Researchs On Deterioration Mechanism Of Seabuckthorn Plantations

Seabuckthorn (Hippophae rhamnoides) is important water conservation trees and crop trees in Loess Plateau. In recent years, large-scale seabuckthorn plantations appeared decline or death in many parts of northern of China, which resulted in huge losses to the local ecological environment and regional economic development。Although it has aroused great attention and been studied, the reason and mechanism remained open.This study researches on seabuckthorn plantations at different growth stages (8a, 13a, 18a) in Malian Beach River of Yongshou County in Shaanxi Province. According to the analysis of the soil physical and chemical properties, root vigor, stand diameter class structure, understory species diversity, soil microbial functional diversity, natural regeneration, we look forward to reveal the mechanism of the decline of seabuckthorn. The main results are as follows:1. 8a seabuckthorn plantations are shrubby and it's/the height and diameter are minimum. Although the coverage are large (88% or so), trees grow well and tree diameter class structure showes"growth-type"with good light transmission. There are the most abundant understory plant species and the highest concentration. Although 8a have a higher proportion of lived root nodules (85% or so), the individual nodules are minimum ( diameter <0.95cm, longitudinal diameter< 0.60cm);13a seabuckthorn plantations grow best, tree height, diameter and canopy density are moderate (77% or so), diameter class structure present"stabilization-type".Understory plant species are richer and more concentrated. There are larger (diameter> 1.40cm, longitudinal diameter> 0.85cm),and the highest percentage of individual nodules (93% or so);The tree height and diameter of 18a seabuckthorn plantations are largest.What's more,the canopy density are large (86% or so),but the tree growth and light transmission are worst, and tree diameter class structure present "recession-type". Understory plants are small and more scatter. Although 18a have the largest lived individual nodules(diameter> 1.80cm, longitudinal diameter> 1.15cm), the proportion (63% or so)are lowest.2. Both absorbing roots and tiller roots of seabuckthorn plantations with different ages are concentrated in the surface. The biomass of absorbing roots (0.5-2mm) is 18a>13a>8a, The biomass of tiller roots is 13a>18a>8a.The surface area of absorbing roots (0.5-2mm) is 13a> 18a> 8a, The surface area of tiller roots is18a>13a>8a.8a seabuckthorn plantation have more suckering root system, less roots, and roots tend to tiller,. So there is the smallest root biomass,. But the root activity is stronger than 18a Health Seabuckthorn,and the roots concentrated and there is the maximum density of understory sprouted seedlings (1,566·hm-2); 13a seabuckthorn have most suckering root system more roots, and roots tend to tiller and thicken, So there is the largest root biomass, strongest root activity, and stronger natural regeneration ability (833·hm-2); 18a Health Seabuckthorn have less suckering root system, more roots, and roots tend to thicken.But as part of the coarse root rot, the root biomass is between the former two, and root activity and natural regeneration ability decline.3. 18a Health Seabuckthorn strains are significantly higher than the 8a, 13a .The resistance of 18a Health Seabuckthorn shows decline, and pest and disease infection was significantly aggravated. Soil moisture difference of 8a, 13a and 18a Seabuckthorn was not significant.4. Forest age does not exert too much influence on surface soil microbial community carbon source utilization characteristics. Main carbon source that is utilized by microbial community in surface soil with different ages: amine / amide and 1 species, three kinds of amino acids, carbohydrates 4, carboxylic acid 7, parents, class 2 compounds, polymer 3. While the ability of using a single carbon substrates reaches a steady state, the average absorbance (AWCD) values is 13a> 8a> 18a.13a shows single carbon substrate utilization capacity of the surface soil microbial seabuckthorn the strongest, and the highest total number of microbial communities; 8a Seabuckthorn surface soil microbial carbon substrate using a single relatively strong, and the total number of microbial communities in the middle; 18a seabuckthorn surface soil microbial ability to use single carbon substrates weakest, the lowest total microbial community. It indicates that there is a certain relationship between differences in microbial decay and Hippophae rhamnoides.5.13a Seabuckthorn Health and surface soil microbes ability to use acid compounds is strong, and 8a, 18a Health Seabuckthorn surface soil microbes using carbohydrates ability are strong.but other compounds used in the capacity of surface soil microorganisms of different forest age Seabuckthorn did not show significant differences. It illustrate that Seabuckthorn plantations of different forest age have many types of top soil microorganisms, alone with a small number of specific features of micro-organisms.6.13a Health Seabuckthorn surface soil microbial diversity (number, type) is maximum, while the surface soil microbial diversity between 8a,and 18a Health Seabuckthorn is different, but little difference. Noting that the number of 8a, 18a surface Health Seabuckthorn specific function of soil microbial flora is instability, and the soil surface of 13a Health Seabuckthorn specific features of microbial community has relatively stable number of bacteria, it reflects the growth conditions of Hippophae rhamnoides (13a seabuckthorn forest growth status is best).In short, with the stand age increased, especially since 13a, competition among the forest plants is stronger , which result in reduction of seabuckthorn plantation density, root activity decreasion, ability to update recession, reduction of the total microbial community and understory plant diversity, diameter class structure "recession,"and the trend of degree of pest and nodule decay increasing. The above results, we can reach conclusion that the mechanism leading to the decline is the combined result of multiple factors that eventually lead to the decline or death of seabuckthorn forest there. To the seabuckthorn which has entered decline period, we should take reasonable measures thinning or stand improvement Stump, as to improve forest temperature and light conditions,and promote the initiation renewal and rejuvenation.