Quantitative study of below- and above-ground competitions in mandchurican ash seedlings

Competition among individual trees in a stand is an important ecological and physiological feature in term of its role on the pattern of carbon allocation and productivity. Competition happens between above- and bellow-ground parts of trees for light and soil resources. Some species are stranger above-ground competitors, some others are more able with their below-ground parts. Both above- and bellow-ground competitions vary with stand densities, development stage, and the allocation of available resources to above- and below-ground spaces. Quantitative study on the contribution of below- and above-ground competitions to general competition among trees is challenging, and is much more important for understanding the pattern of carbon allocation and productivity in forest stands. To quantify the competition from above- and bellow-ground among individual plants, we conducted an experiment with seedlings of Mandchurican ash (Fraxinus mandshurica Rupr.). Seedlings were grown in sandboxes with three spacing (20,30,and 40cm, respectively) for 3 years, path analysis was employed to study both relationship between below- and above-ground competitions, and their contribution to general competitions among individual seedlings. The results show that there were close relationships between root mass, shoot mass and total mass of ash seedlings and the growing space. With wider spacing, the average increment of fine roots, coarse roots, and shoots of the seedlings was relatively higher than that with narrow spacing, and the biomass of each component was greater. As the distances among individual seedlings extended from 20cm to 30cm, and 20cm to 40cm, the fine root mass and leaf mass increased by 25.0% and 28.9%, 28.8% and 47.1%, respectively, suggesting that fine roots and leaves were sensitive to spacing. The competition index, calculated by root mass, shoot mass and total mass, decreased as the spacing increased. The index of above- and below-ground competition decreased by 14.62% and 14.24%, respectively, as the spacing varied from 20cm to 30cm, 15.75% and 38.80% from 20cm to 40cm. At the same spacing level, below-ground competition was greater than above-ground competition, the competition index were over 30% higher with below-ground competition index in all the three spacing levels. Significant correlation was found among below-ground, above-ground, and general competition. The below- and above-ground competitions comprised about 95% of the general competition, involving direct and indirect competitions. Coefficients of path analysis, in three spacing levels, showed that the direct influence intensity of below-ground competition on general competition was from 0.5543 to 0.7426, and correspondingly, 0.2851 to 0.5282 for the above ground competition. In all the three spacing levels, below-ground competition contributed a larger proportion to general competitions. Mean direct effect of below-ground competition was much higher than that of above-ground competition in term of their path coefficient (i.e., 0.7405, and 0.2883, respectively). The relatively higher ratio of fine root mass to leaf mass in narrower spacing level suggested that more carbon be allocated to fine roots, this might be important to understand the intraspecific competition.