Partial Harvesting Research Articles

Effects of structural retention harvesting on seed production and seed characteristics of old-growth eastern white pine (Pinus strobus L.) stands in northern Ontario

Seed production and seed characteristics were examined during a mast seeding year in unmanaged, old-growth eastern white pine (Pinus strobus L.) stands located in northeastern Ontario and compared with those in adjacent stands partially harvested 16 years earlier using a structural retention system. Seed yields from old-growth stands were comparable to those of mature, second growth white pine stands but seed production assessed relative to unit area (# ha-1) and pine basal area (# m-2) was lower in partially harvested stands. In both unmanaged and harvested stands, seed production rate of trees growing in localized areas of lower pine basal area was higher. Seed characteristics and seed viability did not differ between harvest treatments. Although structural retention harvesting reduced seed production, results suggest that supply and viability of seed are unlikely to limit seedling recruitment in managed or protected old-growth white pine forests. Key words: germination, old-growth forest, partial harvest, seed mass, seed production

A multi-scale assessment of amphibian habitat selection: Wood frog response to timber harvesting

To understand changes in habitat selection in response to timber harvesting, we used radio-telemetry data from 82 adult wood frogs (Lithobates sylvaticus, formerly Rana sylvatica) and logistic regression modelling to assess habitat selection in response to an unharvested control and 3 forest management techniques: clearcutting (with removal of all merchantable timber > 10 cm diameter), clearcutting with coarse woody debris (CWD) retention, and partial harvesting with retention of ~50% canopy cover. At the home range scale, frogs selected the partially harvested treatment in spring 2005 and avoided the CWD-retained treatment in fall 2006. Frogs spent 5 ± 2 d (mean ± SE) longer in forested treatments than in both clearcut treatments, but certain individuals were able to specialize on the clearcut treatments. At the weekly activity centre scale, the best-supported models indicated that frogs were more likely to occupy locations with more complex ground structure, especially coarse woody debris, warmer temperatures, moister substrates, and greater canopy cover than random. Resource use among frogs overlapped substantially at both the weekly activity centre and daily microhabitat scales. Frogs selected daily microhabitats with more complex ground structure, greater canopy cover, and moister substrates than random. Selection at coarser scales may be mechanistically linked to finer scale resource selection by the physiological processes of thermo- and hydro-regulation. Our results support recommendations for minimizing the impact of logging by retaining coarse woody debris in clearcuts and partial harvesting with retention of ~50% canopy cover.

Comparative physiological responses of Rhododendron groenlandicum and regenerating Picea mariana following partial canopy removal in northeastern Quebec, Canada

In boreal black spruce ( Picea mariana (Mill.) BSP) dominated stands, careful logging around advance regeneration contributes to protect the shrub layer while increasing light availability. Therefore, it may promote expansion of bog Labrador tea ( Rhododendron groenlandicum (Oeder) Kron & Judd), which can impair black spruce growth by direct competition for nutrients and by alteration of soil biochemical conditions. Such harvesting approaches may not be suited for irregular, uneven-aged black spruce stands in the context of ecosystem-based management. Our objectives were to evaluate the effects of different intensities of partial harvest on critical resource availability and to compare the physiological and morphological responses of black spruce and Rhododendron to different intensities of partial harvest treatments. Two years after harvest, photosynthesis rate of black spruce was unchanged and was lower than that of Rhododendron, which was 63% higher after harvest. Nitrogen and water use efficiency were, respectively, 171% and 42% higher in Rhododendron than in black spruce after logging. Following harvest, black spruce physiology appeared to be regulated by the atmospheric humidity deficit, but for Rhododendron, physiology was regulated by light availability. The high plasticity of Rhododendron leaf traits could be indicative of the future advantage of this species in these stands following canopy removal associated with harvest.

Effects of diameter-limit by-laws on forestry practices, economics, and regional wood supply for private woodlands in southwestern Ontario

The agriculturally dominated Counties of Huron and Perth in southwestern Ontario regulate forest harvesting on private land through diameter-limit-based tree conservation by-laws. The rates of harvesting, along with the volume and value of timber sales and the type and quantity of tree marking were examined for the years 1997 to 1999. Although these harvests may form an important part of periodic farm income, at only 13% forest cover, these landscapes maybe further degraded by unsustainable forest harvesting practices. Based on the three study years, the mean annual area of forest harvested was found to be 4.4% of the total private forest landbase. The mean volume harvested from upland and lowland deciduous forest was 4666 and 6148 fbm/ha, respectively. Over-harvesting under a diameter-limit or hybrid method occurred in 8% of woodlot area with removal rates in excess of 10 000 fbm/ha. The most severe over-harvesting disproportionately targeted lowland woodlots, possibly compromising the ecological health of these often sensitive areas. Sugar maple, red/silver maple and ash were most commonly harvested at 33%, 31% and 21% of total species volume, respectively. On average, for standing timber, landowners received $680/Mfbm in the upland hardwood forests and $281/Mfbm in the lowland hardwood forests. On an area basis, mean price paid was $3680/ha and $1956/ha respectively on upland and lowland forests. Only 8% of the private land was harvested using single-tree selection or stand improvement (92% harvest was diameter-limit or a hybrid of same). Using a simple model, we found that woodlot owners comprising at least 74% of private woodland area would need to participate in forest harvesting in order to maintain the 1997 to 1999 partial harvest area rate of 2349 ha/yr. This rate may not be sustainable, given poor forest conditions in some areas, past management practices and a reduction in landowners interested in forest harvesting. Improvements are needed to bring the level of good forestry practice up by 62% to meet the rates that were being performed under pre-1994, free, provincial government private land forestry programs. Key words: private land forestry, forest harvesting, forest conservation by-laws, sustainable forest management, diameterlimit harvest, private woodlots

Effect of partial harvesting strategies on Artemia biomass production in Vietnamese salt works

The effect of partial harvest strategies on the production of Artemia biomass was evaluated for 12 weeks under Vietnamese salt farm conditions. The initial stocking density was 100 nauplii L−1. After 3 weeks of inoculation, Artemia adults were partially harvested at intervals of 1, 3, 6 and 9 days starting with an initial quantity of 30 kg ha−1 day−1 at first harvest, and then the quantity of harvestable biomass was adjusted according to the standing stock present in the culture pond, combined with the time needed to harvest these quantities and with the weight of biomass harvested in each pond. The results showed that in most cases, the total densities were not significantly different among harvesting frequencies (P>0.05). However, a relatively higher Artemia adult density and its standing stock were better maintained in the 3-day than in the 1-day interval, and were significantly higher compared with the other two harvesting frequencies. The total biomass yields were the highest (1587 kg ha−1) in the 3-day harvesting interval, followed by 1-, 6- and 9-day harvesting interludes, corresponding to 1323, 1091 and 975 kg ha−1 respectively. However, no statistical difference was observed between the 1- and the 3-day interval as well as between the 6- and the 9-day harvest schemes (P>0.05). The results of this study suggest that partial harvest of Artemia biomass performed every 3 days appears to be an appropriate strategy to enhance biomass productivity.

Effects of upland clearcutting and riparian partial harvesting on leaf pack breakdown and aquatic invertebrates in boreal forest streams

Summary1. Leaf litter breakdown and associated invertebrates were compared among three logged and three reference stream reaches 2–3 years before and 3–4 years after logging to assess the environmental impacts of partial‐harvest logging as a novel riparian management strategy for boreal forest streams.2. Partial‐harvest logging at three sites resulted in 10, 21 and 28% average basal area removal from riparian buffers adjacent to upland clear‐cut areas.3. Leaf litter breakdown rates were not significantly different between reference and logged sites after logging, but litter breakdown was significantly different from year to year at all sites.4. Significant post‐logging differences in aquatic invertebrate communities were detected at only one of the three logged sites. These differences were largely the result of increases in some leaf‐shredding stoneflies and a detritivorous mayfly and a decrease in a chironomid group 2–4 years after logging. This site where significant change was detected had the lowest intensity of riparian logging (average 10% removal) but the highest proportion of the catchment area that was clear cut (85%).5.The post‐logging differences in invertebrate communities at this site were more related to catchment‐wide influences (e.g. weather patterns, water yield, possibly upland clearcutting) than to reach‐level disturbances from riparian logging.6.The study indicates that partial‐harvest logging in riparian buffers at up to 50% removal should pose little risk of harm to leaf litter breakdown processes or aquatic invertebrate communities beyond any impacts that might arise from upland logging disturbance or catchment‐wide influences. However, the results should be viewed in the context of the natural disturbance (summer drought conditions) through the post‐logging assessment period of this study. Post‐logging summer drought conditions may have masked or confounded logging impacts on streams.

Response of a boreal forest to canopy gap openings - assessing vertical and horizontal tree growth with multi-temporal lidar data

Fine-scale height-growth response of boreal trees to canopy openings is difficult to measure from the ground, and there are important limitations in using stereo-photogrammetry in defining gaps and determining individual crowns and height. However, precise knowledge on height growth response to different openings is critical for refining partial harvesting techniques. In this study, we question whether conifers and hardwoods respond equally in terms of sapling growth or lateral growth to openings. We also ask to what distance gaps affect tree growth into the forest. We use multi-temporal lidar to characterize tree/sapling height and lateral growth responses over five years to canopy openings and high resolution images to identify conifers and hardwoods. Species-class-wise height-growth patterns of trees/saplings in various neighborhood contexts were determined across a 6-km2 matrix of Canadian boreal mixed deciduous coniferous forests. We then use statistical techniques to probe how these growth responses vary by spatial location with respect to the gap edge. Results confirm that both mechanisms of gap closure contribute to the closing of canopies at a rate of 1.2% per annum. Evidence also shows that both hardwood and conifer gap edge trees have a similar lateral growth (average of 22 cm/yr) and similar rates of height growth irrespective of their location and initial height. Height growth of all saplings, however, was strongly dependent on their position within the gap and the size of the gap. Results suggest that hardwood and softwood saplings in gaps have greatest growth rates at distances of 0.5–2 m and 1.5–4 m from the gap edge and in openings smaller than 800 m2 and 250 m2, respectively. Gap effects on the height growth of trees in the intact forest were evident up to 30 m and 20 m from gap edges for hardwood and softwood overstory trees, respectively. Our results thus suggest that foresters should consider silvicultural techniques that create many small openings in mixed coniferous deciduous boreal forests to maximize the growth response of both residual and regenerating trees.

Evaporative demand across a range of microsites in partial-cut boreal forests

Relative rates of evaporative demand were measured across a range of microsites in partial-cut boreal mixed-wood forests in northern Alberta using atmometers (evaporimeters); evaporation was measured by water loss from porous ceramic cones. The atmometers provided a consistent index of evaporative demand, i.e. a measure of water loss from a moist surface, when compared to potential evapotranspiration calculated from meteorological data collected at the study sites. The average evaporative demand at 2 m height was similar in two different patterns of partial harvest, but these were intermediate between that of nearby uncut forests and clear-cut harvests. There were differences in evaporative demand between 5 m wide extraction trails and adjacent partial-harvested areas. Rates of evaporative demand were related to small-scale spatial differences in basal area of the residual trees. This study demonstrates that these inexpensive instruments are suitable for replicated measurements of relative evaporative demand across a variety of forest habitats.

The influence of partial timber harvesting in riparian buffers on macroinvertebrate and fish communities in small streams in Minnesota, USA

Relatively few evaluations of aquatic macroinvertebrate and fish communities have been published in peer-reviewed literature detailing the effect of varying residual basal area (RBA) after timber harvesting in riparian buffers. Our analysis investigated the effects of partial harvesting within riparian buffers on aquatic macroinvertebrate and fish communities in small streams from two experiments in northern Minnesota northern hardwood-aspen forests. Each experiment evaluated partial harvesting within riparian buffers. In both experiments, benthic macroinvertebrates and fish were collected 1 year prior to harvest and in each of 3 years after harvest. We observed interannual variation for the macroinvertebrate abundance, diversity and taxon richness in the single-basin study and abundance and diversity in the multiple-basin study, but few effects related to harvest treatments in either study. However, interannual variation was not evident in the fish communities and we detected no significant changes in the stream fish communities associated with partially harvested riparian buffers in either study. This would suggest that timber harvesting in riparian management zones along reaches ≤200 m in length on both sides of the stream that retains RBA ≥ 12.4 ± 1.3 m 2 ha −1 or on a single side of the stream that retains RBA ≥ 8.7 ± 1.6 m 2 ha −1 may be adequate to protect macroinvertebrate and fish communities in our Minnesota study systems given these specific timber harvesting techniques.

Short-term demographic response of an old forest specialist to experimental selection harvesting

The brown creeper (Certhia americana) was recently identified as one of the forest bird species most sensitive to partial harvesting in North America. However, the processes underlying this sensitivity are poorly known. In this study, we quantified the immediate, post-treatment demographic response of this species to experimental selection harvesting in plots of northern hardwood forest in northwestern New Brunswick, Canada. We mapped individual detections and nest locations in 5 pairs (1 control and 1 treatment) of 25-ha plots in the first 2 y after single-tree selection harvesting. Linear mixed models with site and landscape context as random effects showed a significant negative effect of treatment on nest density and seasonal reproductive success. The density of large-diameter trees (≥ 30 cm dbh) was significantly lower in treated plots than in controls (mean: 77 versus 112 stems-ha−1), whereas the density of potential nesting substrates (snags with peeling bark) did not decrease significantly following treatment. Hence, the density of suitable foraging substrates may represent a limiting factor for both nest density and reproductive success, and partial harvesting may not be compatible with the persistence of breeding populations of brown creeper. Patches of untreated forest should be maintained in managed forest landscapes at all times for this and other taxa requiring old forest conditions.

The preferences of saproxylic beetle species for different dead wood types created in forest restoration treatments

Restoration by imitating natural disturbances is widely practised in boreal forests to increase the availability of habitats for specialized species. We studied the abundance and species richness of saproxylic beetles on different types of created dead wood during 2 years after restoration. The study was conducted on areas of a large-scale experiment in which Norway spruce ( Picea abies (L.) Karst.) forests were restored by controlled burning and partial harvesting with down wood retention in southern Finland. More beetle species were attracted to spruces than to birches and more species were attracted to burnt trees than to unburnt trees killed by girdling. Birch-living species consistently benefited from fire, but on spruce, the abundance of cambium consumers and their associates was negatively affected by fire. Trees at harvested sites attracted more beetles in the first year, but the volume of down wood retention had only minor effects. Beetle assemblages were strongly altered by burning and harvesting. We conclude that burning and harvesting are efficient tools to promote species richness within a short time period, but there is a risk that the dead wood resource may be rapidly exhausted. Moreover, many saproxylic species of spruce forests may not be adapted to open habitats formed by stand-replacing disturbances.

Comparison of tree size structure and growth for partially harvested and even-aged hemlock-spruce stands in southeast Alaska

The effects of partial cutting on tree size structure and stand growth were evaluated in 52 plots in 13 stands in southeast Alaska that were partially harvested 53–96 years ago and compared with 50-year-old even-aged stands that developed after clearcutting. The net basal-area growth was greater in the partially cut plots than in the uncut plots, and basal-area growth generally increased with increasing cutting intensity. However, the basal-area growth of all of the partially harvested stands was significantly less than the growth of 50-year-old even-aged stands, and net basal area growth over the 50 year period since partial harvesting was about 33–43% of the growth of the even-aged stands. Partial cutting maintained stand structures similar to uncut old-growth stands, and the cutting had no significant effect on tree species composition. The tree size distribution of the partially harvested stands was far more complex and well distributed in comparison with the 50-year-old even-aged stands, and included the presence of several trees with diameters of more than 100 cm. These trees included both large-diameter spruce and hemlock trees and were a distinctive structural feature that was noticeably lacking in the even-aged stands.

Planted stock performance 10 years after partial cutting in west-central British Columbia

Mature lodgepole pine (Pinus contorta) forests were harvested using group selection (GS) (0.02-ha openings) and irregular group shelterwood (IGS) (0.05-ha openings) systems to maintain arboreal and terrestrial lichens in the winter range of northern woodland caribou (Rangifer tarandus caribou). Ten years after planting, lodgepole pine showed excellent survival, but were smaller in the partial cut openings compared to the clearcuts. Pine grew less in the Sub-Boreal Pine–Spruce biogeoclimatic subzone (SBPSxc) than in the Montane Spruce subzone (MSxv), and trees were smaller in the GS versus IGS treatment within the MSxv subzone. Interior spruce (Picea glauca × engelmannii) grew best in the MSxv and partial cut treatments, but was significantly affected by summer frost in the clearcuts. In an operational-scale Adaptive Management trial, openings were enlarged to 0.15 ha, and both pine and spruce showed excellent survival, minimal frost damage, and 10-year size similar to clearcut conditions. This study suggests that lodgepole pine and interior spruce can be successfully regenerated in partial cut openings with acceptable growth in gaps of 0.15 ha. Key words: caribou, group selection, interior spruce, irregular group shelterwood, light level, lodgepole pine, Montane Spruce zone, partial harvest, soil moisture, soil temperature, Sub-Boreal Pine Spruce zone, summer frost

The effectiveness of different buffer widths for protecting water quality and macroinvertebrate and periphyton assemblages of headwater streams in Maine, USA

We evaluated the effect of timber harvesting on water quality and macroinvertebrate and periphyton assemblages in first-order streams in Maine, USA. Fifteen streams were assigned to one of five treatments: clearcutting without a stream buffer, clearcutting with 11 m buffers, clearcutting with 23 m buffers, partial harvesting with no designated buffer, and unharvested controls. Harvest blocks on both sides of the stream were 6 ha and partial harvesting within buffers was allowed. Specific conductivity, pH, dissolved oxygen, turbidity, and soluble reactive phosphorus did not change significantly for 3 years after harvesting in all treatments. Unbuffered streams had significantly elevated concentrations of chlorophyll a as well as increased abundance of algal feeding organisms (Diperta Cricotopus and Diptera Psectrocladius ). Streams with 11 m buffers had substantial (10-fold) but nonsignificant increases in chlorophyll a. No other significant changes were detected in other treatment groups. In all treatment groups, the dominant taxa (periphyton Achnanthes minutissimum and macroinvertebrate Chironomidae) are adapted to disturbed environments. We attribute the limited harvest-induced changes to lack of soil disturbance within 8 m of the stream, the small (≤40%) proportion of watersheds harvested, and the resilient nature of aquatic organisms. However, small-scale changes may not be detected due to the small sample size, an inherent limitation of field-based studies.

Advances in intensifying the cultivation of the shrimp Litopenaeus vannamei in floating cages

Shrimp farming in floating cages has advantages over conventional cultivation systems and is a potential production alternative for low-income communities. We present advances in intensifying the cultivation of L. vannamei in floating cages by testing high stocking densities and a large cage-size not previously used, and introducing partial harvesting for the management of the high shrimp biomass. A 38-day nursery trial made during 2006, using 9-m 2cages showed that the final weight of shrimp and specific growth rate (SGR) were significantly ( P < 0.05) greater when postlarvae was stocked at 950 and 1300 PL/m 2, rather than at 1600 PL/m 2. However, the highest number of juveniles per cage was obtained when the later stocking density was used. A 62-day grow-out trial in 2006, using 9-m 2 cages, resulted in a significantly lower growth rate (0.17 g/day) when juveniles were stocked at 300/m 2, as compared to 0.18 and 0.18 g/day obtained at 200 and 250/m 2. The final shrimp biomass varied significantly among densities and ranged from 1.9 to 2.6 kg/m 2. There were no significant differences in final weight (11.3–12.0 g), survival (77% to 81%), and feed conversion ratio (FCR) (0.84–0.93). A 58-day grow-out trial in 2006 using 200-m 2 cages showed that the final weight and growth rate were significantly greater (17.7 g and 0.27 g/day) when juveniles were stocked at 45/m 2, as compared to 13.3 g and 0.20 g/day obtained at 180/m 2. This density did produce the highest biomass yield (1.9 kg/m 2). A nursery trial was made during 2008, stocking at 1500 PL/m 2 and obtaining results that were consistent with those from the trial of 2006. A 93-day grow-out trial was made during 2008 stocked with 420 and 560 juveniles/m 2 in 9-m 2cages. A partial harvest was made after 62 days when shrimp weighed 8.9 to 9.5 g. A significantly higher biomass yield (1.5 kg/m 2) was obtained at the higher density when compared to 1.2 g/m 2 harvested from the lower density. After the final harvest, the weight of shrimp differed significantly between the lower (14.4 g) and higher (13.2 g) densities, and the corresponding total biomass yields were 4.4 and 5.0 kg/m 2. The values of the zootechnical variables were acceptable when compared to those reported for shrimp cultivation in floating cages, intensive ponds, or raceways, thus indicating the feasibility of further intensifying the cultivation of L. vannamei in cages.

The role of gaps and tree regeneration in the transition from dense to open black spruce stands

Black spruce forests growing on clay soils in northwestern Quebec change structure from dense even-aged stands to open uneven-aged stands such that almost all forests older than 200 years have an open canopy. These forests become unproductive over time because they are prone to paludification. The main goal of our study was to document the transition between dense and open stands in terms of gap dynamics, with a focus on tree regeneration. Our objective was to determine whether forests remain open due to a lack of regeneration, a lack of growth or both. Nine stands along a 50–250-year-old time since fire gradient were sampled with the line intersect sampling method. Gap fraction increased with stand age and reached a maximum of 77% in the oldest site. In old-growth stands, gaps were interconnected due to the low density of these forests. Most of the gap makers were found with broken stems. Regeneration was dominated by black spruce layers and was relatively abundant (1.71stems/m2). However, the majority of gap fillers were smaller than 1m in height in stands of all ages. Instead of a lack of regeneration, the opening of the forests is due to a lack of growth associated with cold and wet organic deposits. Partial harvesting could be implemented on the most productive sites, while management techniques including soil disturbances will be required on low productivity sites to recreate good growth conditions.

Preliminary investigation of the effects of timber harvesting on the activity status of beaver lodges in central Ontario, Canada

Beavers (Castor canadensis) are widely considered a keystone species in boreal and northern temperate forest ecosystems and are seasonally dependent on intolerant hardwood tree species for food. We used existing data to investigate the effects of timber harvesting on the activity status of beaver lodges in central Ontario, Canada. Beaver lodges were initially visited from 1976 to 1979 and active lodges were revisited in 1994. We analyzed a sample of 100 lodges. Fifty had some timber harvesting within 400 m of the lodge, 41 of which had harvest within 100 m of the shoreline (subsequently referred to as shoreline harvest). We differentiated timber harvest by type (clearcut vs. partial cut), years since harvest, and location for each lodge. Clearcut timber harvesting appeared to have a positive effect on the occupancy of beaver lodges. Seventy-three percent of lodges adjacent to shoreline clearcut areas were active, whereas only 34% of lodges with no shoreline harvest were active. We developed logistic regression models to evaluate the relationship between presence of timber harvest and activity status based on harvest location, harvest type, years since harvest, and water feature (pond, stream, lake, or wetland). Using Akaike’s Information Criterion, our modeling suggested that the presence of a 21- to 35-year-old shoreline clearcut adjacent to a lodge, combined with the associated water feature type, was the best predictor of lodge activity. However, our dataset included only a small sample of lodges in this harvest category (n = 11). While these results are viewed as preliminary, they do suggest that further investigation into the effects of shoreline timber harvest and shoreline reserves on habitat suitability for beavers is warranted. Key words: beaver, beaver pond, boreal, clearcut, forestry, Great Lakes – St. Lawrence, natural disturbance, Ontario, partial harvest, riparian, shoreline

Modelling stand development after partial harvests: An empirically based, spatially explicit analysis for lowland black spruce

Across forest ecosystems worldwide, alternative silvicultural practices such as partial or “structural retention” harvesting are gaining increasing use as means to balance economic and ecological management goals. Recent studies have documented highly elevated rates of residual-tree mortality and growth, both of which are likely to influence post-harvest stand development. Explicit consideration of residual-tree responses to partial harvest is therefore required to make accurate growth and yield predictions. In this paper we incorporate empirically based, spatially explicit tree growth and mortality predictors into a forest simulation model in order to quantify stand dynamics following a range of simulated partial harvests. We consider lowland black spruce boreal forests and use a common strip-cutting technique, Harvest with Advance Regeneration Protection, as a reference against which to compare alternative scenarios. Predicted stand development proceeded most rapidly in scenarios with reduced skidding impacts and relatively high levels of retention, since these treatments were associated with lower residual-tree mortality rates and larger growing stocks respectively. Stand development was also influenced on a proportional basis by residual-tree growth rates, which increased with declining neighbourhood competition. Across a range of harvest scenarios with equivalent retention levels (20%), differences in residual-tree responses among treatments led to a 35-year range in projected basal area return intervals (70–105 years). Our results demonstrate that short-term responses of residual trees affect stand development rates over long (>100-year) time scales, and show that post-harvest stand recovery rates are influenced not only by the retention level but also by the specific parameters of partial harvest prescriptions, including the layout of skid trails and the spatial arrangement of residual trees. Results from this study provide essential information for evaluating partial harvests in the black spruce boreal forest and supply modelling tools for assessing alternative silvicultural treatments in other forest ecosystems.

Effects of Timber Harvest on Amphibian Populations: Understanding Mechanisms from Forest Experiments

Harvesting timber is a common form of land use that has the potential to cause declines in amphibian populations. It is essential to understand the behavior and fate of individuals and the resulting consequences for vital rates (birth, death, immigration, emigration) under different forest management conditions. We report on experimental studies conducted in three regions of the United States to identify mechanisms of responses by pond-breeding amphibians to timber harvest treatments. Our studies demonstrate that life stages related to oviposition and larval performance in the aquatic stage are sometimes affected positively by clearcutting, whereas effects on juvenile and adult terrestrial stages are mostly negative. Partial harvest treatments produced both positive and weaker negative responses than clearcut treatments. Mitigating the detrimental effects of canopy removal, higher surface temperature, and loss of soil-litter moisture in terrestrial habitats surrounding breeding ponds is critical to maintaining viable amphibian populations in managed forested landscapes.

Threshold effects of variable retention harvesting on understory plant communities in the boreal mixedwood forest

We studied the effects of six levels of dispersed green-tree retention (GTR) harvesting (clearcut (0%), 10%, 20%, 50%, and 75%, and unharvested reference (100%)) on understory plant communities in the 8th growing season post-harvest in the mixedwood boreal forest in northwestern Alberta. For the partial harvest treatments (10%, 20%, 50%, 75%) sample plots were located in the partially harvested (retention) strips as well as in the intervening machine corridors used by the harvesting equipment. The understory plant community was significantly influenced by the gradient of retention level. The cover of understory vegetation, especially graminoids, increased with increasing harvesting intensity for the retention strips and overall considering both plots types. Species richness was unaffected by retention level but did decrease as tree density increased. Lower levels of retention lead to increased abundance of early successional, shade-intolerant species. The results suggest a threshold in understory response to GTR harvesting between the 10% and 20% retention treatments. In terms of understory cover and composition, machine corridors within partially harvested forests resembled clearcuts. The results suggest that retaining more than 10% during GTR harvesting could have significant benefits in terms of maintaining understory plant communities more similar to unharvested reference forest.