Fine root interactions in subtropical mixed forests in China depend on tree species composition

Belowground interactions can greatly modify fine root (≤2 mm in diameter) traits to increase soil resource acquisition for tree growth. We examined how mixed forests alter fine root traits compared to pure forests. A pseudo-experimental tree cluster design was used to select small-area plots of single and mixed species in a Pinus massoniana–Lithocarpus glaber (PM–LG) forest and a L. glaber–Cyclobalanopsis glauca (LG–CG) forest. In each plot, soil cores were sampled down to 30 cm at a 0.5 m interval between target and neighbouring trees. Fine roots in soil cores were then divided by species to determine biomass and morphological traits. The mixed PM–LG plots exhibited significantly higher fine root biomass while the mixed LG–CG plots had no significant differences in fine root biomass compared to their respective pure plots. In pure plots, P. massoniana had higher fine root biomass and lower specific root length (SRL) and specific root area (SRA) than L. glaber, whereas fine root traits were similar for L. glaber and C. glauca. Compared with pure plots for a given species, fine root biomass in the entire soil profile decreased for P. massoniana but increased for L. glaber in mixed PM–LG plots. In mixed LG–CG plots, fine root biomass decreased for each species at all soil depths. Whether positive interactions of fine roots occur is dependent on tree species composition. Fine root biomass was greater in mixed forests where tree species showed contrasting growth strategies and root traits, such as PM-LG forests, thus suggesting positive interactions.