Effects of thinning-induced changes in structural heterogeneity on growth, ingrowth, and mortality in secondary coastal Douglas-fir forests

Thinning is believed to accelerate the development of late-successional attributes, thereby enhancing stand structural heterogeneity in young, secondary forests. By making use of a large-scale experiment implemented in 40- to 60-year-old coastal Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) forests, we addressed the following objectives: (i) determine the effect of three thinning treatments on the temporal dynamics (first 11 years after thinning) of key forest structure measures, (ii) evaluate the relationships between spatially explicit structural diversity measures and spatially nonexplicit stand metrics, and (iii) test the relationships between stand structure and observed periodic stand volume growth, ingrowth, and mortality. Treatments consisted of high-density, moderate-density, and variable-density thinnings-from-below, as well as a control. Differences in stand structural heterogeneity between treatments were mostly nonsignificant. However, our results suggest that variable- density stands displayed structural enrichment as tree size and tree species diversity increased throughout the study period as a result of continuous ingrowth of species other than Douglas-fir. Simple spatially nonexplicit metrics could not be used to reliably model spatially explicit structural diversity measures. The inclusion of structural and species diversity measures only rarely improved accuracy of sample plot level growth, ingrowth, and mortality prediction models. Despite the short-term nature of this study, we conclude that variable-density thinning shows promise in increasing structural heterogeneity in young even- aged stands. The inclusion of structural diversity measures in growth and mortality models may be beneficial, but further work is needed to clarify the underlying relationships, particularly at the individual-tree level. Key words: variable-density thinning, tree species diversity, tree size diversity, spatial tree arrangement, forest structure. L'eclaircie est censee accelerer le developpement des attributs de fin de succession, ameliorant ainsi l'heterogeneite structurelle des jeunes peuplements de seconde venue. A l'aide d'une experience a grande echelle etablie dans des forets cotieres de douglas de Menzies (Pseudotsuga menziesii (Mirbel) Franco) agees de 40 a 60 ans, nous avons poursuivi les objectifs suivants: (i) determiner l'effet de trois traitements d'eclaircie sur la dynamique temporelle (11 premieres annees apres l'eclaircie) des mesures cles de la structure forestiere, (ii) evaluer les relations entre des mesures spatialement explicites de diversite structurelle et des mesures de peuplements non spatialement explicites, et (iii) tester la relation entre la structure du peuplement et les valeurs observees de croissance periodique du volume, du recrutement et de la mortalite dans le peuplement. Les traitements etaient des eclaircies par le bas a densites forte, moderee et variable ainsi qu'un temoin. Les differences d'heterogeneite structurelle des peuplements entre les traitements etaient generalement non significatives. Cependant, nos resultats indiquent qu'un enrichissement structurel s'est produit dans les peuplements a densite variable puisque la diversite en taille et en espece d'arbre a augmente pendant la periode d'etude a cause du recrutement continu d'especes autres que le douglas de Menzies. Des mesures simples, non spatialement explicites, n'ont pu etre utilisees pour modeliser de facon fiable les mesures de diversite structurelle spatialement explicites. L'inclusion de mesures de diversite structurelle et specifique a rarement ameliore la precision des modeles de prevision de la croissance, du recrutement et de la mortalite a l'echelle de la placette echantillon. Malgre la courte echelle temporelle de cette etude, nous concluons que l'eclaircie a densite variable semble prometteuse pour augmenter l'heterogeneite structurelle dans les jeunes peuplements equiennes. L'inclusion de mesures de diversite structurelle dans les modeles de croissance et de mortalite peut etre benefique, mais d'autres travaux sont necessaires pour clarifier les relations sous-jacentes, particulierement a l'echelle de l'arbre individuel. [Traduit par la Redaction] Mots-cles: eclaircie a densite variable, diversite des especes d'arbre, diversite de la taille des arbres, distribution spatiale des arbres, structure forestiere.
1. Introduction Restoring structural heterogeneity in secondary forests as a means to increasing the provision of ecosystem services has become a major objective in forest management and a much-noticed topic [...]