Competition effects on fine root survival of Fagus sylvatica and Fraxinus excelsior

Abstract

Belowground competition has been identified as a major force structuring plant communities, but it is not well known how inter- and intraspecific root competition are influencing the survivorship of individual roots. We investigated the impact of inter- and intraspecific competition between European ash (Fraxinus excelsior) and European beech (Fagus sylvatica) on fine root survivorship, root system size and plant productivity in a competition experiment with direct fine root growth observation. Ash and beech saplings were grown either in mixture, monoculture or in isolation (single plant) in rhizoboxes with a transparent observation window that allowed quantifying root growth as well as root longevity dependent on neighbour presence. Root survival was analysed using Cox proportional hazards regression and Kaplan–Meier estimations. Standing root biomass and root productivity were quantified at a final harvest, allowing the calculation of competition indices and biomass partitioning in the plant. With competition indices indicating asymmetric competition in favour of ash, our experiment supports earlier findings on the competitive superiority of juvenile ash over beech plants. Mean root lifespan differed significantly among species (higher longevity of ash fine roots) and also in dependence of the competition treatment. The risk of fine root mortality increased when beech roots grew in mixture with ash or in beech monoculture as compared to beech plants growing in isolation. In contrast, ash fine roots had a lower mortality in mixture with beech than when grown in isolation. Our data indicate that ash fine roots apparently profit from the presence of beech roots while beech root growth and survival are negatively affected, indicating size-asymmetric belowground competition. Competition may represent an important force influencing the fine root lifespan of these tree species.