Soil fungi lead to stronger ‘diminishing returns’ in fine‐root length versus mass allometry towards earlier successional tropical forests

Decreasing returns in resource acquisition ability with increasing leaf mass investment is called ‘diminishing returns’, which provides important insights into plant economy. Yet, whether this is true for fine roots and how root resource acquisition strategies change with forest succession remain unclear. We investigated the scaling relationship between fine‐root length (L) and mass (M) for 215 topsoil cores from 24 plots across four successional stages in tropical forests of Xishuangbanna, southwestern China. We also assessed the relative effects of edaphic conditions, leaf functional traits, tree species diversity and soil fungal factors on L versus M scaling relationship using hierarchical variation partitioning. Our results revealed the existence of diminishing returns in root length (L vs. M scaling exponent <1), and that the exponent was higher in late‐ than early‐successional forests, corresponding to a strategy shifting from ‘do‐it‐yourself’ in the late‐successional stage to ‘outsourcing’ resource uptake by soil fungi in the early‐successional stage. Soil fungal abundance was the main driver of changes in the L versus M scaling exponent across plots (explained 58% of variances), with root endophytic fungi the strongest predictor (22.11%), followed by mycorrhizal fungi (10.41%), while other factors (leaf functional traits, edaphic nutrient conditions and tree species diversity) exerted weak effects. Our results suggest that root endophytic and mycorrhizal fungi act as key modulators of root economy changes during forest succession, but the former has received less attention previously. L versus M scaling exponent may be a better indicator for shifts in root resource acquisition strategy than the commonly used specific root length. Read the free Plain Language Summary for this article on the Journal blog.