Research On The Distribution Characteristics Of Large Roots Of Populus Hopeiensis In The Loess Area Of Northern Shaanxi Based On GPR Technique

Populus hopeiensis was a native tree species in the Loess Plateau of Northern Shaanxi Province, which can come into being a natural forest with root sprouting and become the main tree species for soil and water conservation in the area. Therefore, the research of Populus hopeiensis coarse roots distribution have an important significance for exploring the formation of natural forest and the function of soil and water conservation. In order to overcome the difficulty of the traditional root observation methods, such as mining method, ground-penetrating radar technique as a non-destructive method is used. At first, we explore the relationship between the ground penetrating radar parameters and diameter of Populus hopeiensis coarse roots under controlled experiment, and establish the estimation model of the root diameter. Secondly, the distribution characteristics of Populus hopeiensis coarse roots were studied through field experiments, and the influence of topographic factors on the distribution of coarse root was studied. At last, the results showed that:(1) The multivariate linear model which is established in the total time interval, the first periodic wave and the maximum amplitude is best. First cycle parameters of reflected wave and root diameter show a significant positive correlation (p<0.01), while other cycle parameters of root diameter has no significant correlation with root diameter (p>0.05); the maximum and sum of the waveform parameters which include amplitude, time interval and amplitude area are very significant (p<0.01). Through stepwise regression method, the total time interval, the first periodic wave and the maximum amplitude are chosen to establish model for eatimating roots diameter.(2) Coarse roots of Populus hopeiensis mainly concentrate in the range of 10-50cm soil depth. The density of coarse roots increased first and then decreased with the increase of soil depth. The distribution of different diameter class of coarse roots listed in the following:the proportion of 0.5-1.5 cm diameter coarse root is the highest in 0-50 cm siol layer. Besides, the proportion of 1.5-2.5 cm diameter coarse root is the highest in 50-70 cm soil layer. The proportion of 2.5 cm diameter coarse roots in the root distribution layers has always been minimal, which is less than 10%.(3) The coarse roots density showed a decreasing trend of power function with the increase of the distance from the tree trunk. The distribution of different diameter class of coarse roots listed in the following:the proportion of 0.5-1.5 cm diameter coarse root decreased first and then increased, while the proportion of 1.5-2.5 cm and 2.5 cm diameter coarse root showed first increased and then decreased. The increased of 0.5-1.5 cm diameter coarse root density is caused by the split of the 1.5- 2.5cm diameter coarse roots in a certain extent.(4) The coarse roots distribution of Populus hopeiensis is not only related with roots’ splitting, but also influenced by the terrain factor which is mainly the slope. Through the principal component analysis and double factor analysis of topographic factors, slope factors has the maximal influence on the coarse roots density of Populus hopeiensis. The coarse roots density in different slopes have significant differences (P< 0.05) and the size sorting:shady slope> semi shady slope> semi sunny slope. Larger root density tends to appear in the slopes that soil moisture condition is better. Additionally, the influence of slope factor on the coarse roots density of 1.5-2.5cm diameter is relatively smaller than the others.