The lateral spread of tree root systems in boreal forests: Estimates based on 15N uptake and distribution of sporocarps of ectomycorrhizal fungi

In nutrient poor environments, such as boreal forests, many of the most important interactions between plants take place belowground. Here, we report the results of two approaches to obtain estimates of the lateral spread of tree roots.In a Pinus sylvestris forest and a Picea abies dominated forest, lateral root spread was estimated by analysing the distribution of ectomycorrhizal sporocarps on tree-girdled and adjacent non-girdled plots and by a 15N uptake experiment, where the soil in circular 1 m2 plots was labelled and the uptake by surrounding trees was studied.Both methods gave similar estimates of lateral root spread, i.e. between 4 and 5 m, and both studies indicated overlapping of root systems. Up to 11 trees took up tracer from a 1 m2 labelled area, although not all trees close to the 15N-labelled area took up tracer.The study clearly showed the dependence of ectomycorrhizal fungi on photosynthate and indicated a high potential for belowground competition between trees due to substantially overlapping root systems.