| The relationship of forest and water is a hot topic in eco-hydrology.In this study,the Robinia pseudoacacia plantations with different densities(750·hm-2,1600·hm-2,2196·hm-2,4563·hm-2),topographic locations?tableland,beam slope,ditch platform?and slope directions?shady slope,semi-shady slope,sunny slope and semi-sunny slope?were taken as the research objects in Zhonggou small watershed on the Loess Plateau of Eastern Gansu Province.By combining outdoor field observation and indoor experiment and taking the soil moisture changes as the focus to analyze the structure of Robinia pseudoacacia plantation,soil physical characteristics and the seasonal soil moisture variation to provide theoretical basis for water resources management,ecological vegetation restoration and construction of Robinia pseudoacacia Plantation in this area.The research results showed that:?1?The stand density of Robinia pseudoacacia plantation had significant effects on the average DBH,tree height and under-branch height?P<0.05?,which all showed in negative correlation.High density will lead to smaller average of DBH,tree height and height under branches.There was a negative correlation between stand density and mean crown width,but the effect was not significant.In topographic position factor,the growth of locust forest on the plateau?top?was the best,followed by locust forest on the plateau?bottom?,locust forest on gully platform was poor and locust forest on Liangpo was the worst one.Concerning from the slope aspect,Robinia pseudoacacia plantation grew best on semi-shady slope,followed by sunny slope and shady slope and the worst one was on semi-sunny slope.?2?The study on soil hydrophysical properties of Robinia pseudoacacia plantation showed that there were significant differences in soil bulk density and soil porosity among different stand densities?P<0.05?.With the increase of stand density,soil bulk density was decreased and the total porosity,capillary porosity and water holding capacity increased.The soil bulk density was750 plants·hm-2(1.26g·cm-3)>1600 plants·hm-2(1.24g·cm-3)>2196plants·hm-2(1.21g·cm-3)>4563 plants·hm-2(1.16g·cm-3).The average total porosity was 750·hm-2?47.83%?<1600·hm-2?49.17%?<2196·hm-2?50.38%?<4563·hm-2?51.64%?.The non-capillary porosity was1600·hm-2?6.52%?<750·hm-2?7.14%?<2196·hm-2?8.59%?<4563·hm-2?10.19%?.And the water-holding capacity of soil was 4563·hm-2>2196·hm-2>1600·hm-2>750·hm-2;The soil bulk density,total porosity and non-capillary porosity of Robinia pseudoacacia plantations at different topographic locations were significantly different?P<0.05?but the capillary porosity was not affected too much by the slope position.The ranking of soil bulk density was locust forest on the plateau?top?(1.24g·cm-3)>locust forest on the plateau?bottom?(1.20g·cm-3)>locust forest on gully(1.18g·cm-3)>locust forest on Liangpo(1.16g·cm-3).The ranking of total porosity was Robinia pseudoacacia forest in Liangpo?51.64%?>Robinia pseudoacacia forest in Goutai?50.94%?>Robinia pseudoacacia forest in tableland?lower??50.38%?>Robinia pseudoacacia forest in tableland?upper??49.17%?.The ranking of capillary porosity was locust forests?above??42.65%?>locust forests?below??41.79%?>locust forests on Liangpo?41.44%?>locust forests?41.30%?on gully plateau and the ranking of non-capillary porosity was locust forests on Liangpo?10.19%?>locust forests on gully plateau?9.64%?>locust forests on gully plateau?below??8.59%?>locust forests on gully plateau?above??6.52%?.Soil water holding capacity was Robinia pseudoacacia forest on Liangpo>Robinia pseudoacacia forest on gully platform>Robinia pseudoacacia forest on tableland?lower?>Robinia pseudoacacia forest on tableland?upper?.Slope direction has significant impact on the soil bulk density and porosity?P<0.05?shown as soil bulk density was sunny slope(1.29g·cm-3)>semi-shady slope(1.27g·cm-3)>shady slope(1.24g·cm-3)>semi-sunny slope(1.15g·cm-3)and the total porosity was semi-sunny slope?52.17%?>shady slope?49.17%?>sunny slope?47.67%?>semi-shady slope?47.38%?.The soil water holding capacity of semi-sunny slope was the best,followed by shady slope and semi-shady slope and the worst one was on sunny slope.?3?The research on seasonal variation of soil moisture in Robinia pseudoacacia plantation during the annual growth season had shown that the soil moisture in Robinia pseudoacacia plantation had obvious seasonal variation difference characteristics.Soil moisture showed a similar trend with the change of rainfall.The seasonal variation of soil moisture in Robinia pseudoacacia Plantation at different densities and different topographical locations could be divided into four periods,namely,consumption period?May-June?,replenishment period?July?,recession period?August-September?and stable period?October?.While Robinia pseudoacacia plantations with different slope orientation were stable period of soil moisture in August-September and recession period of soil moisture in October.During the annual growing season from May to October,the soil moisture content of Robinia pseudoacaciacia plantation plots with different density showed as 1600 plants·hm-2?18.75%?>2196 plants·hm-2?15.93%?>750 plants·hm-2?15.92%?>4563 plants·hm-2?11.87%?in the annual growing season from May to October.Soil moisture content in different topographic locations was best in the locust forest on the plateau?top?,Robinia pseudoacacia forest on the plateau?lower?and Robinia pseudoacacia forest on gully plateau was in the middle and Robinia pseudoacacia forest on Liangpo was the worst.Concerning from the slope aspect,the shady slope was the best,the semi-shady slope and the sunny slope were the middle and the semi-sunny slope of Robinia pseudoacacia plantation was the worst.?4?According to the variation coefficient and standard deviation,the vertical variation of soil moisture in 0-120cm soil layer of Robinia pseudoacacia plantation could be divided into four layers:fast-changing layer?Cv>0.3,S>4?,active layer?0.2<Cv<0.3,3<S<4?,secondary active layer?0.1<Cv<0.2,2<S<3?and relative stable layer?Cv<0.1,S<2?.In terms of different densities,the variation coefficient of 1600·hm-22 Robinia pseudoacacia plantation sample plots were the smallest and the soil moisture condition was the best.And the4563·hm-22 Robinia pseudoacacia plantation sample plots were all in the water velocity change layer from 10 cm to 100 cm and the soil moisture content was the lowest.For different topographic locations,the coefficient of variation of soil moisture in the top layer of locust forest on the plateau was the smallest,soil moisture was stable relatively while the soil moisture velocity layer of Robinia pseudoacacia forest in Liangpo was 90cm thick and the water condition was also the worst and Locust forest on the plateau?lower?was in the middle with Robinia pseudoacacia forest on gully platform.Concerning into the slope direction factor,the changes of soil moisture in sunny slope and semi-sunny slope were more active than that in shady slope and semi-sunny slope.More specifically,the changes of soil moisture in sunny slope was more stable than in semi-sunny slope and in shady slope was more stable than in semi-sunny slope. |
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