<b>Soil fungi lead to stronger “diminishing returns” in fine-root length vs. mass allometry towards earlier successional tropical forests</b>

The concept of “diminishing returns”, namely decreasing efficiency in resource acquisition with increasing leaf mass, is supported by leaf economy studies. However, it is unclear whether this also applies to fine roots and how root strategies change during forest succession. To address this knowledge gap, we investigated the scaling relationship between fine-root length and mass for 215 topsoil cores from 24 plots across four successional stages in tropical forests of Xishuangbanna, southwestern China. We then assessed the relative effects of edaphic conditions, leaf functional traits, tree species diversity, and soil fungal factors on the scaling relationship. Our results revealed the existence of diminishing returns in root length (scaling exponent &lt; 1). The scaling exponent was higher in late- than early-successional forests, corresponding to a root strategy shifting from “do-it-yourself” to “outsourcing” resource uptake by soil fungi. Furthermore, our results demonstrated that soil fungal abundance, particularly root endophytic fungi and mycorrhizal fungi, were the main drivers of root strategy changes. Overall, our study suggests 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. Fine-root length and mass scaling exponent is a crucial indicator of shifts in root resource-acquisition strategy.