Study On Spatio-temporal Change Of Eucalyptus Puplwoods Soil Water And Its Ecological Effect In West Hainan

With the rapid increase in consumption of forest products, developing artificial forestsbecome an important means for countries to solve the contradiction between supply anddemand of forest products. In Mainland China, there are over two million square hectometersof artificial Eucalyptus forests planted in Guangdong, Guangxi, Hainan and other provincesand autonomous regions, which is in the front rank of all countries. Despite the significanteconomic benefits brought by the planting of Eucalyptus trees, arguments about a series ofrelevant ecological problems have arisen. In the west and southwest of Hainan Island, theannual rainfall is about1000mm, and the drought severity index is over2. With theprogressing of Eucalyptus pulp plantation project, forest areas joined together to formlarge-scale cultivation belts. A regular repeated felling, i.e., all trees are cleared at one time in5to6years, and multi-generation continuously planted rotations are exerting a significantimpact on regional hydrology and soil ecology.In this paper, the research target area sited in Danzhou where a large scale of Eucalyptusplantation distributed. We selected1-to-4-year old Eucalyptus and Cocos trees with similarspatial and natural conditions as research objects. After continuous observation in fixed sitesand the extended experimental time sequence by the way of space alternative time of method,we analyze the change pattern of soil water and its vertical change features and compare thedifferences of soil water among short-rotation Eucalyptus plantations, non-short-rotationEucalyptus plantation and Cocos plantation in order to explore the internal relationshipbetween soil moisture and growth of short-rotation Eucalyptus trees and provide a scientificbasis for the ecological management and sustainable development of artificial Eucalyptusforests in Hainan, and a reference for similar regions to develop their pulp wood plantations.The results showed that:(1) There were significant changes in the soil moisture in different seasons. In the dryseason (from January to May) the soil moisture gradually decreased, while it graduallyincreased in the rain season (from June to October) and the peak value occurred in Septemberand October. From November, the soil moisture declined again. This indicates thatprecipitation is the most important factor to Eucalyptus soil moisture content. The averagemoisture content of sample C (three generations of4Eucalyptus plantation in age) in0-150cm soil layer is5.47%,5.42%,4.89%,4.88%,4.81%,4.81%,4.79%,7.46%,7.32%,8.15%,6.86%, and5.90%from January to December respectively. According to the changesin soil moisture, the whole year can be classified into two phases of accumulation and depletion.(2) In the0-50cm depth soil layer, the variation of moisture content over time becamemore dramatic with decreasing depth in soil. This showed that the external environment ofthe air had a large effect on the surface layer of the soil. The soil moisture of50-100cm layerwas relatively stable compared with that of the surface layer (0-50cm). The soil moistureunder100cm was inversely proportional to the depth which showed that precipitation onlymarginally influence this layer and a small number of tree roots in this layer consume a smallamount of water.(3) The soil moisture varied significantly for Eucalyptus in different ages. The older theforest wa, the less soil water there was.(4) The breast height of Eucalyptus changed seasonally. The breast height growth ofEucalyptus in different age was different and the average growth of breast height for olderEucalyptus was slower. In addition, the Eucalyptus breast height growth increased with moresoil moisture.(5) In the long term, there was no significant difference between the soil moisture of thenon-short-rotation and continuously planted Eucalyptus forests and artificial Cocos forestsand their variations over time was similar. The average soli moisture of non-short-rotationand continuously planted Eucalyptus plantation and artificial Cocos plantation was9.96%(2010),10.80%(2011) and9.65%(2010),10.92%(2011) respectively which showed thatnon-short–rotation and continuously planted Eucalyptus plantations had little influence onsoil water ecology. However, for short-rotation and continuously planted Eucalyptus forests,the soil moisture almost dropped to the same valley value, about8.0%, before the rain seasonwithout regard to the different precipitation, which showed that the planting of Eucalyptushad a negative affects on the soil water ecology.