In forests, the herbaceous understory vegetation plays an important role for ecosystem functioning as it represents a significant component of total aboveground productivity. In addition, the herb layer contributes to overall forest species richness and controls tree species regeneration. Vice versa, trees in the overstory control understory herb and shrub growth through competition for resources. Using an experimental forest plantation with manipulated tree richness we asked to which degree tree species richness and identity affect herb layer composition, richness and productivity and how these relationships across strata change with abiotic environmental conditions and competition intensity. In the context of the Biodiversity-Ecosystem Functioning project in subtropical China (BEF-China), we made use of the integrated BEFmod experiment arranged along a tree species richness gradient at two sites, with additional subplot treatments of phosphorus addition, herb layer weeding and no weeding. We recorded the understory vegetation and determined herb layer biomass production on a total of 201 subplots.

In all plots in BEF-China (including C, P, F and I treatment) the herb layer has been continuously weeded twice a year, (since 2009 and 2010, for site A and site B, respectively) and since 2014 once a year on both sites to increase the establishment of planted trees. In each VIP, subplots were subjected to a manipulation of resource supply and biotic interactions (BEFmod experiment), which started in 2014. On the northern edge of each VIP, 5 subplots were established, each measuring 5.16 x 5.16 m and containing 4 × 4 trees. Subplots were randomly assigned to five different treatments: “No weeding“ (NW, i.e., admitting all types of weeds, already established in 2011), “Phosphorus fertilization” (P, i.e., applying 40 g triple super phosphate (H2PO4)2 per tree, corresponding to 100 kg P/ha once in April 2014), and “Control” (C, i.e., weeding as on the total plot level together with monthly H2O application). The BEFmod comprises two further treatments (F, i.e. Fungicide application, and I, i.e. Insecticide application), which we do not refer to in the present study. Herb layer data were collected in the NW, P and C treatments. Herb layer species composition was recorded in 33 and 34 VIPs at site A and B, respectively, (three plots had to be omitted for accidental weeding and due to missing environmental data leading to 201 sublots in total). All herb layer species were identified and cover was estimated separately for each species. The herb layer was defined as all vascular plants including herbaceous species, climbers, ferns as well all woody seedlings and saplings with < 1.0 m height.

VIP Plots of the main experiment on Site A