| Hippophae rhamnoides is the pioneer species for ecological restoration in arid and semi-arid loess areas in China.It has the natural characteristics of drought resistance,cold resistance and barren resistance,and its strong sprout-tillering ability can guarantee the "eternal life" of the population.It plays an extremely important role in ecological environment construction and economic development However,since 2010,large-scale decline and death of sea-buckthorn occurred in Wuqi County,Shaanxi Province,the largest plantation area of hippophae rhamnoides.Thus,this research to mortality,and death,renewal rates as the decline of hippophae rhamnoides plantation density and renewal measures,using the one-way ANOVA,grey correlation analysis,change-point analysis method,analyzes the region of hippophae rhamnoides plantation recession death and natural regeneration response mechanism of opposition to factor,the results are as follows:(1)Slope aspect,slope gradient and micro-topography were significantly correlated with death of hippophae rhamnoides plantation(P<0.05).The mortality rate of different slope aspect shows as sunny slope>semi-sunny slope>semi-shady slope>shady slope.With the increase of gradient,the mortality of hippophae rhamnoides plantation increased,and the decline of hippophae rhamnoides on steep slope was the most serious.The death rate of platform was significantly higher than that of other micro-topography types.Slope position had no significant effect on hippophae rhamnoides mortality(P>0.05)(2)Slope aspect had significant influence on natural renewal of declining hippophae rhamnoides forests,and the condition of shady solpe’s declining hippophae rhamnoides forests was the best.Micro-topography was significantly correlated with the natural renwal rate of hippophae rhamnoides,and the renewal rate presented as scarp<ephemeral gully<platform<original slope.Slope position and slope gradient had no significant influence on the renewal rate of declining hippophae rhamnoides plantation(P>0.05),and the shade slope and gentle slope located in the middle slope position were most suitable for the renewal of budding(3)The grey correlation analysis to the four topographic factors and mortality and renewal rate showed that slope aspect and slope gradient could be used as the screening indexes of the antigenic factors of the declining death of hippophae rhamnoides plantation,and the micro-topography and slope aspect could be used as the screening indexes of the antigenic factors of the declining natural renewal of hippophae rhamnoides plantation(4)The death rate and renewal rate of hippophae rhamnoides plantation had obvious topographic thresholds,and the stable state of death or renewal would suddenly change at the thresholds.The gradient-threshold of death was 24°～260 and 40°～41°.When the gradient was 17°～18°,the death rate also jumped.The death threshold value of slope aspect of hippophae rhamnoides plantation on gentle slope was 1250 and 285°.The death threshold of slope aspect on steep slope was 85° and 255°.With the increase of gradient,the natural renewal rate of the declining slope aspect forest on the sunny slope would suddenly drop sharply,and the threshold point was 31°;the renewal rate of the declining slope aspect forest on the sunny slope would suddenly drop sharply and jump once,and the threshold point was 25° and 36° respectively.Under gentle slope condition,the renewal threshold point of slope aspect was 90°.Under the condition of steep slope,the rrenwal rate would drop sharply at 215° and jump to 300°.(5)When planting Hippophae rhamnoides artificial forests in arid and semi-arid loess Plateau region,the thresholds of topographic factors should be mastered.The slope gradient which is less than the threshold range of 18°～31° and slope aspect which is less than the threshold range of 85°～215° or bigger than 255°～300° should be preferred,so as to obtain better forest formation effect. |
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