Soil data and soil mapping are indispensable tools in sustainable forest management. In northern boreal ecosystems, paludification is defined as the accumulation of partially decomposed organic matter over saturated mineral soils, a process that reduces tree regeneration and forest growth. Given this negative effect on forest productivity, spatial prediction of paludification in black spruce stands is important in forest management. This paper provides a description of the soil database to predict organic layer thickness (OLT) as a proxy of paludification in northeastern Canada. The database contains 13,944 OLT measurements (in cm) and their respective GPS coordinates. We collected OLT measurements from georeferenced ground plots and transects from several previous projects. Despite the variety of sources, the sampling design for each dataset was similar, consisting of manual measurements of OLT with a hand probe. OLT measurements were variable across the study area, with a mean ± standard deviation of 21 ± 24?cm (ranging from a minimum of 0?cm to a maximum of 150?cm), and the distribution tended toward positive skewing, with a large number of low OLT values and fewer high OLT values. The dataset has been used to perform OLT mapping at 30-m resolution and predict the risk of paludification in northeastern Canada (Mansuy et al., 2018) [1]. The spatially explicit and continuous database is also available to support national and international efforts in digital soil mapping.

In northern boreal ecosystems, paludification is defined as the accumulation of partially decomposed organic matter over saturated mineral soils, a process that reduces tree regeneration and forest growth. Given this negative effect on forest productivity, spatial prediction of paludification in black spruce stands is important in forest management. Here, we used the Random Forest approach to predict organic layer thickness (OLT) as a proxy of paludification in northeastern Canada, where forests tend to paludify naturally. The RF approach involved regression and classification models using a suite of 20 environmental predictors derived from multiple sources. The performance of each model was evaluated using cross-validation and an independent dataset based on conventional ecological survey maps from a provincial forest inventory. Importance measures of the predictors indicated that slope, topographic position index, spectral bands 4 and 5 from Landsat, latitude, and PALSAR_HH were the most important variables explaining the spatial distribution of OLT for both models. Cross-validated relative root mean square error (± standard deviation) for the regression model was estimated at 20.66%?±?0.576, with R2 of 0.41?±?0.020, whereas the average out-of-bag error for the classification model was estimated at 44.75%. However, both models performed better in predicting high risk of paludification (OLT values >40?cm). With predicted OLT values averaging 44.07?±?16.80?cm (range 4.25–104.58?cm), the spatial patterns were in accordance with the results of previous studies at the national and landscape scale. These results highlight that ecological types such as black spruce–sphagnum on thin-to-thick organic deposit, with ombrotrophic drainage, are particularly prone to paludification (OLT depth?>?40?cm) throughout the study area. Limitations of the models and applications for decision-making in forest management are discussed.

Conserving species-at-risk requires quantifiable knowledge of the key drivers of population change. Non-linear demographic responses to habitat loss have been documented for many species and may serve to establish quantitative habitat thresholds for management purposes. In Canada, boreal populations of woodland caribou are considered threatened; Environment Canada’s empirical model of calf recruitment–range disturbance suggests that at least 65% undisturbed habitat is required to ensure viability. We tested the relationship upon which this conservation guideline is based by pairing demographic estimates with range conditions over a 10-year period for three boreal caribou populations. Our objectives were (1) to evaluate evidence of intra-population demographic responses to fluctuations in range quality over time; (2) to evaluate inter-population differences in demographic responses to cumulative range disturbances; and (3) to evaluate the sensitivity of disturbance tolerance thresholds to variation in local population demography. We found strong evidence in support of the disturbance–recruitment relationship for within-population responses over time (R2 = 0.77). Mixed effects logistic regression modeling revealed variations in local population responses to cumulative habitat depletion. Range-specific disturbance thresholds derived from Monte Carlo simulations were highly elastic in response to observed variation in local population demography, suggesting that 65% undisturbed habitat is insufficient when adult female survival and/or sex ratio is suboptimal. Study populations were determined to be not self-sustaining (Pr(? ? 0.99) = 37–47%). Adult survival was comparable to estimates reported elsewhere despite Aboriginal harvesting for subsistence purposes. Results underscored potential trade-offs between forest harvesting and wildlife habitat conservation. Protection and restoration of sufficient quantities of undisturbed habitat, particularly via road reclamation, is essential for caribou population recovery.

In the absence of fire in black spruce-feathermoss stands, a thick forest floor layer dominated by bryophytes and sphagnum accumulates. This layer is associated with wet, cool and nutrient-poor soil conditions conducive to the paludification process and pushing the ecosystem towards an unproductive open black spruce forest. The presence of Populus tremuloides in theses stands may halt this process because this species has a high nutrient cycling rate and a litter that represses moss cover. The main hypothesis of this study is that, despite similar abiotic conditions (slope and drainage), the presence of Populus tremuloides in a stand dominated by Picea mariana affects surface soil nutrient availability, total N, pH as well as the decomposition process. The abundance of Populus tremuloides trees was associated with higher exchangeable cations, cationic exchangeable capacity and pH of the forest floor layer on all sites. A decrease in organic matter thickness with increasing aspen presence was also found on all sites, suggesting that this species affects the decomposition process by the quality of its litter as well as by a general improvement of soil physical and chemical properties. The decomposition rate of a standard substrate as well as in vitro potential net nitrogen mineralization were positively related to Populus tremuloides on only one of the three sites, and non-significant on the other sites. Strong immobilization of added nitrogen during incubation was observed on all sites and was not related to aspen, which suggested that in these stands, the soil microbial community is uniformly and strongly nitrogen limited. The zone of influence of Populus tremuloides was evaluated in areas around the soil sampling plot ranging from 3 to 7 m. The results revealed that this zone varies with soil properties. The results suggest that the presence of Populus tremuloides accelerate nutrient cycling, which could affect stand productivity to some extent.

La productivité des pessières voisinant le 49e parallèle à l’ouest du Québec est généralement inférieure à un mètre cube par hectare par année alors que la productivité des peuplements mixtes dans la même région oscille entre deux et trois mètres cube par hectare par année. Plusieurs facteurs peuvent expliquer cet écart, en particulier le sol mais aussi l'effet de la végétation sur la fertilité du sol. Pour des conditions de sols semblables, certaines études ont montré le potentiel des conifères à favoriser un couvert de mousses et d'éricacées qui peut réduire la température du sol, occasionner une remontée de la nappe phréatique et produire une litière de faible qualité favorisant ainsi une baisse de la fertilité du sol. D'autre part, la litière feuillue, notamment de peuplier faux-tremble, semble empêcher la croissance de mousses et accélérer le cycle des éléments nutritifs. Dans la forêt boréale du sud-ouest du Canada, la présence de peuplements mixtes d’épinette noire et de peuplier faux-tremble évoluant sur des conditions abiotiques similaires a offert l’opportunité d’étudier l’influence du peuplier faux-tremble sur le cycle des éléments nutritifs et la croissance de l’épinette noire. Un échantillonnage du sol minéral suivi par des analyses en laboratoire a permis de vérifier l’homogénéité des propriétés du sol minéral le long du gradient de surface terrière de peuplier faux-tremble. Bien qu’il y ait absence de corrélation entre la présence de tremble et les propriétés physiques et chimiques du sol minéral, la présence croissante du peuplier faux-tremble dans des peuplements dominés par l’épinette noire est associée avec une augmentation de la disponibilité des éléments nutritifs (Ca, Mg, K), du pH, du taux de décomposition et de la concentration en azote totale dans le sol organique, ce qui suggère que la présence de tremble pourrait augmenter la fertilité du site. De plus, la composition de la strate de sous-bois change en fonction de la présence de peuplier faux-tremble. Notamment, le recouvrement de mousses et de sphaignes diminue le long du gradient croissant de tremble suggérant que la présence de peuplier faux-tremble pourrait ralentir le processus de paludification rencontré dans les peuplements dominés par l’épinette noire sur la ceinture d’argile de l’Abitibi-Témiscamingue.

Un inventaire de la végétation arborescente ainsi que des analyses de tige révèlent qu’il y a une relation significative entre la surface terrière en peuplier et le volume marchand total du peuplement. Cependant, la relation entre la surface terrière en peuplier faux-tremble et le volume marchand en épinette noire n’est pas significative, suggérant un gain net en fibre de peuplier. L’effet positif du peuplier faux-tremble sur le dhp et la hauteur de l’épinette noire est présent seulement lorsque le peuplier faux-tremble couvre de 0 à 40 % de la surface terrière du peuplement. Ces résultats suggèrent que les deux espèces utilisent une niche écologique relativement différente et que la proportion de tremble influence l’intensité de la compétition interspécifique. De plus, l’augmentation significative de la hauteur de l’épinette noire le long du gradient de peuplier faux-tremble suggère que la présence du peuplier augmente la fertilité du sol par son influence sur la disponibilité des nutriments. L’aménagement de peuplements mixtes, qui sont abondamment représentés dans le paysage forestier, pourrait offrir une situation où l’utilisation économique la forêt pourrait être en accord avec l’aménagement écosystémique. © 2005 UQAT tous droits réservés.

In the western boreal forest of Quebec, black spruce stand productivity is approximately 1 m3/ha/year. The low productivity of these stands is often attributed to the paludification process, which is sustained by low quality black spruce litter and the influence of black spruce on soil moisture. In contrast, aspen hasten nutrient cycling, suggesting that the presence of aspen in black spruce-dominated stands could offset the effect of black spruce on soil processes and positively affect stand productivity. We hypothesised that aspen in black spruce-dominated stands could: (1) increase black spruce DBH, height and volume per stem, (2) increase black spruce productivity without affecting black spruce volume in the stand, and (3) increase total stand volume. In 2001, twelve 14 m diameter plots were sampled for DBH and height of every stem on three black spruce-dominated sites containing various proportions of aspen. Using stem analysis, the time to grow from a height of 5 m to a height of 10 m was determined on three dominant black spruces in each plot. Statistical analyses revealed that DBH, height and volume per black spruce stem were not affected by aspen. However, total black spruce volume decreased with increasing aspen basal area in sites 2 and 3, suggesting that the presence of aspen reduced black spruce density. In site 1, black spruce volume was not affected by aspen, indicating, for total stand productivity, a net gain in aspen fibre. Along a gradient of increasing aspen basal area, the time to grow 5 m decreased in sites 1 and 3. These results suggest that the presence of aspen influences black spruce productivity, although this influence is site-specific and could be dependent on the proportion of aspen, its hierarchical position in the canopy, and the nutrient status of the site. To some extent, this could explain the absence of a general trend concerning mixed stand productivity. © 2005 Elsevier B.V. All rights reserved.

Over time boreal black spruce forests on fine-textured soils in western Quebec, Canada develop very thick forest floors composed of poorly decomposed litter created by the tree and understory layers. These paludified soils are typically waterlogged and cold, and in this fire-mediated landscape, are at least partially consumed by stand replacing fires, which facilitates the establishment of the next generation of trees. Within a context of ecosystem-based management, forest harvest should mimic the dual effects of high severity fire on tree and forest floor biomass. This study was designed to investigate potential factors of forest floor thickness in order to determine the impact of removing only a tree layer, and to suggest strategies to limit paludification in this important forestry region. Forest floor thickness, fire severity, basal area, canopy closure, cover of Sphagnum spp. and ericaceous spp. were measured in black spruce stands across a chronosequence from 50 to 350 years after fire. Fire severity was determined to be a key factor in determining forest floor thickness by path analysis. After high severity fires forest floor thickness was primarily dependant on stand age, but was also positively influenced by Sphagntan spp. cover and negatively influenced by the presence of trembling aspen (Populus tremuloides). These results suggest that forest interventions that do no remove the organic layer may be mimicking low severity fires and promoting poor tree growth and regeneration. Forest floor thickness may be limited by avoiding interventions that open the canopy and may promote the presence of Sphagnum spp. and ericaceous spp., and or by practicing mixed silviculture of trembling aspen and black spruce. However, a balance needs to be maintained between the application of these techniques and the preservation of paludified forests in the landscape. (c) 2005 Elsevier B.V. All rights reserved. © 2005 Elsevier B.V. All rights reserved.

In the southeastern boreal forest of Canada, the presence of mixed stands of black spruce (Picea mariana (Mill.) BSP) and trembling aspen (Populus tremuloides Michx.) growing in similar abiotic conditions offers the opportunity to study the influence of aspen on stand volume and spruce growth. A regression analysis performed on field data from the ministère des Ressources naturelles du Québec showed a significant relationship between the relative basal area of aspen (aspen relative basal area was determined by the ratio of aspen basal area to total basal area of the stand) and the total stand merchantable volume after accounting for stand density. However, the relationship between total black spruce volume and relative basal area of aspen was not significant, implying that the volume gain was, in fact, aspen fibre. The positive effects of aspen on black spruce DBH and height were only present when the proportion of aspen in the stand ranged between 0% and 41% of the total stand basal area. These results suggest that aspen uses a different niche than black spruce. Furthermore, the significant increase in black spruce dominant height along the aspen gradient suggests that aspen enhances soil fertility by its influence on nutrient availability. The management of mixed stands, which make up an important proportion on the landscape, offers an example as to how commercial management of the forest can be in agreement with ecosystem management.

Dans la forêt boréale du Sud-Est du Canada, la présence de peuplements mixtes d'épinette noire (Picea mariana (Mill.) BSP) et de peuplier faux-tremble (Populus tremuloides Michx.) croissant dans des conditions abiotiques similaires a offert l'opportunité d'étudier l'influence du peuplier faux-tremble sur le volume du peuplement et la croissance de l'épinette noire. Une analyse de régression effectuée sur des données d'inventaire du ministère des Ressources naturelles du Québec révèle qu'il y a une relation significative entre la surface terrière relative en peuplier et le volume marchand total du peuplement. Cependant, la relation entre la surface terrière relative en peuplier faux-tremble et le volume marchand en épinette noire n'est pas significative, suggérant un gain net en fibre de peuplier. L'effet positif du peuplier faux-tremble sur le DHP et la hauteur de l'épinette noire est présent seulement lorsque le peuplier faux-tremble couvre de 0 à 41 % de la surface terrière du peuplement. Ces résultats suggèrent que les deux espèces utilisent une niche écologique différente. De plus, l'augmentation significative de la hauteur de l'épinette noire le long du gradient de peuplier faux-tremble suggère que la présence du peuplier augmente la fertilité du sol par son influence sur la disponibilité des nutriments. L'aménagement de peuplements mixtes, qui sont abondamment représentés dans le paysage forestier, pourrait offrir une situation où la mission commerciale de la forêt pourrait être en accord avec l'aménagement écosystémique.©2004 NRC Canada

Ectomycorrhizal (ECM) diversity was measured in 12 mixed-wood stands in the Abitibi region of north-western Quebec. Stands were of similar age and were situated on similar mineral soil deposits, but supported varying proportions of ECM host trees. Host roots were sampled in a manner that enabled their separation into species on the basis of wood anatomy. Shannon diversity indices for the ECM colonizing each host species were determined on the basis of ECM anatomy. The diversity of overstory trees, understory plants and host roots, as well as overstory tree composition, root density and pertinent abiotic factors were measured and used as independent variables in multiple regressions against ECM diversity. We found a positive relationship between overstory tree diversity and ECM diversity, which appears related to fungal host specificity. Although no direct relationship was seen between ECM diversity and soil factors, levels of exchangeable base cations were related to ECM fungal species composition which correlated with ECM diversity at the scale sampled.

Forest overstory composition influences both light and nutrient availability in the mixed boreal forest. The influence of stand composition on understory cover and biomass was investigated on two soil types (clay and till deposits). Four forest composition types were considered in this study: aspen (Populus tremuloides Michx.), paper birch (Betula papyrifera Marsh.), jack pine (Pinus banksiana Lamb.) and a mixture of balsam-fir (Abies balsamea (L.) Mill.) and white spruce (Picea glauca (Moench) Voss). The cover of all understory species was recorded while the biomass of two important and ubiquitous species was measured: mountain maple (Acer spicatum Lam.) of the shrub layer and large-leaved aster (Aster macrophyllus L.) of the herb layer. Soil analyses were conducted to evaluate the influence of overstory composition on understory biomass through its influences on soil characteristics. Analyses of variance showed a significant effect of forest canopy type on mountain maple biomass, understory cover and shrub cover but not on herb cover and large-leaved aster biomass. Path analysis was performed to explore the relationships between canopy type, nutrient availability and understory biomass. Contrary to what was expected, the variation in plant biomass associated with forest composition was weakly related to soil nutrient availability and more strongly related to stand structural attributes.

Variation in canopy composition can influence ecosystem processes, such as nutrient cycling and light transmittance, even when environmental soil conditions are similar. To determine whether forest cover type influences species composition of the understory vegetation (herbs and shrubs), the composition of this layer was studied on two different surface deposits, clay and till, and under four different forest cover types dominated, respectively, by Populus tremuloides Michx. (aspen), Betula papyrifera Marsh. (white birch), Pinus banksiana Lamb. (jack pine), and Picea glauca (Moench) Voss - Abies balsamea (L.) Mill. (spruce-fir) over similar environmental conditions. Detrended correspondence analysis and analysis of variance performed on the ordination scores revealed that understory plant composition was highly affected by surface deposit and forest cover. The gradient observed in the correspondence analysis proceeds from aspen, white birch, spruce-fir, to jack pine. Indicator species were identified for each surface deposit and cover type, and most of them were associated with either jack pine or aspen. The richness, evenness, and diversity of the understory vegetation did not vary between cover types, but were affected by surface deposit. By controlling ecosystem processes such as light transmittance and nutrient cycling, forest cover influences understory composition.