Belowground facilitation and competition in young tree species mixtures

Abstract

Belowground interactions in diverse plant communities may be decisive for the performance of individual species and community stability. Here we assessed the effect of tree species richness on belowground fine-root morphology and belowground competition between four different species in a 6-year-old field biodiversity experiment to test the hypotheses: (i) overall fine-root exploitation (total fine-root length and surface area) increases with tree species richness; (ii) belowground interspecific competition is size-symmetric. Overall fine-root length and surface area in the centre of neighbourhoods of four saplings were initially low (1.03 km m −2 and 2.00 m 2 m −2), but reached 3.13 km m −2 and 6.50 m 2 m −2, respectively, across all species combinations after two growing seasons in the ingrowth cores. However, no significant differences were found among the different tree species richness levels. The saplings of different tree species grew in proportion to their initial sizes with respect to aboveground basal area increments. For belowground fine-root growth in mixed neighbourhoods, however, Pseudotsuga menziesii and Picea abies had higher fine-root growth rates in ingrowth cores than in monocultures, whereas the reverse was true for Fagus sylvatica and Quercus petraea. After two years of root ingrowth, the competitive ability indexes ( P. abies = 0.07, P. menziesii = 0.08, F. sylvatica = −0.19, Q. petraea = −0.18) revealed that belowground competition in this sapling stand was size-asymmetric and that conifers showed a higher competitive ability, when fine-root growth was related to aboveground standing basal area. Nutrient enrichment in ingrowth cores did not affect proliferation rates and morphology of fine roots significantly. Fine-root morphologies of different species were remarkably different, but within each species the morphology was not significantly influenced by tree species richness of neighbourhoods. Our results show that belowground competition may occur earlier than aboveground in mixed forest stands and fine-root growth of dominant species benefitted more from mixing with other species than that of inferior species.