林窗中植物竞争强度随林窗发育的变化

CHANGES IN PLANT COMPETITION WITH THE DEVELOPMENT OF GAPS

通过野外移植外来种尾叶桉 (Eucalyptusurophylla)、大叶相思 (Acaciaauriculaeformis)和本地种荷木 (Schimasuperba) 3种幼苗 ,挖沟排除根竞争和砍树创造林窗排除地上竞争的实验 ,来测定林窗形成后植物地下竞争和地上竞争强度的变化以及这些竞争对 3种幼苗生长的影响。在林窗内 ,尾叶桉、大叶相思和荷木受到的地下竞争强度和地上竞争强度均呈现出随林窗年龄的增长而逐渐增大的趋势。外来种尾叶桉和大叶相思受到的地上竞争强度大于本地种荷木受到的地上竞争强度 ,而荷木受到的地下竞争强度大于两种外来种受到的地下竞争强度。在只有地下竞争和地上竞争时 ,这 3种幼苗的相对生长速率均小于没有竞争时 ,这反映了地下竞争和地上竞争对 3种幼苗的生长有抑制性影响 ,而且这种影响在本地种和外来种间存在差异。

Juvenile plants in a community usually compete with the established vegetation within that community. In general, plants are likely to compete for three kinds of essential resources: photosynthetically active radiation, water and essential nutrients. After the formation of treefall gaps, the competition between plants in gaps and in adjacent understory may change accordingly. Two fundamental questions are how the intensity of competition on juvenile plants changes with the development of gaps and whether the intensity of competition on different species is different or not. We used a field experiment to measure the intensity of competition on juvenile trees in a mixed forest community with infertile soil on an island in Southeastern China. We separated competition into its above_ and belowground components in the field. We reduced aboveground competition by felling large trees to create three treefall gaps, and removed belowground competition by trenching. Thus we created three neighborhood treatments: with roots of neighbors only, with shoots of neighbors only and with no neighbors. Seedlings of three tree species were transplanted into the three gaps with and without trenches cut around the gaps. These species included the two exotic species Eucalyptus urophylla S.T.Blake and Acacia auriculaeformis A.Cunn., and one native species, Schima superba Gardn. et Champ. We measured the biomass, the relative growth rate, and competition intensity of seedlings of the three species over two years. Results based on the average of the three species showed that the intensity of above_ and belowground competition on the seedlings of the three species increased gradually with the development of gaps. The intensity of aboveground competition was greater for the two exotic species than that for the native. However, for the native species, the intensity of belowground competition was greater than that for the two exotics. The relative growth rates of seedlings of the three species with roots of neighbors only and with shoots of neighbors only were all less than with no neighbors. The results showed that the competition of shoots and roots of neighbors in gaps imposed negative impacts on the growth of the three species. The impacts differed between exotic and native species.