Long-term response of planted conifers, natural regeneration, and vegetation to harvesting, scalping, and weeding on a boreal mixedwood site

This study reports 14th-year response of a boreal mixedwood stand to different harvest intensities (uncut, 50% partial cut with and without removal of residuals after 3 years, and clearcut), spot site preparation treatments (none and scalped), and chemical weeding frequencies (none, single, and multiple) in northeastern Ontario. The response variables include the survival and growth of planted white spruce (Picea glauca [Moench] Voss) and jack pine (Pinus banksiana Lamb.), height and density of natural regeneration and shrubs, and cover of shrubs and non-woody vegetation. Harvesting and weeding generally improved survival and growth of planted trees, although white spruce survival did not significantly differ among the three weeding frequencies. Harvesting tended to increase heights of hardwood (mostly trembling aspen (Populus tremuloides Michx.)) and conifer (largely balsam fir (Abies balsamea (L.) Mill.).) natural regeneration, cover and density of shrubs, and cover of herbs, lichens, and ferns. Chemical weeding reduced height, density and cover of shrubs, height and density of hardwood regeneration, and fern cover, but increased moss and lichen cover. Spot scalping did not significantly affect planted seedling, natural regeneration, or the vegetation.Maximum survival and growth of planted white spruce and jack pine were achieved using a combination of clearcutting and multiple weeding. However, partial cutting followed by a single weeding produced acceptable survival and reasonable growth of planted trees, particularly for white spruce. Partial canopy removal alone substantially reduced the amount of hardwood regeneration, relative to clearcutting, but did not adequately suppress understory shrubs. Significant improvement in seedling growth following multiple weedings was evident primarily in the complete canopy removal treatments: 50% partial cut with removal of residuals after 3 years and clearcut. While the effects of harvesting and weeding on planted crop trees found in the 5th-year assessments generally persisted at year 14, survival decreased, likely due to light competition from developing hardwood and shrubs.     

Post-fire natural regeneration of young stands on clearcut and partial-cut and uncut sites of boreal mixedwoods

Boreal mixedwoods are an important element and the most productive forest type in the Canadian boreal forests. However, they experience frequent disturbances. In order to better understand the responses of boreal mixedwoods to different combinations of anthropogenic and natural disturbances, we investigated the natural regeneration of boreal mixedwoods that were previously subjected to three different harvesting treatments (clearcut, partial-cut and uncut control) and naturally regenerated, but subsequently burnt by a severe natural fire 6 years later. The study was conducted 8 years following the fire. Significant interactions were found among harvesting method, species and block in several regeneration variables. There were a total of 12 woody species (trees and shrubs) regenerated, but not all the species were present in all the sites. In general, the species richness and species diversity of the new stands were lowest on clearcut sites while the differences between partial-cut and control varied with blocks. However, the combined total density for all species was lowest on uncut control sites. Density and regeneration index data show that trembling aspen was the predominant tree species in all stands except at one uncut control site where jack pine was the dominant species. The density of trembling aspen generally declined from clearcut to partial to the uncut control. Pincherry, beaked hazel and mountain maple were the dominant shrub species in the new stands, but no general patterns were found in terms of variations in density with harvesting methods for any of the shrub species. Jack pine and white birch were the tallest tree species in the clearcut treatment while white birch was taller than jack pine in the partial-cut and control. The results suggest that active measures are necessary to restore the complex structure of the initial mixedwoods.     

Climate and site characteristics affect juvenile trembling aspen development in conifer plantations across southern British Columbia

In southern British Columbia, juvenile trembling aspen is managed primarily as a competitor with conifers rather than for its ecological and economic value. As a result, brushing treatments have been applied on a widespread basis and this practice is likely to continue in the near future. Given the potential for climate change to affect our valuation of aspen, we require a better understanding of factors that affect its development, its competitive ability with conifers and its responses to brushing. We used data from 11 aspen management experiments to examine the influence of climate and site factors on aspen height, cover, and density in 17–24 year-old control stands and 9–16 years after manual cutting or girdling. Models explained 64% and 89% of the variation in aspen height in control and manually brushed stands, respectively, but were poor for girdling. Increasing length of the frost-free period was associated with increasing aspen height in control stands, whereas drier summer conditions on cool aspects favoured height growth of aspen suckers following manual cutting. We also examined the influence of climate and site factors on three simple competition indices that describe the height and density of aspen relative to conifer height, and then tested how well these indices predicted conifer growth. The density of aspen taller than conifers accounted for 39% of the variation in lodgepole pine diameter and the ratio of aspen/conifer height accounted for 33% of the variation in Douglas-fir height, suggesting that aspen competition was only moderately important to conifer growth. Our findings imply that aspen may become more productive with warmer summers provided it is not limited by summer moisture availability and that mixed stand management is a viable option in southern interior stands.     

The influences of conifer succession, physiographic conditions and herbivory on quaking aspen regeneration after fire

Fire suppression over the last century has increased conifer expansion and dominance in aspen-conifer forests, which appears to be a driving force behind aspen decline in some areas. The primary objective of this study was to examine how increasing conifer dominance affects aspen regeneration vigor following the return of fire. The influence of physiographic features and herbivory on aspen regeneration vigor were also examined. The study was conducted in the Sanford fire complex located in the Dixie National Forest in southern Utah, USA, where more than 31,000 hectares burned in the summer of 2002. Seven years after the burn (at 66 locations) we measured aspen regeneration density and height as response variables and former stand composition and density (the burned trees were still standing), soil characteristics, slope, aspect and presence or absence of herbivory as independent variables. Aspen regeneration (root suckering) densities ranged from <500 to 228,000 stems/hectare with an average of 37,000 stems/hectare. Post-fire aspen regeneration density was most strongly correlated with pre-fire stand successional status (as measured by stand composition and species abundance), with percent conifer abundance (R² = −0.55) and overstory aspen density (R² = + 0.50) being the most important. Average aspen suckering densities ranged from approximately 60,000 stems/hectare in what were relatively pure aspen stands (>90% aspen) to less than 5000 stems/hectare in stands where conifer abundance was greater than 90%. Soil C, N, and P showed positive correlations (R² = 0.07 to 0.17) with aspen regeneration vigor, while soil texture had a relatively weak influence on sucker regeneration. Aspen regeneration densities were 15% lower on north facing aspects compared to east, west and south facing aspects with slope steepness showing no correlation with regeneration vigor. Regeneration density was significantly lower (8%) at sites with evidence of herbivory versus sites where herbivory was absent. Overall, the aspen regeneration response in the Sanford fire complex was strong despite high wildlife densities, which may be related to disturbance size. Where the maintenance of aspen is desired in the landscape we recommend promoting fire when the percentage of overstory conifer stems is greater than 80% or overstory aspen density is less than 200 overstory stems/hectare.     

Effects of Various Silvicultural Systems on Regeneration in Mixed Hardwood Stands of the Southern Appalachians

This study investigated the effects of clear-cutting and several other commonly used silvicultural systems on regeneration at seven sites in the Appalachian Mountains of Virginia and West Virginia. These even-aged oak dominated stands ranged in age from 63 to 100 yr and were located on medium quality oak sites (site indices from 18 to 23 m, base age 50). The treatments evaluated included a clear-cut, commercial harvest, shelterwood, leave-tree, group selection, preharvest herbicide, and control.

Mixed model ANOVA was used to analyze treatment response in five species groups: (a) oak (Quercus spp.); (b) maple (Acer spp.); (c) black cherry (Prunus serotina Ehrh.), and yellow-poplar (Liriodendron tulipifera L.); (d) miscellaneous overstory hardwoods; and (e) midstory species that typically do not occupy main canopy positions in mature stands. Response variables included stem density, importance value, average height for all regeneration, and the tallest 365 stems ha^?1. Comparisons were made among treatments, species groups, and between regeneration of sprout and seedling origin.

Alternative systems with residual trees reduced the regenerations overall mean height growth compared to the clear-cut by 0.34 to 0.74 m. Current conditions indicate oak will be a lesser component of the future stand, represented mostly through stump sprouting, and maple will likely increase in proportion among all treatments.     

Does mechanical site preparation affect trembling aspen density and growth 9–12 years after treatment?

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.     

Control of rhododendron (Rhododendron ponticum and R. flavum) in the eastern beech (Fagus orientalis) forests of Turkey

Because purple-flowered rhododendron (Rhododendron ponticum L.) and yellow-flowered rhododendron (R. flavum Don.) significantly reduce seedling growth and regeneration of eastern beech (Fagus orientalis Lipsky) as well as local floristic diversity in northern Turkey, effective and cost-efficient woody control is needed. Various manual (cut and grub) and herbicide (foliar and cut-stump spray) woody control techniques were used in two different beech-rhododendron sites in the Black Sea Region (BSR) of Turkey. Two years later, biological and economic effectiveness were substantially greater with foliar-applied herbicide and grubbing treatments than with the cutting, cut-stump, and check treatments. Yet, grubbing showed a great potential for sprout crown reduction. Cut-stump spray was intermediate in effectiveness. Cutting had the least woody control and economic effectiveness among all of the treatments. Foliar-applied Arsenal SL (imazapyr) had significantly greater rhododendron control than foliar-appliied Garlon 4 (triclopyr ester). Increasing rates did not enhance herbicide efficacy. Reducing rates further may provide the same level of control at less cost.     

Regeneration dynamics of planted seedling-origin aspen (<Emphasis Type="Italic">Populus tremuloides</Emphasis> Michx.)

Aspen (Populus tremuloides Michx.) is a foundational tree species of the North American boreal forest. After disturbance, clonal aspen stands quickly achieve canopy closure by sending up numerous clonal shoots (root suckers) from their lateral root system. Controlled aboveground disturbance will commonly induce prolific root suckering and thereby increase stem density in clonal aspen stands, but it is unclear if increases in stem density will be observed in planted seedling-origin aspen stands. The objectives of this study were to determine (1) overall root suckering response of planted aspen to aboveground disturbance; (2) if different cut heights of the stem or infliction of root damage impact the number of root suckers produced. We found that planted aspen regenerated readily after disturbance, averaging five root suckers per cut tree. However, individual response was highly variable, ranging from zero to 29 root suckers per root system. Of the cut trees, 75% produced at least one root sucker and 60% produced at least one stump sprout. Cutting trees close to the soil surface produced more root suckers than leaving a 25 cm stump. While root system size (mass and length) was well correlated with aboveground measures of planted aspen, root suckering was not related to root system size. As a result of increased forest reclamation efforts in the boreal forest region the planting of aspen has become a more common practice, necessitating a better understanding of the regeneration dynamics and root suckering potential of these planted and seedling-origin aspen forests.     

Regeneration behavior of competing plants after clear cutting: implications for vegetation management

Regeneration characteristics and population dynamics of four major competing plants, trembling aspen (Populus tremuloides Michx.), pin cherry (Prunus pensylvanica L.f.), green alder (Alnus viridis spp. crispa (Aiton) Turril) and beaked hazel (Corylus cornuta Marsh.) were studied from a seven-year-old clearcut in northwestern Ontario, Canada. The site was planted with jack pine two years after clearcutting. Regeneration strategies and population dynamics of these plants were studied by determining their crown diameter, stem density, stem height, stem age, depth of sprouting center, inter-sprout distance, oven-dry weight of shoots, roots plus rhizomes by nondestructive and destructive sampling. Stem density of trembling aspen and pin cherry in 1992 was 4580 and 3600 stems per ha respectively. Much higher stem density was obtained in green alder and beaked hazel during the same time, 27580 and 14600 stems per ha respectively. Substantial reduction in stem density was recorded in trembling aspen (45%) and pin cherry (69%) over two years, 1992–1994. However, reduction in stem density of green alder and beaked hazel for that period was very little (6 and 2%, respectively). Comparison of species' clonal characteristics of above- and below-ground components indicates that trembling aspen and pin cherry possess similar vegetative regeneration strategies that differ from those of green alder and beaked hazel. Ordination of the results of canonical variate analysis of the eight vegetative parameters of the four species arranged the species into two significantly different groups. Based on species regeneration strategies, two potential competition strategies were identified: a vertical competition strategy (VCS) and a horizontal competition strategy (HCS). We argue that the degree and duration of competition can be predicted from the density and ratio of VCS and HCS plants on a site once sufficient empirical data on the species' competitive abilities are gathered. We suggest that future studies should relate the regeneration strategies, population dynamics and competitive abilities of competing plants to competition tolerance of crop trees. This will fine tune our prediction about species interaction based on the present model and better justify the need for vegetation control intervention.     

Spatial impacts of soil disturbance and residual overstory on density and growth of regenerating aspen

We examined spatial aspects of harvesting impacts on aspen regeneration at 25 sites in northern Minnesota. These sites had been clearcut or partially harvested 4–11 years ago. At each site, residual overstory, which was composed of trees other than aspen, soil disturbance, and tree regeneration were determined along transects leading away from skid trails into the neighboring stand. We characterized spatial extent of soil disturbance as soil strength using an Eijkelkamp soil cone penetrometer. Soil disturbance dropped off very quickly at the edge of skid trails, suggesting that the impact of harvesting traffic on areas adjacent to skid trails is minor. On skid trails, disturbance levels were higher on sites harvested in summer than on sites harvested in winter. Even after adjustment for differences in soil disturbance, stands harvested in winter had higher regeneration densities and greater aspen height growth than stands harvested in summer, suggesting that aspen regeneration was more sensitive to a given level of soil disturbance on summer-harvested sites versus on winter-harvested sites. Soil disturbance and residual overstory interactively reduced aspen regeneration densities and height growth, indicating that avoidance of soil disturbance is even more critical in partially harvested stands. Predictions based in the spatial patterns of impact found in this study indicated that harvesting conditions may have a great impact in future productivity of a site.     

Regeneration of Populus nine years after variable retention harvest in boreal mixedwood forests

Aspen and balsam poplar regeneration from root suckers were assessed in boreal mixedwood forests nine years after logging in a variable retention experiment (EMEND Project—Ecosystem Management Emulating Natural Disturbance) located north of Peace River, Alberta, Canada. Five levels of retention of mature trees (2%, 10%, 20%, 50% or 75% of the original basal area) were applied in stands dominated by aspen, white spruce or mixtures of the two species. Basal area of aspen (or that of aspen plus balsam poplar combined) prior to logging strongly influenced sucker density of aspen (or aspen + balsam poplar combined) and in some cases their growth. Nine years after harvest there was a decline in sucker density and volume ha⁻¹ with increasing retention levels of aspen (or both poplars combined); sucker density declined by 50% when only 20% of the original basal area was left in the stand. Retaining mature spruce trees in the stand had little influence on the number of suckers but did affect their total volume ha⁻¹. Thus, we suggest that by knowing stand aspen and balsam poplar density prior to logging and varying levels of retention of aspen and balsam poplar or conifers at harvest, the density of Populus regeneration can be predicted by managers, thereby allowing them to create a range of mixedwood conditions.     

Conifer expansion reduces the competitive ability and herbivore defense of aspen by modifying light environment and soil chemistry

Disturbance patterns strongly influence plant community structure. What remains less clear, particularly at a mechanistic level, is how changes in disturbance cycles alter successional outcomes in plant communities. There is evidence that fire suppression is resulting in longer fire return intervals in subalpine forests and that these lengthened intervals increase competitive interactions between aspen and conifer species. We conducted a field and greenhouse study to compare photosynthesis, growth and defense responses of quaking aspen and subalpine fir regeneration under light reductions and shifts in soil chemistry that occur as conifers increase in dominance. The studies demonstrated that aspen regeneration was substantially more sensitive to light and soil resource limitations than that of subalpine fir. For aspen, light reductions and/or shifts in soil chemistry limited height growth, biomass gain, photosynthesis and the production of defense compounds (phenolic glycosides and condensed tannins). Biomass gain and phenolic glycoside concentrations were co-limited by light reduction and changes in soil chemistry. In contrast, subalpine fir seedlings tended to be more tolerant of low light conditions and showed no sensitivity to changes in soil chemistry. Unlike aspen, subalpine fir increased its root to shoot ratio on conifer soils, which may partially explain its maintenance of growth and defense. The results suggest that increasing dominance of conifers in subalpine forests alters light conditions and soil chemistry in a way that places greater physiological and growth constraints on aspen than subalpine fir, with a likely outcome being more successful recruitment of conifers and losses in aspen cover.     

针叶树育苗除草剂效果试验

用除草剂对针叶树(落叶松、油松、樟子松)播种苗进行土壤处理和茎叶处理,试验表明5月上旬(针叶树播种)后喷施1.25g/m2的果尔,除草效果最好、最明显,且对出苗没有影响.     

Root carbohydrates and aspen regeneration in relation to season of harvest and machine traffic

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 (120 m × 120 m) were selected within two large mature aspen stands slated for summer harvest. Plots (50 m × 50 m) 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.     

Growth and physiological response of eastern white pine seedlings to partial cutting and site preparation

We examined how white pine (Pinus strobus L.) seedlings planted under a mature cover of white and red (Pinus resinosa Ait.) pine in eastern Ontario (Canada) responded to treatments aimed at improving light and soil conditions for seedling growth. The treatments were: (a) three levels of partial cutting (no cut or CS0, cut to one-crown spacing between residual trees or CS1, cut to two-crown spacing or CS2); (b) two levels of vegetation control (without herbicide or H0, with herbicide or H1); and (c) two levels of soil scarification (S0 and S1). On the third growing season after planting, total growth of seedlings was lowest in CS0 treatment and similar in CS1 and CS2 treatments. The CS2 created better growing light conditions than the CS1, with and average of 50% of full light at seedling height, which corresponded to the maximum height and diameter growth rates of seedlings. However, CS2 also stimulated the growth of competing woody vegetation (both understory trees and shrubs), and resulted in greater microsite heterogeneity of light availability. Scarification warmed the soil (approximately 1–3 °C in the middle of the growing season), decreased the density of competing trees, but increased the shrub density, with no impact on white pine seedling growth. The treatments had no effect on light-saturated photosynthetic rate (A) of current-year foliage of seedlings, nor on their midday shoot water potential. Leaf N was higher in partial cuts and with vegetation control, but the relationship between N and A was weak to non-existent for the different foliage classes. Measures of the proportion of aboveground biomass allocated to foliage (leaf-mass ratio) suggest an acclimation response of young white pine that improves growth under moderate light availability and compensates for the lack of leaf-level photosynthetic plasticity. We suggest a combination of soil scarification under a one-crown spacing partial cut (corresponding to 14 m² ha⁻¹ of residual basal area, or an average of 32% of available light at seedling height) as an establishment cut. This should provide optimum growth conditions for planted understory white pine, while also favoring natural regeneration and providing some protection against damage from insects and disease.     

A comparison of lodgepole pine responses to varying levels of trembling aspen removal in two dry south-central British Columbia ecosystems

We used manual cutting to manipulate trembling aspen (Populus tremuloides Michx.) density and spatial arrangement in relation to crop lodgepole pine (Pinus contorta Dougl. Ex Loud. var. latifolia Engelm.) on two sites in contrasting dry, cool to cold ecosystems of south-central British Columbia. In the dry, cool interior Douglas-fir ecosystem (IDFdk3), we reduced the density of tall aspen (aspen at least as tall as target pine) to 0 (broadcast removal), 1000, 2500, or 4000 stems/ha when the planted lodgepole pine was 6 years old. Eight years later, pine height/diameter ratio (HDR) was significantly lower in the broadcast removal and 1000 stem/ha treatments than in the control. There were no other significant growth responses and pine survival and vigour were good regardless of treatment. In contrast, in a dry, cold sub-boreal pine spruce ecosystem (SBPSxc) where treatments were applied at a stand age of 11 years, naturally regenerated lodgepole pine stem diameter increased significantly in the broadcast removal treatment relative to the untreated control within 2 years. After 4 years, HDR had declined significantly relative to the control where tall aspen density was ≤1000 stems/ha. There were no significant pine responses where 2500 tall aspen stems/ha were retained or where tall aspen were removed only within a 1-m radius around pine. The greater difference in height (height differential) between aspen and pine at the SBPSxc than the IDFdk3 site may partly explain the differing response of lodgepole pine to treatment. Trends of decreasing sucker density with increasing aspen retention were evident at both sites, but differences were significant (p ≤ 0.05) only at the SBPSxc site.     

Quantifying successional rates in western aspen woodlands: Current conditions,future predictions

Stands of quaking aspen (Populus tremuloides) rank among the most biologically diverse plant communities across the intermountain region of western North America. Marked declines of aspen have occurred in recent decades, likely due to a combination of effects from changes in fire regimes, herbivory, climate (e.g. drought), and interspecific competition with conifer species. However, it is poorly understood how the effects of these factors are manifested at a landscape scale over decadal time periods. Analysis of field data combined with topographic information collected across the 500,000 ha Owyhee Plateau in southwestern Idaho revealed that aspen in the area occur in three different biophysical settings; First, aspen stands exist at high altitudes on south-facing slopes where local conifer species are not likely to occur because of limiting temperature or precipitation levels under current climate conditions. In these areas aspen is the potential vegetation type rather than conifers. Second, aspen grow on anomalously wet microsites (e.g. near springs), and third, aspen grow within upland mixed aspen/conifer stands, which are experiencing rapid rates of conifer establishment. Based on a paired t-test (α = 0.05) we conclude that stands growing on wet microsites show significantly slower successional rates of conifer establishment relative to upland aspen stands. We developed a conceptual state-and-transition model for upland aspen/conifer stands occurring across a range of topographic positions. We then parameterized the model using extensive field data in the vegetation dynamics computer simulation model Vegetation Dynamics Development Tool (VDDT), and examined the current and future aspen distribution under varying fire regimes. Model results indicate that average fire return intervals of 50–70 years are desirable for maintenance of aspen in upland areas where conifers are present. Under the current fire regime in the area many upland aspen/conifer stands will likely be lost within 80–200 years. Thresholds for the effect of conifer encroachment and browsing on aspen regeneration identified through this research are similar to those described by others across the West. We therefore suggest that the results presented for the Owyhee Plateau are likely applicable to semi-arid aspen woodlands across the American West where succession to conifers is a cause of aspen decline.     

Establishing trees in an Appalachian silvopasture: response to shelters, grass control, mulch, and fertilization

In order to successfully introduce trees into existing pastures, it is important to determine and recommend a whole range of tree establishment practices. In the spring of 1995, approximately 350 bare-root seedlings each of black walnut (Juglans nigra L.) and honeylocust (Gleditsia triacanthos L.) were planted in six randomized paddocks within a silvopastoral study area at the Agroforestry Research and Demonstration Site in Blacksburg, Virginia. Three seedling establishment studies were tested, including (1) a tree protection study, (2) a water retention study, and (3) a fertilization study. Seedlings were planted using two different tree shelters (60 cm-tall poultry wire cage and 1.2 m-tall plastic Tubex), two water retention treatments (mulch and herbicide spray), and one fertilizer treatment. All treatments were compared to untreated controls. Tree survival, damage, and stem volume were compared for each species. Tree survival was comparable among all studies over three growing seasons. Tree establishment using poultry wire and Tubex shelters resulted in significant reduction of deer damage and significant increase in stem volume from 1996 to 1998. Tubex shelters had a pronounced positive impact on tree height and also on stem form; height of both black walnut and honeylocust was twice the height of control seedlings. Mulch and herbicide treatments for moisture control resulted in significant stem volume increases over thecontrol treatment from 1997 to 1998. However, mulching was less effective than the herbicide treatment. There was no significant tree growth response resulting from fertilization during this same period. This revised version was published online in June 2006 with corrections to the Cover Date.     

Response of aspen stands to forest tent caterpillar defoliation and subsequent overstory mortality in northeastern Ontario,Canada

Overstory mortality, understory tree recruitment, and vegetation development were assessed in trembling aspen (Populus tremuloides Michx.) stands following two recent episodes of forest tent caterpillar defoliation (Malacosoma disstria Hbn.) in northeastern Ontario. The results suggest that poplar (aspen and balsam poplar (Populus balsamifera L.)) mortality increased with consecutive years of insect defoliation occurring from the mid-1980s to mid-2000s and the proportion of poplars in the overstory, but decreased with improved pre-defoliation tree vigour (DBH increment). The first outbreak, which lasted from the mid-1980s to early 1990s, was more severe in terms of insect defoliation and contributed more to poplar mortality and decline. The decline began in the late 1990s and peaked in early 2000s. Poplar regeneration and understory shrubs responded rapidly to foliage loss to insect defoliation and mortality of overstory poplars. The regenerated poplars were able to maintain their growth under developing shrubs and residual overstory canopy and numbers were sufficient to compensate for the poplar trees lost to insect infestation. The defoliation-induced overstory decline will accelerate the transition of aspen stands to conifer dominance through enhanced conifer recruitment and growth, and reduced hardwood overstory in aspen-dominated stands, while hardwood dominance will persist in pure aspen stands. From a timber supply perspective, the decline caused by forest tent caterpillar defoliation could delay the availability of aspen stands for harvesting by 40–50 years.     

Effects of overstory retention and site preparation on growth of planted white spruce seedlings in deciduous and coniferous dominated boreal plains mixedwoods

Survival and growth of planted white spruce was assessed under partial harvest treatments and different site preparation techniques in mixedwood forests of two compositions prior to logging: deciduous dominated (d-dom) – primarily comprised of mature trembling aspen (Populus tremuloides Michx.) and coniferous dominated (c-dom) – primarily comprised of mature white spruce (Picea glauca (Moench) Voss). Levels of overstory retention were 0% (clearcut), 50% and 75% of original basal area, and site preparation techniques were inverted mounding, high speed mixing, scalping and control (no treatment). The survival and growth of white spruce were assessed seven years after planting. The experiment was established as a part of the Ecosystem Management Emulating Natural Disturbance (EMEND) experiment located in northern Alberta, Canada. In the c-dom, the 50% and 75% retention of overstory resulted in reduced growth and survival of white spruce seedlings compared to clearcuts. In contrast, in the d-dom, the seedlings performed best in sites that had 50% of the overstory retained. For the c-dom, the mounding and mixing treatments yielded the best growth of spruce seedlings, while scalping yielded the worst. In the d-dom, spruce growth was highest in sites with the mixing treatment. In the d-dom, growth and survival of the planted spruce was greater than in the c-dom. The natural regeneration of deciduous trees was suppressed by the retention of canopy regardless of original composition.