Intraspecific and interspecific interactions between three boreal conifer species seedlings: a greenhouse experiment

The growth dynamics of seedlings in mixed conifer plantations or early stages of natural regeneration can be influenced by interactions among seedlings and their capacity to quickly acquire soil resources. We carried out a greenhouse experiment to investigate the growth and N uptake of three widespread conifer species in mono- and multispecific pots at different density levels. Tamarack, black spruce and jack pine seedlings were grown in pots with one to three seedlings per pot, in the absence (N−) or presence of 15N-enriched fertilization (N+). In monospecific pots, tamarack seedlings had higher apical growth and benefited more from N fertilization than the two other species. Unexpectedly, increasing seedling density did not reduce seedling growth and even stimulated jack pine’s growth and N uptake. In two-species mixtures, jack pine seedlings (1) produced up to 56% and 26% greater biomass than black spruce and tamarack, respectively; (2) achieved ~ 35% higher biomass than in two-seedling monocultures, indicating a stronger intraspecific than interspecific competition for this species; and (3) caused a ~ 20% decline in the biomass of both black spruce and tamarack seedlings relative to monocultures. The three-species mixture achieved similar yield as the most productive three-seedling monoculture and absorbed up to 15% more fertilizer than the three-seedling monocultures. Seedlings in the three-species mixture of the N+ treatment had their biomass increased by on average 16% relative to the three-seedling monocultures. Although these results cannot be extrapolated to predict the future dynamics and structure of mature stands, they clearly indicate that jack pine seedlings are more competitive than the other species in mixtures, particularly under conditions of high nitrogen availability.