在植物幼苗生长过程中,总是受到包括地下根竞争在内的各种竞争影响。植物间的竞争主要同有效光辐射、水分和各种营养相关。当外来植物侵入森林群落时,可能受到群落中其它植物竞争的影响。该文通过移植尾叶桉(Eucalyptus urophylla)、大叶相思(Acacia auriculaeformis)两种外来种和本地种荷木(Schima superba)幼苗,挖沟排除根竞争和砍树创造林窗来排除地上竞争的野外实验,研究植物竞争对幼苗生长的影响。有根竞争时,荷木、尾叶桉和大叶相思幼苗的生物量和净初级生产力均小于没有根竞争时,可见地下根竞争对3种幼苗生长有抑制性影响。尾叶桉、大叶相思和荷木3种幼苗受到的总竞争强度分别是0.357 9、0.338 3和0.198 9,受到的地下根竞争强度则分别是0.104 3、0.053 04和0.118 8,受到的地上竞争强度则分别是0.285 1、0.277 0和0.090 85。尾叶桉和大叶相思两种幼苗受到的总竞争强度间的差异不显著,但都显著大于荷木;3种幼苗受到的地上竞争强度间的差异同总竞争强度情况相似;尾叶桉和荷木两种幼苗受到的地下根竞争强度间的差异不显著,但都显著大于大叶相思。地上竞争对阳性树种尾叶桉和大叶相思两种幼苗的生长影响大,而地下根竞争则对耐阴性强的荷木幼苗影响大。尾叶桉和大叶相思两种外来种幼苗受到的总竞争强度均大于本地种荷木幼苗,这反映了这两种外来树种侵入次生林这样的群落受到竞争影响大。

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.