Forest productivity may be determined not only by biodiversity but also by environmental factors and stand structure attributes. However, the relative importance of these factors in determining productivity is still controversial for subtropical forests. Based on a large dataset from 600 permanent forest inventory plots across subtropical China, we examined the relationship between biodiversity and forest productivity and tested whether stand structural attributes (stand density in terms of trees per ha, age and tree size) and environmental factors (climate and site conditions) had larger effects on productivity. Furthermore, we quantified the relative importance of environmental factors, stand structure and diversity in determining forest productivity. Diversity, together with stand structure and site conditions, regulated the variability in forest productivity. The relationship between diversity and forest productivity did not vary along environmental gradients. Stand density and age were more important modulators of forest productivity than diversity. Synthesis. Diversity had significant and positive effects on productivity in species‐rich subtropical forests, but the effects of stand density and age were also important. Our work highlights that while biodiversity conservation is often important, the regulation of stand structure can be even more important to maintain high productivity in subtropical forests.

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Historically, height-diameter models have mainly been developed for mature trees; consequently, few height-diameter models have been calibrated for young forest stands. In order to develop equations predicting the height of trees with small diameters, 46 individual height-diameter models were fitted and tested in young black spruce (Picea mariana) and jack pine (Pinus banksiana) plantations between the ages of 4 to 8 years, measured from 182 plots in New Brunswick, Canada. The models were divided into 2 groups: a diameter group and a second group applying both diameter and additional stand-or tree-level variables (composite models). There was little difference in predicting tree height among the former models (Group I) while the latter models (Group II) generally provided better prediction. Based on goodness of fit (R 2 and MSE), prediction ability (the bias and its associated prediction and tolerance intervals in absolute and relative terms), and ease of application, 2 Group II models were recommended for predicting individual tree heights within young black spruce and jack pine forest stands. Mean stand height was required for application of these models. The resultant tolerance intervals indicated that most errors (95%) associated with height predictions would be within the following limits (a 95% confidence level): [-0.54 [-17.1%, 18.6%] for jack pine. The recommended models are statistically reliable for growth and yield applications, regeneration assessment and management planning.Key words: composite model, linear model, model calibration, model validation, prediction interval, tolerance interval RÉSUMÉ Historiquement, les modèles hauteur-diamètre ont été élaborés surtout pour des arbres mûrs; en conséquence, rares sont les modèles hauteur-diamètre qui ont été calibrés pour les jeunes peuplements forestiers. Dans le but d' élaborer des équa-tions prédisant la hauteur d'arbres de petit diamètre, 46 modèles différents de hauteur-diamètre ont été ajustés et mis à l' essai dans des jeunes plantations d' épinette noire (Picea mariana) et de pin gris (Pinus banksiana) âgées entre 4 et 8 ans, dont les données provenaient de 182 parcelles situées au Nouveau-Brunswick au Canada. Les modèles ont été divisés en deux groupes : un groupe basé sur le diamètre et un deuxième groupe utilisant à la fois le diamètre et des variables additionnelles provenant du peuplement ou de l'arbre (modèles composites). Nous avons obtenu peu de différence dans la pré-diction de la hauteur des arbres parmi les modèles antérieurs (Groupe I) tandis que les modèles subséquents (Groupe II) ont permis d' obtenir généralement une meilleure prédiction. En fonction de l'homogénéité de la distribution (R 2 et carré de l' erreur moyenne), de la capacité de prédiction (le biais et ses intervalles associés de prédiction et de tolérance en termes absolus et relatifs) et la facilité d'utilisation, deux modèles du Groupe II sont recommandés pour la prédiction de la hauteur d'arbres individuels dans le cas de jeunes peuplements d' épinette n...

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