| Soil moisture is the dominant factor restricting vegetation restoration and reconstruction in arid and semi-arid region. It is significant for improving stability and productivity of forest vegetation, and accelerating vegetation restoration in ecological fragile area to study the dynamic equilibrium between tree water consumption and soil water supply. In this study, four typical stands on gentle slopes in low mountains and hills of western Liaoning, namely mixed forest of Pinus tabulaeformis and Robinia pseudoacacia (P.×R.F), Pinus tabulae for mis forest(P.F), Robinia pseudoacacia forest(R.F) and Prunus sibirica forest(S.F), were taken as test objects, and the soil moisture dynamics,tree water consumption characteristics and vegetation carrying capacity were analyzed by fixed observation test and pot culture test.The fixed test fields in the 4 types of stands(non-forest stand as control) were located at Beipiao Tree Breeding Center, in which the soil moisture, sap flow and meteorological factors etc were observed. Pot culture tests of Pinus tabulaeformis, Robinia pseudoacacia and Prunus sibirica were set up to determine the parameters of water physiology and photosynthetic physiology under different water gradient.The main results are as follows:(1) Physical properties related to soil moisture in different types of standOverall, the water holding capacity, permeability the soil organic matter content and the soil particles(d<0.01mm) of the mixed forest are higher than those of the pure forests, and those of the pure forests are higher than non forest in 0-20 cm and 20-40 cm soil layers, so the mixed forest is the excellent stand type for soil and water conservation in the reserch area.The soil moisture characteristic curves of 4 types of stand and non-forest stand were simulated by the exponential model:θ=ae-bs. The wilting coefficients were determined, and the values of P.×R.F, R.F, P.F, S.F and N.F are 3.95%,4.30%,4.70%,4.00% and 4.80%, respectively.(2)Temporal dynamic characteristics of soil moisture in different types of standIn the diurnal variation of soil moisture in 0-20 cm layer,soil moisture keeps at a high level at cloudless night and present "V" shape curve during sunny day. The change trend is similar on cloudy day, but the variation is greatly reduced. The diurnal variation of soil moisture content is negatively correlated with atmospheric temperature highly significantly(p <0.01), but positively correlated with relative humidity of atmosphere highly significantly(p <0.01).Based on seasonal variation characteristics of soil moisture, one year can be divided into 4 periods:soil moisture consumption period(from Apr.to May), the soil moisture content is low and the variation range is small; soil moisture recovery period(from Jun.to Aug.), the moisture content high and variation range big; soil moisture regressive period(from Sep. to Oct.), the content gradually declines to low level; soil moisture stable period(from Nov. to Mar. of next year), the soil is frozen, the moisture content low and variation range very small.The influencing factors of surface soil moisture were analyzed by the gray correlation analysis method. The result showed that on rainy day, the correlation degrees between the meteorological factors and the seasonal variation of the surface soil moisture content are:the temperature in forest>the air temperature>the relative humidity>wind speed,0.88,0.81, 0.77 and 0.72 respectively, but on sunny day, wind speed> the relative humidity> temperature in forest> air temperature,0.92,0.86,0.74 and 0.61, respectively.The time sequence models-ARIMA(1,1,1) of soil moisture content in 0-20 cm layer of the 4 types of stand and non-forest stand were built.(3) Soil moisture distribution characteristics in profile for different types of standOn the whole, it was found that the descending order of soil moisture content in the profiles is:P.×R.F>S.F,P.F>R.F>N.F, and variation coefficient:N.F>P.F> R.F> S.F> P.×R.F.The linear regression equations for estimating soil moisture content of 30～100 cm layers were built according to the soil moisture contents of 0-10 cm,10～20 cm,20～30 cm layers, which can significantly reduce the workload of observation.By cluster analysis,soil profile could be divided into three layers with different water change characteristic:the quickly changing layer in moisture (0-30 cm), the mean value of soil moisture is higher and the coefficient of variation is larger in one year; The moisture stable layer(60-100 cm),the mean value of soil moisture higher, the coefficient of variation small; Moisture transition layer(30-60 cm), the mean value of soil moisture low, the coefficient of variation between the former layers. Semivariance function models simulated horizontal spatial variation characteristics of forest soil moisture in 0-10 cm and 10-20 cm layers..(4)Water consumption characteristics of different tree speicesThe sap flow of Pinus tabulaeformis, Robinia pseudoacacia and Prunus sibirica was observed by thermal probe diffusion method (TDP), which exhibits a significant diurnal variations:the sap flow density change showes a single wide peak curve on sunny day, a jagged peak curve on cloudy day, little sap flow on rainy day,and that of night decreases to the minimum(even to 0).The peak of daily average sap flow density on sunny day in every month of growth season is:Pinus tabulaeformis> Robinia pseudoacacia> Prunus sibirica. The seasonal density variance of the tree species sap flow was simulated by the time sequence model.The sap flow density is positively correlated with air temperature, sunlight intensity and vapor pressure deficit significantly, but negatively correlated with RH significantly. Sap flow prediction models of three tree species were constructed based on environmental factors.(5) Effects of soil moisture on sap flow and photosynthetic physiologyThe dynamic of profile soil moisture has obvious different effects on the sap flow in different tree species. For Pinus tabulaeformis and Robinia pseudoacacia, the correlation degree between sap flow density and soil moisture in 90～100 cm layer is the maximum.and the correlation has the trend which becomes bigger with soil layer depth in 0～100 cm. For Prunus sibirica, correlation degrees between the soil moisture in 0～60 cm layers and sap flow are bigger, and that between the soil moisture in 40～50 cm layer and sap flow biggest.The Threshold Effect of soil moisture on main photosynthetic indexes can be observed, for example,net photosynthetic rate(Pn) and water use efficiency(WUE) of 3 tree species increase with soil moisture content, but increase gradually after soil moisture reaches to the certain threshold value. By cluster analysis,soil moisture was classified to 4 classes, namely, soil moisture content of high productivity and high efficiency, that of middle productivity and middle productivity, that of middle productivity and middle productivity, and that of low productivity and middle efficiency.(6)Vegetation carrying capacity of soil moistureThe relationship between diameter of trunk and daily water consumption of single tree was simulated by the exponential model:y=Aebx(y, water consumption of single tree; x, diameter of trunk), based on which the annual water consumption of single tree with different diameter class in 3 tree species was predicted.The dynamic evaluation of test forest soil water availability was undertaken by the soil water productivity classification standard and wilting coefficient.The result showed that the soil moisture in the test forests can reach the level of medium efficiency water and high efficiency water in soil moisture recovery period. In soil moisture consumption period and regressive period, the soil moisture contents are lower, but higher than wilting coefficient, so soil moisture consumption period and regressive period become crucial period for the decision of vegetation carrying capacity.A method of vegetation carrying capacity determination was proposed, which is based on the existing stand density, and the increase or decrease of the density is determined by effective water storage in soil moisture consumption period and single tree water consumption in soil moisture consumption period and regressive period. The maximum retention density of Pinus tabulaeformis, Robinia pseudoacacia and Prunus sibirica with different diameter was estimated, namely,1621～294 plants·hm-2 from 10cm to 22cm diameter classes of Pinus tabulaeformis,2069～287 plants·hm-2 from 10cm to 24cm diameter classes of Robinia pseudoacacia,4186～1059 plants·hm-2 from 6cm to 14cm diameter classes of Prunus sibirica,respectively, which can provide reference for stand density management.To conclude, in this study, rules of the soil moisture temporal dynamics and distribution in profile were revealed,4 periods and 3 layers of soil moisture dynamics were divided, water consumption characteristics of main tree species was specified, and quota of forest water requirement were quantified; The grading standard of forest soil water productivity was put forward, which will provide theoretical basis for the evaluation of soil moisture availability; A new method of vegetation carrying capacity determination was proposed, and the maximum retention density of Pinus tabulaeformis, Robinia pseudoacacia and Prunus sibirica with different diameter class was estimated. The research results provide scientific basis for highly efficiency utilisation of forest soil moisture and sustainable management of forest in the research area. |
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