| Water is key limiting factor for vegetation restoration in Loess Plateau of China. In recent years, soil aridization has turned into an important restricted factor on ecological construction in Loess Plateau and has been paid more attention by researchers. Robinia pseudoacacia, Platycladus orientalis and Pinus tabulaeformis are the planting tree species that are widely planted in this region to control soil erosion and land desertification. There is much research on vertical distribution and seasonal dynamics of fine root biomass. However, the importance of fine root surface area for nutrient and water uptake is apparent and the seasonal dynamics of fine root surface area remain poorly understood. The study of tree fine root distribution, seasonal dynamics and its relationship with environmental factors, is significant to the revealing of the formation mechanism of soil dried layer, to the scientifically selecting tree species in forestation, to the speeding up construction of the Northwest Landscape Beautification Project.During the growing season in 2006 and 2007, we selected Changwu county, Jingchuan county and Ansai county as the study area, according to the different characteristics of precipitation in Loess Plateau region. Ansai county lies in the semiarid region of the Loess Plateau of China, but Changwu county and Jingchuan county lie in the semihumid region of the Loess Plateau of China. In study area, by using soil coring method, we got the samples of tree fine root and the soil samples in R. pseudoacacia, P. orientalis and P. tabulaeformis forests. The impact of soil moisture on the seasonal dynamics of fine root surface area of main planting tree species in Loess Plateau was studied from the view of ecological zones large-scale and the forest environment small-scale. The main results were below:(1) Characteristics of vertical distribution and seasonal dynamics of fine root surface area and soil water content of main planting tree species in Loess PlateauIn Changwu county and Jingchuan county, during the growing season in 2006 and 2007, respectively, the soil moisture actively changed in the 0-150 cm soil layer where more than 87.2% of the total fine root surface area were concentrated in the R. pseudoacacia plantation. In Changwu county, the mean soil water content in the R. pseudoacacia plantation changed in order of April>October>July and the mean total fine root surface area changed in order of April > July > October. In Jingchuan county, the mean soil water content in the R. pseudoacacia plantation changed in order of October>April>June>August and the mean total fine root surface area changed in order of April>June>August>October.In Ansai county, during the growing season in 2007, the soil moisture actively changed in the 0-200 cm soil layer where more than 82.4% of the total fine root surface area were concentrated in the R. pseudoacacia, P. orientalis and P. tabulaeformis plantations, respectively. The mean soil water content in various forests all changed in order of October>April>June>August. The highest value of fine root surface area was in June for R. pseudoacacia and P. tabulaeformis, while it was in June and October for P. orientalis.In Ansai county, Changwu county and Jingchuan county, the values of fine root surface area of different tree species all decrease slowly along the soil profile. The model S =Ah~B(C+Dh+Eh~2+Fh~3) can well described the fine root surface area vertical distribution of the R. pseudoacacia, P. orientalis and P. tabulaeformis plantations, the fit decide coefficient (R~2) was more than 0.85. During the growing season, there was more fine root surface area in the soil layer where the soil moisture actively changed. But In Ansai county, Changwu county and Jingchuan county, there lay differences in the seasonal dynamics of fine root surface area and soil water content.(2) Difference of surface area of fine roots of R. pseudoacacia in the different climate regions of Loess PlateauThe vertical depth of fine root distribution, the fine root area and seasonal dynamics of fine root area of R. pseudoacacia were different between Ansai County and Jingchuan county. The 86.5% of total fine root area was in the 0～200 cm soil layer in Ansai county , but the 87.6% of total fine root area was in the 0～150 cm soil layer in Jingchuan county. In the growing season in 2007, the highest value of fine root area was observed in April in Jingchuan county, while it was in June in Ansai county. The difference of cumulative fine root area between Ansai and Jingchuan was not significant in June. By comparing the characteristic of fine root area of R. pseudoacacia in Ansai County with the characteristic of fine root area of R. pseudoacacia in Jingchuan county, the R. pseudoacacia in Ansai County was to reduce the density of fine root area, to increase the vertical depth of fine root distribution and to maintain the cumulative fine root area for tree growth.(3) Correlation between soil water content and vertical distribution, seasonal dynamics of tree fine root surface areaIn Ansai county, Changwu county and Jingchuan county, there was a significant positive correlation (p<0.05) between vertical distribution of fine root surface area and soil water distribution in the profile. But there was not significant correlation (p>0.05) between the seasonal dynamics of fine root surface area and soil water content. The results indicates that combined and integrated soil moisture, temperature, available nitrogen and genetic characteristics of tree species should be considered in research on seasonal dynamics of fine roots.(4) Correlation between tree fine root surface area and seasonal dynamics of soil water content in the deep soil layerThe coefficient of variation of soil moisture in the soil layer (0-200 cm soil layer for Changwu and Jingchuan county, 0-300 cm soil layer for Ansai county) where the tree fine roots distributed was bigger than it in the deep soil layer (220-380 cm soil layer for Changwu, 220-500 cm soil layer for Jingchuan county, 320-500 cm soil layer for Ansai county). In Changwu, Jingchuan and Ansai county, the soil profiles of woodland were divided into two layers based on the vertical distribution of coefficient of variation of soil moisture. One was the soil moisture actively changing layer where the soil moisture changed actively. The other was the soil moisture stably changing layer where the soil moisture changed stably. The soil moisture actively changing layer was in the 0-200 cm soil layer for Changwu and Jingchuan county, but was in the 0-300 cm soil layer for Ansai county. The soil moisture stably changing layer was in the the 220-380 cm, 220-500 cm and 320-500 cm soil layer for Changwu, Jingchuan and Ansai county, respectively.There was a significant negative correlation (p<0.05) between the coefficient of variation of fine root surface area in the soil moisture actively changing layer and the coefficient of variation of soil moisture in the 220-380 cm, 220-500 cm and 320-500 cm soil layer for Changwu, Jingchuan and Ansai county, respectively, but there was not significant correlation (p>0.05) between the coefficient of variation of fine root surface area in the soil moisture actively changing layer and the coefficient of variation of soil moisture in the 420-500 cm for Jingchuan county. The results indicates that tree root affected on the soil moisture status in the deep soil layer, but the affected depth of soil moisture status was different in the different climate regions of Loess Plateau. The affected depth of soil moisture status in Ansai was bigger than it in Jingchuan.This study indicates that soil water is the important factor to effect the growth and distribution of fine root surface area of main planting tree species in Loess Plateau, combined and integrated soil moisture, temperature, available nitrogen and genetic characteristics of tree species should be considered in research on seasonal dynamics of fine roots.
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