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A dynamic volume and biomass growth model system for even-aged downy birch stands in south-western Europe

Gómez-García, Esteban, Crecente-Campo, Felipe, Tobin, Brian, Hawkins, Michael, Nieuwenhuis, Maarten, Diéguez-Aranda, Ulises
Forestry 2014 v.87 no.1 pp. 165-176
Betula pubescens, aboveground biomass, basal area, decision support systems, equations, even-aged stands, forest management, growth models, heteroskedasticity, land use, mortality, prediction, trees, Spain
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 number of 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 m3 ha−1 year−1 in volume and by 2–8 Mg ha−1 year−1 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.