Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

While nitrogen supply is known to enhance the growth of boreal conifers, species-specific plastic responses of biomass partitioning and functional traits to nitrogen availability in seedlings – critical factors for their growth and survival in a changing environment – remain understudied. Here, we conducted a 15-month-greenhouse experiment to investigate how nitrogen availability affects growth, biomass partitioning, bud production, and root traits in seedlings of three widespread boreal conifer species. Black spruce seedlings had intermediate biomass partitioning values between tamarack and jack pine. Tamarack had the highest stem mass fraction (SMF) and relative height growth, but the lowest aboveground mass fraction (AGMF). Jack pine had the highest leaf mass fraction (LMF), LMF/SMF ratio and AGMF. Specific root length was higher in black spruce (10-13 m g-1) than in tamarack (8-9 m g-1). Nitrogen fertilization 1) decreased root mass fraction in all species, particularly in tamarack, consistent with the optimal partitioning theory; 2) increased root diameter by 20-25% and decreased root tissue density by 17-33% in all species; 3) significantly increased apical growth in tamarack only (+140%); and 4) increased lateral bud production in black spruce by 50% in the first growing season and in tamarack by 150% in the second, whereas no effect was found in jack pine. Unlike in black spruce and tamarack, no positive linear relationship was observed between apical growth and lateral bud production in jack pine, indicating predominant bud preformation in this species. Overall, the three species exhibited distinct biomass partitioning patterns with great phenotypic plasticity, particularly in tamarack.