Growth Response of Northern White-Cedar (Thuja occidentalis) to Natural Disturbances and Partial Cuts in Mixedwood Stands of Quebec, Canada

Abstract

Northern white-cedar (Thuja occidentalis) is a species of high commercial and ecological value, the abundance of which has been declining since the middle of the 19th century. Very little information regarding its silviculture in mixedwood stands is currently available, even though a significant portion of wood resources comes from these stands. The present study is a retrospective analysis of white-cedar growth in partially harvested mixedwood stands of western Quebec, Canada. Eight stands distributed across two regions were analyzed. Dendrochronological approaches examined long-term diameter growth for sample white-cedar trees and stems of associated species. These approaches were used to reconstruct stand characteristics at the time of harvesting, together with local harvesting intensity. The study demonstrated white-cedar’s capacity to maintain good growth for long periods of time and at large tree sizes. Accession to the upper canopy positions occurs through repeated episodes of suppression/release, most of which seem to be associated with spruce budworm (Choristoneura fumiferana) outbreaks. White-cedar response to partial harvesting varies with tree size, residual basal area and species composition. Growth response was generally stronger for small trees, even though large trees still maintained the best diameter growth. Growth of white-cedar was negatively affected by an increase in softwood proportion in basal area. Growth responses to harvesting could be sustained for a period of 20 years.

Development of Northern White-Cedar Regeneration Following Partial Cutting, with and without Deer Browsing

“…In the presence of these species, northern white-cedar could then become overtopped. Northern white-cedar can survive extended periods of suppression [8,12,35], and we know that merchantable stems of this species can respond well to release [10]. Hence, the growth of northern white-cedar to maturity could involve a succession of suppression-release periods.…”

“…No major natural disturbance was reported, except for the 2 most recent spruce budworm (Choristoneura fumiferana (Clem.)) outbreaks [10,20]. The understory was mostly composed of balsam fir, maples, northern white-cedar, yellow birch, and hemlock (except on the OR site).…”

“…For details, see Ruel et al [10]. Regeneration monitoring of northern white-cedar by 5-cm height classes was done in 3 equally spaced, circular 4-m 2 plots on the circumference of the sapling plots (n = 216 seedling plots).…”

Development of Northern White-Cedar (Thuja occidentalis L.) Plantations within and outside Deer Yards

“…This growth decline suggests that non-suppressed Age-25 cedars could benefit from the thinning of their overtopping deciduous competitors, which would allow for a growth release and leave dense, nearly pure, and well-established cedar stands. This thinning intervention is not necessary for cedar survival, as cedar can survive for extended periods in a suppressed position and still react well to release [13,32,33]. However, in the context of cedar production, thinning should significantly shorten the duration of the rotation by avoiding the suppressed stage altogether.…”

“…It was a better predictor of cedar growth than the canopy closure metric. The ratio of softwood basal area to total basal area (RATIO) was tested as a factor affecting cedar growth rates because softwood cover and browse are preferred during the winter by both deer and hare [55] and because an increase in softwood content has been linked to a decrease in diameter growth after partial cutting [33]. However, it was not identified as a contributing factor to variations in cedar growth, which can be explained in part by the high RATIO values found on most sites and by the lower influence of RATIO values on sites with open canopies.…”