Juvenile stand responses and potential outcomes of conifer release efforts on Alberta's spruce–aspen mixedwood sites

Abstract

Twelve Alberta forest regeneration blocks, situated on representative white spruce (Picea glauca (Moench) Voss) -trembling aspen (Populus tremuloides Michx.) boreal mixedwood sites, planted to white spruce, and operationally released with glyphosate herbicide, were surveyed in the fall of 2002. Stand structure and composition were quantified and compared for treated and untreated portions of each block. The Mixedwood Growth Model (MGM, Department of Renewable Resources, University of Alberta) was used to project these stands over a 100-year horizon and to model the outcomes of several additional silvicultural treatments that could be applied to these blocks. A single release treatment provided 17% and 43% gains in planted white spruce height and stem diameter, respectively, an average of five years after treatment. Treatment shifted stands from being deciduous-dominated, with only 12% conifer basal area, to more than 75% conifer basal area, increasing conifer volumes per hectare nearly three-fold, but retaining conifer-deciduous mixture. Model projections suggest that these stands will produce similar total volumes over an 80-year rotation and that conifer release essentially trades deciduous volume for conifer volume, the degree of release dictating the extent to which this trade-off takes place. A single conifer release treatment led to an average simulated mature stand that contains 21% deciduous basal area, likely meeting mixedwood rather than conifer regeneration criteria. Model simulations of additional silvicultural interventions in these stands suggested that a variety of options exist to satisfy a range of stand or landscape management objectives for spruce-aspen mixedwoods, all within a relatively fixed volume production envelope. A clearer understanding of how early stand conditions translate into stand and landscape management objectives seems prerequisite to solving management conflicts on boreal mixedwood sites.Key words: boreal mixedwoods, vegetation management, conifer release, Mixedwood Growth Model, white spruce, trembling aspen Douze blocs de régénération forestière de l'Alberta, situés dans des stations représentatives des peuplements mélangés nordiques d'épinette blanche (Picea glauca (Moench) Voss) et de peuplier faux-tremble (Populus tremuloides Michx.), plantés avec de l'épinette blanche et dégagés de façon opérationnelle avec de l'herbicide à base de glyphosate, ont été mesurés au cours de l'automne 2002. La structure et la composition du peuplement ont été mesurées et comparées en fonction des portions traitées et non traitées de chaque bloc. Le Mixedwood Growth Model (MGM, Département des Ressources renouvelables de l'Université de l'Alberta) a été utilisé pour projeter ces peuplements sur une période de 100 ans et pour modéliser les retombées de certains traitements sylvicoles supplémentaires qui pourraient être appliquées à ces blocs. Un traitement de dégagement unique a résulté en des gains de 17 % et de 43 % au niveau respectivement de la hauteur et du diamètre des épinettes blanch...

Early vegetation control for the regeneration of a single-cohort, intimate mixture of white spruce and aspen on upland boreal sites – 10thyear update

“…Thus, it appears that two to four years of complete vegetation control within a 2-m radius around spruce planted at 5-m spacing is capable of maximizing the early performance of both spruce and aspen as a single cohort, intimate mixture. With the exception of higher-than-anticipated mortality on the spruce, this strategy appears to have the potential to provide an early rotation of aspen (~30 years) and a longer-term (> 60 years) sawtimber crop of spruce, as modelled by Pitt et al (2004b). Such a strategy could conceivably be targeted at suitable microsites to mitigate the spruce mortality observed in the experiment.…”

“…Research and operational experience have shown that fairly rigorous vegetation management is required to counter the natural propensity of upland boreal sites to regenerate aspen following harvest (Wood and von Althen 1993, Cole et al 2003, Pitt and Bell 2005, Boateng et al 2006 and that unintentional, intimate mixedwoods are often the result of conventional conifer regeneration efforts (Navratil et al 1991, Pitt et al 2004a2004b, Pitt andBell 2005). In 2000, we worked with a coalition of forest industry partners to formulate and test a practical regeneration strategy for a singlecohort, intimate mixture of spruce and aspen that would capitalize on the natural propensity of these sites to regenerate aspen following harvest.…”

Free-to-grow regeneration standards are poorly linked to growth of spruce in boreal mixedwoods

“…There have been many studies of vegetation management treatments applied to white spruce and results range from large increases in growth following treatment (Yang 1991, Jobidon 2000, Dampier et al 2006, through moderate increases (Biring et al 1999, Comeau et al 2003, Pitt et al 2004 to relatively small responses (Brand and Janas 1988, Groot 1999, Simard and Hannam 2000, Posner and Jordan 2002, Heineman et al 2005. We speculate that deciduous trees may be solely competitive in some situations while, in others, their presence facilitates spruce growth by ameliorating effects of summer frosts (Groot andCarlson 1996, Posner andJordan 2002), reducing evaporative demand (Bladon et al 2006), or improving nutrient cycling (Man andLieffers 1999, Simard andHannam 2000).…”

“…By giving all the credit for size asymmetry to competition, we create unrealistic expectations of improved growth once the neighbours are removed. Accurate determination of the effects of neighbours on tree growth requires density manipulation experiments (e.g., Vanclay 1994, Pitt et al 2004, Bokalo et al 2007.…”

Review of herbicide use for forest management in Alberta, 1995–2009

“…Herbicides are a highly effective means of assuring investments in reforestation (Pitt et al 2004, Wagner et al 2006. Decades of research have shown herbicides to be an effective means of renewing forests around the world.…”