Effects of topography and stand spatial structure on the diameter at breast height growth of major pioneer tree species of natural broad-leaved mixed forests in Zhejiang Province, China.

Based on the continuous inventory data of forest resources in Zhejiang Province in 2019 and 2021, we used statistical methods such as polynomial regression to analyze the impacts of topography and forest spatial structure on average annual diameter at breast height (DBH) growth of main pioneer tree species in natural broad-leaved mixed forests. The results showed that DBH of Schima superba , Quercus glauca , Quercus fabri , Lithocarpus glaber , Castanopsis eyrei , and Castanopsis sclerophylla were between 5-50.8, 5-41.5, 5-50.8, 5-43.9, 5-55.5, and 5-46.1 cm, respectively. We classified all the trees into three classes based on DBH: small (6-12 cm), medium (12-14 cm), and large (>26 cm). The average annual DBH growth of S. superba and Q. glauca was the highest on semi-shady slope and shady slope, with increases of 2.9%-15.7% and 1.1%-41.2%, respectively. The average annual DBH growth of large-diameter S. superba , L . glaber , C . eyrei and C . sclerophylla decreased with the increase of slope, with a maximum decrease of 27.0% for S. superba , with no significant difference among small- and medium-diameter trees as a whole. The slope position did not affect the annual DBH growth of small-diameter trees, while that of medium- and large-diameter S . superba , Q. glauca , and large-diameter Q . fabri , L . glaber decreased with the change of slope position from downhill, mesoslope, uphill to ridge, with a maximum decrease of 28.1% for L . glabe , and the major-diameter C . eyrei was on the contrary. Appropriate increase in the mingling was beneficial to the average annual DBH growth of medium- and large-diameter trees. Moderate mixing was suitable for S . superba , while low degree mixing and moderate mixing was suitable for Q . glauca , Q . fabri and L . glaber , and intensive mixing was suitable for C . eyrei and C . sclerophylla . No significant difference was observed for minor-diameter trees under the mingling. The neighborhood comparison only had a significant effect on the average annual DBH growth of large-diameter Q . glauca, Q . fabri , and L . glaber , which was significantly higher under subdominance-moderation than moderation-inferiority. The average annual DBH growth in the study area was mainly affected by aspect and mixing degree.