Competition intensity affects growing season nutrient dynamics in Korean pine trees and their microhabitat soil in mixed forest

Exploring the response mechanisms of the understorey replanted target trees to competitive pressure from upper neighbouring trees in mixed forests is the scientific basis for growth promotion of the target trees. Taking a mixed forest of artificial planted Korean pine trees (target species) and natural broadleaved trees as the studying case, six competition intensity gradients to Korean pine trees were established according to the Hegyi competition index and the responses of growth, hormones and nutrition status of the target Korean pine trees to different competition intensities were analysed. Competition intensities had significant effect on the growth of Korean pine trees, and the more significant the competition intensity was, the more pronounced the inhibition effect. When the Hegyi competition index was greater than 15, the neighbourhood trees had a significant inhibition effect on the growth of Korean pine trees. With the increase of the competition intensity, the content of gibberellin and indoleacetic acid in Korean pine trees decreased; the content of abscisic acid increased; the moisture content of needles, the soluble sugar content and C/N ratio of needles and roots decreased; the chlorophyll, P and K and starch content, and total non-structure carbohydrates (NSC) of needles and roots increased at first and then decreased; the P and K content and C/N ratio in Korean pine microhabitat soil decreased. Excessive adjacent tree competition reducing the living space, the endogenous hormone content, nutrient content, and NSC of the target Korean pine trees, resulting in the growth of the target trees and subsequent nutrient synthesis to be inhibited. Appropriate adjacent tree competition increased the target tree chlorophyll, NSC, and nutrient content and had no excessive inhibitory effect on the growth of the target Korean pine trees. There was certain trait correlationship between Korean pine needles and fine roots, but there was an endostatin equilibrium mechanism for changes in the surrounding soil of the target trees. The above results indicated that competition intensity of the upper neighbourhood trees significantly influenced the growth and internal nutrient situation of the replanted target trees and their microhabitat soil conditions. In this case, when the Hegyi competition index of neighbouring trees in the surrounding area of the understorey planted target Korean pine trees was greater than 15, the growth space of target trees should be regulated to ensure their healthy development.