Water Balance And Suitable Stand Density Selection Under Half-mature Forest Of Pinus Slvestris In Yulin Sandy Area

The arid and semi-arid region in northern China is characterized by rare precipitation,heavy evaporation,dry climate,serious soil desertification,and sensitive and fragile ecological environment.In order to improve the deteriorating desertification and restore the ecological,a large area of Pinus sylvestris var.Mongolica（P.sylvestris）plantation was built with the help of“the Three-North Shelter Forest Programme”and other ecological construction project.However,P.sylvestris plantation began to decline and characterized as poor seedling renewal,weakened growth and death occurred.Although preliminary scientific research showed that the main reason of P.sylvestris plantation decline in northern sandy is water imbalance,water use strategies and characteristics of different ages and different regions,and the water adaptation mechanism of stand structure still need further discussion.We performed a field investigation and experiment in P.sylvestris plantation in Yulin semi-arid sandy area to monitor the transpiration,rainfall distribution,water balance and stand structure of P.sylvestris plantation,in order to explore the hydrological characteristics of P.sylvestris plantation and help to determine the suitable stand density for half-mature forest.The main findings and conclusions are the following:（1）The transpiration water consumption of P.sylvestris plantation results showed that the diurnal changes of sap flow was a single peak curve,and showed significant differences in different grow season.The average canopy transpiration per leaf area（E_L）was 0.60 mm·d^-1,and the average daily transpiration water consumption was 0.86 mm.The E_L was obviously affected by air temperature,potential evaporation,photosynthetic active radiation（PAR）,and vapor pressure deficit（VPD）.The average values measured for the dimensionless coefficient（Ω）is 0.15,it shows that the canopy and the atmosphere were highly coupled.The canopy stomatal conductance（g_s）was more limited by VPD than PAR.The average g_s was105.26 mmol·m^-2·s^-1 and the average aerodynamic conductance was 1152.30 mmol·m^-2·s^-1 in the growing season.（2）The measured penetrating rain（T_f）,trunk runoff（S_f）,and canopy interception（I）accounted for62.82%,1.13%,and 36.05%of rainfall（P）.The ratios of saturated evaporation rate（E_c）to rainfall intensity（R）was 0.0260（0.0458）,coefficient of canopy water holding capacity（S_c）was 1.210 mm（2.006mm）,coefficient of trunk runoff(p_tc)was 0.0630,and coefficient of trunk water holding capacity(S_tc)was 0.0486 mm.The simulation results of canopy interception of P.sylvestris plantations can be obtained accurately when E_c was 0.182 and S_c was 1.210 mm.The changing rate of canopy interception was analyzed for each input parameter based on the optimal modified Gash model.It showed that the parameter of P,S_c,S_tc,canopy closure（c）,E_c and R were the most sensitive and p_tc was relatively insensitive.（3）The density of plantation had a great influence on diameter at breast height.The degrees of neighborhood comparison can be compared by the neighborhood comparison of height,crown and the branch angle score.The speed of vertical growth was faster,but the crown growth did not decrease significantly.It means the neighborhood comparison of crown showed no obvious differences in order to compete for ecological niche and water resources.（4）The thickness of forest litter layer increased with the stand density.The sample plot with the highest effective water holding capacity of litter was 1250 plants·hm^-2,in which the non-decomposed layer was 3.66±0.21 t·hm^-2 and the semi-decomposed layer was 2.66±0.22 t·hm^-2.In terms of effective of storage capacity,1250 plants·hm^-2 sample plot was also the highest,and the non-decomposed layer was 3.31±0.42 t·hm^-2,the semi-decomposed layer was 2.12±0.22 t·hm^-2.The effective storage capacity of non-decomposed layer was greater than that of semi-decomposed layer.The water storage capacity was strong at the initial stage of rainfall,which has good forest hydrological characteristics and ecological significance.（5）There was an obvious difference between soil saturated water capacity and capillary water capacity.Among them,the capillary water holding capacity of sample plot with stand density of 2250plants·hm^-2 was the highest among all plots.The sample plots with the highest effective water holding capacity and the maximum water holding capacity were 1100 plants·hm^-2,which was 53.12 t·hm^-2 and284.56 t·hm^-2.In terms of soil nutrients,925 plants·hm^-2 sample plot had the higher content of total N,P and hydrolytic N quality percentage in each layer of soil,and the content of soil organic matter in 1250plants·hm^-2 sample plot and 2250 plants·hm^-2 sample plot was higher than the other’s.From the perspective of fractal dimension value,the largest fractal dimension was found in 925 plants·hm^-2 sample plot.（6）Based on the relationship of water balance in forest land,the forest water had surplus when the stand density was less than 1100 plants·hm^-2 and it had the highest water adaptation degree.We obtained the forest structure and rainfall distribution function variables of different density plantation sample land with the factor analysis method.Combined with the typical analysis method,the results showed that the sample plot of 925 plants·hm^-2 had the best effect in structure,function and rainfall distribution.Our study revealed the water balance of P.sylvestris plantation with different stand density in Yulin sandy area.The main of the study is to find the suitable planting density based on water balance,forest structure and rainfall distribution function.Our research results can provide the theoretical basis for afforestation density in desertification control.