Additive volume-equation systems for Pinus ayacahuite and Pinus douglasiana in temperate forests of the Sierra Norte, Oaxaca, Mexico

Volume models are essential tools for quantifying timber stocks and optimising forest utilisation. This study aimed todevelop additive volume systems based onone- and two-entry simultaneous equations for <i>Pinus ayacahuite</i> Ehrenb. ex Schltdl. and <i>Pinus douglasiana</i> Martnez. Destructive sampling of55 <i>P. ayacahuite</i> trees and 65 <i>P.douglasiana</i> trees was conducted inthe communal forest ofIxtln de Jurez, Oaxaca, southern Mexico. The additive systems were fitted using non-linear seemingly unrelated regression toestimate tree-volume components: stem and branch volumes, with whole-tree volume being the sum ofboth. The systems were evaluated using the relative ranking method, considering statistical indicators ofaccuracy, variability, and relative errors. Additionally, the predictive capacity ofthe equations was assessed through linear regression between observed and predicted values for each volume component, and the biological consistency was verified. The results indicate that two-entry additive systems provide greater accuracy inestimating stem, branch, and whole-tree volumes for both species. These equations are based onthe Schumacher-Hall model, and their recommended range ofapplication for both species isfor diameter atbreast height (<i>DBH</i>) between 9 cm and 75 cm, and for total height (<i>H</i>) between 9 m and 34 m. Therefore, their application isrecommended for forest inventories and the planning of sustainable forest management.