Assessment of the “root-flow reciprocal relationship” within soil layers: Insights from the northern sub-tropical forest stands

Preferential flow is a critical soil infiltration process in the vadose zones. Root systems are also the key belowground component in such zones and could co-evolve with preferential flow paths. The spatial distribution of root systems and preferential flow paths suggests a functionally coupled relationship: the “root-flow reciprocal relationship”. Yet, we know very little about how root systems and preferential flow govern this reciprocal relationship across different forest stands. In this study, four forest stands (bamboo, pine, oak, and mixed forest stands) were selected. The results showed that the preferential flow intensity decreased with increasing soil depth in the monoculture forest stands (bamboo, pine, and oak), whereas the opposite trend was observed in the mixed forest stands. The spatial concentration index analysis showed that root systems were more concentrated within preferential flow paths than outside them. The generalised additive model analysis revealed that the most important root morphological traits governing the preferential flow intensity were the root length (pine- and mixed forest stands), root surface area (bamboo forest stands), and root dry weight (oak forest stands). The results also showed that there were critical values of those traits that influenced the preferential flow intensity, that is, below the critical value (157.73cm/100 cm 3 of root length for the pine-, 127.30 cm 2 /100 cm 3 of root surface area for the bamboo-, 0.185 g/100 cm 3 of root dry weight for the oak-, and 109.49cm/100 cm 3 of root length for the mixed forest stands), the preferential flow intensity increased with root content, whereas above the critical value, the preferential flow intensity decreased. Root systems had the strongest direct effect ( DE ) on the preferential flow intensity in the oak forest stands ( DE = 0.625), followed by the pine ( DE = 0.487), bamboo ( DE = 0.482), and mixed forest stands ( DE = 0.098). The total effects ( TE ) were highest in the oak forest stands ( TE = 0.630), with pine and bamboo forest stands showing TE values of 0.444 and 0.156, respectively, while mixed forest stands had the lowest total effect ( TE = 0.114).