L’épinette noire (Picea mariana [Mill.] BSP)possède une spécificité racinaire : elle développe majoritairement un système racinaire adventif. Les racines adventives émergent à la base de la tige et remplacent progressivement les racines initiales issues de la germination de la graine, jusqu’à devenir prépondérantes au stade mature de l’arbre. Bien que ce phénomène ait été longtemps observé, il reste encore largement inexpliqué. Cette étude visait ainsi à déterminer le rôle des racines adventives lors de la croissance de l’épinette noire au stade de semis, en se focalisant sur leurs aptitudes à capter l’eau et les nutriments, et sur leur capacité d’adaptation à des conditions limitantes en ressources. L’hypothèse générale était que les adventives parviendraient à mieux capter l’eau et les nutriments que les racines initiales et que leur développement permettrait aux semis d’avoir une plus forte croissance et de meilleures performances physiologiques. Pour essayer d’y répondre, cette étude a été réalisée en trois volets. Lors du premier volet (Chapitre II), les contributions des racines adventives et initiales à la croissance de semis âgés de 4 ans ont pu être testées grâce à un système de doublepot permettant une irrigation et une fertilisation indépendantes pour chaque type de racines. Après une saison de croissance en environnement contrôlé, la plupart des paramètres mesurés (croissance, physiologie et nutrition) ont été semblables pour un même traitement au niveau des racines adventives ou initiales. La principale différence a été observée au niveau de la biomasse racinaire avec un développement de racines adventives favorisé lorsque les semis ont reçu un apport en eau ou en nutriments important, excepté en cas d’irrigation exclusive aux racines initiales qui a résulté en un développement équivalent de racines initiales et adventives. Le deuxième volet (Chapitre III) consistait à tester l’impact du système racinaire sur la croissance de semis de 2 ans lors d’une plantation en forêt boréale. La comparaison des semis avec ou sans racines adventives a permis d’observer une croissance semblable mais une plus forte captation de nutriments de la part des semis avec racines adventives, en particulier de l’azote marqué ajouté au début de la deuxième saison post-plantation. Enfin le troisième volet (Chapitre IV) a permis de tester l’influence d’un système racinaire composé de racines adventives et initiales ou uniquement de racines initiales, sur la croissance de semis de 4 ans en conditions limitantes en eau, nutriments ou oxygène. Les semis ayant développé des racines adventives ont eu une plus forte croissance en hauteur que les semis n’ayant que des racines initiales, associée à de meilleures performances physiologiques, en particulier lorsque la disponibilité en eau a été restreinte. Les racines adventives ont présenté une plus grande plasticité cellulaire en conditions limitantes en eau et en oxygène par raport aux racines initiales. En conclusion, cette étude a permis de relier la présence de racines adventives à de meilleures performances en termes de croissance, de physiologie et de nutrition, chez l’épinette noire au stade de semis. Ces résultats laissent ainsi entrevoir plusieurs pistes de recherches futures mais également de possibles retombées pour la production de semis. En effet, il pourrait être intéressant d’envisager l’enfouissement de la base de la tige lors de la production de semis d’épinette noire, afin de favoriser le développement de racines adventives, en particulier pour les semis destinés à des sites où la disponibilité en eau et/ou en nutriments peut être limitante.

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.

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.