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

Abstract

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.Résumé : L'éclaircie est censée accélérer le développement des attributs de fin de succession, améliorant ainsi l'hétérogénéité structurelle des jeunes peuplements de seconde venue. À l'aide d'une expérience à grande échelle établie dans des forêts côtières de douglas de Menzies (Pseudotsuga menziesii (Mirbel) Franco) âgées de 40 à 60 ans, nous avons poursuivi les objectifs suivants : (i) déterminer l'effet de trois traitements d'éclaircie sur la dynamique temporelle (11 premières années après l'éclaircie) des mesures clés de la structure forestière, (ii) évaluer les relations entre des mesures spatialement explicites de diversité structurelle et des mesures de peuplements non spatialement explicites, et (iii) tester la relation entre la structure du peuplement et les valeurs observées de croissance périodique du volume, du recrutement et de la mortalité dans le peuplement. Les traitements étaient des éclaircies par le bas à densités forte, modérée et variable ainsi qu'un témoin. Les différences d'hétérogénéité structurelle des peuplements entre les traitements étaient généralement non significatives. Cependant, nos résultats indiquent qu'un enrichissement structurel s'est produit dans les peu...

Evaluation of 10-year temporal and spatial variability in structure and growth across contrasting commercial thinning treatments in spruce-fir forests of northern Maine, USA

“…A graphical depiction of the approach is provided in Zenner (2000). SCI has a minimum value of 1 when all trees have the same size, regardless of spatial pattern, and has no upper bound (Kuehne et al 2015). In order to calculate SCI as a means to quantify spatial variability in individual tree size and growth, we used stem DBH and annual basal area increment (ΔBA, cm 2 year −1 ), i.e., absolute growth rates.…”

“…In addition to the direct effect on individual trees, thinning also modifies overall forest stand structure. Depending on the method (Nyland 2002), thinning can alter variation in stem diameter, homogenize canopy stratification, or increase species mingling (Pretzsch 1998;Barbeito et al 2009;Kuehne et al 2015). More recently, thinning has been promoted as one means of enhancing structural heterogeneity in secondary forests due to thinning-induced changes in stand dynamics (Bauhus et al 2009).…”

“…Traditionally, the interconnectivity between these factors has been evaluated via the classical stand density-forest growth relationship (e.g., Assmann 1970). Other, more recent studies directly linked forest structure and stand (e.g., Lei et al 2009;Kuehne et al 2015) as well as tree growth (Mainwaring and Maguire 2004;Dănescu et al 2016), or evaluated growth and stand dynamics in different structural forest settings (e.g., Ngo Bieng et al 2013;Pamerleau-Couture et al 2015). These studies also included the examination of stand conditions shaped and defined by spatially restricted, hence small-scale structural, features such as artificial canopy openings (Arseneault et al 2011) and harvester trails (Pukkala and Kolström 1991;Boivin-Dompierre et al 2017).…”

Restoration thinning enhances growth and diversity in mixed redwood/Douglas‐fir stands in northern California, U.S.A.

“…Local competition has been shown to influence individual tree growth, and may be particularly important in projecting growth and yield in diverse stands (Roberts & Harrington ; Berrill & Dagley ; Berrill et al ; van Mantgem & Das ; Kuehne et al ). Most promising appears to be the use of spatially explicit models that utilize stem‐mapped data to examine the competitive environment of each tree.…”

Regeneration Dynamics of Coast Redwood, a Sprouting Conifer Species: A Review with Implications for Management and Restoration

“…VDT attempts to diversify stand structures by thinning stands to have greater horizontal, vertical, or species diversity. Methodologies for VDT are developing and many alternative methods have been tested (e.g., [91][92][93][94][95]). Generally, the more successful methods for creating stand complexity, the more difficult they are to implement [93].…”