| Mass plantation of Eucalyptus spp. has significant impacts on local biodiversity. In this study,89 plots of E. grandis×E. urophylla in the a industrial plantation and 8 plots of original communities were compared in similar, adjacent stands set up in Puer City, Yunnan Province. Synecology sampling, soil analysis, andαbiodiversity data were analyzed using the software SPSS. The community structure of the E. grandis×E. urophylla industrial plantation and the undergrowth's biodiversity changes, patterns, and driving factors were discussed. A comparison study of biodiversity in the original endemic communities and the E. grandis×E. urophylla communities was carried out. The findings of the study will provide theoretical support for improving under-forest biodiversity of artificial E. grandis×E. urophylla forests and their sustainable management.In the subtropical mountains of southern Yunnan, undergrowth's species diversity of young-middle stands of artificial E. grandis×E. urophylla forests can be very high. However, a large portion of the species composition is composed of pioneer heliophyte, adaptable dispersed species, secondary and exotic species. Original elements of the endemic floristic composition are rarely found in undergrowth of E. grandis×E. urophylla industrial plantation, largely due to the alternation of habitats during mass forestation. such as light and water ecological factors, to be accommodated with themselves bionomics. Additionally, these exotic species have strong diffusion and invasiveness. All above factors produced advantageous conditions for these exotic species to invade.At the same time, human disturbances have caused species invasion. Eupatorium adenophorum and Eupatorium odoratum etc. have already dominated the herbaceous layers and major companion species of regional communities.This paper summarizes the characters of the undergrowth species diversity and the changing patterns. Within the studied area, the herbaceous layer is generally higher than shrub layer in terms of undergrowth species richness and lower in terms of evenness and diversity. This shows the environmental heterogeneity of the E. grandis×E. urophylla plantation. The herbaceous layer was dominated by secondary and exotic species, forming a"consociation". Yet the shrub layer lacks such dominant species. This shows that the secondary subtropical herb species have higher adaptability than shrubs, and can enter and dominate new communities after human disturbances. Compatibly, shrub genuses are stricter to their habitat. Their recovery after human disturbances is much slower than herbs.The correlation analysis of species diversity and environmental factors showed that the stand situation largely determines the undergrowth species diversity. The soil type is positively pertinent to the Simpson indicator of the herbaceous layer, Lateritic red soil has high impacts on species richness, while yellow-brown soil has high impacts on species biodiversity indicators.Another key factor is the aspect of a stand. showing that"aspect"is the most important factor that decides the undergrowth species diversity of E. grandis×E. urophylla forest。Thus, the forestation and management of E. grandis×E. urophylla in subtropical mountainous stands of south Yunnan should not have just one uniform plan, design and methodology, like that in big plains. It should be designed according to different aspects, slopes, and gradient. The forestation on slope >25°should be avoided. Additionally, soil type should also be taken as an important factor.Undergrowth species diversity of E. grandis×E. urophylla industrial plantation has a close relationship to soil fertility. The concentration of soil organic matter, especially the hydrolyzable nitrogen, is a key factor of undergrowth species diversity. In order to maintain the fertility of soil and improve the undergrowth species diversity, scientific and reasonable fertilization conforming to the characteristics of the soil will be important to build a stable ecosystem with high species diversity.A spectrum of monsoon evergreen broadleaved forests, mixed forests, Pinus kesiya var. langbianensis (Simao Pine) forests, and dry-hot shrub-grasslands are distributed along the great rivers and through the mountain ranges of the studied area, which has a 1000m elevation difference. Large scale forestation of E. grandis×E. urophylla forests for industrial purposes will replace the original natural or sub-natural vegetation and have the following large-scale impacts on local ecosystem and species diversity: 1) the original south-subtropical mountains have very diverse ecosystems, which will be lost and altered into unitary artificial vegetation. 2) The original natural or sub-natural vegetation with high species diversity will be replaced by artificial vegetation. The undergrowth species diversity will decrease with significant species richness and diversity loss.For South Yunnan's mountains that have such high ecosystem and species diversity, there are several methods to delimitate the impacts of E. grandis×E. urophylla industrial plantation on undergrowth species diversity. For example, do not use the original forest land; use just a few of the secondary shrub lands; arrange different tree species for different aspects and soil types of the stand; build mixed forests; maintain the biological corridors; manage the stand density. The subsequent management will promote the undergrowth biodiversity preservation and realize the ultimate goal of"sustainable forest".A species diversity assessment indicators system and the standards for the undergrowth of E. grandis×E. urophylla industrial plantation were set up using the following five factors: species richness, species diversity, importance value of exotic species, undergrowth vegetation biomass and coverage. The system was validated through case studies found to be scientifically sound. The proposed biodiversity assessment indicators system has provided a useful tool to assess the E. grandis×E. urophylla forestation, and to consider and involve the biodiversity amelioration and recovery issues during planning, forestry design, forestation and management of planting Eucalyptus forests.
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