Root functional traits vary independently from edaphic factors at local scales

Background and Aims Roots are the primary organs in plants responsible for nutrient and water uptake. Root traits have been shown to vary predictably with respect to soil fertility gradients, primarily at regional and global scales where climatic conditions and soil nutrient variability vary greatly. However, it remains unclear whether local-scale heterogeneity in soil nutrients also drives similar trends in root trait variation. Methods We characterised the fine root traits and bulk soils of 33 species across an old-growth and secondary forest in Singapore to test how interspecific root trait variation responded to differences in soil nutrient availability within a single forest site. Results The relationship between fine root and soil nutrient contents differed depending on the nutrient tested, indicating the possibility of multiple nutrients co-limiting among species. Species’ root morphological traits varied independently from soil fertility gradients, but most species expressed consistently high root phosphomonoesterase activity and low mycorrhizal colonisation rates, regardless of mycorrhizal and forest type. We consolidated these findings to propose a new conceptual framework to explain why root trait-soil nutrients relationships differ across local and regional scales. Conclusion Our study highlights the large belowground functional space in tropical ecosystems, driven by diverse tree species with contrasting belowground trait combinations within a narrow soil fertility range. We propose that soil nutrient limitation may not necessarily filter for species with specific belowground trait combinations as previously anticipated, and biotic processes may impose a stronger effect on belowground trait expression within highly biodiverse tropical ecosystems instead.