)
Summary
The Betula genus is distributed throughout most of Europe, where it is represented by four species, two of which are trees: downy birch (Betula pubescens Ehrh.) and silver birch (B. pendula Roth.). The re-measurement of such plots has provided information about the trajectories of the main stand variables over time, which allows the construction of dynamic growth equations These equations ( referred to as transition functions in this study) have the general form (omitting the vector of model parameters) of Y2 1⁄4 f(t2, t1, Y1), where Y2 is the value of the function at age t2, and Y1 is the reference variable defined as the value of the function at age t1. These dynamic models will offer a better description of the development (according to specified management intensities) of a stand over time than static models (Garcıa, 1988), and are preferable for simulation purposes
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