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Drought induces an increase in a tree's vulnerability to a loss of its hydraulic conductivity in many tree species, including two common in western Canada, trembling aspen ( Populus tremuloides) and balsam poplar ( Populus balsamifera). Termed 'cavitation fatigue' or 'air-seeding fatigue', the mechanism of this phenomenon is not well understood, but hypothesized to be a result of damage to xylem pit membranes. To examine the validity of this hypothesis, the effect of drought on the porosity of pit membranes in aspen and balsam poplar was investigated. Controlled drought and bench dehydration treatments were used to induce fatigue and scanning electron microscopy (SEM) was used to image pit membranes for relative porosity evaluations from air-dried samples after ethanol dehydration. A significant increase in the diameter of the largest pore was found in the drought and dehydration treatments of aspen, while an increase in the percentage of porous pit membranes was found in the dehydration treatments of both species. Additionally, the location of the largest pore per pit membrane was observed to tend toward the periphery of the membrane. [ABSTRACT FROM AUTHOR]

Drilling pads that are used to explore petroleum reserves in the boreal forest are often only used for a short time. We studied the vegetative regeneration potential of aspen ( Populus tremuloides ) through root suckers as a means to rapidly recover forest vegetation in these disturbed sites. We compared protecting the original forest floor under a layer of subsoil during the leveling of drilling pads, with the current practice of stripping off the forest floor and topsoil and placing it back on the site (Rollback) in the re-contouring of the reclamation phase. We also tested three techniques of delineating the forest floor so that it can be effectively uncovered during the reclamation phase. After re-contouring and top soil placement on the sites, we assessed the extent of surface disturbance, soil temperature, soil bulk density, and the density and height of aspen regeneration. Aspen suckers were tallest, had the highest density and had better survival when the forest floor was protected compared to the standard Rollback treatment. When protecting the forest floor, delineating the original forest floor from the subsoil cover resulted only in small differences in the aspen regeneration among delineation treatments with little impact on soil compaction and only moderate effects on soil surface disturbance. The study indicates that protection and the careful uncovering of the forest floor with or without using a delineation layer should be a preferred strategy for temporary drilling pad construction and their subsequent reclamation in aspen-dominated boreal forests. [ABSTRACT FROM AUTHOR]

Highlights: [•] Aspen root fragments of different sizes were buried at different soil depths. [•] Sucker initiation was not affected by soil depth, root size, or presence of fine roots. [•] Initial fragment carbohydrate reserves affected sucker growth and fragment survival. [•] Suckers did not emerge above ground at burial depth of 40cm. [ABSTRACT FROM AUTHOR]

Abstract: Increasingly, trembling aspen (Populus tremuloides Michx.) is being planted in stressful situations such as forest land reclamation, afforestation and forest restoration in North America. This is due to its fast potential for growth and high resiliency, but its indeterminate growth strategy provides a special challenge in creating suitable planting stock for these sites. Clues from naturally established aspen seedlings suggest that root total non-structural carbohydrate (TNC) reserves and root:shoot ratio could be strongly related to subsequent seedling establishment and growth. Carbon allocation in plants is generally partitioned between reserve accumulation and growth and development. Under conditions in controlled nurseries, the allocation of carbohydrates to reserves could be manipulated in order to produce seedlings designed for specific environmental conditions. To manipulate seedling characteristics we attempted to induce premature bud set during nursery culture while allowing continued photosynthesis. Treatments included applying different fertilizer regimes, light intensities, reducing photoperiod and use of a shoot growth inhibitor. The shoot growth inhibitor was the most reliable treatment and resulted in complete bud set, while blackout was successful only when seedlings were grown outside. Low nutrient treatments also resulted in early bud set; however, leaves were abscised much earlier. There was a distinct trade-off between growth and reserve accumulation with the late bud set. A longer period of height growth reduced root and total TNC reserves, while early bud set allowed for continued photosynthesis and produced the highest levels of root and total TNC reserves in seedlings. Increased TNC reserves of aspen planting stock was positively related to height growth after outplanting, with root TNC reserves as high as 33% of dry weight and root:shoot ratios greater than 2 associated with the best growth. [Copyright &y& Elsevier]

1. Variable retention harvest is an increasingly popular management alternative to clear-cutting in boreal forest ecosystems. This harvest system retains a portion of the mature trees to maintain structural elements of the forest and preserve biodiversity. Residual trees must, however, survive in the post-harvest environment to realize these benefits. Here, we investigate which tree and stand characteristics are associated with high survival to aid foresters in designing harvesting prescriptions. 2. We conducted an operational-scale experiment broadly representative of western North American boreal forests. Treatments were four overstorey compositions (ranging from deciduous to conifer dominated) and five rates of canopy retention (10%, 20%, 50%, 75% and a 100% control). Factors affecting mortality were analysed for the leading tree species (aspen Populus tremuloides Michx. and white spruce Picea glauca (Moench) Voss 5 and 10 years after harvest. 3. Mortality of residual aspen was highest in 10% and 20% retention treatments with c.30% mortality after 5 years and c. 50% after 10 years. At year 5, most of the dead trees remained upright as snags (87%), but by year 10, c.40% of dead trees had fallen. By contrast, mortality was much lower in the 50% and 75% retention treatments, and similar to that of the unharvested control. Results for white spruce mortality were similar, except the shallow-rooted spruce that was more susceptible to be blown down. 4. Regression tree analysis was used to identify individual tree attributes that predict 5- and 10-year survival. Mortality was higher for trees with a low percentage of live crown and high height/diameter ratio, when trees were very tall or were close to machine corridors. Smaller residual spruce trees from deciduous-dominated stands had the lowest mortality rates. 5. Synthesis and application. Where live trees are critical elements of wildlife habitat, forest managers should maintain ≥50% in retention harvest systems. At lower levels of retention (≤20%), mortality can be reduced if foresters select retention trees that are stout, possess large live crowns and are small to intermediate in size. These factors can be used to help design partial harvest systems for effective biodiversity conservation strategies. [ABSTRACT FROM AUTHOR]

Copyright of Botany is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Two experiments were conducted to examine the effects of growing conditions, duration of cold storage, and distinct phenological periods of root donor plants on the propagation success of aspen rootlings. Root donor plants were produced either under greenhouse or open grown conditions. Root cuttings were periodically collected from donor plants that had been stored for various lengths of time in cold storage (up to 180 days), or that were stored dormant in cold storage (up to 150 days) and then grown for another full growing season. Longer storage of donor plants produced only slightly smaller rootlings and resulted in slightly lower establishment success. Rootling establishment success was severely depressed (down to 18% establishment success) when cuttings were collected during the active growth period of donor plants. Carbohydrate reserves did not influence rootling establishment success but did affect root and shoot growth performance. It appears that other factors, such as hormone levels, may be more important in rootling establishment success. [ABSTRACT FROM AUTHOR]

Abstract: In a growth chamber, we tested how the seasonal timing of placing a physical barrier (simulating a possible effect of log storage) and inflicting root damage impacted aspen (Populus tremuloides Michx.) root systems and their suckering capability. Roots from 4-year-old saplings were used, and one half of these root systems had the above-ground portion cut in the winter (dormant) while the other half was cut during the growing season in the summer. Damage was inflicted to the roots by driving a large farm tractor over them, and a covering treatment was applied using a polystyrene board to prevent suckers from emerging from the soil. Soil temperatures for the winter-cut root systems were kept at 5°C over the growing season, using a water bath, while for the summer-cut root systems soil temperatures were maintained at 17°C over the growing season. In the winter-cut root systems, both log storage and root wounding caused a 40% reduction in living root mass and carbohydrate reserves, as well as reducing sucker numbers and their growth performance. In the summer-cut root systems log storage and root wounding reduced living root mass by approximately 35% as well as sucker growth, but had less of an impact on the number of suckers produced. [Copyright &y& Elsevier]

Copyright of Forestry Chronicle is the property of Canadian Institute of Forestry and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The dynamics of aspen (Populus tremuloides Michx.) species composition (SC, defined as the ratio of aspen basal area to total stand basal area) in boreal mixedwoods were modelled in this study using the difference equation method, based on datasets collected from repeatedly measured permanent sample plots (PSPs) across Alberta. The aspen SC measured at time 1 was taken as a key predictor variable to project SC values at future ages. Since site quality was found to impact the aspen SC, site index (SI) was incorporated into the model. To correct the autocorrelation and heteroskedasticity problems associated with PSPs data, the non-linear mixed-model technique was applied to accommodate the variance-covariance structure of the error terms and to estimate model parameters. The final model was evaluated on an independent dataset collected from a different region in Alberta, based on a number of statistical measures including a goodness of prediction index (GOPI) from forward and backward projections, and examinations of residual and studentized residual plots. The low prediction bias and high GOPI value obtained from forward and backward projections on the validation data suggest that the model fitted the data well and can be reliably applied to predict changes in aspen SC in boreal mixedwoods across Alberta. The model showed a decline in the SC after year 30, but the decline was steeper in sites of low SI. The model can be used for modelling the transitions of forest compositions in boreal mixedwood forests. [ABSTRACT FROM AUTHOR]

Question: What is the effect of leaf litter on growth and mortality of feather mosses. Three experiments were conducted to isolate the shading effect from the effect of leaching of soluble compounds from the leaf litter effect on feather moss mortality and growth. Location: Edmonton, Alberta and the northern boreal coniferous forest, west-central Alberta, Canada. Methods: In a field experiment Populus tremuloides (aspen) leaf litter was applied to beds of feather mosses dominated by Hylocomium splendens. Treatments were one layer of leaves held in place with netting; one layer of leaves coarsely ground, sprinkled over the moss layer; a shade cloth equivalent to one layer of leaves; and a control. In two growth chamber experiments, the application of aqueous extracts of P. tremuloides and Pinus contorta leaf litter and the effect of shade and soluble carbohydrates were tested on the growth and mortality of Ptilium crista-castrensis. Results: Mortality of Hylocomium was greatest under the intact leaves, followed by the treatments using shade cloth and ground leaves and finally the control. The application of aqueous extracts of aspen leaf litter resulted in senescence or death of nearly all Ptilium shoots compared to no effects for the control or for similar extracts of pine needle litter. Extracts from aspen litter had greater concentrations of phenolic compounds and soluble sugars than pine extracts. Addition of sugars to Ptilium allowed it to grow and accumulate carbohydrates, even in low light conditions. Conclusions: Results suggest that broad-leaved deciduous overstory species can limit the growth of feather mosses through their leaf litter and by implication affect the humus form, nutrient cycling, and understory composition of these forests. [ABSTRACT FROM AUTHOR]

Abstract: Season of harvest has often been suggested as a driver for the erratic success of aspen (Populus tremuloides) sucker regeneration, partially due to root carbohydrate reserves and soil conditions at the time of harvest. A field experiment in western Manitoba, Canada, assessed root suckering and root carbohydrates of aspen in response to season of harvest and machine traffic. Six sites (120m×120m) were selected within two large mature aspen stands slated for summer harvest. Plots (50m×50m) were hand-felled (without machine traffic) in mid-summer, late summer, winter, and one plot was left uncut as a control. Season of cut with no traffic had no effect on sucker density, height or leaf dry mass per sucker. During the dormant season, root starch reserves were highest in the winter cut plots, however, just prior to suckering, this difference in carbohydrate reserves among the three seasons of harvest disappeared and by the end of the first growing season root reserves in all three seasons of cut had recovered to near control levels. Adjacent plots that were conventionally harvested in the summer and impacted by logging traffic had similar sucker densities but had 19% less height growth of suckers and 29% less leaf dry mass per sucker compared to suckers in plots harvested at the same time without traffic. After one growing season, root carbohydrate levels were similar whether or not machine traffic was used; however, the reduction in leaf dry mass in plots with machine traffic could have negative implications for carbohydrate accumulation and growth. The study suggests that the phenological state of the mature aspen plays a very small role in aspen regeneration and that harvesting practices and site conditions are likely the main drivers of aspen regeneration success. [Copyright &y& Elsevier]

Marsh reed grass (Calamagrostis canadensis (Michx.) Beauv.) is a common, highly competitive grass native to the boreal mixedwood forest. This grass increases in abundance after clear-cut logging but little is known about its effects on trembling aspen (Populus tremuloides Michx.) sucker regeneration. The effects of Calamagrostis sod and its litter on aspen regeneration were studied in two separate greenhouse studies. Calamagrostis sod did not affect the initiation of suckers, but resulted in 30 per cent fewer suckers emerging above the soil that were smaller and had 40 per cent less leaf area. Calamagrostis litter had little effect on the initiation and number of emerged suckers; however, it delayed emergence by 10 days. The physical barrier by roots and litter of Calamagrostis reduced or delayed the expansion of suckers and therefore prolonged their dependence on root reserves. By the time the suckers reached the surface, they had to compete for light with Calamagrostis shoots that had emerged a week earlier. This, coupled with low soil temperatures associated with Calamagrostis in other experiments, will significantly reduce the number and growth of suckers. Any reduction and delay in sucker emergence will decrease aspen regeneration and productivity since the growing season in the boreal forest region is short. [ABSTRACT FROM AUTHOR]

Trembling aspen (Populus tremuloides Michx.) density and growth were assessed 9-12 years after stand establishment to determine whether mechanical site preparation (MSP) affects crop tree quality. Study sites were either treated with disc trenching or ripper ploughing and planted with white spruce (Picea glauca (Moench) Voss) seedlings immediately after harvest (treated) or were undisturbed since harvest (control). Stands were surveyed during the summer of 2002 with standard regeneration survey plots. Results show that aspen stem density and height were lower in MSP-treated areas relative to untreated areas. Diameter growth rates were unaffected by treatment, however the percentage of stem discolouration was higher in untreated control stands compared to site prepared areas. The results of this study indicate that there are no long-term benefits to carry out MSP for aspen promotion. However, as MSP does not appear to seriously harm the aspen crop, we suggest that this treatment can still be used on sites where aspen densities may be low without treatment (e.g., sites with extremely low soil temperatures, poor soil aeration, or vigorous competitive vegetation) or where a mixture of aspen and planted spruce are desired. [ABSTRACT FROM AUTHOR]

Abstract: In the absence of chemical vegetative management tools, young regenerating trembling aspen (Populus tremuloides) is typically removed mechanically from sites intended for white spruce (Picea glauca) regeneration. In many cases the aspen vigorously re-suckers and re-sprouts after cutting all the stems. A field study was established to test whether the regeneration of aspen can be reduced by maintaining residual saplings and/or by the timing of cleaning. It was further tested how these treatments affect root carbohydrate reserves. Nine healthy, 10-year-old aspen stands were manually cleaned to three residual aspen densities (0, 500, and 1500 stems per hectare) at three times of the year (late summer, winter, and spring). Retaining residual aspen reduced regeneration by 23–39% relative to complete removal; however, the amount of regeneration was not affected by season. Height growth and leaf area was reduced in spring cleanings compared to late summer and winter and in both residual trees treatments compared to the complete removal. The vast majority of regeneration was from stump sprouting. Generally, as the residual density increased, regeneration decreased; however, regeneration after the first growing season was still too abundant for these treatments to be a viable option for density management of aspen. Maintaining residual stems reduced the root carbohydrate reserves in root associated with regeneration compared to reserves in roots associated with residual saplings or in the control plot. Only in the complete removal treatment did root reserves recover to control levels within one growing season. [Copyright &y& Elsevier]

The mechanisms regulating stomatal response following exposure to low (5°C) soil temperature were investigated in aspen (Populus tremuloides Michx.) seedlings. Low soil temperature reduced stomatal conductance within 4 h, but did not alter shoot xylem pressure potential within 24 h. The xylem sap composition was altered and its pH increased from 6.5 to 7.1 within the initial 4 h of the low temperature treatment. However, the increase in abscisic acid (ABA) concentration in xylem sap was observed later, after 8 h of treatment. These changes were accompanied by a reduction in the electrical conductivity and an increase in the osmotic potential of the xylem sap. The timing of physiological responses to low soil temperature suggests that the rapid pH change of the xylem sap and accompanying changes in ion concentration were the initial factors which triggered stomatal closure in low temperature-treated seedlings, and that the role of the more slowly accumulating ABA was likely to reinforce the stomatal closure. When leaf discs were exposed to xylem sap extracted from low soil temperature-treated plants, stomatal aperture was negatively correlated with ABA and positively correlated with K+ concentrations of the xylem sap. The stomatal opening in the leaf discs linearly increased in response to exogenous KCl concentrations when K+ concentrations were in the similar range to those detected in the xylem sap. The lowest concentration of exogenous ABA to induce stomatal closure was several-fold higher compared with the concentration present in the xylem sap. [ABSTRACT FROM AUTHOR]

Sudden dieback and deterioration of mature aspen stands is commonly observed throughout North America. This dieback process has tremendous ecological and economic importance, yet remains poorly understood. This paper summarizes our understanding of aspen dieback in North America, identifies potential processes that contribute to reduced vigour and dieback of aspen stands, and examines the scales (stand, ecosite, regional) at which these processes operate. Many factors including pathogens, nutrition, or successional changes may be involved in the decline of aspen vigour and thereby contribute to the dieback process. However, insect defoliation, drought, and thaw–freeze events appear to be the most likely factors initiating dieback in mature aspen stands. Further study is clearly needed to elucidate the mechanisms and landscape patterns of dieback. Information needs related to identifying processes and modeling landscape patterns of dieback are indicated. [ABSTRACT FROM AUTHOR]

In early May, 1-m sections of trembling aspen (Populus tremuloides Michx.) roots in a forest cutblock were carefully exposed and examined for damage. Undamaged roots were subjected to one of three wounding treatments (scrape, sever, or uninjured control) and were then reburied to either the full normal organic layer depth or to one third of the normal depth. Following one growing season, the roots were reexposed and assessed for aspen sucker numbers and growth rates. Results indicate that injured roots produced suckers nearly twice as often as uninjured roots. Further, injured roots produced more suckers per root, and these suckers were taller and had greater leaf area. Roots buried under shallow organic layers also generated more suckers, regardless of injury type. The side of injury (distal or proximal) did not affect any of the measured variables. The present study suggests that moderate wounding of aspen roots increases initial sucker numbers and growth rates. [ABSTRACT FROM AUTHOR]

Analyzes the sucker regeneration of trembling aspen in the boreal mixedwood forests in North America. Factors that control aspen suckering; Description of the physiological processes of sucker initiation; Role of temperature in determining early sucker development; Information on the dynamics of sucker stands; Impact of the number of mature aspen trees retained after logging on the number of suckers formed.

This study examines light competition between aspen and spruce during the sequence of aspen development. Leaf area index and light transmission were measured or estimated for aspen stands from 2 to 125 years old. Light transmission was lowest at 15-25 years, and in some stands, transmission was less than 5% of above-canopy light. Hypothetical aspen stands with various stem configurations and heights were developed, and positions were identified that would meet or fail Alberta free-to-grow (FTG) standards. Light transmission was estimated at each position with the MIXLIGHT forest light simulator. Positions in canopy gaps or at the northern sides of canopy gaps had higher light. In general, however, there was little difference in available light between positions that met or failed FTG criteria. Stand density and size of aspen trees appears to be a better index to predict light transmission and spruce success in juvenile aspen stands than current FTG criteria. [ABSTRACT FROM AUTHOR]

Low root temperatures significantly reduced root hydraulic conductivity and increased resistance to water flow through the roots of aspen (Populus tremuloides Michx.) seedlings. Increased resistance to water flow could not be fully explained by the corresponding increase in water viscosity at low temperatures. The shapes of Arrhenius plots of root water flow and the activation energies were dependent on the direction, sequence and extent of temperature change. The Arrhenius plots suggested that the effect of low root temperature on root water flow was mediated by an effect on root metabolism. The low root temperatures tested did not induce root electrolyte leakage normally associated with cell membrane injury. Although a decrease in root temperatures to 7 or 4 °C induced a reduction in stomatal conductance, this reduction lagged the decline in root water flow by several hours. In contrast, when soil temperatures were raised from 4 or 7 °C to 25 °C, root water flow presumably increased, and stomatal conductance responded rapidly and was temporarily higher than before the cold treatment was imposed. [ABSTRACT FROM AUTHOR]

In northeastern Alberta, the current biomass and periodic annual biomass increment (PAI) was measured in 29 stands of maturing aspen (Populus tremuloides)-white spruce (Picea glauca), aged 48 to 105 years. Plots in pure aspen were paired with nearby plots of aspen growing on a similar landform but with a spruce understory. Biomass was estimated by diameter at breast height and allometric equations. Totalled over both species, there was 10.5% greater PAl and 10.0% greater biomass in the mixed plots than in the pure aspen plots. Pure aspen plots, however, had 12.9% greater aspen biomass and 25.2% greater aspen PAl than the aspen component of mixed plots. The apparent decline in productivity of aspen in the mixed stands, however, could not be related to the variation in spruce abundance in these mixed stands. [ABSTRACT FROM AUTHOR]

In boreal mixedwood forests, aspen (Populus tremuloides) and white spruce (Picea glauca) commonly grow in mixture. These species may avoid competition through differential shade tolerance, physical separation of canopies, phenological differences, successional separation, and differences in soil resource utilization. Aspen may also be able to positively affect the growth of white spruce by improving litter decomposition and nutrient cycling rates, controlling grass and shrub competition, ameliorating environmental extremes, and reducing pest attack. These positive relationships likely make mixed-species stands more productive than pure stands of the same species. The evidence regarding the productivity of pure versus mixed aspen/white spruce stands in natural unmanaged forests is examined in this paper. [ABSTRACT FROM AUTHOR]