Relationship Between Microtopography Habitat And Forest Stand Structure On The Loess Plateau, North Shaanxi

Since the1980s, reforestation was designed according to the site type scale and along the contours with certain planting distance spaced arrangement in the loess plateau. Some plantation points were lainon the poor soil moisture habitat conditions of the slope. It led to the survival rate of artificial vegetation was low in the slope, and even formed a "small old trees", which ignores the effects of microtopography and microhabitat in the slope.Therefore, the soil and water conservation forest and microtopography were selected as the research object, and we considerd Wuqi County as the study area.Combined with the field survey of nutrients and soil water data, the3D laser measurement and high resolution remote sensing image interpretation technology were used, using ArcGIS, standardized precipitation index, Mann-Kendall trend test and Ripley ’K function, to reveal the area distribution of the microtopography and soil water and nutrient characteristics, and the response characteristic to regional drought evolution in recent56years.Finally, the effects of microtopography on growth characteristics and the spatial distribution pattern of forest standswere analyzed.The results showed that:(1)The difference among the area distribution of the microtopography was significant, and the area percentage of differerent microtopography types,from large to small, were gully>platform>ephemeral gully>scarp>sink hole.Meanwhile, the area distribution was effected by slope, aspect and the land use types.(2) The catchment of gully, platform and sink hole was about twice ability of scarp and uniform slope in the rainy season; there was positive correlation between the SPI of growing season and the change value of soil water layer thickness of microtopographies, and the dependence of soil moisture of microtopography with the precipitation, from large to small, was scarp, sunny uniform slope and half sunny uniform slope> gully, platform and ephemeral gully> sink hole.(3) The soil moisture of gully, sink hole and platform ranged from15%to18%at the layer of0-120cm, but that of scarp changed from13%to16%at the layer of0-80cm. The dry soil layers of platform, gully and sink hole appeared at the layer of120-140cm, but that of scarp appeared at80-100cm.(4) The soil nutrient content of available nitrogen, available P, available potassium, total N and organic matters displayed significant difference among the microtopography types, and the soil nutrient content of scarp had obvious topsoil accumulation.(5) In shady slope,height, DBH and litter depth had significantly positive correlations with the slope position index and significantly negative correlations with the slope index; while in sunny slope,height and litter depth had significantly negative correlations with the slope index; Conditions of same tree species in different microtopography types displayed the significant difference, especially,the growth rate of trees height with the DBH displayed the sequence, from large to small:Robinia pseudoacacia Linn., Salix matsudana Koidz and Populus simonii Carr in gully>Armeniaca sibirica Lam. in sink hole>Ulmus pumila Linn. in scarp.(6) The maximal aggregation radiuses of tree species on the microtopography slope fell in20-30m, but those of tree species on the uniform slope were larger than40m; andon the microtopography slope, Robinia pseudoacacia Linn. varied from strong negative association at microtopography scales (0-30m) to positive association (60-100m) with Pyrus betulifolia Bunge, Armeniaca sibirica Lam., Populus simonii Carr.and Ulmuspumila Linn., and Populus hopeiensis Hu&Chow-U. pumilashowed the same association pattern,while the others species pairs including P. betulifolia-P. hopeiensis,P. betulifolia-A. sibirica,P. betulifolia-P. simonii, P. betulifolia-U. pumila, P. hopeiensis-A. sibirica,P. hopeiensis-P. simonii, andA. sibirica-U. pumila showed no association.(7) Ulmuspumila-scarp, Armeniacasibirica+Pyrusbetulifolia and Populussimonii-platform, and Robiniapseudoacacia+Populus hopeiensis-gully could be appropriate forestation pattern in shady slopes respectively.Robiniapseudoacacia-platform and Ulmuspumila-gully could be appropriate forestation pattern in sunny slope respectively.