Aspen (Populus tremuloides Michx.) is a clonal tree species that commonly regenerates via root suckering after disturbance. This paper reviews the literature and identifies critical gaps in our understanding of the dynamics of aspen root suckering. The role of plant growth regulators (e.g., hormones, carbohydrates), environmental conditions (e.g., soil moisture, temperature, nutrient availability), overstory disturbance (e.g., harvesting, wildfire), ground disturbance (e.g., soil compaction, wounding or severing of roots), vegetation competition, predisturbance stand condition, and clonal (genetic) differences are discussed as they relate to sucker initiation, sucker growth, and (or) patterns of site establishment. The paper presents a series of conceptual figures summarizing our knowledge of the factors controlling suckering dynamics and identifies areas of future research.

This study evaluated the contribution of different ages of foliage to the nutrient and carbon balance of black spruce (Picea mariana (Mill.) B.S.P.) from a nutrient-poor peatland in Alberta. Seasonal patterns of foliar nitrogen and phosphorus concentration and content were examined in six needle cohorts up to 10 years old. Trees were treated to simulate excess nutrient deficiency (removal of all one-year-old foliage), nutrient excess (fertilized with 250, 50, 100 kg ha NPK split application in June and July), or left as controls. Gas exchange (net assimilation-Na, stomatal conductance-g, mesophyll conductance-g, water-use efficiency-WUE, dark respiration-RS) was measured on six different needle cohorts in several control trees in 1989 and 1990. Nitrogen and phosphorus concentration decreased with needle age. Foliar nutrient concentration fell from April to June and then was stable until September except for the fertilized trees where it increased. There was no evidence of greater than normal retranslocation of nutrients from older needles for defoliated trees or greater than normal nutrient loading in older needles of fertilized trees. NA, g, g, WUE, and RS were similar for all needles up to six or eight years old, these older needles having NA of 65% of current needles and similar RS. The results do not support to conclusion that older needles of black spruce are retained as an adaptation to nutrient stress. It does not appear that older needles serve as a nutrient storage site in conditions of excess nutrient availability or a greater than normal nutrient source during times of excess nutrient deficiency. It appears that the maintenance of long-livedfoliage in black spruce does not provide for greater flexibility in tree nutrient allocation. Their contribution to the carbon balance of the tree seems to be sufficient to explain their retention.

A large component of the boreal mixedwood forest is comprised of aspen and white spruce mixtures of varying proportions and ages. The slower growing white spruce usually starts as an understory component but will succeed to a white sprucedominated stand after aspen break-up. Since both species are utilized by the forest industry, one method of maximizing total yield is to protect the unmerchantable white spruce understory while harvesting the merchantable aspen overstory. Although some of the white spruce understory is lost when the machine corridors are harvested, future conifer yield is augmented by the accelerated growth of the protected spruce component, a result of increased light levels. In a 10 year trial comparing the growth of released versus control understory spruce, annual height growth, diameter growth and volume increment were 76%, 152% and 83% higher, respectively, for the released conifer compared to the control. In order to account for the yield implications in timber supply analysis, accurate forecasts of future stand development can only be obtained through the use of a forest growth model since long-term data are not available. The Mixedwood Growth Model (MGM) has a unique architecture that allows for the modeling of various strata in understory protection stands. This "multi-strata" modeling approach was used to forecast the combined yield of all the strata, including the impact of adjacent strata with regards to light availability. Operational examples of understory protection, data on white spruce release and aspen regeneration, as well as modeled volume forecasts are presented.Keywords: forest management, boreal mixedwood, harvesting systems, white spruce understory protection, white spruce release, growth and yield, Mixedwood Growth Model résumé Une grande partie des forêts mixtes boréales est formée de regroupements de tremble et d' épinette dans diverses proportions et d'âges variés. L' épinette blanche, à croissance plus lente, se présente d'abord en sous-étage, mais formera à terme un peuplement dominé par l' épinette blanche après le déclin du tremble. Étant donné que ces deux espèces sont utilisées par l'industrie forestière, il serait possible de maximiser le rendement total en protégeant le sous-étage d' épinette blanche, sans valeur marchande, lors de la récolte commerciale de la strate dominante de tremble. Même si une partie du sous-étage d' épinette blanche devait être détruite dans les sentiers aménagés pour la machinerie au moment de la récolte, le rendement futur des résineux en serait accru dû à la croissance accélérée de la strate d' épinettes protégée, en raison d'une plus forte luminosité. Dans un essai sur 10 ans destiné à comparer la croissance du sous-étage d' épinette dégagé par rapport au témoin, la croissance annuelle en hauteur, la croissance en diamètre et l'accroissement en volume ont été respectivement de 76 %, de 152 % et de 83 % plus élevés chez les épinettes dégagées comparativement au témoin. Pour tenir compte des implications qu'auront ces résult...

Meta-analysis was used to summarize the research results on the growth response of understory white spruce to release from overstory aspen from different studies available from published and unpublished sources. The data were screened for the suitability for meta-analysis. Treatment effect sizes were calculated using response ratio from mean cumulative increments of released and control trees since release in height, diameter, and volume and modeled using a polynomial mixed effect regression procedure. Predictor variables include linear, quadratic, and cubic components of three independent variables — initial tree height, number of years after release, and residual basal area at release — and their linear interactions. Models with a reasonable predictive power were developed for height, diameter, and volume response, but no significant model was identified for survival. The models developed in this study can be applied to predict the growth response of understory white spruce to release, based on the growth of unreleased control trees, initial tree height, residual basal area at release, and time since release. The individual tree prediction can be easily scaled up to stand level if residual tree density and distribution is known. Key words: meta-analysis, boreal mixedwood, mixed model, polynomial regression, response ratio, growth, survival

Calamagrostis canadensis is present in the understory of most boreal forest mixed-wood stands and can rapidly dominate an area once the overstory is removed. The nature of the photosynthetic response to light was studied by investigating the stomatal response to cycles of high and low fluence light, and by developing photosynthetic light response curves for sun- and shade-grown plants. Photosynthetic light response curves did not differ between sun- and shade-grown plants. However, higher stomatal conductances, at all light intensities, were found in shade-grown plants. Stomatal conductance tracked changes in light under intermittent high and low fluence light. Average rates of stomatal opening after a change from low to high light were greater than closing rates (23 vs. 17 mmol m−2 s−1/min). The faster opening response was negated by a 1 – to 2-min time lag between the onset of high light and the initiation of stomatal opening. Thus the time to reach 80% of maximal response did not differ between opening or closing. The photosynthetic system of C. canadensis is not particularly well adapted to intermittent light conditions. The poor performance in the understory of this apparently meadow-adapted photosynthetic system is offset by the gain in the ability to rapidly recolonize the site after disturbance. Key words: gas exchange, stomatal conductance, Calamagrostis canadensis, non-steady state photosynthesis, variable light.