Relationship of under-story vegetation diversity with stand age and soil factor of eucalypt plantations in central-western Guangdong

Commercial eucalypt plantation forest has become a unique ecosystem in the wide-range of the tropical and subtropical regions in China. In this unique ecosystem,the under-story vegetation is identified as an important concern for biodiversity conservation and sustainable management for a long run. In the past decade,efforts have been made to understand the species composition and diversity of the vegetation associated with eucalypt and other fast-growing tree plantations. However,much work is still needed to cumulate the knowledge and to provide an overall coverage of this topic and the relevant issues. In the present study,we investigated the under-story vegetation community of short-rotation eucalypt plantations in Gaoyao,Guangdong to compare species composition and diversity at three different age classes,and to study the relationship of species distribution and diversity with soil factors by using canonical correspondence analysis( CCA) and correlation analysis. The result showed that difference in species number significant to a certain extent was observed among three stand age classes when the number of the sampling plots was equal or more than 9,and a size of 24 sampling plots per age class was sufficient for analyzing the species composition. Totally,there appeared 136 vegetationspecies belonging to 54 families and 107 genera in the stand plots sampled,among which there were 80 tree and shrub species,36 herbs and 20 lianas. Although tree and shrub species were rich in number, two herbaceous species( Dicranopteris dichotoma( Thunb.) Bernh. and Miscanthus sinensis Anderss.) were always dominant in abundance and coverage over the stand plots studied. Species number did not significantly differ among stand age classes,but was consistently lower in stands at 1—2 years old than at other two age classes,attributed to the abrupt change and fast restoration of vegetation during and after the establishment stage of stands,respectively. The diversity measures( Simpson index,Shannon index and richness) of the vegetation species were highest for eucalypt stands at 3—4 years old. The ordination analysis indicated that the under-story vegetation species was closely related to the first two ordinations of CCA,with correlation coefficients of 0. 856 and 0. 943,respectively,and the cumulated variance proportion of 51. 1%. While the first ordination axis was obviously controlled by the organic matter and total phosphorus,the second ordination axis was mainly contributed by the total potassium and soil bulk density. The species distribution pattern was significantly influenced by these soil factors,and also related to the pH value in a way. In contrast,the vegetation species diversity was consistently shaped by the total nitrogen and pH value,increasing with the total nitrogen and pH value from low to neutral level. Fertile forest soils with neutral pH value favored to restore or maintain the vegetation diversity associated with short-rotation and intensively managed plantations. We also discussed the negative effects on under-story vegetation of developing shortrotation plantation in large scale and intensive silvicultural operations such as forest road network building and maintenance,burning,site clearance and preparation,weeding,clear-cut and log transportation,etc. Eucalypt cultivation reduces species diversity through changed forest environments and soil properties such as physical structure,moisture and nutrient.