Research On Soil Moisture Dynamics And Simulation Under Different Planting Densities

Soil water content(SWC)is one of the most indispensable factor in hydrological processes and vegetation growth,especially in arid and semi-arid areas.In this study,six plots with different densities and different site conditions were employed to analyze the soil moisture content under two different hydrological conditions,using the combination method of field survey and hydrological model.This study was intended to investigate the effects of Chinese pine density on SWC and its temporal stability,and to analyze possible causes and vegetation water processes by means of modeling,further to provide a theoretical basis for sustainable management of regional forest.The main conclusions of the study are as follows:(1)Judging from the mean soil water content along different densities,SWC among different densities on the shady aspect showed no significant difference(P>0.05)both in high(2013)and normal flow(2014)years.On the sunny aspect,the plot with a low density(SUL)had the highest SWC and presented a significant difference(P<0.05)with other densities,and this difference was more obvious in the normal flow year(2014);as for the other densities on sunny aspect(SUM,SUH),significant differences were only presented in 2014 when compared with other plots.It shows that the effect of density on SWC was only appeared under the poor hydrological conditions,such as sunny aspect or less rainy years.When the soil moisture was relatively abundant(such as shady aspect),the SWCs of different densities had no significant differences.(2)The influence of density on SWC could be also affected by other factors such as slope aspect.On the shady slope,SWC of the near-surface layer(0-20cm)decreased with the increase of density.In the sub-surface layer(20-40cm)and the deep layer(40-60cm),mainly due to the competition of water uptake,root distribution changed dynamically into deeper layer,so the SWCs in these layers are less affected by planting density.On the sunny slope,the SWCs of each layer decreased with the increase of density.(3)High density tends to have a lower value of STD,i.e.the soil moisture dynamics is more stable,and this phenomenon is more obvious in the sunny aspect.The index of temporal stability(ITS)in low density plots was positively correlated with the soil water content,i.e.a wetter plot always showed a higher ITS value which means less stable.However,the relationship between soil water content and its index of temporal stability(ITS)value was not obvious in the medium and high density samples and a higher density tends to be more stable.This is partly related to the self-regulating mechanism of water formed by vegetation under long-term drought environment.(4)The simulation model of SWC was established,which can basically simulate the SWC on the surface layer(0-10cm).The SWCs at the shady aspect tend to decrease and then increase with the increasing of leaf area index,while the SWCs of sunny aspect was decreased with the increasing of density.It is concluded that the SWC conditions of low-density and medium-density at the shady slope are relatively good.We conclude that:in the rocky mountain areas of northern China,SWC under high planting density or on the sunny aspect was tend to be insufficient and stable(precipitation pulse cannot improve SWC),which indicated that long-term water deficit condition in this area was not conducive to the growth of vegetation.In order to promote the performance of forest ecosystem,it is not appropriate to plant higher density vegetation in this area.