Using machine learning algorithms to estimate stand volume growth of Larix and Quercus forests based on national-scale Forest Inventory data in China

Estimating the volume growth of forest ecosystems accurately is important for understanding carbon sequestration and achieving carbon neutrality goals. However, the key environmental factors affecting volume growth differ across various scales and plant functional types. This study was, therefore, conducted to estimate the volume growth of Larix and Quercus forests based on national-scale forestry inventory data in China and its influencing factors using random forest algorithms. The results showed that the model performances of volume growth in natural forests (R2 ​= ​0.65 for Larix and 0.66 for Quercus, respectively) were better than those in planted forests (R2 ​= ​0.44 for Larix and 0.40 for Quercus, respectively). In both natural and planted forests, the stand age showed a strong relative importance for volume growth (8.6%–66.2%), while the edaphic and climatic variables had a limited relative importance (<6.0%). The relationship between stand age and volume growth was unimodal in natural forests and linear increase in planted Quercus forests. And the specific locations (i.e., altitude and aspect) of sampling plots exhibited high relative importance for volume growth in planted forests (4.1%–18.2%). Altitude positively affected volume growth in planted Larix forests but controlled volume growth negatively in planted Quercus forests. Similarly, the effects of other environmental factors on volume growth also differed in both stand origins (planted versus natural) and plant functional types (Larix versus Quercus). These results highlighted that the stand age was the most important predictor for volume growth and there were diverse effects of environmental factors on volume growth among stand origins and plant functional types. Our findings will provide a good framework for site-specific recommendations regarding the management practices necessary to maintain the volume growth in China's forest ecosystems.