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.     

Impacts of hemlock looper defoliation on growth and survival of balsam fir, black spruce and white birch in Newfoundland, Canada

Hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) is an economically damaging defoliator that undergoes periodic outbreaks in Newfoundland, Canada. It defoliates and causes extensive tree mortality to its primary host, balsam fir (Abies balsamea [L.] Mill.). We quantified tree survival using data from permanent sample plots (PSPs) and growth reduction or release using dendrochronology, and related these impacts to defoliation severity determined from annual aerial defoliation survey data. Such impact relationships are necessary as a fundamental input to a Decision Support System. Growth and survival of balsam fir, black spruce (Picea mariana (Mill.) B.S.P.) and white birch (Betula papyrifera Marsh.) were assessed from 1996 to 2008 in 48 Newfoundland Forest Service PSPs, selected based on four classes of defoliation severity. Two years of severe (71-100%) defoliation resulted in almost complete mortality for balsam fir, 10 years after defoliation, whereas survival was 70-80% for black spruce and white birch. Lower defoliation severity (1-2 years of moderate (31-70%) or 1 year of severe) resulted in approximately 60% survival for balsam fir and no reduction in survival for black spruce and white birch. Maximum growth reduction of balsam fir was 10-15% with 1 year of moderate-severe defoliation, 35-40% with 2 years of moderate defoliation, and about 50% with 2 years of severe defoliation. Growth recovered to pre-defoliation rates 5 years after defoliation ceased in all severity classes. Growth reduction and recovery of black spruce were more variable and lower than for balsam fir, and white birch exhibited only minor (<10%) growth reduction during the defoliation year or 1 year after defoliation. Control measures should focus on avoiding severe defoliation for two consecutive years.     

The impact of hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) on balsam fir and spruce in New Brunswick, Canada

In 1989, the first recorded outbreak of hemlock looper (Lambdina fiscellaria fiscellaria (Guen.)) occurred in New Brunswick, Canada. Data were collected from ten plots established in an area infested from 1992–1994, to assess impacts of hemlock looper. Ocular and branch sample assessments of current defoliation and ocular assessments of total defoliation (all age classes of foliage) were conducted for balsam fir (Abies balsamea [L.] Mill.), white spruce (Picea glauca [Moench] Voss), and black spruce (Picea mariana (Mill.) B.S.P.). Stand response was assessed and related to cumulative defoliation. Ocular assessments were found to accurately estimate defoliation, which was significantly related to tree mortality. Ninety-two percent of balsam fir trees that had cumulative defoliation >90% died. Mortality of balsam fir was significantly (p < 0.05) related to tree size, in both lightly and severely defoliated plots; trees with DBH <11 cm sustained 22–48% higher mortality than larger trees. Mortality of balsam fir, in terms of both percent stems/ha and m³/ha merchantable volume, increased exponentially in relation to three estimates of cumulative (summed) plot mean defoliation. The strongest relationships (r² = 0.75–0.79) were between mortality and the ocular defoliation assessment for 1990–1993 foliage. Tree mortality caused by the looper outbreak ranged from 4–14% stems/ha in lightly defoliated and from 32–100% in severely defoliated plots; merchantable volume killed was 3–14 m³/ha and 51–119 m³/ha, respectively. Relationships between mortality and defoliation were similar when defoliation was assessed for 1987–1993 and 1990–1993 foliage age classes.     

Can the impact of deer browsing on tree regeneration be mitigated by shelterwood cutting and strip clearcutting?

Silvicultural treatments creating large canopy openings failed to restore regeneration of balsam fir (Abies balsamea (L.) Mill.) due to browsing pressure from white-tailed deer (Odocoileus virginianus Zimmermann). Consequently, we tested two alternative silvicultural treatments aimed at improving balsam fir establishment on Anticosti Island (Québec, Canada). In 1998 and 1999, we set up shelterwood seed cutting using three harvest intensities (0, 25 and 40% of basal area) and strip clearcutting with scarification using three different strip widths (15, 30 and 45 m), both with fenced and unfenced regeneration plots, in balsam fir stands. After 8 years, shelterwood seed cutting did not allow the establishment of new balsam fir seedlings, nor the development of unbrowsed balsam fir seedlings. In the strip clearcutting, deer browsing suppressed growth of palatable species in all strip widths. This favoured the development of unpalatable species, especially white spruce (Picea glauca (Moench) Voss). Our study demonstrates that the use of silvicultural treatments alone is unlikely to restore balsam fir regeneration on Anticosti Island, as long as the deer population remains higher than 20 deer/km².     

Availability of standing trees for large cavity-nesting birds in the eastern boreal forest of Québec,Canada

Large cavity-nesting birds depend on large-diameter trees for suitable nest sites. The increased spatial extent of commercial timber harvesting is modifying forest structure across the land base and may thus compromise the availability of large trees at the landscape scale. In this study, our objectives were to (1) characterize the availability of large living and dead trees in old-growth stands dominated by different tree species and surficial deposits that encompass the range of natural cover types of eastern Québec's boreal forest; (2) analyze the distribution of trees among decay-classes; and (3) compare the availability of large trees in unharvested, remnant, and harvested stands for the entire range of decay-classes. A total of 116 line transects were distributed across unharvested forests, remnant linear forests, and cutblocks in cutover areas. Unharvested forest stands (black spruce [Picea mariana], balsam fir [Abies balsamea]–black spruce, balsam fir–white spruce [Picea glauca] and balsam fir) reflected a gradient of balsam fir dominance. The remnant forests selected were isolated for 5–15 years. Analyses were performed at two diameter cut-off values. Trees with DBH ≥20 cm were considered for availability of total trees whereas trees with DBH ≥30 cm were considered for availability of large trees. Forest stands comprised high proportions of standing dead trees (33% of all stems, 8% were large dead stems). Availability of total and large standing trees increased with the dominance of balsam fir in stands. Forest stands located on thick surficial deposits showed higher densities of large dead trees for every stand type suggesting a higher productivity on those sites. Availability of stems according to decay-classes showed a dome-shaped distribution with higher densities of snags in intermediate decay stages. However, for large stems, black spruce stands showed a significantly lower availability that was consistent across all decay-classes. In linear remnant forests, pure balsam fir stands were absent. Remnant stands thus showed a much lower availability in large trees when compared with unharvested balsam fir stands. Clearcuts had the lowest densities of dead trees across sampled stands. Current even-aged management practices clearly affect availability and recruitment of large trees, therefore forest-dwelling wildlife relying on these structures for breeding is likely to be affected by large-scale harvesting in coniferous boreal forests.     

Does tree seedling growth and survival require weeding of Himalayan balsam (Impatiens glandulifera)?

The effect of Himalayan balsam (Impatiens glandulifera) on survival and growth of naturally regenerated silver birch (Betula pendula) and planted Norway spruce (Picea abies) and silver fir (Abies alba) seedlings was studied in a weeding experiment over 3 years. Three different treatments were applied: control, mowing, and hand weeding by pulling out the entire plant. There were no consistent treatment effects on height and diameter of the tree seedlings. The coverage of Rubus fruticosus had a negative impact on diameter increment of Norway spruce and silver fir. As opposed to growth, treatment effects on seedling survival could be found for planted Norway spruce and silver fir. However, it is very likely that these effects, namely higher seedling survival after mowing, have to be attributed to the control of bramble (Rubus fruticosus) rather than to that of Himalayan balsam. It is concluded that Himalayan balsam is not able to seriously affect the growth of already established seedlings.     

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.     

Impact and recovery of western hemlock following disturbances by forestry and insect defoliation

Impact and recovery of juvenile western hemlock stands were measured 5 years after the end of an outbreak of western blackheaded budworm Acleris gloverana (Walsingham) (Lepidoptera: Tortricidae) and hemlock sawfly Neodiprion tsugae Middleton (Hymenoptera: Diprionidae) on Haida Gwaii, British Columbia, Canada. Overall, whole-tree mortality was infrequent (3.6–6.8%) but aggregated in a few, severely defoliated stands. Top-kill of the apical leader of surviving trees was more frequent (7.2–22.1%) and many of these trees had not recovered a growing dominant leader 5 years after the original damage. These severe impacts were most common in the largest trees in the youngest stands which had been spaced just before the outbreak. Radial growth was also reduced for 2 years following defoliation. However, mean annual ring width and basal area increment in surviving trees recovered quickly and was even enhanced in post-defoliation stands, particularly in spaced stands that had been moderately defoliated. Management recommendations include planning for likely outbreaks by avoiding creation of expanses of young regenerating stands of western hemlock during harvest and modifying spacing treatments when an insect outbreak is imminent to reflect the greater risk to trees <20 years of age.     

White-tailed deer activity reconstructed from tree-rings in eastern boreal Canada

We evaluated the potential of tree-ring techniques for the reconstruction of recent and past seasonal activity of introduced white-tailed deer in a boreal environment of eastern Canada. Hoof scrape scars on balsam fir stems and trampling scars on roots were used to reconstruct deer activity during the winter and snow-free seasons, respectively. Tree damage showed that there was continuous deer activity in the north-central part of Anticosti Island since the mid-1960s. High scrape scars along tree stems (3–3.5 m from the ground) indicate that 1975, 1976, 1981, 1983 and 1985 were years of intensive food search by deer on high balsam fir foliage. The annual number of hoof scrape scars was low between 1982 and 1985, when severe defoliation by the spruce budworm, combined with deer browsing, led to high fir sapling mortality, food depletion, degradation of the winter shelter forests and a decrease in deer activity. The lowest scrape scars 50 cm above ground correspond to the mean height of the residual snowpack in the shelter forest at springtime, when deer start searching for food in nearby open sites and use logging roads, where dead and bonsai-like fir predominate due to overbrowsing. The trampling scar age frequency distribution from two sites indicated that deer activity during the snow-free season started synchronously in the late 1960s. In response to degradation of winter shelter forests, deer may have moved from the southern part to the north-central part of the island and other sectors to survive. Deer-induced tree damage and tree-ring techniques can thus be used to reconstruct past seasonal activity of white-tailed deer.     

Broadleaf competition interferes with balsam fir regeneration following experimental removal of moose

Changes to ecosystems caused by introduced herbivores can be predictable, stepwise transitions or unpredictable and even irreversible state changes. This study's objectives were to explore effects on forest succession and soil development 5 years after moose (Alces alces L.) were fenced out of areas within and adjacent to a national park in Newfoundland, Canada. Study plots spanned a range of understorey broadleaf plant associations with regenerating balsam fir (Abies balsamea (L.) Mill.), an important winter forage plant for moose and a dominant canopy tree throughout Newfoundland. After 5 years, height–diameter ratios were significantly larger for larger basal diameters of understorey balsam fir in unfenced, but not in fenced subplots, suggesting that growth of the conifer is compromised within the exclosure. In contrast, for most broadleaf trees and shrubs, moose removal by fencing results in greater heights and basal diameters than in control subplots. The competitive advantage of broadleaf trees and shrubs over balsam fir in the short-term may be a result of past sustained heavy moose browsing benefiting plants that are better at investing resources into below-ground growth or benefiting plants that have broader leaf canopies. It is not clear how long the broadleaf transition state we document will continue. Restorative actions intended to mimic usual patterns of forest regeneration in this region of Newfoundland might best consider moose removal with site preparation and/or planting to historic densities.     

Stand structure interacts with previous defoliation to influence herbivore fitness

The intensity and duration of Neodiprion abietis outbreaks have recently increased in forests of North America that were precommercially thinned more than a decade earlier. We tested the hypotheses that changes in stand structure following thinning increase the fitness (i.e., survival rate × fecundity) of N. abietis by either (a) increasing foliar availability and/or quality (i.e., increased availability of primary metabolites and/or reduced foliar defenses) or (b) by reducing any negative effects on foliar quality and/or availability resulting from herbivory that occurred during the preceding season(s). Effects of thinning and previous herbivory on N. abietis and its host plant (Abies balsamea) were determined through (i) a manipulative field experiment that evaluated the effects of experimental defoliation on N. abietis in a thinned stand, (ii) a manipulative field experiment that examined the effects of thinning on N. abietis in undamaged and naturally defoliated stands, and (iii) a field survey to estimate survival of N. abietis in natural populations.Defoliation caused reductions in the availability of different-aged foliage available to larvae and in the fitness of a subsequent N. abietis generation feeding on defoliated branches, but decreases in fitness were smaller in thinned than unthinned stands. In thinned stands, defoliation was associated with increases in foliage production and foliar contents of monoterpenes and nitrogen, as well as with a decrease in foliar contents of water. Conversely, only small changes in plant growth and foliar contents of nutrients and secondary chemicals were observed in defoliated unthinned stands. This suggests that deleterious effects of defoliation on sawfly fitness were offset by an increase in the foliar content of nitrogen, a primary compound known to improve larval growth in sawflies, which supports the hypothesis that thinning moderates negative effects of previous defoliation on sawfly fitness. The present study demonstrates that forestry practices that alter stand structure by reducing tree density may increase herbivory by affecting the way trees respond to insect attack, even after crown closure, with consequences on the buildup of herbivore populations in attacked trees.     

Ecophysiology and growth of advance red spruce and balsam fir regeneration after partial cutting in yellow birch-conifer stands

We investigated ecophysiological and growth responses of short (0.4 to 1.3 m in height) advance regeneration of red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea L.) six years after removal of 0, 40, 50, 60 and 100% of the overstory basal area (BA) in two yellow birch-conifer stands. Partial cuts significantly increased stomatal conductance of red spruce only. Light-saturated photosynthesis (leaf-area basis) of both species increased with BA removal, but unlike red spruce, specific leaf area (SLA) of balsam fir decreased with increased cutting intensity. Partial cuts appreciably increased the concentration of N and Ca in red spruce and balsam fir foliage, respectively, and resulted in decreased foliar concentrations of K in red spruce and Mg in balsam fir. The height and lateral growth of both species increased with BA removal, although partial cuts were more beneficial to balsam fir. The data suggest that short advance regeneration of red spruce and balsam fir can coexist under partial overstory conditions, but balsam fir has physiological characteristics and a capacity for morphological adjustment (SLA) that places it at an advantage when in competition with red spruce.     

Forest fragmentation and duration of forest tent caterpillar (Malacosoma disstria Hübner) outbreaks in northern hardwood forests

In northern forests dominated by aspen (Populus spp.), the duration of outbreaks of forest tent caterpillar (Malacosoma disstria Hübner) has been reported to increase with forest fragmentation. This relationship has not been tested in other forest types affected by this widespread native defoliator. From 2002 to 2007, a large-scale outbreak of this insect in the northeastern United States defoliated millions of hectares, with sugar maple (Acer saccharum Marsh.) the primary host. We used digital defoliation maps generated from aerial surveys and national land cover data to assess the effect of fragmentation on outbreak duration in areas of NY, MA, VT, and NH. We found that outbreak duration increased with forest cover and decreased with the forest edge, in opposition to the pattern previously reported for aspen-dominated forests in Canada. This pattern was significant from plot sizes ranging from 100 m to 1000 m in radius. The relationship between FTC and its natural enemies, which was postulated to underlie the effect of fragmentation on outbreaks in aspen forests, may be affected differently in northern hardwood forests, or other factors may be more important in determining outbreak duration in this forest type.     

Interactions of insects, trees and air pollutants

Hypotheses of forest decline are summarized. Stressed trees may be a more suitable food source for invertebrate herbivores than unstressed trees because stress causes an increase in the tissue content of soluble nitrogenous compounds. There is reasonable correlative evidence that air pollutants influence the outbreak patterns of forest insect species. In some cases, formerly innocuous insects may become pests. The occurrence of specific insect pests with specific air contaminants is reviewed. Emphasis is placed on the decline of Fraser fir (Abies fraseri (Pursh) Poir.) in the southern Appalachians and the occurrence of the balsam woolly adelgid. Variable mortality patterns of Fraser fir may be caused by locally distributed air pollutants acting in combination with other types of stress including that caused by the adelgid.     

Long-term realised and projected growth impacts caused by autumn gum moth defoliation of 2-year-old Eucalyptus nitens plantation trees in Tasmania,Australia

Insect damage to production forests has the potential to reduce financial returns by retarding tree growth and causing mortality, however, long-term realised quantification of these losses is rare. In order to help elucidate economic damage thresholds for making spray decisions we capitalised on a natural outbreak of autumn gum moth, Mnesampela privata, in a 2-year-old Eucalyptus nitens plantation. Following the partial chemical control of this insect outbreak we measured the tree growth variables diameter at breast height over bark and height of five differing tree defoliation classes for 75 months following tree damage. At the end of this period a threshold model was fitted to describe the relationship between tree defoliation and realised tree wood volumes. The model revealed that realised stand wood volume was not significantly affected up until defoliation exceeded 60% and then declined sharply after this defoliation level was reached. Further support for this defoliation threshold was evident from multiple comparisons among defoliation classes that showed 50% defoliated trees did not have significantly different wood volume compared to more lightly defoliated trees, but did have significantly greater wood volume compared to trees that were 72% or more defoliated. To determine if the realised differences in wood volume resulted in differences in yield over a plantation rotation the E. nitens growth model NITGRO was used to on-grow trees to age 15 years for a ‘best case’ (type 1 growth response, constant growth rates from last inventory until harvest) and ‘worst case’ (type 2 growth response, divergent growth rates from last inventory until harvest) scenario. The threshold model was then fitted to the outcomes of both scenarios and the economic consequences of defoliation were clearly dependent on the growth function assumed.     

Seasonal changes in soil and tissue nutrition in balsam fir and influence on postharvest needle abscission

Postharvest needle abscission in balsam fir varies based on time of harvest, but little is known about the effect of soil and tree nutrition on postharvest needle abscission. From April to November, 18 balsam fir samples were obtained each month and evaluated for postharvest needle abscission characteristics. Soil samples were collected simultaneously and evaluated for moisture, pH, organic matter, cation exchange capacity, and mineral nutrition. Needle tissue samples were also evaluated for mineral nutrition. Many parameters were found to vary based on month throughout the study, but not all were associated with needle abscission. Soil organic matter, cation exchange capacity, Ca, and Mn all had a significant (P?P?R²?>?.50), suggesting that higher concentrations of tissue Ca were linked to superior needle retention in balsam fir. Overall, this study provides evidence that changes to soil management, such as the inclusion of organic matter and certain minerals, may improve needle retention in balsam fir.     

Forest regeneration 50 years following partial cutting in mixedwood ecosystems of southern Quebec,Canada

The impact of winter harvesting on regeneration 50 years after an experimental diameter-limit cutting was examined in mixed deciduous–coniferous ecosystems of southern Quebec, Canada. The study was conducted in La Mauricie National Park, Quebec, Canada. Regeneration data in two balsam fir (Abies balsamea (L.) Mill.), red spruce (Picea rubens Sarg.), sugar maple (Acer saccharum Marsh.), and yellow birch (Betula alleghaniensis Britt.) ecosystem types were analyzed. Comparisons between uncut and cut stands were obtained from a total of 63 sample plots. For both ecosystems, there were no significant differences between uncut and cut plots for regeneration density and stocking. The most abundant regeneration species were balsam fir, red spruce, sugar maple, red maple (Acer rubrum L.), yellow birch and American beech (Fagus grandifolia Ehrh.). The type of diameter-limit cutting described in the study did not affect regeneration density and stocking but its impact on productivity, timber quality and genetics is still unknown.     

Comparison of model forms for estimating stem taper and volume in the primary conifer species of the North American Acadian Region

? The performance of ten commonly used taper equations for predicting both stem form and volume in balsam fir [Abies balsamea (L.) Mill], red spruce[Picea rubens (Sarg.)], and white pine[Pinus strobus (L.)] in the Acadian Region of North America was investigated.

? Results show that the Kozak (2004) and Bi (2000) equations were superior to the other equations in predicting diameter inside bark for red spruce and white pine, while the Valentine and Gregoire (2001) equation performed slightly better for balsam fir.

? For stem volume, the Clark et al. (1991) equation provided the best predictions across all species when upper stem diameter measurements were available, while the Kozak (2004) and compatible taper equation of Fang et al. (2000) performed well when those measurements were unavailable.

? The incorporation of crown variables substantially improved stem volume predictions (mean absolute bias reduction of 7–15%; root mean square error reduction of 10–15%) for all three species, but had little impact on stem form predictions.

? The best taper equation reduced the predicted root mean square error by 16, 39, and 45% compared to estimates from the widely used Honer (1965) regional stem volume equations for balsam fir, red spruce, and white pine, respectively.

? When multiple taper equations exist for a certain species, the use of the geometric mean of all predictions is an attractive alternative to selecting the “best” equation.

     

Development and activity of early saproxylic fungal communities in harvested and unmanaged boreal mixedwood stands

Limited scientific information is currently available regarding saproxylic fungal communities in the boreal forest of North America. We aimed to characterize the community development, richness and activity of saproxylic fungi on fresh wood in harvested and unmanaged boreal mixedwood stands of northwestern Québec (Canada). Fresh wood blocks (n = 480) of balsam fir (Abies balsamea (L.) Mill.) and trembling aspen (Populus tremuloides Michx.) were placed on the forest floor in a range of stand conditions (n = 24). Blocks were harvested every 6 months for up to 30 months and characterized for species composition and richness (PCR–DGGE, DNA sequencing), respiration, wood density and lignin and cellulose content. Colonization by a wide range of functional groups proceeded rapidly under different stand conditions. We detected a total of 35 different fungal operational taxonomic units, with the highest species richness at the wood block level being observed within the first 12 months. No differences in community composition were found between wood host species or among stand conditions. However, the variability in fungal communities among blocks (β diversity) was lower on trembling aspen wood compared with balsam fir and decreased over time on trembling aspen wood. Also, fungal activity (respiration and wood decomposition) increased on trembling aspen wood blocks and species richness decreased on balsam fir wood over time in partial-cut sites. The overlap in tree composition among stands, the high volume of logs and the recent management history of these stands may have contributed to the similarity of the saproxylic fungal community among stand types and disturbances.     

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.