Annie DesRochers

The coarse-root system of mature Populus tremuloides in declining stands in Alberta, Canada

The coarse-root dynamics of ramets of Populus tremuloides (aspen) were investigated with respect to persist- ence 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. Par- ent 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 becom- ing independent of the parent root system the ramets incorpo- rated the parent roots into their own root systems, remaining interconnected.

A comparison of pre-planting treatments on hardwood cuttings of four hybrid poplar clones

Rooting and early growth of four hybrid poplar clones (Populus spp.) planted in a greenhouse were examined after applying 40 pre-rooting treatment combinations to dormant cuttings. Treatments included 2 cutting lengths (5 and 10 cm), 5 soaking times (0, 2, 4, 8, and 12 days), and 4 dips (chitosan, rooting hormone powder, liquid rooting hormone added to the soaking water, and none). Significant differences in both rooting percentages and growth were shown between clones after 7 weeks of growth. Ten cm cuttings had 29% greater rooting success, 28% more above-ground growth, and 12% lower root/shoot ratios than 5 cm cuttings. Cuttings planted without soaking had the lowest rooting success, at less than 45% on average. Commercial rooting hormones decreased the number of rooted cuttings but increased root/shoot ratios. For optimal rooting, we recommend using 10 cm cuttings, soaked for 2 days in water (4 days for the Jackii10 clone) without any additional dipping/hormone substance.

Molecular and dendrochronological analysis of natural root grafting in Populus tremuloides (Salicaceae)

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 aspens 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 significantly affect interpretations of diverse ecophysiological processes in forests of trembling aspen.

How does drought tolerance compare between two improved hybrids of balsam poplar and an unimproved native species

Poplars are one of the woody plants that are very sensitive to water stress, which may reduce the productivity of fast-growing plantations. Poplars can exhibit several drought tolerance strategies that may impact productivity differently. Trees from two improved hybrids, Populus balsamifera × Populus trichocarpa Torr. & Gray (clone B × T) and P. balsamifera × Populus maximowiczii A. Henry (clone B × M), having P. balsamifera L. as a parent and trees from native and unimproved P. balsamifera were subjected to a 1-month drying cycle in a growth chamber and then rewatered. The unimproved and native B clone maintained higher stomatal conductance (g(s)) than the hybrids, and high photosynthetic activity and transpiration, even when soil water content was nearly zero. As a result, both instantaneous water use efficiency (WUE(i)) and leaf carbon isotope composition (δ(13)C) indicated that this clone was less affected by drought than both hybrids at maximal drought stress. However, this clone shed its leaves when the drought threshold was exceeded, which implied a greater loss of productivity. The B × M hybrid showed a relatively conservative response to water stress, with the greatest decrease in transpiring versus absorbing surface (total leaf area to root biomass ratio). This clone was also the only one to develop new leaves after rewatering, and its total biomass production was not significantly decreased by drought. Among the two hybrids, clone B × T was the most vigorous, with the greatest transpiration (E(i)) and net CO(2) assimilation (A) rates, allowing for high biomass production. However, it had a more risky strategy under drought conditions by keeping its stomata open and high E(i) rates under moderate drought, resulting in a lower recovery rate after rewatering. The opposite drought response strategies of the two hybrids were reflected by clone B × T having lower WUE(i) values than clone B × M at maximal drought, with a very low Ψ(min) value of -3.2 MPa, despite closed stomata and stopped photosynthetic activity. Positive linear relationships between A and g(s) for the three hybrids indicated strong stomatal control of photosynthesis. Moreover, the three poplar clones showed anisohydric behaviour for stomatal control and their use under long-term drought should be of interest, especially the B × M clone.

Plant secondary metabolites: a key driver of litter decomposition and soil nutrient cycling

A broad and diversified group of compounds, secondary metabolites, are known to govern species interactions in ecosystems. Recent studies have shown that secondary metabolites can also play a major role in ecosystem processes, such as plant succession or in the process of litter decomposition, by governing the interplay between plant matter and soil organisms. We reviewed the ecological role of the three main classes of secondary metabolites and the methodological challenges and novel avenues for their study. We highlight emerging general patterns of the impacts of secondary metabolites on decomposer communities and litter decomposition and argue for the consideration of secondary compounds as key drivers of soil functioning and ecosystem functioning.Synthesis. Gaining a greater understanding of plant-soil organisms relationships and underlying mechanisms, including the role of secondary metabolites, could improve our ability to understand ecosystem processes. We outline some promising directions for future research that would stimulate studies aiming to understand the interactions of secondary metabolites across a range of spatio-temporal scales. Detailed mechanistic knowledge could help us to develop models for the process of litter decomposition and nutrient cycling in ecosystems and help us to predict future impacts of global changes on ecosystem functioning.

Effect of natural root grafting on growth response of jack pine (Pinus banksiana; Pinaceae)

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-m(2) 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.

Home Field Advantage of Litter Decomposition in Pure and Mixed Plantations Under Boreal Climate

Tree species influence the litter decomposition process by influencing litter quality and soil microclimate. Furthermore, over the long term, trees could promote soil communities that are particularly capable of degrading the litter they encounter most often. Thus, plant litter could decompose faster when placed in the habitat from which it was derived than in a foreign habitat, which has been termed home field advantage (HFA) of litter decomposition. In mixed-plant species environments however, it is not known whether a specific decomposer community under one tree species is affected by the presence of another tree species in the vicinity. To address this question, we tested if spruce and poplar litters showed HFA in mono-specific and in mixed species plantations under each tree species by reciprocally transplanting litter in the two plantation types. Decomposition rates, as well as the composition and ability of decomposer communities to degrade the different types of litter, were monitored during two years. Only spruce litter exhibited a faster decomposition rate at home. This HFA could be explained by higher abundance of decomposers. Furthermore, cellulose and poplar litter decomposed less or similarly in spruce plantations, suggesting that soil communities of that environment were capable of specifically degrading spruce litter. In mixed plantations, HFA was in the same direction as in mono-specific plantations, but was not as strong, indicating that HFA is sensitive to the surrounding plant community. Furthermore, this “mixed environment” had synergistic effects on decomposition rates under poplar trees. These ‘tree environment-specific’ results highlighted the possible importance of spatial distribution of each litter on decomposition rates in mixed stands. Thus, the influence of litter dispersal should be taken into account in future studies.

Revegetation of non-Acid-generating, thickened tailings with boreal trees: a greenhouse study.

Tree planting presents clear advantages for mine reclamation that is aimed at achieving rapid reclamation of forested landscapes. A greenhouse study was conducted to evaluate the capacity of non-acid-generating, thickened tailings to support six boreal tree species during two growing seasons. One treatment was thickened tailings alone fertilized with inorganic N, P, and K fertilizer or chicken () manure. A thin layer of overburden topsoil was used to cover the tailings and was compared with topsoil alone, where normal tree growth was expected. Two amendments were also tested: overburden topsoil and vermicompost from food wastes. The presence of alkaline thickened tailings under the thin layer of acidic topsoil had a positive effect on tree height and root biomass (broadleaved and jack pine [ Lamb.]) by increasing topsoil pH and available Ca concentrations, which decreased Al, Zn, and Mn phytoavailability to trees; however, root contact with the tailings also increased their Cu concentrations. In thickened tailings that were mixed with topsoil, C/N ratios increased along the experiment from 21 to 40, a value where N immobilization by microorganisms occurred, as suggested by low N concentrations in tree tissues. In consequence, tree height growth (broadleaved) and biomass (conifers) were reduced. Amendment with compost raised the electrical conductivity (3.4 dS cm) to thresholds limiting broadleaved survival, while conifers showed a generalized decrease in biomass production. No trace metal contamination of the trees occurred in the mixtures, probably due to the near-neutral pH conferred by the tailings.

Planting Trees in Soils above Non-Acid-Generating Wastes of a Boreal Gold Mine

Abstract: Tree planting is a useful means of integrating reclaimed mine sites into natural forested landscapes. The main objective of this study was to evaluate the effect of soil thickness and nature on the establishment and metal accumulation of trees planted in low sulfur mine wastes under boreal conditions. Two field experiments were conducted to evaluate survival, growth, and nutrient and trace metal concentrations of several trees, including 2 fast-growing species (Pinus banksiana, Larix laricina, Populus maximowiczii × P. balsamifera, P. × canadensis × P. maximowiczii, and Salix viminalis), planted in soils (topsoil or subsoil, 50 or 20 cm thick) above waste rock and thickened tailings, respectively. As expected, tree growth increased (×2) in the topsoil compared to the subsoil above waste rock, despite mineral fertilization. Tree growth also decreased in thin topsoils, concomitantly with a decrease in foliar N concentrations, but soil thickness had no effect on tree survival. The basket willow appeared well adapted and multi-purpose for mine-waste revegetation over the short term since its survival remained maximal whatever the nature (topsoil or subsoil) or thickness (50 versus 20 cm) of the soil layer or waste type (waste rock versus tailings). The tamarack showed good survival and growth on both waste types (except in subsoil alone). By contrast, the survival (waste rock) and growth (thin soils above tailings) of hybrid poplars was poor under the tested conditions. On these non-acid-generating wastes with low total trace metal concentrations, none of the tree species accumulated trace metals from soil in their foliage, but basket willow survival should be followed over the longer term to check for deleterious effects of Zn accumulation.

Herbivore-simulated induction of defenses in clonal networks of trembling aspen (Populus tremuloides)

Trembling aspen (Populus tremuloides Michx.) as a clonal tree species possesses a complex root system through which trees of the same or different clones are connected. Root connections have been studied with respect to resource sharing, but the nature, quantities or extent of what is shared between trees is relatively unknown. In this study, we posed the hypothesis that systemic defense induction signals could also spread through these root networks and trigger defenses in neighboring ramets before arrival of pests. Temporal expression pattern of Kunitz trypsin inhibitor (KTI) and dihydroflavonol reductase (DFR) genes, two markers of poplar defense, was followed by quantitative real-time polymerase chain reaction. The expression was quantified in systemic leaves of wounded and healthy plants that shared the same parental root and in untreated controls grown in separate pots. Untreated interconnected plants did not show induced resistance upon herbivore-simulated attack. Although wound-treated ramets induced defense genes, untreated interconnected plants produced an expression pattern similar to non-connected controls. Root connections do not automatically lead to induction of defensive traits that are expressed in plants directly under damage thought to simulate herbivory. Rather, it seems that other communication means such as airborne volatiles can serve as signal transmission pathways among neighboring plants.