The effect of different disturbance treatments on tree production, competition, shortroot presence and ectomycorrhizal colonisation in Ericaceae-feathermoss boreal forests

Abstract: In many northern boreal stands, forest floor vegetation constitutes a considerable part of total biomass and production, and consequently is likely to be critical in determining tree seedling growth. This study was conducted in a greenhouse with seedlings grown in intact soil profiles consisting of a minimum coverage of 90% of Pleurozium schreberi and 30% of Vaccinium myrtillus vegetation. I investigated the effect of three different disturbance treatments, (I) burning using a propane flame thrower to remove all vegetation and the uppermost portion of the humus of the profile; (II) trimming of all mosses and V. myrtillus at the height of the interface between the brown and green parts of mosses; and (III) control left intact, on the performance of monocultures and tree seedling mixtures of Pinus sylvestris, Betula pendula and Populus tremula by assessing shoot and root biomass, short root occurrence and mycorrhizal colonisation. The results of the different disturbance treatments on seedlings response variables was generally weak although P. sylvestris and P. tremula showed significantly higher values of biomass in fire treated vegetation compared to intact vegetation. B. pendula did not show any effects on total or root biomass across treatments but showed an increased shoot length and shoot to root ratio in intact vegetation compared to other treatments. The effect of disturbance in seedling mixtures on relative performance of the different tree species was not strong enough to influence the strength of their interactions. P. tremula seemed to be quite uncompetitive with other species and this was especially evident for seedlings grown in association with B. pendula. The production of short roots was generally higher in seedling mixtures compared to monocultures and in more intense disturbed treatments. Higher short root production in seedling mixtures possibly indicates lower nutrient availability when grown in association with different neighbours compared to monocultures. More than 95 % of seedling short roots independent of tree species were colonised by mycorrhiza and most of the EM morphotypes found showed great host specificity. The results of this study shows that ground vegetation of P. schreberi interfere with performance of P.sylvestris and P. tremula seedlings while the effect on B. pendula is more unclear and more study on this topic and the effect of seedling mixtures is needed.