Assessing lead (Pb) and cadmium (Cd) concentration in tree rings may provide historical records of environmental contamination, capturing temporal changes and spatial distribution. This study examined heavy metal (HM) bioaccumulation in tree rings to assess the impact of smelting activities using urban trees. Tree rings from urban conifers were collected within a 5 km radius of the smelter to estimate spatiotemporal trends of Pb and Cd concentrations, evaluating the outcomes of contamination reduction measures and interspecies bioaccumulation patterns. Pb isotopic ratios (206Pb/207Pb, 208Pb/206Pb) were also measured to evaluate the Pb origin. Results showed no clear spatial pattern in relation to distance to the smelter, which may be due to the small sampling area. However, Cd bioaccumulation in urban pine was 4 to 7 times higher than in the distant site, 80 km away, indicating a local impact of industrial contamination in the urban area. Temporal analysis for pine showed a decrease of 0.27 mg/kg Cd (47%) between 1990 and 2020, reflecting the potential influence of contamination-reducing measures, while Pb concentrations in pine were 2.7 times higher (increased by 0.06 mg/kg) for the same period. Pine bioaccumulated more Cd than spruce, while spruce accumulated higher levels of Pb compared to pine. Isotope measurement confirmed that the copper smelter is the primary source of Pb. These findings underscore the complex nature of HM uptake in urban trees and suggest that further research is needed to understand the spatiotemporal effects on HM bioaccumulation patterns and which species are best suited for phytoremediation.

Windthrow is a natural disturbance affecting forest dynamics, characterized by tree uprooting or stem breakage when wind forces surpass tree anchorage strength or stem resistance. Windthrow mortality has been related to several ecological biotic and abiotic factors. However, the influence of natural root grafting on windthrow mortality remains unknown. This research evaluated the influence of root grafting on windthrow mortality by excavating root systems of jack pine (Pinus banksiana) in four windthrow-affected riparian buffers and analyzing root grafts using a dendrochronological approach. Our results revealed that natural root grafting decreased the uprooting likelihood but increased the propensity for stem breakage. In addition, root grafting occurred more frequently in trees closer to one another. These results suggest that root grafting influences the windthrow mortality type, with tree proximity being a good predictor for root grafting. This study provides valuable insights into windthrow dynamics, particularly relevant for managing windthrow mortality following partial harvesting and riparian buffers, conserving soil, and mitigating the impacts of windthrow events in the face of climate change.

In recent years, plant–microorganism interactions and their impact on plant growth and health, and in turn on plant-driven ecosystem services (e.g., carbon storage, phytoremediation) have gained a lot of interest in the scientific community. This interest has been stimulated by the development of high-throughput sequencing technologies that greatly improved the capacity of scientists to efficiently characterize microbiomes, which are defined as the pool of genomes of the microorganisms (e.g., bacteria, fungi) present in a particular habitat. Among trees, the poplar microbiome is one of the most extensively studied. The goal of this chapter is to summarize the current literature on the poplar microbiome, first by describing the microbiomes of belowground and aboveground parts of poplars, and second by highlighting links between poplar genetic and the microbiome. This second part includes a review of studies specifically addressing the impact of transgenesis on the poplar microbiome. Overall, this chapter highlights the diversity of the poplar microbiome and the importance of further assessing links between tree genetic and the microbiome.

Fine roots play a crucial role in soil nutrient and water acquisition, significantly contributing to tree growth. Fine roots with a high specific root length (SRL) and small diameter are often considered to help trees grow fast. However, inconsistencies in the literature do not provide a clear basis on the effect of root functional traits, such as SRL or root mass density (RMD), on tree growth rate in phylogenetically related trees. Our aim was to examine relationships between tree growth rate and root functional traits, using clones displaying different growth rates in a hybrid poplar plantation located in New Liskeard, ON, Canada. Fine roots (diameter < 2 mm) samples were collected using soil cores at depths of 0–20, 20–40 and 40–60 cm, and analyzed for morphological, chemical and architectural traits. High SRL and thin fine roots were associated with the least productive clones, which is not consistent with the root economics spectrum (RES) theory. However, the most productive clone had larger fine root diameter and higher root lignin concentrations, probably reducing root construction and maintenance costs and carbon losses. Therefore, at the 0–20 and 20–40 cm depths, tree growth rates showed positive correlations with root diameter and root lignin concentrations, but negative correlations with SRL and root soluble compounds concentration. Increasing RMD at the 0–20 cm depth promoted tree growth rates, showing the importance of soil exploration in the topsoil for tree growth. We conclude that fine root variation does not always follow the RES hypothesis and argue that the rapid growth rate of trees may also be driven by fine root growth in diameter and mass in phylogenetically related trees.

Mixedwood stands containing aspens (Populus tremuloides or P. grandidentata) often convert to hardwood-dominated stands after harvesting due to the rapid regeneration of aspen from root suckers, even when sites are promptly replanted with conifer seedlings. Without the use of herbicides, this problem is usually dealt with several passes of motor-manual (“manual”) release of overtopped seedlings. The aim of this study was to test a variation of thinning from below treatment (thinning; only 20% of the largest aspens are retained), and to compare it against two traditional release treatments: broadcast brushing (brushing; 100% removal of aspen) or crop tree release (CTR; removal of competing vegetation 60 to 90 cm around planted spruce) and an un-treated control. The thinning treatment left the 20% larger aspen stems in place, in order that they continue exerting apical dominance on smaller suckers and limit re-suckering of the treated plots. Aspen suckering and growth of planted black spruce seedlings (Picea mariana (Mill.) B.S.P) were measured two- and four-growing seasons following treatments. Results four years after release application showed that the thinning and CTR treatments reduced aspen density by 61% compared to the brushing treatment. In addition, aspen individual stem volume in the thinning treatment was almost 10 times larger than the brushing and twice that of the CTR treatments. Spruce height and ground collar diameter (GCD; 5 cm aboveground line) were both measured, and while height increment was similar in all treatments, diameter increment was greater in the thinning treatment (+42%) compared to the control, brushing and CTR treatments (+17%). Thinning yielded better short-term results than the brushing release in terms of aspen re-suckering and aspen sawlog potential, highlighting the need for adapted silvicultural treatments based on the species’ ecology.

Although monocultures are important for timber production, they are often associated with lower biological diversity than mixtures. Thus, mixed plantations have been suggested as a way to enhance biodiversity because of their inherent compositional diversity. However, the effects of monocultures versus mixtures on understory diversity and composition can vary in different ecosystems. The objective of this study was to assess how monocultures and mixed plantations influence understory vegetation diversity and composition in the boreal forest region of southern Quebec. We sampled plantations established with deciduous Populus trichocarpa Torrey & A. Gray × balsamifera L. and P. maximowiczii Henry × balsamifera L. and coniferous Picea abies (L.) Karst. and Picea glauca (Moench) species planted in monocultures and in mixed plantations on abandoned farmlands and a forest site. We assessed understory vegetation diversity and composition in each canopy type (coniferous, deciduous, mixed) and in each plantation type. We evaluated bryophyte and lichen diversity and composition specifically in tree microhabitats: soil, tree bases, and tree trunks. We found that vascular plant and lichen species richness was similar in all plantation types, while bryophyte species richness was higher in spruce monocultures and in mixed plantations compared to poplar monocultures. Our results also highlight how land-use history influenced vascular plant composition as abandoned farmland sites were composed of more ruderal vascular plants, while the previously forested site was composed of species found in natural forests. In the context of reforestation and plantations, our study suggests mixing spruce with poplars to maximize understory vegetation diversity as the addition of spruce in mixed plantations promoted the establishment of terrestrial bryophytes, while poplars favored the establishment of epiphytic lichens.

Increasing soil organic carbon (SOC) storage is one of the promising solutions to mitigate climate change. Fast-growing trees are a potential tool in this context as they rapidly accumulate C in their biomass and could transfer more organic matter (OM) into the soil. However, the relationship between aboveground productivity and SOC storage remains poorly understood. Five clones with different growth rates were selected from a 14-year-old hybrid poplar plantation located in New Liskeard, ON, Canada. We collected soil cores at 87.5 and 175.0 cm distance from the stem and at 0–20, 20–40 and 40–60 cm soil depth for soil C concentration analysis. The most productive clone DN2 (Populus deltoides × P. nigra) stored less SOC (83 Mg ha−1) between 0 and 60 cm depth than the mid-productive clones 1079 (Populus × jackii (P. balsamifera × P. deltoides)) and 915005 (P. maximowiczii × P. balsamifera) (95 and 96 Mg ha−1 respectively), while the least productive clone 747210 (P. balsamifera × P. trichocarpa) also had a lower SOC stock (85 Mg ha−1) compared to the other clones, but not significantly. There was no relationship between aboveground productivity and SOC stocks and total SOC stocks increased by 6 % when the sampling distance was closer to the tree stems. The difference in SOC stocks between clones was mostly observed at the 20–40 cm depth suggesting the significant effect of roots on SOC storage. Soil C/N ratios were significantly different between clones at 0–20 and 20–40 cm depths suggesting differences in OM decomposition rates between clones. There could be a trade-off between aboveground productivity and litter decomposition rate to increase SOC storage.

Knowledge of the morphological and physiological responses of trees to planting density is important for adjusting plantation establishment practices to attain the desired size and characteristics of wood products. In this review, we provide an overview of how planting density is managed as a silvicultural practice and summarize recent advances, trends, and gaps to guide future research in this field. We applied a PRISMA methodology to select papers published between 1981 and 2020 that tested how monospecies planting density affects the morphological and physiological characteristics of planted trees; we observed an increasing trend in the number of relevant publications over this period. These studies focused mainly on species used for timber production (Eucalyptus and Pinus), and research was concentrated in the United States, China, and Brazil. In general, the 255 selected studies identified that planting density has a major impact on stem diameter and volume, resulting in greater stand productivity of high-density stands, especially for trees having a smaller individual volume. Studies evaluating the physiological response of trees to planting density also showed that species ecology, solar radiation, water requirements, and site-specific characteristics have a greater influence on plant growth than planting density. Considering the physiological aspects in high-density plantations, water relations based on transpiration and water use efficiency are the main aspect guiding plants growth rate. Despite major advances in intensive silviculture around the world, there remains a knowledge gap for tree species grown for their leaves, fruits, or seeds and a lack of data related to the physiological response of trees to planting density. The results of this literature survey can improve forest management decisions in regard to forest stand use and develop novel study avenues for silviculture.

Natural root grafts (anastomoses) are morphological unions formed between roots of different trees. Common root systems allow translocation of water, nutrients and photosynthesis products between grafted trees, affecting their growth and their physiology. As carbohydrates are redistributed among grafted trees, the formation of a common root system could reduce the negative effect of intraspecific competition for light or soil resources within stands. The aim of this study was to investigate the role of root grafting on intraspecific competition and growth of balsam fir Abies balsamea. We studied inter-tree relationships in three natural balsam fir stands of the boreal forest of Quebec (Canada) that contained an average 36% of grafted trees. At each stand, ring width and basal area of trees were measured using dendrochronology techniques. We used mixed linear models to test the effect of root grafting and intraspecific competition on annual basal area increment of trees. Trees before grafting had higher growth rates than trees once grafted. Thus, root grafting did not improve tree growth. Growth of grafted trees was more negatively affected by intraspecific competition than growth of non-grafted trees. Thus, grafted trees cannot be considered as better competitors than non-grafted trees. Under high intraspecific competition, growth of larger grafted trees was less affected than that of smaller trees suggesting that they were able to divert resources at their advantage within a union. Our study demonstrated that grafted trees acted on each other's growth and provides support for the idea that grafted trees respond to competition for resources more as a community rather than as individual trees.

Aims
While fungi are key drivers of the carbon cycle and obligate symbionts of trees, the link between plant-fungal interactions and landscape vegetation changes has been largely overlooked. Our aim was to test whether a local difference in dominant tree species would shape the composition of soil fungi communities.

Methods
Fungal communities were described using next-generation DNA sequencing. Composite soil samples were collected in four paired sites (represented by one pure aspen stand and one pure spruce stand) and soil nutriments were measured.

Results
Of the more than 1119 OTUs, 31.6% were Ascomycota while 27.8% were Basidiomycota, 15% were ectomycorrhizal fungi whereas 19.7% were saprotrophic. Communities displayed high species turnover among forest types rather than differences in species richness. Among tested predictors, the dominant tree species explained around 11% of fungal community variation. pH and soil nutrients were also strong predictors of fungal communities.

Conclusions
Our study revealed strong correlations between dominant tree species and fungal communities at a local scale and raised questions regarding the impact of fungal communities on forest soil nutrient dynamics.

Les caractéristiques photosynthétiques des feuilles peuvent être déterminantes pour identifier les clones les plus productifs de peupliers (Populus spp.) et de saules (Salix spp.) hybrides. L’acclimatation photosynthétique de clones de peupliers et de saules hybrides a été étudiée en serre. Sept clones de Populus et cinq clones de Salix ont cru pendant trois mois selon trois espacements [20×20, 35 × 35 et 60 × 60 cm] et deux niveaux d’azote (20 et 200 ?g g?1). Il n’y avait pas d’effet significatif de l’espacement sur la biomasse aérienne sans feuilles par arbre (AGBT) et sur la hauteur. L’acclimatation des clones à des densités plus élevées était associée à une augmentation de 347% de l’indice de surface foliaire (LAI) et de 13% de la surface foliaire spécifique (SLA) malgré une diminution de 31% du contenu foliaire en azote par unité de surface foliaire (Narea). Il n’y a pas eu de changement du taux net d’assimilation de CO2 (A) et de l’efficacité d’utilisation photosynthétique de l’azote sous conditions de lumière ambiante (PNUEamb) et selon différents espacements. L’ajout d’azote a diminué les effets de la compétition en maximisant la surface foliaire (LA) et la SLA. Comparativement aux clones moins productifs, les clones plus productifs avaient des SLA 28% plus élevées, ainsi que des LA et AGBT plus élevées par unité d’augmentation de Narea dans tous les traitements. L’augmentation de LA et de SLA dans les plantations à forte densité est un indicateur clé de clones plus productifs. © 2017 Université Laval.

We developed allometric equations for small-diameter woody species growing on mixed forest marginal lands, which are potential sources of biomass for bioenergy. Eleven species of trees and shrubs were sampled from a site located in eastern Canada. Equations derived in this study generally performed better than equations from the literature. Also, fixed-area plots (FAP) and line-intersect sampling (LIS) methods using both random or systematic selection of sampling units were compared to determine which method required the lowest number of measurements to estimate stand biomass for the same precision.

The fixed-area plots method was successfully used to estimate relatively accurately oven-dry biomass per hectare. Results indicated that potentially harvestable woody biomass (oven dry basis) varied between 33-41 and 12–13 t ha?1 for the most and least productive marginal sites respectively. On the most productive site, LIS estimates (between 20 and 42 t ha?1) were usually lower than those obtained using different FAP sampling methods (i.e. systematic or random, small (50 m2) or large (100 m2) plots), but similar on the more open sites (between 10 and 14 t ha?1). Small FAP resulted in a plot without measurements in one case. Moreover, estimates based on small FAP were generally higher, even if not significantly different from larger plot estimates. We therefore suggest using FAP with 100 m2 plots to estimate small-diameter woody biomass on marginal lands with dense vegetation, while LIS, even if promising for open stands, needs further evaluation before recommendation.

As a black spruce (Picea mariana (Mill.) BSP) tree expands its adventitious rooting system, its root collar is amalgamated into the stump. Total tree age determined by ring counts at ground level or 1 m height on the stem is thus underestimated. This age underestimation would increase with stand age because of the continuous growth of adventitious roots and the sinking of the stem under its own weight. Tree age underestimation could lead to productivity overestimation. The goal of this study was to evaluate the effect of stand age on aging error of trees and productivity estimation. Three trees within each of fifteen non-paludified and naturally regenerated black spruce stands aged 37–204 years were excavated to harvest the stump and locate the root collar. Stump sections were cross-dated down to the root collar to obtain trees’ total biological age which was then compared to a ring count done at 1 m height. Height and volume growth curves obtained by stem analysis were compared with those derived from Pothier and Savard’s (1998) models. Age difference between total biological age and age at 1 m (aging error) ranged 9–58 years and significantly increased with stand age. Site indices (SI; height of trees at 50 years of age) were significantly over-estimated by using age at 1 m, and the overestimation significantly increased with stand age and aging error. The use of age at 1 m lead to poor modeling of height- and volume-growth trajectories, as early height and volume-growth obtained through stem analysis was slower than predicted and stand senescence also occurred later. Due to their period of horizontal growth, seedlings don’t accumulate any height during the first years. Additionally, early tree growth of our oldest trees seems to have been slower than early growth of our younger trees, probably because of less favorable growing conditions. Despite large differences between volume and height growth trajectories, predicted volumes from theoretical SI calculations were not significantly different from observed values using stem analysis, at the time of sampling. Predictive models should nevertheless be adjusted by using total biological age or time since the last fire, to consider the first years of slow growth and to obtain more accurate productivity estimations.

Intensively managed plantations are being established in a wide range of environmental conditions to satisfy the high demand for wood products and reduce the exploitation pressure on natural forests. In this study, we investigated the plasticity of four hybrid poplar (Populus spp.) clones established in 2005 along a latitudinal gradient in northwestern Quebec, Canada. The effect of latitudinal gradient on maximum rates of electron transfer (Jmax) and carboxylation (Vcmax), dark respiration (Rd), spring and fall bud phenology, net photosynthesis (Pn), specific leaf area (SLA), per mass nitrogen leaf concentration (Nm) were assessed in order to evaluate if clonal plasticity would result in increased overall productivity.

Growth season duration between the southernmost to the northernmost sites ranged 21–32 days, and was positively correlated to stem volume and negatively correlated to bud burst and bud set duration. Growth increment (stem volume) along the latitudinal gradient ranged 100–184% between the least and most productive clone. Clone 747215 had the most stable but the slowest growth. Leaf net photosynthesis decreased or did not change northwards except for the most productive clone for which it increased slightly likely due to a significant decrease in SLA. Maximum rates of carboxylation and photosynthesis electron transfer (Vcmax and Jmax) decreased northwards for three of the four clones, suggesting that photosynthesis of trees did not acclimate to lower temperatures from south to north. Plasticity of photosynthetic variables, measured with trait plasticity index was usually greater than that of SLA and Nm.

Background and Aims

Abandoned lands are increasingly used to establish fast-growing tree plantations, and are often rapidly colonized by a high density of herbaceous undergrowth. These weeds are generally removed since they compete with trees for resources, in particular soil nutrients. However, mixing herbaceous litter with the litter of planted trees could also stimulate the activity of decomposers and associated nutrient release due to an increase of litter quality (lower C:N ratio), plant diversity (more diverse litter traits) and water holding capacity. The objective was to determine the impact of herbaceous litter on the litter decomposition process of white spruce and hybrid poplar litters alone or in mixtures.

Methods

Litter mass loss rate, nutrient release and decomposer communities were monitored on single and mixed-species litters using litterbags during two years in three plantations types (hybrid poplar, white spruce and mixed plantations). Litters within mixtures were separated by species to identify species-specific responses of leaf mass loss.

Results

N release of all litter types increased with the presence of herbaceous litter. This finding could be linked to the greater abundance of decomposers and fungal biomass brought about by the herbaceous litter. Addition of herbaceous litter had no effect on spruce litter mass loss but had positive effects on poplar and mixed spruce/poplar litter mass loss. Abundance of fungi and mites was more affected by litter quality, whereas the abundance of collembola was more affected by the diversity of resources than by litter quality.

Conclusions

In these 10-year plantations with poplar, increased litter mass loss for poplar and mixed litters and N release associated to the presence of herbaceous litter showed that weeds may change soil C sequestration and N cycling.

• Key message

Pruning one third of crown length in summer produced the least number of epicormic branches after two growing seasons. Epicormic branches can be removed without compromising tree growth.

• Context

The formation of epicormic shoots is often observed following pruning treatments, but their role in the overall tree growth is unknown.

• Aims

The objectives of this study were to examine how pruning intensity and season affect the production of epicormic branches and how their presence (or removal) affects tree growth and total non-structural carbohydrate reserves in the roots.

• Methods

Trees from four hybrid poplar clones were pruned in fall, spring, and summer at two intensities 1/3 and 2/3 of the crown length and an unpruned control. Produced epicormic branches were removed from half the trees.

• Results

Pruning intensity and season were the most important factors affecting the number and biomass of epicormic shoots while clone was not. Pruning 1/3 crown length in summer reduced the emergence of epicormic shoots compared to 2/3 and spring or fall pruning. Two years after pruning, the removal of epicormic shoots did not affect height or diameter at breast height of trees, and their presence did not restore root total non-structural carbohydrates reserves to unpruned levels.

• Conclusion

We concluded that pruning should be done in summer at 1/3 of the crown length to reduce epicormic shoot formation and to avoid decreases in stem growth. If epicormic shoots appear, they can be removed without compromising tree growth.

• Premise of study: Trees are traditionally considered as distinct entities even though they can share a communal root system through root grafts, which are morphological unions between two or more roots. Little is known regarding the ecological significance of natural root grafting, but because grafted trees can share resources and secondary compounds, growth of linked trees can be affected directly by the presence of root grafts. Traditional forest ecology concepts may have to be revised to include direct interactions between connected trees.

• Methods: We hydraulically excavated six 30–50-m2 plots (three natural stands and three plantations). We measured yearly radial growth and determined the influence of root grafting on radial growth of grafted trees.

• Key results: During periods of root graft formation, root grafting tended to reduce radial growth of jack pine trees, after which growth generally increased. The influence of root grafting on growth was more significant in natural stands, where root grafting was more frequent than in plantations.

• Conclusions: These results suggest that root grafting initially is an energetically costly process but that it is afterward nonprejudicial and maybe beneficial to tree growth. The use of a communal root system allows for a maximum use of resources by redistributing them among trees, leading to increased tree growth.

Commercial thinning is a silvicultural treatment used to increase the merchantable yield of residual trees. Growth response to thinning, however, is highly variable and discrepancies between studies remain largely unexplained. The objective of this study was to demonstrate the effect of natural root grafting on growth response after thinning. We excavated root systems of jack pine (Pinus banksiana) in five naturally regenerated stands, in which three had been commercially thinned 6 and 9 years earlier. Radial growth before and after thinning was examined using dendrochronological techniques. Thinning increased radial growth of trees, however growth increments were significantly less for trees that had root grafts with removed trees, while growth of grafted trees was better in unthinned stands. Furthermore, radial growth response of trees grafted to removed trees was smaller than that of non-grafted trees 4 years and more post-thinning. On average, non-grafted stumps survived less than 1 year (0.4 year), while grafted stumps lived 2.0 years after the stem was removed. Differences in growth response to thinning between grafted and non-grafted trees thus appear to be linked to the support of roots and stumps of removed trees by live residual trees.

Planting stock type and quality can have an important impact on early growth rates of plantations. The goal of this study was to evaluate early growth and root/shoot development of different planting materials in typical heavy clay soils of northwestern Quebec. Using one-year-old bareroot hybrid poplar dormant stock, four planting materials were compared: (1) regular bareroot stock, (2) rootstock (stem pruned before planting), (3) whips (roots pruned before planting), and (4) cuttings (30 cm stem sections taken from the basal portion of bareroot trees, i.e. roots and shoot pruned). Rooted stock types (bareroot and rootstock) produced, on average, 1.2 times larger trees than unrooted stock types (cuttings and whips). However, shoot-pruned stock types (rootstocks and cuttings) reached similar heights and basal diameters as unpruned stock types (bareroots and whips), during the first growing season. Shoot pruning reduced leaf carbon isotopic ratios, suggesting that unpruned stock types were water-stressed during the first growing season. The stress was most likely caused by early leaf development while root growth occurred later in the summer. We conclude that shoot pruning bareroot stock is a useful management option to reduce planting stress without compromising early growth rates of hybrid poplars.

Trembling aspen ( Populus tremuloides ) is a clonal tree species, which regenerates mostly through root suckering. In spite of vegetative propagation, aspen maintains high levels of clonal diversity. We hypothesized that the maintenance of clonal diversity in this species can be facilitated by integrating different clones through natural root grafts into aspen ’ s communal root system. To verify this hypothesis, we analyzed root systems of three pure aspen stands where clones had been delineated with the help of molecular markers. Grafting between roots was frequent regardless of their genotypes. Root system excavations revealed that many roots were still living below trees that had been dead for several years. Some of these roots had no root connections other than grafts to living ramets of different clones. The uncovered root systems did not include any unique genotypes that would not occur among stems. Nevertheless, acquiring roots of dead trees helps to maintain extensive root systems, which increases the chances of clone survival. Substantial interconnectivity within clones as well as between clones via interclonal grafts results in formation of large genetically diverse physiological units. Such a clonal structure can signifi cantly affect interpretations of diverse ecophysiological processes in forests of trembling aspen.

Balsam poplar (Populus balsamifera L.) can easily be grown when planted as dormant, unrooted cuttings. The first part of this research project consisted of a greenhouse study to identify the best combination of pre-planting treatments to maximize rooting of hardwood cuttings for large-scale greenhouse propagation. Eighty-four treatment combinations were tested on 10 cm-long cuttings, including seven soaking lengths of time (0, 1, 2, 3, 4, 5 and 14 days), three dips (none, powder and liquid rooting hormone), two collection dates (fall and spring), and two cutting selections (average trees and a superior clone).

Secondly, we tested the performance of unrooted cuttings on typical forestry sites lacking natural regeneration, using 30 cm-long cuttings and full-length whips (>60 cm). Three pre-planting treatment combinations were used (soaking for 1 or 3 days, or 3 days with a dip in rooting hormone). Other factors tested were two collection dates (fall and spring), two cutting selections (average trees and a superior clone), two storage methods (stored as full whips or cut to size), and two cutting locations (basal or top 30 cm of stools). © 2004 Elsevier B.V. All rights reserved.

Seedlings from three open-pollinated aspen (Populus tremuloides Michx.) families were grown in a greenhouse with four nitrogen (N) sources, each at two N levels and three pH levels. Nitrogen sources were ammonium sulfate ((NH4)(2)SO4), calcium nitrate (Ca(NO3)(2)), ammonium nitrate (NH4NO3), and urea ((NH2)(2)CO); N levels were 50 and 200 mg.L-1 (100-mL aliquots three times per week); and pH levels were 5, 6, and 7. Shoot dry mass and leaf area increased with pH in (NH4)(2)SO4 and (NH2)(2)CO treatments, but were greatest at pH 6 in the NH4NO3 and Ca(NO3)(2) treatments as a result of a pH x N source interaction. N level was directly related to growth, net assimilation, water-use efficiency, and leaf carbon isotope ratio, while N source had no effect on these parameters. Seedlings from families 2 and 3 grew larger than seedlings from family 1, which allocated relatively more dry matter to roots. Growth of the three families interacted with soil pH so that family 1 was largest at pH 7, but families 2 and 3 were largest at pH 6. Seedling boron (B) uptake was reduced by increasing pH and by the Ca(NO3)(2) fertilizer. These results were interpreted to show that although the three families seemed well adapted to all the N and pH conditions to which they were exposed, growth could be increased by selection of a fertilizer most suitable to the pH and availability of other nutrients in the soil.©2003 NRC Canada

The coarse-root dynamics of ramets of Populus tremuloides (aspen) were investigated with respect to persistence of the original root connections (roots of parent trees from which the ramets originated), the time of establishment of new roots at the base of the stem and the fate of the communal root system after death of individual trees, Parts of the root systems of three declining stands of aspen ramets were hydraulically excavated. From each stand, sections of all structural roots were collected at the base of live and dead trees and were analysed using dendrochronology techniques. Parent roots were identified in the root system of every tree. The trees initiated new structural roots shortly after suckering. Live roots were often connected to the stump of dead and decayed trees. Grafting was common, especially at or near the stumps. Death of trees along the parent roots over time did not seem to favour the entry of significant decay. Nor promote breakage of the original root connections. Instead of becoming independent of the parent root system the ramets incorporated the parent roots into their own root systems, remaining interconnected.

In north-central Alberta, 12 plots (9 m²) were hydraulically excavated in young trembling aspen (Populus tremuloides Michx.) stands (5-10 years old) of different sucker density to quantify the effects of sucker density on the parental root system, on the formation of new roots and on the growth of suckers. All roots were collected and divided into live and dead parental roots and new root categories. Size and age of parent roots at the time of suckering were determined. Total biomass ranged from 1 to 18 t/ha of live roots. Living root biomass was proportional to stand density and leaf area index (LAI). Low-density stands had a higher proportion of dead roots. Suckers in plots with more parental root biomass/sucker had greater height growth. Root/shoot ratios ranged from 0.46 to 3.52 but were not correlated with stand densities. Stands with larger basal area of suckers and greater mean parent root diameter produced more biomass of new roots. This research suggests that young sucker-origin aspen stands support a large underground biomass and that high sucker densities and LAI are required to prevent loss of parental root biomass. ©2001 NRC Canada

Trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) seedlings were grown at uniform air temperatures but different soil temperatures (5, 15, and 25 degreesC), and gas-exchange and growth characteristics were examined during active growth and early dormancy. At 5 degreesC, Populus tremuloides had no root growth and limited growth in leaf area and shoot mass compared with the large increases in leaf and shoot mass at 25 degreesC. In contrast, Picea glauca had some root growth at 5 degreesC and moderate growth of roots at 15 and 25 degreesC; however, there were no differences in aboveground mass at the different soil temperatures. Net assimilation and stomatal conductance in Populus tremuloides were reduced with decreasing soil temperatures, while in Picea glauca soil temperatures did not affect these gas-exchange variables. In both species, root mass was higher in the dormant period than during the growing period, while root volume remained constant. Generally, the growth variables of Populus tremuloides were more suppressed by cold soils than in Picea glauca. Root total nonstructural carbohydrates (TNC) decreased between the active growth and dormancy period by nearly 50% in Populus tremuloides, while there was no change in TNC in Picea glauca. Results suggest a more conservative use of TNC reserves in Picea glauca combined with a tolerance to cold soil temperatures compared with Populus tremuloides. ©2001 NRC Canada