Abstract:

Objective: The main purpose of this study is to investigate the effects of distances to and diameters at breast height (DBH) of neighboring trees on fine root biomass with different stocking densities, and to reveal the main factors affecting root biomass, for providing a theoretical basis for formulating a reasonable root sampling plan. Method: In Fraxinus mandschurica plantations with four stocking densities (Treatment Ⅰ to Ⅳ: 3 572, 3 128, 2 215 and 1 468 hm^-2, respectively), we employed the method of random sampling to estimate the biomass respectively of absorptive roots (diameter ≤ 0.5 mm) and fine roots (diameter ≤ 2.0 mm) in soil layers of 0-10, 10-20, and 20-30 cm and the totals of all soil layers, and measured the distance to and the DBH of the tree closest to the sampling point and four nearest neighbor trees. Linear regression analysis was used to examine the relationship between root biomass and the distance to and the DBH of the neighbor trees. Result: The total biomass of absorptive and fine roots (0-30 cm soil depth) significantly varied with stocking density, both of which showing the maximum in the stand with the lowest stocking density. From stocking density Ⅰ to Ⅳ, the proportions of biomass of absorptive roots in fine roots were 61.6%, 54.3%, 52.9%, and 63.4%, respectively. In all stands, more than 50% biomass of the fine roots and absorptive roots are distributed in the soil layer of 0-10 cm. In all the four stocking densities, there were no significant correlations between the total absorptive root or fine root biomass and the distance from the sampling point to the nearest one or four trees (P>0.05), except for the significantly positive correlation (P < 0.05) between fine root biomass in 10-20 cm soil layer and the mean DBH of the nearest four trees in the treatment Ⅲ. Compared with the total fine root biomass, the total absorptive root biomass showed a more general correlation with the DBH of neighbor trees, but the significance level of the correlation was related to the specific stocking densities. Both absorptive root and fine root biomass was positively correlated with the DBH of the nearest tree in the treatment Ⅰ (both R² >0.19), and with the mean DBH of the four neighbor trees in the treatment Ⅱ (both R²> 0.21). The correlations between the absorptive root biomass and the DBH of the nearest tree or the mean DBH of the four neighbor trees were significant in the treatment Ⅲ (both R²> 0.16), while no correlation was found in the treatment Ⅳ. Among the 3 soil layers, significant correlations between the biomass of absorptive roots and fine roots and the DBH of neighboring trees mainly occurred in the 0-10 cm soil layer, which showing similar patterns to that of the totals in 0-30 cm soil layer. Conclusion: According to our findings in the variations of the fine root biomass in F. mandschurica plantations, the distance between the sampling point and the neighboring trees can be set flexibly. It is necessary to consider the potential impact of the size of the neighboring trees around the sampling point, thus set the sampling points around the standard trees is an appropriate approach

Key words: fine root, absorptive root, Fraxinus mandshurica, stocking density, root biomass