ALBERT

Description: Precommercial thinning is often used to control stand density in naturally regenerated balsam fir (Abies balsamea (L.) Mill.) stands. Early stand density control could have beneficial effects on longer term stand stability through a modification of stem shape and root development. To assess the effect of precommercial thinning, two thinned and two unthinned stands were selected. Root sections were collected at 25 cm from the centre of the stem for all major roots (diameter greater than 2 cm). A disk was also cut at breast height level. From these disks, response in radial growth was determined. Detailed root measurements were taken on the sample disks to assess treatment effect on the following variables: asymmetry in root cross-sectional area (CSA) distribution, individual root shape, as well as changes in root shape over time. A quick and pronounced response in root growth occurred. This response was greater than that observed in the trunk. No asymmetry in root CSA distribution was observed at the stand level. Trees allocated more to radial growth above the biological centre of the root both in treated and control stands, but this trend was increased by thinning. Roots also tended to develop T-beam shapes over time, both in control and thinned stands. Most roots initially did not possess an I-beam shape and did not develop one during the course of the study. According to our results, trees respond quickly to the new growing conditions created by thinning by increasing biomass allocation to parts of the roots where mechanical stresses are greater.

Description: This paper reviews aspects of the functional ecology of naturally established tree seedlings in the boreal forests of North America with an emphasis on the relationship between light availability and the growth and survival of shade tolerant conifers up to pole size. Shade tolerant conifer species such as firs and spruces tend to have a lower specific leaf mass, photosynthetic rate at saturation, live crown ratio, STAR (shoot silhouette area to total needle surface area ratio), and root to shoot ratio than the shade intolerant pines. The inability of intolerant species such as the pines and aspen to survive in shade appears to be mainly the result of characteristics at the shoot, crown, and whole-tree levels and not at the leaf level. Although firs and spruces frequently coexist in shaded understories, they do not have identical growth patterns and crown architectures. We propose a simple framework based on the maximum height that different tree species can sustain in shade, which may help managers determine the timing of partial or complete harvests. Consideration of these functional aspects of regeneration is important to the understanding of boreal forest dynamics and can be useful to forest managers seeking to develop or assess novel silvicultural systems.

Description: Black spruce (Picea mariana (Mill.) BSP) seedlings were planted on eight drained forested wetland cutblocks to study the effects of soil type, distance from drainage ditch, and microtopography on the physical conditions of the rooting zone and on seedling growth, survival, and physiology. After two growing seasons, providing a raised planting spot had a greater impact on seedling growth than locally intensive drainage. Less saturated, better aerated, and warmer rooting zone on hummocks increased foliar N and Ca concentrations, which led to significantly greater relative growth rate, terminal shoot height growth, diameter, and survival compared with the seedlings in hollows. The effect of drainage on seedling growth is not conclusive. Depth of the aerobic layer and soil water content at 10 cm depth were similar at all distances to the ditch despite a significantly lower water table level in the 5-m plot. Thus, no significant differences were observed as a function of distance to drainage ditch for water relation and growth parameters except for better height growth in the 5-m plot the second year after planting. Water table levels were identical for both soil types and consequently growth was similar on wet mineral and organic soil types.

Description: Windthrow is a dominant natural disturbance in the boreal forest of eastern Canada. To provide the range of variability of a natural disturbance, its spatial distribution and patch metrics at stand and landscape scales have to be considered, together with the characterization of its severity. Our study characterized both partial windthrow (PW) and total windthrow (TW) spatial distributions at the landscape scale and patchiness within affected stands (stand scale). Landscape scale corresponded to three areas of about 5000 ha. Stand scale was the finest scale of analysis and corresponded to each affected stand within landscapes. In addition, windthrow spatial characteristics were compared with spatial characteristics of harvested areas (CUT). At the landscape scale, our results showed that TW stands were more isolated than PW stands and that mean shape complexity of disturbed stands was low, regardless of whether the disturbance was a windthrow or a harvested area. At the affected stand scale, residual trees covered a significantly higher proportion of PW stands than TW stands and CUT. CUT and TW did not share many spatial characteristics at the stand scale. CUT had a significantly higher proportion of complete canopy openness than TW. Our results showed that PW are spatially heterogeneous at both landscape and stand scales. In an ecosystem management context, i.e., a management that reduces the discrepancy between natural and managed forests, our results showed that forest managers could practice a variety of harvesting methods of different intensities.

Description: La coupe avec protection des petites tiges marchandes est un type de coupe partielle qui consiste généralement à récolter toutes les tiges d’un diamètre à hauteur de poitrine (dhp) supérieur à 15,0 cm, tout en conservant les tiges de plus petites dimensions. Le succès du traitement, appliqué à des forêts résineuses mûres dominées par le sapin baumier ( Abies balsamea (L.) Mill.) ou l’épinette noire ( Picea mariana (Mill.) Britton, Sterns & Poggenb.), repose en partie sur la capacité des tiges protégées à survivre. Un modèle logistique mixte a été calibré à partir de 27 blocs expérimentaux établis au Québec. Ce modèle identifie les variables qui conditionnent les probabilités de pertes des tiges individuelles protégées de 5,1 cm et plus de dhp, par mortalité sur pied ou par chablis, 5 ans après des coupes avec protection des petites tiges marchandes. Les résultats indiquent que les probabilités de pertes après traitement sont largement tributaires des caractéristiques du peuplement avant coupe (surface terrière marchande, densité de gaules, proportion de pin gris ( Pinus banksiana Lamb.)), de la qualité des opérations (procédé de récolte, taux de protection des petites tiges marchandes) et des caractéristiques des tiges protégées au moment de la coupe (inclinaison, dhp, essence). Les variables associées à l’exposition aux vents et à l’écologie des stations n’ont pas permis d’améliorer le modèle. Afin d’éviter des pertes trop élevées, il importe de bien cibler les peuplements à traiter et de réaliser un suivi rigoureux des opérations de récolte.

Description: As climate changes, boreal forest ecosystems may become subject to disturbances that were previously uncommon in some regions. In recent decades, large tracts of northeastern boreal forest of Canada have been affected by different types of climatic events causing a lot of partial and some total stand mortality. Since these disturbances may become more important drivers of forest dynamics, there is a need to document their impact on forest structure. The objectives of this study were to describe temporal dynamics of partial windthrows and determine the effect of partial windthrow on stand composition and understory vegetation. The study was conducted in the North-Shore region of Quebec (Canada). Eighteen plots in closed forests were paired with 18 adjacent windthrow areas, in which trees experienced similar edaphic and climatic conditions. Dendroecological analyses, combined with vegetation sampling, were conducted on each site to determine stand structure and vegetation development through time. Significant increases in balsam fir and shade-tolerant species were observed in windthrow gaps. Tree mortality in windthrown stands was a slow process until the mid-1990s, a period during which spruce budworm defoliation may have played a role in weakening trees and making them more vulnerable to partial windthrow. Greater mortality observed following the mid-1990s was most certainly related to a regional storm. The initial composition of stands plays an important role in driving postwindthrow succession, as balsam fir is more susceptible to treefall. As opposed to stand-replacing windthrow and spruce budworm outbreaks that generate various postdisturbance responses, partial windthrow appears to only create opportunities for pre-established balsam fir to undergo release in gaps.

Description: Boreal ecosystem functioning is largely controlled by disturbance dynamics. There have been efforts at adapting forest management approaches to emulate natural disturbance effects, as this is expected to maintain ecosystem resilience. In many instances, this involves resorting to partial cutting strategies that are likely to increase windthrow losses. The objective of this study was to determine the effects of alternative silvicultural practices on windthrow damage and how these effects vary with the scale of treatment. The study was conducted in the Quebec North Shore region (Canada), an area dominated by balsam fir (Abies balsamea (L.) Mill.) and accompanied by black spruce (Picea mariana (Mill) B.S.P.). Four different silvicultural treatments (overstory removal, heavy partial cutting, and two patterns of selection cutting) and control areas were implemented in 2004 and 2005. The experiment used a nested approach where treatment at the plot level was independent and yet nested within the block-level treatment. At the block level, treatments were applied over 10–20 ha units, leaving a small portion of the block for a smaller application of each treatment (plot scale, 2500 m2). Inventory was carried out before harvesting and monitoring was done yearly after harvesting, with the aim to better understand the plot- and block-level factors that drive windthrow damage levels and the effects of alternative silvicultural treatments. Results after 6–7 years show that basal area proportion windthrown differs substantially between treatments, as well as between treated sites and control sites. Windthrow levels were higher under heavy cuts relative to selection cuts and also increased with balsam fir proportion. Windthrow proportions were better correlated to block-level treatment than plot-level treatment, showing that the environment surrounding the treated plot can have an important effect on windthrow losses. Overall, the selection cutting system, particularly SC2, retains the most green-tree basal area and thus best meets the management objective of retaining old-growth attributes. A simple empirical model was calibrated that could aid in hazard rating.

Description: Large gaps (equivalent to twice the height of the trees, 1590 m2) originating from group selection cutting in mixed tolerant hardwood eastern white pine (Pinus strobus L.) stands of Quebec were used for geostatistical analysis of the seasonal mean of daily percent photosynthetic photon flux density (%PPFD), soil temperature and moisture, regeneration density of white pine, yellow birch (Betula alleghaniensis Britt.), and paper birch (Betula papyrifera Marsh.)), and growth of white pine. %PPFD, which measures the quantity of light above the shrub layer, varied according to the northsouth axis, the maximum being located in the northern portion of gaps. Surficial soil (5 cm) temperature and moisture varied according to both northsouth and eastwest axes. The southwest sector offered favourable conditions for white pine, yellow birch, and paper birch establishment, with maximum soil moisture and minimum soil temperature encountered in this position. There was no evidence of gap partitioning at the early establishment stage, although asymmetry in the microclimate was observed. However, results indicate that spatial patterns could change in the future and a partition resulting from differential survival among species could occur. Finally, results outline a possible discordance between optimal regeneration and growth niches for white pine, since planted seedlings (〉1 m) had the best growth in the central and northern section of gaps.

Description: In this study we compare the mechanical resistance of black spruce (Picea mariana (Mill.) BSP) and jack pine (Pinus banksiana Lamb.) and quantify the effect of species, forest cover type, and soil conditions on tree stability. To measure tree resistance to an applied load, 85 trees were pulled over using a cable and winch system. Predictive equations for the maximum turning moment that a tree can withstand (Mc) were developed with stem mass, and the other factors were used as explanatory variables. The presence of jack pine within the stand negatively affected black spruce resistance. In mixed stands, Mc was significantly influenced by the interaction between tree species and soil type. Jack pine was the only species with significantly lower resistance when grown on shallow and stony soils, which are likely to restrict root development. Black spruce resistance was not affected by soil conditions. Preliminary calculations of critical wind speeds required to cause damage using an adaptation of the ForestGALES model were much lower than those previously published for black spruce.