Aim

The main objective of this study was to evaluate the influence of leachates from three typical boreal forest ground layers on young tree growth and to explore the linkages between the chemical composition of the leachates, tree growth, the allocation between belowground and aboveground parts, and ectomycorrhizal colonization.

Methods

An original 6-month greenhouse experiment was set up to investigate (i) the effects of lichen (Cladonia spp.) and feather moss (Pleurozium schreberii [Brid.] Mitt.) leachates on jack pine (Pinus banksiana Lamb.) growth and (ii) the effects of feather moss and Sphagnum spp. leachates on black spruce (Picea mariana [Mill.] B.S.P.) growth.

Results

Belowground growth and root allocation was reduced by lichen leachates in 2-year-old pine seedlings, while the impact was significant on both below- and aboveground growth in 6-month-old pine seedlings. A substance having the same migration time as usnic acid was detected in the lichen leachates by high-performance liquid chromatography. Sphagnum spp. and feather moss leachates were not found to have any effect on black spruce seedling growth, despite a higher supply of dissolved inorganic N in the feather moss leachates compared to the leachates of Sphagnum spp. and the control.

Conclusions

These results demonstrate that ground layer composition plays a crucial role in shaping the plant community in boreal ecosystems by influencing the chemical composition of the soil solution. They suggest that chemical interference may be another mechanism by which lichens promote the self-perpetuation of open woodlands in the closed-crown boreal forest.

The increase in open-crown forest stands in the closed-crown boreal forest of Quebec over the last 50 years prompts us to identify and understand the drivers responsible for stand opening. To do so, we studied 37 jack pine plots with varying degrees of canopy opening in the Eastern Canadian boreal forest to answer four questions: (1) Does stand opening result from a deficit in pine regeneration, from poor tree growth, or from both processes simultaneously? (2) In the event that pine stand opening results at least in part from poor tree growth, how early following stand initiation does the tree growth divergence occur between unproductive and productive plots? (3) Is poor tree growth in the unproductive plots related to water stress? Finally, (4) are there predisposing site factors and, if so, what are their contributions versus non-permanent factors such as disturbance history, vegetation, and soil dynamics? In the study area, jack pine stand openings resulted from both a poor regeneration density and weak tree growth. Tree growth divergence between productive and unproductive plots occurred very early during the post-disturbance forest succession and is not likely to result from water limitation during the early development of the trees as revealed by ?13C analysis of tree rings. Low-productivity plots were exclusively found on substrates with low base cation reserves. However, because plots of higher productivity were also found on these substrates, we conclude that stand susceptibility to regeneration failures may be greater on sites with such conditions. Variations in tree cover were mainly related to non-permanent environmental variables, suggesting that restoration of forest productivity is theoretically possible in the low-productivity sites investigated.

In the boreal forest, open lichen woodlands have been described as an alternative stable state to closed-crown feather moss forest. In this study, we addressed the role of terricolous lichens in stabilizing open woodlands by hindering tree regeneration and/or growth. Based on field and greenhouse experiments, we compared germination and growth of jack pine (Pinus banksiana) on feather mosses (primarily Pleurozium schreberi) and lichens (primarily Cladonia stellaris), using bare mineral soil as a control. Drivers were investigated by (1) manipulating nutrient supply, (2) simulating shade of a closed canopy on the ground layer with the assumption this would mitigate lichen influence on pine growth, and (3) examining pine root ectomycorrhizal colonization and diversity as indicators of pine ability to take up nutrients. Total growth of 6-month-old greenhouse and 2–3-year-old field seedlings, as well as belowground growth of 2-year-old greenhouse seedlings, was significantly greater in moss than in lichen. Seed germination was not affected by ground cover type. Although field phosphorus and base cation availability was greater in mosses than in lichens, fertilization did not entirely compensate for the negative effects of lichens on pine growth in the greenhouse. Ground layer shading had no impact on pine growth. Lichens were associated with reduced abundance and modified composition of the root ectomycorrhizal community. By suggesting that terricolous lichens constitute a less favorable growth substrate than mosses for pine, our results support the hypothesis that lichens contribute to open woodland stability in the potentially closed-crown feather moss forest.

The composition of the bryophyte layer influences boreal forest regeneration and growth through its effect on soil conditions. Canopy openings in boreal spruce stands can favor the expansion of Sphagnum spp. in the understory at the expense of feather mosses. We use an experimental approach in both paludified field and greenhouse fully-randomized conditions to examine the differential effects of these two ground cover types on black spruce germination and growth, specifically the role of nutrient limitation in generating these effects. We also tested the impact of ground cover shading, simulating the effect of a closed forest canopy on the ground layer, with the assumption that the stress induced to mosses, especially to Sphagnum spp., would have a beneficial impact on tree growth. The two moss types had no differential effects on spruce germination and 0–6-month-old seedling growth in the greenhouse. However, the growth of 2-year-old seedlings in the greenhouse was lower in Sphagnum spp. than in feather mosses. This negative effect was removed by fertilization, suggesting that soil nutrient availability could explain the seedling growth difference between moss types. Greenhouse 2-year-old seedlings also allocated a greater proportion of biomass to roots in Sphagnum spp. than in feather mosses. In the field, feather and Sphagnum spp. mosses had no differential effects on 3-year-old seedling growth, and ground cover shading did not have any short-term positive impact on spruce growth. Although they were not validated in the field, the results we obtained in the greenhouse suggest that the replacement of feather mosses by Sphagnum spp. mosses do not only affect spruce growth through the build-up of an organic layer often associated with low soil temperature and excess water, but also through more direct effects on nutrient availability. Therefore, silvicultural treatments that would favor Sphagnum spp. expansion at the expense of feather mosses, such as partial or total harvesting with protection of regeneration and soils, may result in subsequent tree growth problems even in sites with moderate organic layer accumulation.

In the boreal forest, ground-layer composition may modulate the effects of precommercial thinning (PCT) on stand productivity by affecting tree growth conditions. Based on data from 15 years of PCT monitoring in black spruce (Picea mariana) and jack pine (Pinus banksiana) stands, the objectives of this study were to investigate the effects of PCT on ground-layer composition and the way ground-layer composition is related to tree growth, stand productivity, and the PCT impact on stand productivity. PCT favored lichen expansion in xeric sites. The positive impact of PCT on stand productivity after 15 years was lower in sites with high year-one lichen cover, suggesting that the aboveground positive effect of PCT on growth may have been mitigated by a belowground negative feedback resulting from lichen expansion in xeric sites. Although Sphagnum spp. cover was not affected by PCT, 15-year increase in stand productivity was lower in sites with high year-one Sphagnum spp. cover. These results suggest that xeric stands with high lichen cover should not be targeted for PCT because of either null or negative effects on stand productivity. Subhydric stands with high Sphagnum spp. cover should also be avoided because of lower potential stand productivity. © 2017 Society of American Foresters.

En forêt boréale, les accidents de régénération peuvent mener à l’établissement de deux types de milieux ouverts stables : des landes à lichens sur les sites à drainage excessif et des tourbières à sphaignes sur les sites sujets à l’entourbement. Dans les deux cas, l’ouverture à long-terme de la station forestière s’accompagne d’une modification de la composition de la strate des mousses et lichens. Les mousses hypnacées (e.g. Pleurozium schreberi (Brid.) Mitt.) sont remplacées par les lichens (Cladonia spp.) sur les sites fortement drainés, par les sphaignes (Sphagnum spp.) sur les sites faiblement drainés. Dans la mesure où elle affecte les conditions physico-chimiques et biologiques du sol forestier, la composition de la strate des mousses et lichens contribue à la structuration de la communauté végétale forestière par la facilitation ou non de la régénération des plantes vasculaires. Le rôle de la strate des mousses et lichens dans le déficit de régénération menant à l’établissement et/ou au maintien de milieux ouverts stables est souvent suggéré bien que peu démontré expérimentalement, et les mécanismes sous-jacents restent à identifier. Cette thèse vise à (i) déterminer les effets différentiels des mousses hypnacées, des lichens et des sphaignes sur la régénération et les conditions de croissance du pin gris (Pinus banksiana Lamb.) et de l’épinette noire (Picea mariana [Mill.] B.S.P.), (ii) identifier les mécanismes à l’origine de ces effets, et (iii) évaluer les risques d’accidents de régénération liés à l’ouverture du couvert forestier par éclaircie pré-commerciale. Comparé à l’absence de couvert au sol ou à la présence d’un couvert de mousses hypnacées, les lichens ont inhibé la croissance du pin gris à chacun des stades de développement analysés, c.-à-d. la croissance des plantules (0-6 mois), la croissance des jeunes plants (2 à 3 ans), ainsi que le développement racinaire fin d’arbres adultes (20-40 ans). D’après nos résultats, les effets du lichen sur la croissance du pin gris seraient liés non seulement à une disponibilité moindre des nutriments dans le sol forestier, mais aussi à l’existence d’interférences chimiques plus directes via l’émission de substances dans la solution du sol. La présence de lichens était également associée à une mycorhization moindre des racines de plantules, ce qui suggère un effet négatif des lichens sur la capacité du pin gris à prélever les éléments nutritifs dans le sol. Nos résultats confirment que les sphaignes affectent moins l’établissement que la croissance de l’épinette noire. Bien que le défaut de croissance associé à la présence de sphaignes soit principalement lié à l’accumulation à long-terme d’une épaisse couche organique au sol, nos résultats indiquent qu’un couvert peu épais de sphaignes peut également avoir des effets à court-terme sur la disponibilité des nutriments et donc limiter le potentiel de croissance d’un peuplement forestier. Compte-tenu des effets contrastés des différents types de couvert au sol sur la germination et la croissance ligneuse, il parait tout à fait probable que le remplacement des mousses hypnacées par les lichens ou les sphaignes en cas d’ouverture du couvert forestier contribue à la dégradation des conditions de croissance du pin gris et de l’épinette noire. Nos résultats supportent donc l’hypothèse selon laquelle les changements qui s’opèrent dans la strate des mousses et lichens constituent un moteur de l’établissement et de la stabilisation des milieux ouverts en forêt boréale, en intervenant comme un facteur aggravant du déficit de régénération. Les pratiques sylvicoles qui ouvrent le couvert forestier, telles que l’éclaircie pré-commerciale (EPC), sont susceptibles dans certaines conditions de favoriser l’expansion des lichens ou des sphaignes aux dépens des mousses hypnacées. Etant données les répercussions négatives de ces changements de composition sur la croissance ligneuse, nos résultats suggèrent que les sites fortement drainés à lichens et les sites à drainage lent avec un fort couvert initial en sphaignes ne devraient pas être ciblés dans la mesure où ils sont moins enclins à répondre favorablement à l’EPC et plus à risque de transiter vers un état de clairière ouverte stable. En améliorant notre compréhension des mécanismes sous-jacents aux effets des lichens et des sphaignes sur la croissance des arbres, les résultats de cette thèse offrent également des pistes intéressantes pour l’élaboration de techniques de restauration de la productivité forestière. Ils suggèrent notamment que la plantation et/ou l’ensemencement à forte densité contribuent à améliorer les conditions de croissance ligneuse en favorisant une refermeture rapide du couvert forestier.

In the boreal forest, long-lasting canopy gaps are associated with lichens on dry sites and with Sphagnum spp. on wet sites. We hypothesize that ground layer composition plays a role in maintaining gaps through its effects on fine root biomass (Ø?2mm) and soil nutrient availability. Along gradients of canopy openness in both jack pine-lichen and black spruce-moss forests, the relationships between canopy closure, ground layer composition, tree fine root biomass and soil nutrients were analyzed and decomposed using path analysis. The effects of lichen and Sphagnum spp. removal on tree fine root biomass and soil nutrients were tested in situ. Although variations in pine fine root biomass were mainly explained by stand aboveground biomass, lichen removal locally increased fine root biomass by more than 50%, resin extractable soil potassium by 580% and base cations by 180%. While Sphagnum cover was identified as a key driver of stand aboveground biomass reduction in paludified forest sites, its removal had no short-term effects on spruce fine root biomass and soil nutrients. Our results suggest that lichens, unlike Sphagnum spp., affect tree growth via direct effects on soil nutrients. These two different patterns call for different silvicultural solutions to maintain productive stands.

Plusieurs éléments théoriques sont susceptibles d’expliquer la nature différente des relations entre strates forestières. Les strates sont liées entre elles (i) lorsqu’elles s’affectent mutuellement via des interactions biotiques et/ou la modification endogène de facteurs environnementaux, ou (ii) lorsqu’elles répondent de manière uniforme aux mêmes facteurs environnementaux et/ou perturbations. L’absence de liens étroits entre strates forestières peut résulter quant à elle : (i) de réponses à des facteurs environnementaux opérant à des niveaux différents de la stratification forestière ; (ii) de réponses aux mêmes gradients environnementaux et/ou perturbation mais à des échelles spatiale et/ou temporelle différentes ; (iii) d’un fort recouvrement de niches entre espèces qui affaiblit les associations spécifiques et tend à découpler les fractions de communauté ; (iv) d’interactions biotiques intra-strates qui altèrent la correspondance compositionnelle inter-strate ; (v) de l’intervention de facteurs stochastiques et de leur empreinte historique sur le patron de la communauté végétale forestière. Les publications recensées font état de liens relativement faibles entre strates forestières. Le faible pouvoir prédictif des facteurs environnementaux nous laisse supposer que les processus stochastiques jouent un rôle important lors de la structuration de la communauté végétale. En d’autres termes, les facteurs stochastiques constituent sans doute l’une des principales sources de découplage entre strates.