Precommercial thinning of overtopping aspen to release coniferous regeneration in a boreal mixedwood stand

Abstract

We used precommercial thinning (PCT) to accelerate natural succession in an 8-year-old, even-aged stratified mixture, in which trembling aspen (Populus tremuloides) overtopped a dense understory of balsam fir (Abies balsamea) and spruce (Picea mariana, P. glauca). In addition to an unthinned control, we applied three residual aspen spacings (2.5, 3.0 and 3.5 m) while retaining all understory conifers, and a 3.0-m spacing retaining only small conifers (<1/3 of the aspen crop tree height). PCT improved survival of spruce, increased diameter growth of aspen by 90% and doubled diameter and height growth of conifers, without differences among spacings or between levels of conifer retention after 10 years. The level of conifer retention did not affect aspen growth response, but appeared to influence the occurrence of browsing on aspen and balsam fir. Retaining only small conifers decreased regeneration density of balsam fir in favour of abundant vegetative reproduction of red maple (Acer rubrum) and beaked hazelnut (Corylus cornuta). Live crown characteristics and stand structure indicate that thinning caused a shift from overtopping of aspen to an intimate mixture of species sharing the growing space. This study confirms PCT as a means of accelerating natural succession in a boreal mixedwood stand, thereby securing the coniferous component at the early stage of development.Key words: ecosystem-based forest management; hardwood conversion; stratified species mixture; precommercial thinning; live crown characteristics. RÉSUMÉNous avons utilisé l' éclaircie précommerciale (EPC) afin d'accélérer la succession naturelle dans un mélange équien stratifié de 8 ans, dans lequel le peuplier faux-tremble (Populus tremuloides) surplombait une dense régénération de sapin baumier (Abies balsamea) et d' épinette (Picea mariana, P. glauca). En plus d'un témoin non éclairci, nous avons appliqué trois espacements résiduels entre les trembles (2,5, 3,0 et 3,5 m) en conservant tous les conifères du sous-étage, et un espacement de 3,0 m en conservant seulement les petits conifères (<1/3 de la hauteur du tremble éclairci). L'EPC a amélioré la survie de l' épinette, augmenté la croissance en diamètre du tremble de 90% et doublé la croissance en diamètre et en hauteur des conifères, sans différences entre les espacements ou entre les niveaux de rétention des conifères après 10 ans. Le niveau de rétention des conifères n'a pas affecté la réponse de croissance du tremble, mais a semblé influencer la fré-quence du broutement sur le tremble et le sapin. La rétention des petits conifères seulement a diminué la densité de la régénération de sapin au profit d'une abondante reproduction végétative d' érable rouge (Acer rubrum) et de noisetier à long bec (Corylus cornuta). Les caractéristiques des cimes vivantes et la structure du peuplement indiquent que l' éclaircie a amené le peuplement à passer d'une dominance du tremble à un mélange intime d' espèces se partageant l' espace de croissance. Cette étude confirme que l'EPC est un moyen d'ac...

Mixedwood silviculture in North America: the science and art of managing for complex, multi-species temperate forests

“…Cut-stump, stem injection, or targeted foliar or basal spray application of herbicides could be used to selectively remove competing hardwoods in mixedwoods (Kenefic et al 2014), but non-target deposition can occur (Nowak and Ballard 2005). Among the mixedwood compositions we examined, hardwood species commonly sprout following disturbance, suggesting that properly timed mechanical release (brushing) may provide regenerating softwoods competitive advantage while not eliminating the hardwood component of the stand (Prévost and Charette 2017).…”

“…Managing for mixedwoods provides some assurance that stands will retain value in the face of market fluctuations, particularly if management increases quality and vigor in addition to compositional diversity (Granstrom 2019). Moreover, managing for hardwood -softwood mixtures can help improve crop tree value, especially when hardwoods are stratified above softwoods that act as trainers, shading the lower bole to improve hardwood branch shedding and accumulation of knot-free wood (Prévost and Charette 2017;Puhlick et al 2019).…”