Research highlights: To understand differences in the establishment of balsam fir regeneration observed in the boreal forest, we examined how soil layer and microorganisms explained differences in growth and mycorrhization in three different stand types. Our experiment revealed positive and negative effects on growth of seedlings, and highlights the importance of biotic interactions in balsam fir establishment. Background and Objectives: In a context of climate change, understanding tree migration can be examined through changes in tree regeneration. At the ecotone between mixed and conifer boreal forest, regeneration of balsam fir northward is of particular interest because it thrives better under aspen-dominated stands as compared to adjacent spruce-dominated stands. As the understorey differs between these stands, with more Ericaceae under spruce and different ectomycorrhizal fungal communities in organic and mineral horizons, we hypothesized that biotic factors could explain differences in balsam fir establishment. Materials and Methods: Using a growth chamber experiment, we tested if differences in soil layers and modification of soil fungal communities would affect germination, mycorrhization, and growth of balsam fir seedlings in three different stand vegetation. We compared 12 treatments and followed 120 seedlings over three growth seasons. Results: We found similar survival in soils from aspen- and spruce-dominated stands, and a greater biomass on organic layers. In addition to this, a greater mycorrhization rate was found in aspen soils but improved germination in spruce soils. The presence of Ericaceae in spruce soils was associated with lower mycorrhization but did not affect other traits. Sterilization and therefore microorganisms affected mainly the number of ectomycorrhizae and the investment in root biomass. Finally, mycorrhization and biomass were correlated, but independent from N nutrition measured in needles. Conclusions: Our results highlighted the positive effects of organic soil layers and of mycorrhization on biomass, and showed that mycorrhization was increased under aspen as compared to other stand types. Our experiment also revealed positive effects of spruce soil on fir germination and showed that fir was able to grow and survive in all conditions. Our study suggests that fir establishment is affected by belowground multi-species interactions, and therefore highlights that biotic interactions shall be taken into account to understand and predict future tree migrations in the boreal forest. View Full-Text

Natural root grafts (anastomoses) result from the fusion of the vascular systems of two roots and allow trees to share water, nutrients and photosynthesis products, affecting tree growth and physiology. The aim of this study was to investigate the frequency of root grafting in balsam fir (Abies balsamea (L.) Mill) of the boreal forest of Quebec (Canada), and to identify ecological drivers such as tree proximity or size of stems and roots. One 50 m2 area was hydraulically excavated in each of three natural balsam fir stands of various ages, tree diameters and densities. For each area, we measured the number of roots and grafts per tree, and the diameter and age of all stems, roots and grafts using dendrochronology techniques. Percentages of grafted trees and number of grafts per tree were similar between stands, corresponding to 36% (± 2.86 SE) and 1.30 (± 0.03 SE), respectively. Root grafting occurred at a wide range of tree ages from 12 to 106 years old. Mean distance between grafted trees was 47.91 cm (± 5.10 SE) and we did not observe any graft beyond a 2 m distance between trees. The number of grafts per tree increased with number of roots per tree and decreased with distance between trees. Root grafting also occurred at a wide range of root ages, from 5 to 64 years old. However, roots were relatively small at graft initiation, with an average root diameter of 3.94 cm (± 0.33 SE). These results demonstrate that balsam fir stands are highly connected through root grafting, occurring early in stand development and continuing throughout the life of the stands. The number of roots per tree and distance between trees were the best predictors for root grafting.

Root grafting in trees is a well-recognized phenomenon allowing nutrient and photosynthate redistribution between connected trees. This study assessed the influence of tree spacing on root graft formation in a 15-year-old hybrid poplar plantation with two clones (747215: Populus balsamifera?×?Populus trichocarpa and 915319: Populus maximowiczii?×?Populus balsamifera) planted and two spacings (1?×?1 m, 3?×?3 m) in Amos, Quebec, Canada. Root graft occurrence was characterized and tree growth metrics including tree diameter and height, number of roots, root cross-sectional area and root age were measured after hydraulic excavation of root systems. We showed that root grafts were present between trees distant of 1 m but not between trees planted at the 3?×?3 m spacing. An average of 38% of excavated trees was grafted in the 1?×?1 m plots with a mean of 1.14 grafts per tree, and 86% of the grafted trees were located in the exterior part of plots (the border rows), where the trees had grown larger. Root grafts were relatively young (1–6 years) and most had not yet completed, preventing us from determining the effect of root grafting on tree growth. Root grafts were more prevalent in clone 915319, and in trees that had greater above and belowground growth. These results strengthen the postulate that tree proximity enhances the frequency of root grafting.

Succession is generally well described above-ground in the boreal forest, and several studies have demonstrated the role of interspecific facilitation in tree species establishment. However, the role of mycorrhizal communities for tree establishment and interspecific facilitation, has been little explored. At the ecotone between the mixed boreal forest, dominated by balsam fir and hardwood species, and the boreal forest, dominated by black spruce, several stands of trembling aspen can be found, surrounded by black spruce forest. Regeneration of balsam fir seems to have increased in the recent decades within the boreal forest, and it seems better adapted to grow in trembling aspen stands than in black spruce stands, even when located in similar abiotic conditions. As black spruce stands are also covered by ericaceous shrubs, we investigated if differences in soil fungal communities and ericaceous shrubs abundance could explain the differences observed in balsam fir growth and nutrition. We conducted a study centered on individual saplings to link growth and foliar nutrient concentrations to local vegetation cover, mycorrhization rate, and mycorrhizal communities associated with balsam fir roots. We found that foliar nutrient concentrations and ramification indices (colonization by mycorrhiza per length of root) were greater in trembling aspen stands and were positively correlated to apical and lateral growth of balsam fir saplings. In black spruce stands, the presence of ericaceous shrubs near balsam fir saplings affected ectomycorrhizal communities associated with tree roots which in turn negatively correlated with N foliar concentrations. Our results reveal that fungal communities observed under aspen are drivers of balsam fir early growth and nutrition in boreal forest stands and may facilitate ecotone migration in a context of climate change.

Roots directly affect planted seedling adaptation to new growing conditions at reforestation sites. To test the influence of root characteristics on the short-term response of seedlings to limiting resources (water, nutrient, or oxygen), we conducted two experiments. We compared (1) the growth and physiology of three types of four-year-old black spruce (Picea mariana (Mill.) BSP) seedlings (Containerized, highly developed initial roots restricted to a plug; bareroot, less developed but unrestricted initial roots; deeply-planted containerized, restricted initial and adventitious roots) to different combinations of irrigation and fertilization. We also investigated (2) the cellular plasticity of adventitious and initial roots to three irrigation regimes including flooding. Bareroot seedlings had better relative growth rates in height than containerized seedlings, probably due to their larger initial size. On the other hand, containerized seedlings took better advantage of fertilization, as shown by a higher relative growth rate in diameter compared to bareroot seedlings and were less affected by water limitation, possibly due to the root plug acting as an additional water reserve capacity. For containerized seedlings, the presence of adventitious roots was beneficial to height growth and physiological performances compared to seedlings with initial roots only. Adventitious roots showed great cell plasticity, particularly under flooding conditions.

The mechanistic bases of thermal acclimation of net photosynthetic rate (A n ) are still difficult to discern, and the data sets available are scarce, particularly for hybrid poplar. In the present study, we examined the contribution of a number of biochemical and biophysical traits on thermal acclimation of A n for two hybrid poplar clones. We grew cuttings of Populus maximowiczii × Populus nigra (M×N) and Populus maximowiczii × Populus balsamifera (M×B) clones under two day/night temperature of 23°C/18°C and ?33°C /27°C and under low and high soil nitrogen level. After ten weeks, we measured leaf RuBisCO (RAR) and RuBisCO activase (RARCA) amounts and the temperature response of A n , dark respiration (R d ), stomatal conductance, (g s ), apparent maximum carboxylation rate of CO 2 (V cmax ) and apparent photosynthetic electron transport rate (J). Results showed that a 10°C increase in growth temperature resulted in a shift in thermal optimum (T opt ) of A n of 6.2±1.6 °C and 8.0±1.2 °C for clone M×B and M×N respectively, and an increased A n and g s at the growth temperature for clone M×B but not M×N. RuBisCO amount was increased by N level but was insensitive to growth temperature while RARCA amount and the ratio of its short to long isoform was stimulated by the warm condition for clone M×N and at low N for clone M×B. The activation energy of apparent V cmax and apparent J decreased under the warm condition for clone M×B and remained unchanged for clone M×N. Our study demonstrated the involvement of both RARCA, the activation energy of apparent V cmax and stomatal conductance in thermal acclimation of A n © 2019 Benomar et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Black spruce (Picea mariana [Mill.] BSP) is a boreal tree species characterized by the formation of an adventitious root system. Unlike initial roots from seed germination, adventitious roots gradually appear above the root collar, until they constitute most of mature black spruce root system. Little is known about the physiological role they play and their influence on tree growth relative to initial roots. We hypothesized that adventitious roots present an advantage over initial roots in acquiring water and nutrients. To test this hypothesis, the absorptive capacities of the two root systems were explored in a controlled environment during one growing season. Black spruce seedlings were placed in a double?pot system allowing irrigation (25 and 100% water container capacity) and fertilization (with or without fertilizer) inputs independent to initial and adventitious roots. After 14?weeks, growth parameters (height, diameter, biomass), physiology (net photosynthetic rate, stomatal conductance, shoot water potential) and nutrient content (N, P, K, Ca and Mg foliar content) were compared. Most measured parameters showed no difference for the same treatment on adventitious or initial roots, except for root biomass. Indeed, fertilized black spruce seedlings invested heavily in adventitious root production, twice as much as initial roots. This was also the case when adventitious roots alone were irrigated, while seedlings with adventitious roots subjected to low irrigation produced initial root biomass equivalent to that of adventitious roots. We conclude that black spruce seedlings perform equally well through adventitious and initial roots, but if resources are abundant, they strongly promote development of adventitious roots.

Planting shock can last for several years during which seedling growth may significantly slow down. This duration depends upon the time that is required for the root system to acclimatize to its new environment. We hypothesized that characteristics of the root system are important factors for black spruce (Picea mariana [Mill.] BSP) seedling establishment. To test this hypothesis, we compared growth and physiology of three stock types: containerized seedlings, with a dense initial root system restricted to the volume of a root plug; deeply planted containerized seedlings, with the stem buried during production to develop dense adventitious and initial root systems; and bareroot seedlings, with a less developed but unrestricted initial root system. The three stock types were planted on three microsites that are typical of reforestation sites found in eastern Canadian boreal forest: (1) bare mineral soil; (2) in presence of ericaceous shrubs; or (3) in presence of Sphagnum spp. and ericaceous species. Labelled nitrogen fertilizer was added to half of the seedlings to compare nitrogen uptake. We measured seedling growth (height, diameter, above- and belowground biomass), nutrition (including 15N uptake) and 13C discrimination (as an indicator of water stress) at the end of the second growing season. Except at the root level, there were few interactions between stock types and microsites. For fertilized microsites, we speculate that ericaceous species captured most added nutrients since seedlings that were planted directly on mineral soil had greater height and diameter increments than seedlings with competing vegetation. In unfertilized microsites, Sphagnum spp. and ericaceous species had significant negative effects on seedling growth. For stock type, the presence of a root plug allowed containerized and deeply planted seedlings to have a lower ?13C, suggesting less water stress compared to bareroot seedlings. Strong initial root system development of containerized seedlings resulted in greater total biomass. We hypothesized that the presence of adventitious roots in deeply planted seedlings positively influenced nutrient uptake, particularly N. As nutrient availability is one of the most limiting factors in the boreal forest, we suggest using deeply planted containerized seedlings to reduce planting stress.

Aims
While fungi are key drivers of the carbon cycle and obligate symbionts of trees, the link between plant-fungal interactions and landscape vegetation changes has been largely overlooked. Our aim was to test whether a local difference in dominant tree species would shape the composition of soil fungi communities.

Methods
Fungal communities were described using next-generation DNA sequencing. Composite soil samples were collected in four paired sites (represented by one pure aspen stand and one pure spruce stand) and soil nutriments were measured.

Results
Of the more than 1119 OTUs, 31.6% were Ascomycota while 27.8% were Basidiomycota, 15% were ectomycorrhizal fungi whereas 19.7% were saprotrophic. Communities displayed high species turnover among forest types rather than differences in species richness. Among tested predictors, the dominant tree species explained around 11% of fungal community variation. pH and soil nutrients were also strong predictors of fungal communities.

Conclusions
Our study revealed strong correlations between dominant tree species and fungal communities at a local scale and raised questions regarding the impact of fungal communities on forest soil nutrient dynamics.

L’apprentissage automatique, une branche importante de l’intelligence artificielle, est de plus en plus mis en application dans le domaine des sciences comme l’écologie forestière. Ici, nous examinons et faisons le point sur trois méthodes d’apprentissage automatique généralement utilisées incluant l’apprentissage par arbre de décision, le réseau de neurones artificiels et la machine à vecteurs de support, ainsi que leurs applications au niveau de quatre aspects différents de l’écologie forestière au cours de la dernière décennie. Ces applications incluent : (i) les modèles de répartition des espèces, (ii) les cycles de carbone, (iii) l’évaluation et la prédiction des dangers et (iv) d’autres applications en gestion forestière. Alors que les approches d’apprentissage automatique sont utiles au niveau de la classification, de la modélisation et de la prédiction en recherche dans le domaine de l’écologie forestière, le développement accru des technologies d’apprentissage automatique est limité par le manque de données pertinentes et le « seuil relativement plus élevé » des applications. Cependant, l’utilisation combinée d’algorithmes multiples et de communication et coopération améliorées entre les chercheurs en écologie et les concepteurs d’apprentissage automatique présente toujours des défis importants et des tâches en vue de l’amélioration de la recherche écologique à l’avenir. Nous laissons entendre que les applications futures d’apprentissage automatique en écologie deviendront un outil de plus en plus intéressant pour les écologistes face aux « données massives » et que les écologistes auront accès à plus de types de données tel que le son et la vidéo dans un proche avenir ce qui ouvrira probablement de nouvelles avenues en matière de recherche en écologie forestière. [Traduit par la Rédaction]