Plant Diversity And Ecosystem Functions In A Long-term Continuous Planting Eucalypt Plantation Experiment

Eucalypts belongs to the Myrtaceae family, Eucalyptus genus, with 795 species (including 114 subspecies and varieties). Eucalyptus is the genus of trees is most planted around the world because of its large number of species, fast-growing, high-yield, the ability to adapt to many different sites, and the possibilities of producing wood for many different uses. Up to now, there are 96 countries planted eucalypt, and there are at least 20 million ha of areas of planted eucalypt. Remarkably, eucalypts have become the most widely planted hardwood species in the world.Eucalypts were introduced into China in 1890. Up to now, more than 300 species have been introduced in more than 600 counties of 17 provinces, among the species introduced, 200 species have been tried in afforestation, but only about 10 species were widely planted. Over 2 million of Eucalypts plantations have been established in China. Eucalypts plantation of 60-70% in Southern China belonged to short-rotation forest plantation, in which 50-60% of stand sites had been carried out continuous planting system. With the increasing areas of Eucalypts plantations established and carrying out on continuous planting throughout the country, the environmental problems of eucalypt plantations (include impacts on soil water, soil nutrients, soil erosion and biodiversity) are more and more serious, the "Eucalyptus controversy" has become an important issue in the tropics and subtropics. The developing eucalypt plantation has been a focus of the society's attention. To make high-yield and high-diversity of eucalypt plantations, it is an important task of ecology and a major scientific issue too.The relationship between biodiversity and ecosystem functioning has emerged as a major scientific issue today, a key problem for the sustainable management of eucalypt plantation and sustainable development of the ecological environment. Although the relationship between biodiversity and ecosystem function have been widely studied in recent decades in the world, the multi-rotation continuous planting eucalypt plantations ecosystem have not been studied. This study began at 1998, the . objectives of this study are to examine the relationship between plant species diversity with eucalypt tree growth, productivity, nutrient cycling, nutrient use efficiency, site productivity maintenance, community stability and sustainability in eucalypt plantations, by carrying out a long-term experiment in the different rotations of eucalypt plantations, and at the same time, provide a theory and guidance for high yield and high species diversity maintenance and sustainable management in eucalypt plantations. This study, without a doubt, plays an important role in the sustainable development of eucalypt plantations.Based on the long-term experiment research from 1998 to 2005, the study has main results and conclusions are as follows:By the methods of long-term experiment research, this paper studied the law of growth and development, characteristics of forest structure in Eucalyptus grandis×Eucalyptus urophylla (EGEU) plantations in different rotation stands from 1998 to 2005. The results indicated that the diameter breast height (DBH) and volume increment of eucalypt plantations have 2 growing-peak periods during a period of 1998-2005, the first is after planted 2 years, the second is after planted 5 years, the highest growing period of tree diameter and volume of the plantation was 2 years old and 5 years old, respectively. Both increment change obviously in peak and low during the period of 1998-2005. Eucalypt tree height growth has only one peak during the period of 1998-2005, at after planted 2-year. The curve of volume average increment and current increment was across at after planted 5-year. Based on the study, the growth and development of eucalypt plantation were classified a sapling stage, young stand stage, half-mature stand stage and mature stand stage. The change of diameter and tree height structure of eucalypt plantations with stands age follow the general law of pure forest, but the shewnees of curve of stand diameter distribution and tree height distribution in young stand stage were obviously to right side due to early fast growth of EGEU. The canopy structure of EGEU plantations was simple, LAI also wais low, and the first rotation stand was lower than that of second rotation stand. T-test results showed that there was significant difference in canopy structure (LAI: t=-4.348,α=0.0001; MFIA: t=-2.603,α=0.019) between first rotation and second rotation forest, but DBH increment (t=0.044,α=0.965), height increment (t=-0.280,α=0.781) and volume increment (t=-0.262,α=0.795) was not significant. It is indicated that continuous planting system was not significant effect on growth of the second rotation of eucalypt plantation.The study showed that EGEU is a tree of high-productivity, its average volume productivity and productivity in mature stands (7.3 years) is 20-21m~3/ha·a and 19t/ha·a, respectively. T-test results showed that there was not significant difference in biomass (t=-0.011,α=0.991) and productivity (t=0.077,α=0.940). It is indicated that continuous planting system was not significant effect on biomass and productivity of the second rotation of eucalypt plantation. Biomass and productivity of bole stem, bole bark, branch and root in eucalypt plantations increased with the increasing of stand ages, but leaf increased in the young stand stage and decreased in half-mature stand stage and mature stand stage. At the mature stand stage, the biomass of root and leaf increased slowly even though appeared negative growth that influenced the sustainability of productivity of eucalypt plantation. The biomass ratio of bole bark, branch and leaf increased tendency by influenced continuous planting. Continuous planting of Eucalyptus plantations resulted in a significant reduction in biomass and productivity of understory vegetation. The biomass and productivity of understory vegetation in second rotation stand was less 70% than that in first rotation stand. The biomass and productivity of understory vegetation in first rotation stand appeared an increasing tendency, but decreasing tendency in second rotation stand from voung stand stage (2-3 ages) to mature stand stage (6-7 ages). Continuous planting of Eucalyptus plantations resulted in a significant reduction in plant species diversity, species richness and Shannon-Wiener index in second rotation stand was less 39.39% and 17.76% than that of first rotation stand. Jaccard index and Cody index of first rotation stand significantly was higher than the second rotation inβ-diversity measures of the community. Coefficient of similarity in community was higher within-rotation (60%) and lower inter-rotation (30%) in eucalypt plantations. It is indicated that the continuous planting of eucalypt plantation resulted in many species loss. Continuous planting not only decreased the species diversity but also changed the composition of species, life-form, growth-form in community and plant life-history strategy. The multi-rotation continuous planting of Eucalyptus plantations is advantageous to herbaceous and r-strategy species and disadvantageous to woody plants and k-strategy species, and thus understory vegetation tends to be dominated by light-demanding grasses. With the increased stand age, woody plant cover, herbaceous cover, plant richness (S), Shannon-Wiener index (H'), Simpson index (D) in both rotations increased or recovered, but their recovering rate was obviously different. The woody plant cover, herbaceous cover, S, FT and D in first rotation stand restored more quickly than those of second rotation stand, and the early stage of stand restored faster than the late stage. Although such differences between 1st rotation and 2nd rotation stand become smaller over the stand age, the understory diversity in Eucalyptus plantations may not be sustainable because the understory vegetation in plantations can not be restored the same level of understory vegetation in the pre-rotation. The change regulation of soil seed bank in different rotations was a similar with understory vegetation. Continuous planting resulted in reduction of soil seed bank reserves and density. Soil seed bank and vegetative propagation in the soil is a basis of species diversity maintaining in industrial Eucalyptus plantation. Therefore, a new hypothesis, initial plant diaspora, on the maintaining mechanism of species diversity in eucalypt plantation was put forward. In a rotation of management, species richness in industrial plantation is determined by initial plant diaspora in the soil due to few seeds are introduced to the soil seed-bank in a rotation. The species with high-competed or only with seed (fruit) dispersal was lost by strong disturbance (e.g., the clearing site by burning, the soil preparation by mechanically ploughed) that resulted in reduction of initial plant diaspora in the next rotation stand (land). Therefore, the species richness of the continuous planting forest floor decreased in rotation by rotation.This study indicated that nutrient structure of EGEU belongs to a tree species of Ca>N>K>Mg>P, with strong concentrated Ca in mature individual. The order of tree parts for N, P, K, Mg concentration was the same, leaf>bole bark>branch> root > bole stem, Ca-concentration was very different, followed bole bark > leaf> branch >root >bole stem. Five nutrient elements concentrations, generally, increased with increasing continuous planting rotation, and decreased with increasing with tree age in N, P, K concentration, and increased with increasing tree age in Ca and Mg concentration. EGEU plantation was characterized as a fast of nutrient accumulation, high of nutrient retention ratio (account for 94%) and low of nutrient return ratio (account for 6%). Five nutrient elements (N, P, K, Ca and Mg) bio-cycle ratio of EGEU plantation was 40.40-41.81%. The bio-cycle ratio of N and P was the highest in the nutrient cycle of EGEU plantation, K and Mg was the middle, and Ca was the lowest. The nutrient utilized per biomass of EGEU plantation was lower, to produce per 1t biomass needed to utilize nutrient 5kg including N, P, K, Ca and Mg, and to produce per 1t bole wood only needed to utilize nutrient 2kg above nutrient. Although nutrient utilized per biomass of EGEU plantation was lower, the total biomass of EGEU plantation was higher and hence, the nutrient consumption of eucalypt plantation was higher too. The study results also showed that continuous planting system was not significant effect on nutrient accumulation of the second rotation of eucalypt plantation. On the contrary, continuous planting system was significant effect on nutrient accumulation of shrub layer and herbaceous layer. The nutrient accumulation of shrub layer and herbaceous layer in the second rotation eucalypt plantation was less 115% and 85% than that in first rotation eucalypt plantation. Continuous planting has the effects of decreasing soil bulk density, increasing soil porosity and aeration and retained water in eucalypt plantation, but these effects was not sustainable and generally maintenance 3-4 years after planted. Continuous planting has a positive effect and negative effect, but negative effect was more obviously. Continuous planting resulted in a significant reduction of soil nutrient of 0-20cm soil layers, in which Ca and Mg decreased the most obviously. Continuous planting of eucalypt plantation also decreased soil pH and resulted in soil acidification.Based on measuring the species composition, biomass and nutrient etc. in the 36 plots (each 100m2), the relationship between plant species diversity and ecosystem function has been researched using three measures of species diversity, and four measures of ecosystem function. The results showed that a relationship between species diversity indices and biomass of tree parts of eucalypt plantation was not close, but species diversity indices with biomass of absorbing root (diameter is less 0.6cm) were closely and extreme closely negative relation. It is indicated that the biomass of absorbing root decreased significantly with increasing of species diversity of understory vegetation. Plant species diversity effected significantly on the biomass of shrub layer and herbaceous layer, and species diversity increased a biomass and productivity of understory vegetation. Plant species diversity effected significantly on the nutrient use efficiency (NUE) of tree layer, shrub layer and herbaceous layer. However, different layer of community and nutrient element were different in relationship between species diversity and NUE. Species richness, Shannon-Wiener index and evenness index with NUE of N, P, K and Ca of tree layer were a positive relation of significance and extremely significance, but Mg was a significant negative relation. The result showed that species diversity improved the NUE of N, P, K and Ca of the tree layer, and decreased the NUE of Mg of the tree layer. The study also found that species richness with NUE of plant species of shrub layer were a positive relation of extremely significance. Except for species richness with K element was a closely relation in the herbaceous layer, another were not closely relation. Species richness increased the NUE of plants. The species richness of the community was higher, the species resilience, vegetation covering resilience, biomass resilience and soil nutrient resilience was higher. Species diversity was advantageous to improve a resistance and resilience of community and strengthen community stability. Due to strong disturbance with continuous planting of eucalypt plantation effected obviously on the community stability that threaten a sustainability of species diversity and soil fertility in eucalypt plantation ecosystem. Therefore, a new hypothesis, resource and space classifying management, on the maintaining mechanism of high species diversity and high productivity in eucalypt plantation were put forward.