Epsilon Archive for Student Projects

Abstract

In boreal forest ecosystems, growth of tree seedlings is limited by resources both aboveground (through shading) and belowground (through nutrient limitation), and the ability to tolerate or avoid these conditions is important in determining the relative performance of different tree species. Two of the means to compete for these resources, are through increasing stem-elongation to escape shading by neighbours (shade avoidance), and through mycorrhizal symbiosis that may increase the nutrient- and water up-take. This experiment studies the effects of humus fertility and vegetation shade on the shade avoidance response, productivity, competitive ability and ectomycorrhizal (EM) colonisation of seedlings of the three major boreal tree species [Pinus sylvestris L., Picea abies (L.) Karst. and Betula pendula Roth]. The seedlings were grown in monocultures and in all possible two-species mixtures in pots containing humus collected from low- and high fertility sites in the boreal forest. They were then placed under light filters that were intended to simulate daylight (control) and two levels of partial shade by neighbouring plants by reducing light intensity (PAR) plus reducing light quality; i.e. the red: far-red ratio (R: FR). Three species of ectomycorrhizal (EM) fungi [(Paxillus involutus (Batch) Fr., Amanita muscaria (L.) Hooker and Laccaria bicolor (Maire) Orton] were added to each seedling, in order to assess the effects of the humus, shade, and seedling combinations on the relative EM root colonisation of the seedlings.

The seedlings responded to shade by increasing seedling height, but occasional changes in biomass were also observed. The greatest shade avoidance response was induced in Betula pendula seedlings, which were tallest under the most shaded conditions (i.e. the largest reduction in R: FR) and shortest under the non-shaded conditions. However, shade avoidance of B. pendula seedlings was only significantly induced in the high fertility substrate. Shade avoidance was induced in P. sylvestris seedlings in both the low and high fertility substrate, but was not affected by the level of R: FR. The shade tolerant P. abies did not show any shade avoidance responses. Interspecific competition between seedlings was generally not significantly affected by humus and shade treatments.

Ectomycorrhizal colonisation was unaffected by shade treatments, but was generally higher in the low than in the high fertility substrate. For P. sylvestris seedlings, EM colonisation was stimulated in low fertility when grown in mixture, but not when grown in monoculture. The relative abundance of EM species differed significantly between tree species in the low fertility substrate, and also between B. pendula seedlings grown in the low fertility and B. pendula seedlings grown in the high fertility substrate. Paxillus involutus was the dominant root colonist in all treatments, but its dominance was reduced on B. pendula seedlings in the low fertility substrate, through a relative increase of colonisation by A. muscaria. The results provide evidence that the shade avoidance response is species-specific corresponding to the light- and nutrient strategies of the different tree species, and indicates a higher ecological importance of the EM symbiosis in the low than in the high fertility conditions.