Snag availability for cavity nesters across a chronosequence of post-harvest landscapes in western Newfoundland

We examined the availability and quality of standing dead trees (snags) for nesting habitat in a harvest chronosequence of boreal forests dominated by balsam fir (Abies balsamea) in western Newfoundland. Snag density declined substantially 10–15 years after harvest, then increased to reach its highest level in 81- to 100-year-old forests that had become senescent. Most (55%) of 1260 snags encountered were balsam fir, which contained 41% of the 81 cavities identified. Cavity presence was most strongly positively correlated to snag diameter at breast height (dbh), followed by decay class, time since harvest and height. Less than 40% of snags available throughout the chronosequence had large enough dbh for cavity nesters. Downy Woodpeckers (Picoides pubescens) were responsible for 47% of all cavities identified, excavating balsam fir 50% of the time. Two larger cavity nesters present, Northern Flickers (Colaptes auratus) and Three-toed Woodpeckers (Picoides tridactylus), were more likely to use large-diameter white birch (Betula papyrifera) snags. Northern Flickers excavated the largest cavities, potentially providing habitat for the greatest variety of secondary cavity nesters. Managing for snags with >30 cm dbh, which flickers target for excavation, has the greatest potential to enhance the broader snag cavity-based community in western Newfoundland.     

Snag longevity of Douglas-fir,western hemlock,and western redcedar from permanent sample plots in coastal British Columbia

Snags are important both as structural components and as animal habitat in forests, but abundance is often low and their dynamics poorly understood in young, managed stands. Using a large data set of 19,622 snags from permanent plots in second-growth forests of coastal British Columbia, we modeled snag longevity (time from tree mortality to snag fall) for three species: Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and western redcedar (Thuja plicata). Snag longevity was strongly related to species and snag size (diameter): the median snag longevity was 16 years for Douglas-fir, 11 years for hemlock and 5 years for redcedar. Western redcedar was predominantly in the subcanopy and its rapid fall rate was related to the small size of its snags. In addition to diameter, other attributes (height to diameter ratio, height, and live crown ratio before death) contributed significantly to models for one or two of the species. However, site level variables did not contribute significantly to any of the models. Snags greater than 50 cm diameter, especially Douglas-fir snags, have the potential for persistence well beyond 20 years in these second-growth forests, and could be important for wildlife.     

Biomass conversion factors (density and carbon concentration) by decay classes for dead wood of Pinus sylvestris, Picea abies and Betula spp. in boreal forests of Sweden

For estimating the amount of carbon (C) in dead wood, conversion factors from raw volume per decay class to dry weight were developed using three different classification systems for the species Norway spruce (Picea abies L. Karst), Scots pine (Pinus sylvestris L.) and birch (Betula pendula Roth and B. pubescens Ehrh) in Sweden. Also the C concentration in dead wood (dry weight) was studied. About 2500 discs were collected from logs in managed forests located on 289 temporary National Forest Inventory (NFI) sample plots and in 11 strips located in preserved forests. The conversion factors were based on an extensive data compilation with a wide representation of different site-, stand-, species- and dead wood properties and were assumed to represent the population of fallen dead wood in Sweden. The density decreased significantly by decay class and the range in density for decay classes was widest for the NFI decay classification system, suggesting this to be the most suitable. The C concentration in dead wood biomass increased with increasing decay class and in average Norway spruce (P. abies) showed a lower C concentration than Scots pine (P. sylvestris). The average dead wood C store of Swedish forests was estimated to 0.85 Mg C/ha.     

Dynamics of seedling establishment and survival in uneven-aged boreal forests

The aim of this study was to describe the dynamics of seedling establishment and development in spruce-dominated uneven-aged boreal forests. The study was based on empirical data from 15 stands with permanent plots, which had been intensively monitored for 10 years in southern Finland. All trees (height > 1.3 m) were measured every fifth year. Regeneration was measured on 64 permanent sample plots (4 m²) in each stand. The establishment of first-year seedlings was analysed on a yearly basis. The survival and development of older Norway spruce (Picea abies (L.) H. Karst.) seedlings were analysed based on observations made every five years. The establishment of spruce seedlings was closely correlated with the abundance of seed crops. Seedling cohorts originating from abundant seed crops were clearly detectable in the development of seedling height distributions over time. It took about 15 years for spruce seedlings to reach a height level of 15-30 cm. Local basal area had hardly any effect on the emergence or survival of small spruce seedlings, while the number of higher spruce seedlings decreased with increasing local basal area.     

Snag degradation pathways of four North American boreal tree species

Many studies have highlighted the importance of deadwood, whether standing (snags) or fallen, in boreal ecosystems dynamics. However, a dearth of literature exists regarding the persistence and degradation pathways of these structures in northeastern American boreal species.     

Understory plant communities of boreal mixedwood forests in western Canada: Natural patterns and response to variable-retention harvesting

We examined patterns of variation in richness, diversity, and composition of understory vascular plant communities in mixedwood boreal forests of varying composition (broadleaf, mixedwood, conifer) in Alberta, Canada, before and for 2 years following variable-retention harvesting (clearcut, 20 and 75% dispersed green tree retention, control). Broadleaf-dominated forests differed from mixedwood or conifer-dominated forests in that they had greater canopy cover, litter depth, soil nitrogen, warmer soils, as well as greater shrub cover, herb and shrub richness and diversity (plot scale). In contrast, conifer, and to a lesser extent mixedwood, forest had greater β diversity than broadleaf forest. Overall, mixedwood and conifer forests were similar to one another, both differed from broadleaf forest. Several species were found to be significant indicators of broadleaf forest but most of these also occurred in the other forest types. Understory composition was related to canopy composition and edaphic conditions. Variable-retention harvesting had little effect on understory cover, richness, or diversity but resulted in reduced richness and β diversity at a larger scale. The clearcut and 20% treatments affected composition in all forest types. Early successional species and those common in disturbed sites were indicators of harvesting while evergreen, shade-tolerant understory herbs were indicators of the control forest and 75% retention harvest. We conclude that it is important to maintain a range of variation in canopy composition of mixedwood forests in order to conserve the associated understory communities. The presence of conifers in these forests has a particularly important influence on understory communities. The threshold for a lifeboat effect of variable-retention harvesting is between 20 and 75% retention. Examination of richness and β diversity at a variety of scales can provide interesting information on effects of harvesting on spatial reorganization and homogenization of understory plant communities.     

Sphagnum community change after partial harvest in black spruce boreal forests

In eastern Canada, boreal forests develop structural diversity in association with time since stand replacing fire. In some regions, this is associated with significant changes in the bryophyte community (Sphagnum moss invasion) and paludification (thick waterlogged forest floor development). The bryophyte community responds to opening of the canopy, and increasing moisture by replacement of slow growing species by faster growing Sphagnum spp. (e.g. magellanicum, fallax) that are dependent on constant hydration. Within a forest management context, partial harvest systems have been proposed as a strategy to maintain structural diversity, which is currently not accomplished with low retention systems. However, it is unknown whether these interventions will effectively accelerate community succession. The questions addressed in this study were: (1) is the composition of Sphagnum colonies in partially cut stands more similar to old-growth communities than in control, and low retention cut stands, (2) what aspects of harvest disturbance drive these changes, and (3) is the growth rate of Sphagnum capillifolium (an early successional shade tolerant species) different in partial versus low retention harvest systems? After harvest, Sphagnum patch size was reduced by 19.8% and 11.7% after low retention and partial harvest, respectively. While trends were not constant across three separate partial cut trials, the proportion of Sphagnum magellanicum, Sphagnum fallax and Sphagnum fuscum increased compared to controls and low retention 1–2 years after harvest. Models of percent Sphagnum cover indicated machinery track cover, percent cover of vascular plants, and patch depth were positive factors, while the influence of open canopy varied among species. Despite the inclusion of individual disturbance variables, the summary variable ‘treatment’ was significant in all models. Growth of S. capillifolium in partial cuts was intermediate to growth rates in control and low-retention cuts. Growth was positively influenced by slash cover and, contrary to the patch level, negatively influenced by track cover. These results indicate that partial harvest does represent an intermediate level of disturbance, as direct and indirect harvest effects were reduced, as was Sphagnum death. Change in composition 1 and 2 years after harvest indicates that partial harvests may effectively shift the bryophyte community towards an older community type and may thus be used to create landscape diversity. Long term trends and entire community compositions need to be assessed before this can be stated definitively. However, as paludified stands are less productive, the capacity of these partially harvested sites to produce merchantable timber is questioned.     

Planning timber harvest of residual forest stands without compromising bird and small mammal communities in boreal landscapes

Numerous efforts have been invested in designing and configuring residual forest stands in Canadian boreal forest to preserve their overall biodiversity. Now that several landscapes have been partially logged, the next issue in forest management involves the planning of residual forest stand harvesting without compromising wildlife populations. Residual stands can be cut when adjacent regeneration reaches 3 m in height according to current regulations in several Canadian provinces (e.g., Québec, Ontario, Alberta, and British-Columbia). However, little is known on whether such regenerating habitat (RE-3m) can maintain wildlife communities similar to those found in unharvested mature forest (CO). We estimated the relative abundance of small mammals and forest birds in RE-3m and CO habitats and characterized landscape and stand structures. These variables were then compared between the two contrasting successional stages and were used to build habitat use models (HUMs) for 21 species. CO and RE-3m differed with regard to several landscape characteristics and stand structure variables as a result of logging. Snowshoe Hare, Northern Flicker, Alder Flycatcher, Ruby-crowned Kinglet, White-throated Sparrow and Magnolia Warbler were more abundant in RE-3m than CO, while Red-backed Vole, Brown Creeper and Golden-crowned Kinglet exhibited lower abundances in RE-3m. No significant differences in abundance were observed for the 12 other species. Species HUMs were highly significant and explained between 64.3 and 99.1% of the total variability in abundance. Following variance partitioning, stand structure variables accounted for most of the explained variability (54.2%) while landscape characteristics accounted for only 28.7%. No difference in species richness was observed but community evenness was greater in CO than RE-3m. Our results suggest that current regulations may threaten the maintenance of 3 out of 21 censused species for which abundances were significantly lower in regenerating 3 m tall stands. As stand structure explained a large amount of variability in abundance, it should be considered during timber harvest planning in both mature and regenerating stands. Until we know more on whether the current regulations are suitable for maintaining overall biodiversity, our results suggest that some mature forest stands should be maintained within managed landscapes for a complete logging rotation period.     

Short-term responses of soil decomposer and plant communities to stump harvesting in boreal forests

Recently, in addition to logging residues, stumps have become an important component in energy production since there is growing global interest in the use of renewable energy sources in order to decrease anthropogenic carbon emissions. Harvesting of stumps influences the forest floor by changing vegetation and soil organic layers and exposing mineral soil across large areas. We studied whether stump harvesting after clear felling poses further short-term changes in boreal forest soil decomposer community (microbes and mesofauna) and vegetation when compared to the traditional site preparation practice (mounding). In general, stump harvesting caused decline in enchytraeid abundance but did not induce further major changes in decomposer community otherwise nor in vegetation of each soil micro-habitat (intact soil and exposed mineral soil). However, the abundances of almost all decomposer animals were lower in the exposed mineral soil than in the intact soil. Stump removal increased the area of exposed mineral soil in the clear felled areas, leading to lower amount of high quality habitat for most decomposer organisms. Hence, it is obvious that there are (or will be) differences in the decomposer community dynamics between the treatments at the forest stand level. Both species richness and coverage of plants benefitted from large-scale exposure of mineral soil. Because the stump removal procedure disturbs soil organic layers and negatively affects the decomposer community, it has the potential to alter nutrient dynamics in forests.     

Estimating cumulative defoliation of balsam fir from hemlock looper and balsam fir sawfly using aerial defoliation survey in western Newfoundland,Canada

Both hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) and balsam fir sawfly (Neodiprion abietis (Harris)) undergo periodic outbreaks in eastern Canada and cause significant growth and mortality losses to forests. Tree growth and mortality are closely related to cumulative defoliation estimates, which integrate annual defoliation over multiple years. Our objective was to determine a method to estimate cumulative defoliation of balsam fir (Abies balsamea [L.] Mill) due to these insects in western Newfoundland, using aerial defoliation survey data, as an essential input to modeling impacts for Decision Support Systems. Interpretation of aerial defoliation survey data for hemlock looper and balsam fir sawfly is problematic because both insects feed upon multiple age classes of foliage. Current-year (2008) aerial defoliation survey data were compared with ground estimates of defoliation by age class from 45 plots (450 trees and 395 mid-crown branch samples), representing a range of defoliation severity classes for each insect. Cumulative defoliation was calculated using defoliation per foliage age class, weighted by relative foliage mass for a given age of foliage. Three significantly different severity classes were defined based on cumulative defoliation values derived from aerial defoliation survey: (i) 1-year moderate (30–70%) defoliation, (ii) 1-year severe (71–100%) defoliation with calculated cumulative defoliation values of 19 and 39%, respectively, for balsam fir sawfly, 21 and 34% respectively for hemlock looper; and (iii) 2–3 years of moderate–severe defoliation, with cumulative defoliation ranging between 59 and 64% for balsam fir sawfly and 49% for hemlock looper. Defoliation severity from aerial defoliation survey alone hence can be misleading if defoliation measurements are not converted to cumulative defoliation values.     

Past and present outbreaks of the balsam fir sawfly in western Newfoundland: An analytical review

Historical data of defoliation and population density were examined to determine whether a sustained outbreak of balsam fir sawfly (Neodiprion abietis Harris) in western Newfoundland, Canada is unprecedented in severity and duration. Results indicate that the current outbreak departs substantially from historical trends, covering a surface area twice the sum of all infestations occurring in the preceding 50 years. The current outbreak is also of longer duration due to a northward expansion of the range usually subjected to severe defoliation by this insect. Time-series analysis indicates that balsam fir sawfly dynamics have a strong second-order component, providing testable hypotheses for future studies investigating the factors responsible for population fluctuations.     

Long-term dynamics of large woody debris in a managed boreal forest stream

Little is known about how past forest management in Sweden influenced the quantity and quality of large woody debris (LWD) in streams. The present study provides information of the long-term dynamics of LWD in a reach of a boreal stream intersecting a managed forest. Dendrochronological methods were used to reconstruct mortality years of the pieces of LWD and the general history of fire and cuttings of the surrounding riparian forest. Today, spruce dominates among the living trees, whereas the LWD is dominated by birch in the forest and by pine in the stream. Fire frequency prior to active fire suppression was similar to values reported from boreal forests. Pine trees were more abundant in the riparian forest before selective logging operations and active fire suppression began in the 1800s. Many of the pieces of LWD found in the stream today died more than 200 years ago and derived from a cohort of pines that generated in the early 1600s. Pine LWD in stream channels is highly resistant to decomposition and can reside for more than 300 years. A substantial amount of the LWD found today in managed forest streams in boreal Sweden most likely derives from the time before extensive human influence and is likely to decrease further in the future. Management of riparian forests to ascertain future supply of long-lived LWD in streams should target to increase the proportion of pine trees.     

Quantifying local factors in medium-frequency trends of tree ring records: Case study in Canadian boreal forests

Growth rings of a tree are simultaneously affected by various environmental constraints, including regional factors such as climate fluctuations and also local, gap-scale dynamics such as competition and stochastic mortality of neighbor trees. Although these local effects are often discarded by dendroclimatologists as random variation, the dendroecological trends may provide valuable information on past forest dynamics. Since dendroecological trends arising from local stand dynamics often have medium-term frequencies with persistence of several years to a few decades, it is usually difficult to separate local, gap-scale forcings from regional, medium-frequency forcings such as El Niño Southern Oscillation or North Atlantic Oscillation. Moreover, conventional dendroecological practices have failed to analyze the continuously changing medium frequency trends. In this study, a continuous index of medium-frequency dendrochronological trends was developed, by generalizing previous analytical methods that evaluate relative changes using moving averages. This method was then tested against a tree ring dataset from a site with a known history of release and suppression due to a hurricane disturbance. To quantify the effects of local gap dynamics against the regional, often climatic effects, increments cores of black spruce (Picea mariana) were sampled from boreal forests in Saskatchewan, Canada, using a stratified sampling design. Assuming that regional forcings affect trees in the given stand homogeneously, the relative effect of stochastic heterogeneity within stand was quantified. The results closely agreed with conventional dendrochronological observations. In closed-canopy stands, stochastic local effects explained 12.9–35.4% of the variation in tree ring widths, because interactions between neighbor trees were likely to be intense. In open-canopy stands, on the other hand, the proportion of explained variance was 1.4–10.2%, reflecting the less-intense local tree interactions in low-density stands. These advancements in statistical analysis and study design will help ecologists and paleo-climatologists to objectively evaluate the effects of climate fluctuations, relative to the effects of local, ecological interactions. Moreover, forest managers can apply concepts of filtering medium-frequency trends to assess release and suppression caused by forest management practices, such as selective cutting and forest thinning.     

Effects of management on coarse woody debris volume and composition in boreal forests in northern Sweden

Forest management practices have led to a reduction in the volume and a change in the composition of coarse woody debris (CWD) in many forest types. This study compared CWD volume and composition in reserves and two types of managed forest in the central boreal zone of Sweden. Ten areas were surveyed, each containing clear-cut, mature managed and old-growth stands, to determine the volume of standing and lying CWD in terms of species composition, decay class and size class. Volumes of CWD on clear-cuts and in mature managed forests were high compared with previous studies. Old-growth forests (72.6 m³ ha^?1) contained a greater volume of CWD than mature managed forests (23.3 m³ ha^?1) and clear-cuts (13.6 m³ ha^?1). Differences were greatest for the larger size classes and intermediate decay stages. Despite stand ages being up to 144 years, CWD volume and composition in managed forests was more similar to clear-cuts than to old-growth forests.     

Changes in quantitative patterns of dead wood in maritime pine plantations over time

Although the volume of dead wood is commonly acknowledged as an indicator of biodiversity in sustainable forest management schemes, only few data are available for plantation forests. To evaluate the volume and qualify the diversity of woody debris along a chronosequence of maritime pine plantations, we sampled downed woody debris, snags and stumps in 143 stands of different ages. To test the hypothesis that the pattern of dead wood accumulation mainly results from silvicultural operations, we developed a predictive model. It combined an empirical growth model evaluating the amount of dead wood produced by successive thinnings with a decay function that estimated the loss of dead wood with time.     

Effects of pre- and post-harvest spray with glyphosate and partial cutting on growth and quality of aspen regeneration in a boreal mixedwood forest

In the boreal forest, conifer release treatments can leave a low quality hardwood component, which does not contribute to healthy, productive mixedwood forests. In this study, the growth and quality of trembling aspen (Populus tremuloides Michx.) regeneration were examined 5-7 years after spraying with glyphosate herbicide before and after harvesting. Results were compared to those from a partial cut and clearcut treatment without herbicide treatments. The preharvest spray treatment effectively reduced aspen density and height but did not lower regeneration quality—assessed by stem and crown deformation and stem stain—compared to the postharvest spray and clearcut treatments. Increased stem stain in the postharvest spray treatment was largely associated with the stem section that grew prior to herbicide application—post-herbicide growth was not affected. While the effect of stem stain may be restricted to growth that occurred before herbicide use, stem deformation from stem dieback may have longer term effects. In the partial cut treatments both density and stocking of aspen regeneration were lower, but aspen basal diameter growth, height growth, and quality were similar to those in the clearcut treatment. Thus, preharvest spray should promote conifer growth by reducing the density and growth of aspen regeneration without reducing the quality of aspen; this should be considered a preferred treatment to support for management objectives calling for productive and healthy mixtures of fast-growing aspen and slow-growing conifers.     

Nitrogen dynamics across silvicultural canopy gaps in young forests of western Oregon

Silvicultural canopy gaps are emerging as an alternative management tool to accelerate development of complex forest structure in young, even-aged forests of the Pacific Northwest. The effect of gap creation on available nitrogen (N) is of concern to managers because N is often a limiting nutrient in Pacific Northwest forests. We investigated patterns of N availability in the forest floor and upper mineral soil (0–10 cm) across 6–8-year-old silvicultural canopy gaps in three 50–70-year-old Douglas-fir forests spanning a wide range of soil N capital in the Coast Range and Cascade Mountains of western Oregon. We used extractable ammonium (NH₄⁺) and nitrate (NO₃⁻) pools, net N mineralization and nitrification rates, and NH₄⁺ and NO₃⁻ ion exchange resin (IER) concentrations to quantify N availability along north-south transects run through the centers of 0.4 and 0.1 ha gaps. In addition, we measured several factors known to influence N availability, including litterfall, moisture, temperature, and decomposition rates. In general, gap-forest differences in N availability were more pronounced in the mineral soil than in the forest floor. Mineral soil extractable NH₄⁺ and NO₃⁻ pools, net N mineralization and nitrification rates, and NH₄⁺ and NO₃⁻ IER concentrations were all significantly elevated in gaps relative to adjacent forest, and in several cases exhibited significantly greater spatial variability in gaps than forest. Nitrogen availability along the edges of gaps more often resembled levels in the adjacent forest than in gap centers. For the majority of response variables, there were no significant differences between northern and southern transect positions, nor between 0.4 and 0.1 ha gaps. Forest floor and mineral soil gravimetric percent moisture and temperature showed few differences along transects, while litterfall carbon (C) inputs and litterfall C:N ratios in gaps were significantly lower than in the adjacent forest. Reciprocal transfer incubations of mineral soil samples between gap and forest positions revealed that soil originating from gaps had greater net nitrification rates than forest samples, regardless of incubation environment. Overall, our results suggest that increased N availability in 6–8-year-old silvicultural gaps in young western Oregon forests may be due more to the quality and quantity of litterfall inputs resulting from early-seral species colonizing gaps than by changes in temperature and moisture conditions caused by gap creation.     

Tree species composition affects productivity and carbon dynamics of different site types in boreal forests

The objective was to analyse how differences in the initial proportions of tree species and site fertility affect carbon sequestration in living biomass and soil. We used the individual-based simulation model EFIMOD, which is able to simulate spatially explicit competition between trees for light and nutrients. Simulations were carried out for three site types with distinct initial stocks of soil nutrients. For each site, the 100-years undisturbed dynamics of monocultures and mixtures of three tree species (Betula pendula Roth, Pinus sylvestris L. and Picea abies (L.) H. Karst.) was predicted. Changes in the proportions of competing tree species were dependent on the fertility of the site: on poor sites, pine was the most competent species, while on rich sites, spruce increased its proportion during stand succession. Net primary production (NPP) and soil respiration were the highest in stands of two coniferous species and in stands with a high initial proportion of pine. Mixed stands were more productive than monocultures; the highest overyielding was observed with mixtures of two coniferous species. Simulated NPP and carbon stocks in all pools increased from poor to rich sites. The highest carbon stocks in standing biomass were observed for mixtures of conifer species and three-species mixtures; the greatest accumulation of forest floor occurred in stands with high proportions of pine.     

Soil solution,foliar concentrations and tree growth response to 3-year of ammonium-nitrate addition in two boreal forests of Québec,Canada

Ammonium nitrate (NH₄NO₃) was applied monthly (from June to October) for 3 years in a balsam fir (Abies balsamea (Linné) Miller) and a black spruce (Picea mariana (Mill.) BSP) boreal forest in Québec (Canada). The design was composed of nine experimental units of 10 m × 10 m for each site. Application rates were 3 and 10 times the atmospheric N deposition measured at each site which was 6 and 3 kg ha⁻¹ year⁻¹ for the fir and the spruce sites, respectively. Soil solution composition (30 and 60 cm), tree growth, and foliar concentrations were analysed. The inorganic N in the soil solution of the control plots of both sites was low, particularly at the spruce site indicating that these forests are actively accumulating the atmospheric deposited N. Nitrogen additions regularly caused sudden and large inorganic N increases in the soil solution at both sites, both treatments and both sampling depths. However, these increases were transitory in nature and no persistent changes in inorganic N were observed. It was estimated that more than 95% of the added N was retained above the rooting zone at both sites. Nitrogen addition increased N, Ca, Mg and Mn foliar concentrations at the black spruce site but had no effects at the balsam fir site. After 3 years of N application, tree growth was similar in the control and the treated plots at both sites. Our results show that slow growing black spruce boreal forests with low ambient N deposition are responsive (in term of foliar N, Ca, Mg and Mn concentrations) to even small increases in N inputs, compared to higher growth balsam fir boreal forests with higher N deposition.