Coarse root architecture: Neighbourhood and abiotic environmental effects on five tropical tree species growing in mixtures and monocultures

Abstract

Most biodiversity-ecosystem function research focusses above-ground; if it considers roots, it considers fine roots, while coarse roots remain a “black box”. We utilized the Sardinilla experiment in Panamá, the oldest tropical tree diversity experiment, to test how species mixtures at the plot and neighbourhood scales and abiotic environmental characteristics affected 7 coarse root traits of 5 tropical tree species. Sardinilla was planted in 2001 with a gradient of native tree species richness of two-, three- and five-species mixtures and their respective monocultures. Root systems of 128 mature trees were excavated 0.5 m radially and one coarse root was fully excavated; unexcavated root lengths were predicted using species-specific allometric relationships. Root traits included the observed traits primary root length, diameter, and number, the number of secondary roots per meter of primary root, predicted root system extent and total predicted length of roots, and coarse root biomass. Tree size scaled positively with most root traits. Species identity (conspecific or heterospecific) and size of near neighbours were found to explain more variation (63.0%) in root traits than abiotic environmental characteristics (30.4%). Trees in waterlogged plots grew fewer, longer roots. Trees reduced root system extent but grew more roots when growing with conspecifics or growing in plots of high soil nitrogen and greater depth to water table. These findings suggest strategies employed by tropical tree species to both reduce competition with neighbours and make the most of local abiotic conditions, bringing us closer to understanding the role of coarse roots in the context of BEF research in forests.