| Summary: | To understand the insect resistance mechanism of the larch, <i>Larix olgensis</i>, in a mixed forest, larch (<i>Larix olgensis</i>) seedlings and ashtree (<i>Fraxinus mandshurica</i>) seedlings were planted with mixed banding forests in the proportion of 1:1 (BMF<sub>1:1</sub>), 3:3 (BMF<sub>3:3</sub>) and 5:5 (BMF<sub>5:5</sub>), in pots and in the field. One year later, the content of secondary metabolites in the needles of each larch treatment were tested with an ultraviolet spectrophotometer. The results showed that the allelopathic effect of <i>F. mandshuricas</i> (ashtree) on <i>L. olgensis</i> (larch) could increase the content of secondary metabolites in larch needles. It was found that the flavonoid content in the needles of BMF<sub>5:5</sub> was higher than that in the needles of BMF<sub>1:1</sub> and BMF<sub>3:3</sub> (<i>p</i> < 0.05). The tannin content in the needles of FBMF<sub>3:3</sub> and FBMF<sub>5:5</sub> was significantly higher than that of FBMF<sub>1:1</sub>, whereas the tannin content in the needles of PBMF<sub>3:3</sub> reached 1.27 mg/g, which was the highest (<i>p</i> < 0.05). The lignin content in the needles of FBMF<sub>3:3</sub> reached 2.27 mg/g, which was significantly more increased than that in the control group in a dose-dependent manner, while that in the needles of PBMF<sub>3:3</sub> and PBMF<sub>5:5</sub> was higher than that in the needles of PBMF<sub>1:1</sub> (<i>p</i> < 0.05). The tannin and lignin content in the needles of FBMF was higher than that of PBMF. However, there was no difference in the content of flavonoids in the needles of FBMF and PBMF. These results suggest that banding mixed larches and ashtrees can significantly increase the content of secondary metabolites (phenolic compounds) in the needles of <i>L. olgensis</i> and improve its chemical defense, and the allelopathic effect of ashtrees on larches is related to the mixed proportion. Thus, the effect of mixed banding forests in the proportion of 3:3 and 5:5 is better.
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