| For understanding the hydrological effect of forests and their spatial scale effect within the range of slope, six plots of representative forest types were set up, and a southeast-facing slope(with a horizontal length of 398.2 m) on which 16 plots of 33-years-old Larix principis-rupprechtii plantation were set up continuously was chosen in the small watershed of Xiangshuihe, in the semi-humid part of Liupan Mountains, northwest China. The slope spatial variation and temporal dynamics of vegetation structure, soil physical properties, topography and meteorological factors were investigated. The hydrological processes and the water balance of some plots were measured. Then, the main factors influencing the hydrological processes and the water balance were determined. The variation of vegetation structure, soil physical properties, soil moisture, stand evapotranspiration and its composition, and the water yield with increasing slope length from the slope top was quantified with statistical relations. The approach of up-scaling from a plot to the whole slope was explored for the stand structure and its hydrological effects. This study can promote the development of scale effect theory and scaling techniques in forest hydrology.1. Water balance and water yield of representative forest standsDuring the growing period(May to October) of 2012, the water balance and water yield were measured and compared among six representative forest plots. Under the open field precipitation of 566.1 mm near the 5 arbor forest plots and 507.2 mm near the shurb forest plot, the evapotranspiration follows the order of larch forest(568.2) > shurb forest(480.8) > B. platyphylla forest(480.7) > P. armandii forest(413.2). Q. liaotungensis forest(389.0) > P. tablaeformis forest(377.6); The change of soil water storage within the layer of 0-80 cm varied in the order: B. platyphylla forest(92.1) > P. armandii forest(57.8) > Q. liaotungensis forest(39.1) > shurb forest(10.1) > larch forest(2.8) > P. tablaeformis forest(-6.6); the water balance(mm) followed the order: P. tablaeformis forest(113.1) > Q. liaotungensis forest(54.7) > P. armandii forest(16.6) > shurb forest(12.9) > B. platyphylla forest(-82.1) > larch forest(-85.8). The order of water yield(mm) was as: P. tablaeformis forest(148.0) > Q. liaotungensis forest(125.4) > P. armandii forest(95.9) > B. platyphylla forest(83.7) > shurb forest(57.2) > larch forest(40.0).2. Slope difference and scale effect of the forest structureThe observation carried in the growing season of 2104 showed a remarkable difference in the forest structure index among slope positions. The DBH, tree height, dominant tree height, tree layer biomass and canopy leaf area index(LAI) presented a trend of firstly increase and then decrease with increasing slope length from slope top downwards. These differences were results of tree density and the slope difference of temperature, precipitation and soil properties. Since the tree canopy shading, the herb layer biomass changed along slope position somewhat different from the tree biomass, firstly relatively stable, then increase, and then decrease. The absolute value of changes per 100 m of horizontal slope length was used to evaluate the slope scale effect, and it was 0.40 cm for the DBH, 0.64 m for the tree height, 0.66 m for the dominant tree height, 0.12 for the canopy leaf area index(LAI), 6.12 t/hm2 for the tree layer biomass and 0.04 t/hm2 for the hurb layer biomass of forests on the slope.3. Calculation of stand sap flow density by considering tree height differenceThe sap flow density(Js) of dominant trees(with bigger tree height) was obviously different from the suppressed trees, it started earlier in the morning and ended later at night, had a long time span and a larger peak value. A lineal relation was fitted between the Js of single trees and their height at different slope positions. The stand Js calculated by using these linear relations differed within the range of-0.2%~7.9% when compared with the values calculated using traditional method not considering the tree height difference, with bigger Js difference in stands with bigger tree height difference. Therefore, it is proposed that both the tree dominance(or tree height difference) and the diameter representativeness should be taken into account when up-scaling the Js from individual sample trees to stand.4. Variation of daily stand transpiration by coupling the effects of potential evaporation, LAI and soil moistureBased on the envelope line analysis of observed data, the daily transpiration of larch stand follows logistic equation with increasing potential evaporation(PET) and tree canopy LAI. The daily stand transpiration firstly increased nearly linear and rapidly, and then tended to be stable when PET and LAI reached a certain threshold. The daily stand transpiration increased monotonically and nonlinearly with increasing volumetric soil moisture(SMC). Based on these relations, a more general model covering the whole variation range of PET, LAI and SMC was fitted T.......... This model can well explain the variation of daily transpiration of the larch plantation, and thus can be used to describe and calculate the daily transpiration of other forest stands under the influence of varying PET, LAI and SMC.5. Slope difference and scale effect of the stand transpirationThere was a remarkable difference in the transpiration in the growing season of 2014(May to October) among the stands located at different slope positions, within the variation range of 89.29-156.10 mm. With increasing distance from the slope top, the stand transpiration showed a trend of firstly increase, then decrease. The tree transpiration showed a slope scale effect. With increasing horizontal slope length(X, m) from slope top, the moving average of stand transpiration(Y, mm) showed a trend of firstly increase, then decrease, and finally to be stable, with the relation of Y =2E-06X3+0.002X2+0.4734X+89.914(R2=0.85). The slope scale effect for stand transpiration is a change of 12.10 mm per 100 m of horizontal slope length in the whole growing season, and the corresponding values of slope scale effect are 2.74 in May, 3.13 in June, 2.40 in July, 2.18 in August, 1.49 in September, 0.40 in October. It appears that the stand transpiration had a stronger slope scale effect in the months with dry-hot weather and bigger slope position difference in soil moisture.6. Slope difference and scale effect of the stand evapotranspirationThere was a remarkable difference in evapotranspiration(ET) among the stands at different slope positions, within the variation range of 361.85-440.97 mm during the growing season in 2014. With increasing slope length from the slope top, the stand ET showed a trend of firstly increase(in the slope length of 0-151.84 m), then decrease(in the slope length of 151.84-398.18 m), but there was a small recovery in the slope length range of 251.75-326.27 m. This slope variation was an integrated result of the difference in sapwood area, tree density and the slope position difference of temperature and precipitation. The ET showed a slope scale effect. With increasing horizontal slope length(X, m) from slope top, the moving average of ET(Y, mm) showed a trend of firstly increase, then decrease, with the relation of Y = 2E-06X3 + 0.0021X2+0.5367 X +376.48(R2=0.96). The slope scale effect of stand ET is a change of 12.89 mm per 100 m of horizontal slope length.7. Slope difference and scale effect of the water yieldObvious difference was found in the water yield among the stand plots at different slope positions, within a variation range of 175.64-203.60 mm during the growing season in 2014. With increasing slope length from slope top, the water yield showed a trend of firstly decrease(within the horizontal slope length of 0-225.13 m), then increase(within 225.13-398.18 m), but a slight decrease within the slope section of 251.75-326.27 m. The water yield showed a slope scale effect. With increasing horizontal slope length(X, m) from slope top, the moving average of water yield(Y, mm) presented a trend of firstly decrease and then increase, with the relation of Y=-2E-07X3+ 0.0003X2-0.0874 X +189.08(R2=0.98). The slope scale effect of water yield is a change of 3.44 mm per 100 m of horizontal slope length.8. Up-scaling of stand structure and hydrological effects from stand to slopeThere are obvious difference in stand structure(including vegetation and soil structure) and corresponding hydrological effect(such as ET and water yield) among stands at difference slope positions. Therefore, the representativeness of observed value of any stand varies obviously along slope positions. The statistical relations reflecting the variation of the ratios of stand structure and hydrological effects measured at any stand to the whole slope average(Y) with increasing horizontal slope length from slope top(X, m) were fitted. Using these relations, such as the stand ET(mm) of Y=2E-08X3-1E-05X2+0.0025X+0.92(R2=0.72) and the water yield(mm) of Y=-3E-09X3+4E-06X2-0.0009X1+1.0166(R2=0.79), the whole slope average can be estimated from the measured value at any stand with certain slope position. |
PDF Full Text Request