Juvenile plants usually compete with the established vegetation within a community. Competition among plants takes place both aboveground and belowground. Aboveground competition primarily involves a single resource, light. In contrast to aboveground competition, plants compete for a broad range of soil resources, including water and many essential mineral nutrients that differ in molecular size, valence, oxidation state, and mobility within the soil. Of particular interest is the intensity of competition by invasive exotic plants and how this varies among species. We used a field experiment to measure the intensity of competition on juvenile trees in a secondary forest community with infertile soil on an island in South China. We separated competition into above- and belowground components in the field. We reduced aboveground competition by felling large trees to create a treefall gap and removed belowground competition by trenching. We created three neighborhood treatments: roots of neighbors only, shoots of neighbors only and no neighbors. Seedlings of three tree species were transplanted into the understory with and without trenches cut around plots (with roots and shoots of neighbors and with shoots of neighbors only), and single treefall gap area with and without trenches cut around plots (with roots of neighbors only and with no neighbors). The species used included two exotic species, Eucalyptus urophylla and Acacia auriculaeformis, and one native species, Schima superba. We measured the biomass, net primary productivity (NPP), and total competitive intensity in above- and belowground components over a two-year period.
The biomass and NPP in the plots without root competition were greater than those in the plots with root competition. Comparisons among the three species of seedlings showed that the intensity of total competition was greater for E. urophylla and A. auriculaeformis and lower for S. superba. Difference in the intensity of aboveground competition among the three tree species showed the same pattern as that of intensity of total competition. However, the intensity of belowground competition was the greatest for S. superba and the lowest for A. auriculiformis. Results based on the average of the three species showed that seedlings of the exotic trees, such as E. urophylla, A. auriculiformis, were also influenced by competition with established vegetation within the community during the invasion process. The intensity of total competition on the two exotic species, E. urophylla and A. auriculaeformis, were significantly greater than on the native species, S. superba. These results suggest that competition might pose a greater impact on the two exotic species when they invade a secondary forest community with a complex structure.