| Currently,global climate change has a serious impact on human survival and social development,characterized by increasing CO2concentration,rising temperature,changing precipitation distribution patterns,and the aggravated nitrogen（N）deposition.The primary approach to combat climate change is to improve the carbon sequestration capacity of vegetation through artificial afforestation.China is the word’s largest timber importer and the second-largest timber consumer,where the contradiction between timber supply and demand in the market is highly prominent.Eucalyptus is the main tree species of plantation in China and even in the world’s tropical and subtropical regions.Guangxi has the largest area of Eucalyptus plantations in China.Eucalyptus plantation plays an important role in maintaining ecosystem balance,mitigating climate change,maintaining timber security,as well as providing regional ecosystem services and promoting economic development.However,the expanding scale and scope of planting,continuously declining operation cycle,and highly intensive cultivation mode of operation cause a series of ecological problems,such as deterioration of soil structure in artificial forestland,degradation of physical and chemical properties,decreased nutrient content,decreased in biodiversity,and decline in forestry productivity,seriously restricting the sustainable development of Eucalyptus industrial area.In this paper,South Guangxi,the main production area of the Eucalyptus Plantation,was selected as a study area,choosing five types of Eucalyptus Plantations:young forest（1a）,middle forest（2a）,near mature forest（3a）,mature forest（5a）and over-mature forest（8a）.It analyzed plant diversity,nutrient storage,and stoichiometry in Eucalyptus forests of different ages,and quantitatively examined the relationship between plant diversity and ecological factors（community characteristic factors,soil physical and chemical properties）in Eucalyptus plantations at different ages,exploring the effects of planting Eucalyptus at different ages on the ecological environment.This research provides a scientific basis for soil nutrient maintenance,soil fertility improvement in Eucalyptus plantations,and the sustainable development and management of Eucalyptus plantations in southern Guangxi and the Guangxi Zhuang Autonomous Region.The main conclusions are listed as follows:（1）There are 142 species in 128 genera and 63 families in the Eucalyptus Plantation in Southern Guangxi.Among them,there are 88 species,79 genera,and 35 families in the shrub layer,while 54 species,49 genera,and 28 families in the herb layers.The number of species in the understory of shrubs is highest in the near-mature forest（41 species）,and lowest in the middle-age forest（21 species）.As to herbage,the number of understory species is highest in mature forests（37 species）,and lowest in young forest（6 species）.In terms of species ecological types,most of the understory plants in Eucalyptus Plantations of different ages are neutral plants and native species,while the number of positive plants,negative plants,and alien species is fewer.The species richness and Shannon Wiener index of the shrub and herb layer under Eucalyptus forest initially decreased,then increased,and finally decreased again with the forest age;the shrub species diversity index（species richness,Shannon winner index,Simpson index,and Pielou evenness index）and the stand characteristics（canopy density,tree height,and DBH）,and the physical and chemical properties of soil（total potassium and available potassium）showed a significant trend（P<0.05）.The Shannon winner index of herbaceous plants was negatively correlated only with soil total potassium（P<0.05）.It can be seen that Eucalyptus plantation can maintain high plant diversity and a reasonable community structure during the near and mature stages of Eucalyptus plantation in southern Guangxi.The stand characteristics and soil physical and chemical properties of eucalypt plantation are the main factors affecting plant diversity.（2）The trunk serves as the main carbon,nitrogen,and phosphorus pool in the tree layer of Eucalyptus plantation,accounting for 51.61%-87.24%,19.98%-57.41%,and 42.55%-75.48%of the total carbon,nitrogen,and phosphorus reserves in the tree layer,respectively.Compared to the content of carbon element,the content of nitrogen and phosphorus varies significantly among the five forest ages.The soil layer is the main reservoir of carbon,nitrogen,and phosphorus in the Eucalyptus plantation ecosystem,accounting for more than60%of carbon reserves,and more than 90%of nitrogen and phosphorus reserves.In the 0～100 cm soil layer,the carbon content of the soil layer in the middle-age,near-mature,and mature forest was significantly higher than that in the young and over-mature forest.The nitrogen content in the young forest was significantly lower than that in the other four forest types,whereas the phosphorus content of the soil layer in the middle-aged forest and the over-mature forests was significantly higher than that in the young forest,near-mature forest,and over-mature forest（P<0.05）.In the whole Eucalyptus plantation ecosystem,the middle-aged forest has the highest reserves of total carbon,nitrogen,and phosphorus elements,followed by the mature and over-mature forests,while the near mature and young forest have the lower and lowest reserves,respectively.（3）The leaves and soil layers of Eucalyptus Plantation in South Guangxi showed the distribution pattern of high carbon,low nitrogen low phosphorus,while the litter layer showed high carbon,high phosphorus,and low nitrogen.The contents of carbon,nitrogen,and phosphorus in leaves vary significantly among different forest age（P<0.05）,showing a changing trend of first increasing and then decreasing with the increase of forest age,indicating a stronger demand for nutrients in the early stage of Eucalyptus plantation.The contents of nitrogen and phosphorus in the litter and soil layer,as well as the C/N ratio,C/P ratio,and N/P ratio were also significantly different among different forest ages（P<0.05）.There was a significant positive correlation（P<0.05）between the C/P ratio of the litter layer and the N/P ratio of the leaves,as well as between the N/P ratio of the litter layer and the content of carbon,nitrogen,and phosphorus in the soil layer and leaves.The content of nitrogen and phosphorus in the litter layer is lower than that in leaves,and the ratio of carbon to nitrogen and carbon to phosphorus is higher;the ratio of carbon to phosphorus and nitrogen to phosphorus in the soil layer is lower,which indicates that the decomposition of phosphorus in the soil layer is faster,and the phosphorus fertilizer can be applied to compensate for the lack of available phosphorus in soil layer during afforestation and forest management;the ratio of carbon to nitrogen in soil layer is higher,which indicates that the organic matter in soil layer has slower mineralization.The N/P ratio in the leaves of middle age,near mature,and mature forest was less than 14,which indicated that the growth process of Eucalyptus was limited by nitrogen.Similarly,nitrogen is the main limiting element of litter decomposition in middle age,near mature,and mature forest.Nitrogen-fixing plants or an appropriate amount of nitrogen can be added to improve soil fertility and compensate for the lack of nitrogen.The main limiting element of litter decomposition is phosphorus,which can be applied scientifically to improve soil conditions.（4）The nutrient reserves and leaf stoichiometric characteristics of Eucalyptus plantation ecological system are affected not only by tree height,DBH,canopy density,and other biological factors,but also by chemical properties such as soil organic carbon,total potassium,and available potassium.The carbon storage in the tree layer is positively correlated with tree height,DBH,and canopy density,while being negatively correlated with organic carbon,total potassium,and available potassium in the soil layer;nitrogen storage is positively correlated with tree height,DBH,and canopy density;phosphorus storage is positively correlated with tree height,DBH,canopy density,organic carbon and total potassium in soil layer,while being negatively correlated with available potassium in soil layer.The total P content of leaves was positively correlated with tree height,DBH,canopy density,organic carbon and total K in soil layer,but the influencing factors of C/P and N/P ratio were more complex.It can be seen that the nutrient reserves and stoichiometric characteristics of Eucalyptus plantation ecological system are affected by their biological factors and soil properties.Therefore,to maintaining the stable productivity of Eucalyptus plantation ecosystem and achieve sustainable development of regional Eucalyptus plantation,it is crucial to regulate both the nutrient status of the soil layer and the biological characteristics of the stand,especially the limiting factor of nitrogen. |
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