| Water is the main limited factor in apple yield in the loess region of west Shanxi Province, based on the theory of time series and soil-plant-atmosphere continuum, measurement on potted plant and observation in field are all introduced. With analyses and forecast of precipitation resource in the loess region of west Shanxi Province and its influence to apple yield, studied on water utilization and water use efficiency in different soil water content of apple tree, the root distribution patterns, commutative influences of root and soil water and nutrient in agro-forestry and relationship between root and upper-ground parts, the result as follows was indicated:Occurrence of extreme drought weather is on the trend of increase and the times of flood year is decreased in the loess region of west Shanxi Province; intra-annual rainfall variability arrived 59.2% and it's drought in spring and winter; inter-annual rainfall variability presented increased trend and instability of precipitation is climbing. Apple yield is mainly influenced by water factor and many other meteorological factors, especially the precipitation in March,May and August. The Hurst index in March,May and August of year 2002-2006 is 0.42,0.64,0.41,which forecast the precipitation is on increased trend in March,decreased trend in May and August of year 2007,and the facts accord with the forecasting. The Hurst index can forecast 4-5 years.Transpiration of apple tree in different soil water content is increased with water content and the soil water condition of apple tree can be divided three kinds that is 6%-10%(low soil water content),12%-16%(medium soil water content),18%-20%(high soil water content);the correlation between soil water content and evapotranspiration,transpiration is significant and its correlation coefficient is 0.74,0.65;the regression model of evapotranspiration(Et),transpiration(Tr) to temperature(T),relative humidity(RH),wind velocity(V),sunshine(L) is Et=-26.091+15.754T-2.09RH+25.506V+14.805L and Tr=45.772+8.426T-1.567RH-17.134V+14.082L,in which temperature is the supreme influencing factor.In drought year, the most water utilization of apple tree is in last ten-day of June and the first ten-day of July; July is the most water-need period; The first ten-day of May to the last ten-day of June is the critical water requirement period of apple tree and the April to May is the period sensitive to water. In the normal year, the water utilization of apple tree presented double-peak curve with the first peak in last ten-day of May and second peak in September to October; the first ten-day of May to the last ten-day of June is the critical water requirement period of apple tree and the last ten-day of March to the first ten-day of May is the period sensitive to waterSoil water content is 16.04% which produced maximal photosynthetic rate and the least soil water content to photosynthesis is 4.31%; photosynthetically active radiation is 1030μmol/m2?s which produced maximal photosynthetic rate and the least photosynthetically active radiation to photosynthesis is 50.70μmol/m2?s;water use efficient is the optical in 15.19% soil water content. The optical range of soil water content of apple tree in the loess region of west Shanxi Province is 11%-15%, which indicated that medium to high apple yield can be obtained by lower investment and irrigation in time which maintain soil water content on low to medium status.Apple root of agroforestry on gentle and steep slope distribute deeper than pure apple trees; root system and sucking roots of old apple tree distribute deeper soil than that of young apple tree to absorb more soil water and nutrient, with which root diameter and percentage of coarse root are increased. For young apple tree, protective zone set on the upper ridge of contour ditch with 0.5m width on gentle slope and set on the lower ridge of contour ditch with 0.5m width on steep slope; for old apple tree, protective zone is 0.5-1.0m width either on gentle slope or steep slope.On gentle slope, apple root absorb soil water mainly from field of upper ridge and the correlation between root length,root area,fine root length and soil water is significant; on steep slope, apple root absorb soil water mainly from field of lower ridge, and soil water condition of old apple tree is better than young apple tree, besides, the correlation between soil water and root growth of young apple tree is negative in the range of 6% to 10% whereas the correlation between soil water and root index of old apple tree is positive.Soil nutrient of apple tree, which pH value present alkalescence with content of available phosphorus and kalium are scare whereas content of soil organic matter and total nitrogen are rich, has great influence on root growth. The relative fine root length difference (RFRLD) of apple tree in different soil layer and different distance from apple are all not equal to zero, that indicated root growth present phenomena of proliferation and tend to grow in soil with abundant nutrient and has high percentage.The relation between root system of apple tree and upper parts can be described by fractal model W=CLD,in which D is the fractal dimension, that indicated root biomass of apple tree accumulated with diameter at breast height(DBH),tree heihgt,crown size by power function method,that is accompanied by DBH enhanced one unit the root weight and root length increased by rule of 3.1,2.0476 of power function; accompanied by tree height enhanced one unit the root weight and root length increased by rule of 3.7737,2.7888 of power function; accompanied by crown size enhanced one unit the root weight and root length increased by rule of 1.0324,0.6856 of power function. According to tree height,DHB or crown size with the fractal dimension D, the underground biomass can be calculated.
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