Short-term structure–growth responses to crop tree release across thinning intensities in mixed broadleaf plantations

Aim of study: We tested whether short-term (3-year) responses to crop tree release (CTR) in subtropical mixed broadleaf plantations follow a consistent gradient with realized thinning intensity, and whether growth gains are associated with changes in forest-floor and topsoil-related conditions. Area of study: Beishan Forest Farm, Tongshan County, Hubei Province, China. Material and methods: We established 25 plots (20 m × 20 m) and applied five thinning-intensity classes: control (CK, 0%), weak thinning (WT, 16.3%), moderate thinning (MT, 25.6%), heavy thinning (HT, 31.1%), and super-heavy thinning (ST, 41.8%). Stand structure and tree growth were measured before and three years after thinning; forest-floor and topsoil properties were assessed at year 3. Treatment effects were tested using ANOVA (or Kruskal–Wallis), and structure–growth associations involving forest-floor and topsoil variables were evaluated using plot-level regression models with realized thinning intensity (TI) as a continuous covariate where relevant. This two-tier design separates management-relevant class contrasts (CK–ST) from TI-gradient relationships based on realized thinning intensity, thereby strengthening inference on how intensity mediates coupled structure–growth patterns and their associations with forest-floor and topsoil variables. Main results: Increasing TI reduced competition (CI) and stand density (SD) while increasing openness (O). In the growth-association analyses involving forest-floor and topsoil variables, total nitrogen (TN) was positively associated with diameter at breast height (DBH) and stand-volume growth, organic matter (OM) was positively associated with DBH growth, and humus thickness (Hu) was negatively associated with DBH and stand-volume growth. At the treatment level, ST showed lower CI and SD but the thinnest forest-floor layers, whereas MT maintained relatively higher organic-matter-related values and thicker litter/humus layers. Research highlights: Management is goal-dependent: ST showed the highest short-term DBH growth, whereas MT had the numerically highest stand-volume average annual increment (AAI) while better maintaining forest-floor thickness and organic-matter-related values, although stand-volume AAI did not differ significantly among treatments. These short-term results reflect statistical associations; baseline and repeated soil measurements are needed to strengthen causal inference.