Are tree roots in the canopy ecologically important? A critical reassessment from a case study in a tropical montane rainforest

Abstract

Background: The occurrence of adventitious roots in the crowns of rainforest trees has been reported occasionally. These roots have been suggested to serve as an ecological mechanism that increases nutrient supply to trees. Aim: We investigated the biomass and ecomorphology of tree fine roots in the canopy and in the terrestrial soil in a nutrient-poor montane rainforest in Costa Rica. Methods: Crown humus and terrestrial soil organic layer samples were analysed for tree fine root mass, root morphology and chemical properties. We surveyed crown humus abundance and quantitatively estimated the stand's tree fine root biomass in the canopy. Results: Soil chemical properties were similar in crown humus and terrestrial organic layer samples. Fine root biomass density and live-to-dead ratio were higher in the terrestrial soil than in the crown humus. Tree fine roots in the canopy represented less than 0.1% of the fine root biomass in the terrestrial organic layer. Canopy roots completely lacked ectomycorrhizal colonisation, while terrestrial fine roots were entirely colonised. Conclusions: The negligible biomass of adventitious fine roots in the canopy clearly suggests that the ecological benefit from canopy roots for the host tree must be marginal even under particularly poor growth conditions.