Influence of competition on the relationship between tree growth and climate of main tree species in the broadleaved Korean pine forest under different thinning intensities in northeastern China

  Objective  This paper aims to explore whether the competition environment difference of trees under different thinning measures affects the trend of tree growth and its response strategies to climate factors, so as to provide scientific basis for forest management under the background of climate change.

  Method  Four dominant tree species in the broadleaved Korean pine forest of Forestry Experimental Administration in Jiaohe, Jilin Province of northeastern China, Pinus koraiensis, Fraxinus mandshurica, Acer mono, and Tilia amurensis, were selected in four permanent monitoring sample plots with an area of 1 ha, with thinning intensity of 0 (control, CK), 15% (light treatment, LT), 35% (moderate treatment, MT), and 50% (high treatment, HT). Hegyi competition index was used to quantify competition and divide competition pressure groups, compare and analyze the growth differences of various tree species under different thinning intensities and competition, build the relationship between trees in different competition pressure groups and climate factors, and discuss whether thinning intensity and competition differences affect trees’ response strategies to drought events.

  Result  (1) The growth of trees in the low competition group was significantly higher than that in the high competition group under different thinning intensities (P < 0.05). The growth of trees in the high-competition group did not vary significantly after thinning. Except for Acer truncatum, the growth of trees in the low-competition group remained stable (9.43 cm²/year), and other trees in the low-competition group showed a significant increase trend. With the increasing of thinning intensity, the occurrence of highest growth value of trees in the low-competition group was not consistent. (2) The tree growth of different competition groups of Acer mono in different thinning sample plots was significantly correlated with temperature and precipitation, while the relationship between tree growth of different competition groups of Pinus koraiensis, Fraxinus mandshurica and Tilia amurensis and climate factors was jointly affected by thinning intensity and tree species factors. Under medium thinning, Korean pine in low competition group was more sensitive to climate factors. With the increase of thinning intensity, the stability of positive correlation between Fraxinus mandshurica and minimum temperature in different competition groups decreased, while the stability of the positive correlation between Tilia amurensis growth in low-competition group and precipitation increased (P<0.05). (3) Thinning treatment enhanced the ability of trees to adapt to drought event. The resilience of Pinus koraiensis in high competitive group, as well as the resistance and resilience of Tilia amurensis in low-competitive showed remarkable improvement. The highest resilience of Pinus koraiensis, Acer mono and Tilia amurensis was observed in high thinning sample plot.

  Conclusion  Our results add evidence that the competitive environment of retained trees is a key element to affecting the growth of trees, which change the relationship between tree growth and climate factors. Light and medium thinning can effectively improve the proportion of retained trees in low-competition group and promote the growth of retained trees. Height treatment enhances the resilience of retained trees for drought events. Therefore, thinning can alleviate the growth decline caused by climate warming and drying.