Understory interception contributed to the convergence of surface runoff between a Chinese fir plantation and a secondary broadleaf forest

Litter and vegetation under forest canopy (i.e., understory vegetation) are known to reduce soil erosion and contribute substantially to evapotranspiration. However, previous studies rarely examined understory water interception. We conducted a comprehensive study of rainfall partitioning using a water balance equation that included both overstory and understory components in a Chinese fir (Cunninghamia lanceolata) plantation and a nearby naturally regenerated Castanopsis (C. carlesii) forest of similar age in southeastern China. Understory interception was measured by submerging understory components (vegetation and litter) in water for 24 h, which gave an estimate of the amount of water retained (i.e., water holding capacity). The total interception loss of rainfall events (>9.8 mm) that generated surface runoff converged between the two forests, a result of a smaller overstory interception loss (20%) coupled with a greater understory interception loss (14%) in the Chinese fir plantation relative to the Castanopsis forest (overstory 25% and understory 11%). The convergence of the total interception contributed to the convergence of the surface runoff, 1% of the rainfall in both forests. Furthermore, the throughfall partition (78%) was greater whereas the stemflow partition (2%) was smaller in the Chinese fir plantation relative to the Castanopsis forest (throufhfall 70% and stemflow 5%). These patterns in rainfall partitioning could be explained by the greater development of the understory vegetation (hence greater understory interception loss) in the Chinese fir plantation, lower canopy leaf area index (hence smaller overstory interception loss and greater throughfall), and rougher barks (hence smaller stemflow) of Chinese fir trees compared to that of Castanopsis trees. Our study illustrated that a comprehensive evaluation of interception loss must take into account understory components.