Effects of canopy density on the growth of broadleaved tree species under artificial regeneration of Pinus sylvestris forest

  Objective  The growth differences of Fraxinus mandshurica, Juglans mandshurica and Tilia amurensis artificially planted seedlings under different canopy densities of Pinus sylvestris var. mongolica forest were investigated in this study to provide theoretical basis and technical support for the selection of broadleaved trees suitable for regeneration under different canopy densities of P. sylvestris var. mongolica forest.

  Method  A artificial regeneration experiment was conducted with two-year-old bare root seedlings of F. mandshurica, J. mandshurica, T. amurensis under the P. sylvestris var. mongolica forest. Furthermore, four light environments of P. sylvestris var. mongolica forest were applied, i.e. full light, canopy density of 0.4, 0.6 and 0.8. At the end of the growing season, the changes of seedling height, base diameter, photosynthetic parameters, photosynthetic pigment content and leaf nutrient content of the three seedlings were measured.

  Result  By the end of the growing season, the height growth and ground diameter growth of J. mandshurica seedlings under canopy density of 0.6−0.8 were significantly higher than those under full light and canopy density of 0.4, and those were the largest under canopy density of 0.8. The height growth and ground diameter growth of F. mandshurica and T. amurensis seedlings under canopy density of 0.4−0.6 were significantly higher than those under full light and canopy density of 0.8. The height growth and ground diameter growth of J. mandshurica seedlings were better than F. mandshurica and T. amurensis seedlings under the four treatments. With the increase of canopy density, chlorophyll content of the three tree species showed a significant upward trend, and the trend of photosynthetic rate (P_n) was consistent with the trend of total chlorophyll content (C_t). The stomatal conductance (G_s) and intercellular CO₂ concentration (C_i) of J. mandshurica increased with the increase of canopy density; the G_s of F. mandshurica and T. amurensis showed an upward trend, and the C_i showed an overall trend of first increasing and then decreasing; transpiration rate (T_r) showed a downward trend with the increase of canopy density. The leaf N content of the three tree species increased with the increase of canopy density; there was no significant difference in the total C content of J. mandshurica leaves among the treatments. The total C content of F. mandshurica and T. amurensis leaves showed a downward trend with the increase of canopy density, among which, the total C content of F. mandshurica seedling leaves under full light, canopy density of 0.4 and 0.6 treatments had no significant difference, but they were all higher than that under canopy density of 0.8 treatment; total P content in the leaves of T. amurensis increased first and then decreased, J. mandshurica decreased gradually, while total P content in the leaves of F. mandshurica was not significantly different among the treatments, and total K content decreased first and then increased as a whole. The growth of J. mandshurica was better than F. mandshurica and T. amurensis under different canopy densities of P. sylvestris; the growth of J. mandshurica was better than F. mandshurica and T. amurensis under different canopy densities of P. sylvestris; among them, the photosynthetic rate of T. amurensis under each canopy density did not change significantly, the change of light intensity had little effect on it.

  Conclusion  This study conducted a comparative analysis of the growth of three broadleaved tree species under different canopy densities of P. sylvestris var. mongolica forest in one year of pianting. It was found that F. mandshurica, T. amurensis and J. mandshurica can grow in the soil environment of P. sylvestris var. mongolica forest in the year of planting, and all of them have certain negative tolerance. F. mandshurica and T. amurensis grow better in canopy density of 0.4−0.6 than in full light and canopy density of 0.8, J. mandshurica grows best in canopy density of 0.8, and the growth status of J. mandshurica under different canopy densities is better than F. mandshurica and T. amurensis. With the growth of seedlings, the relationship between J. mandshurica and its surrounding vegetation and its demand for light resources may change. Thus, to provide a solid theoretical basis for the regeneration of P. sylvestris var. mongolica plantation, followed investigations are still needed.