Objective  The niche separation and overlap of underground resources between Pinus koraiensis and common broadleaved tree species in broadleaved secondary forests in Xiaoxing’anling Mountains of northeastern China were explored, in order to provide some reference for the rational selection of tree species in secondary forests and the scientific management of secondary forests in northeastern China.

  Method  The spatial niche of underground nutrition, seasonal niche and quantitative niche of soil moisture absorption, seasonal niche and quantitative niche of soil nutrient absorption and morphological niche of nutrients absorption of Pinus koraiensis and broadleaved tree species such as Betula platyphylla, Populus davidiana, Quercus mongolica, Fraxinus mandshurica and Tilia amurensis in secondary forest were studied.

  Result  There were separations between Pinus koraiensis and broadleaved tree species in the niche of underground nutrition space, seasonal niche for absorbing soil water, seasonal niche and quantitative niche for absorbing soil nutrients, and niche for absorbing soil nutrient forms. The concrete manifestations were as follows: (1) The absorption roots of broadleaved tree species were mainly distributed in the soil surface layer (0−20 cm), which was characterized by “shallow roots”. On the contrary, the distribution ratio of absorbing roots of Pinus koraiensis was lower in the topsoil and higher in the lower soil space (30−60 cm), which was relatively “deep-rooted”. (2) The broadleaved tree species generally began to absorb soil moisture from the middle of May to the end of September in a year; however, Pinus koraiensis still had transpiration from early April to October in a year. (3) The broadleaved tree species absorbed nitrogen from May to September in a year, with a short absorption period, and the peak of nutrient absorption in summer was steep and obvious; however, Pinus koraiensis, which was shaded by broadleaved tree species, had been slowly absorbing nitrogen nutrients in the whole growing season from April to October in a year, and the peak of nutrient absorption in summer was slow and the peak period was not obvious. (4) The broadleaved tree species had a large consumption of N nutrient and a low utilization efficiency, which belonged to high consumption and low efficiency; however, the N nutrient consumption of Pinus koraiensis was relatively small, and its utilization efficiency was 10.7%−36.8% higher than that of broadleaved tree species, which belonged to low consumption and high efficiency. (5) The nitrate reductase activity of Pinus koraiensis leaves was low in the selectivity of N nutrient chemical forms. The nitrate reductase activity in the leaves of broadleaved tree species was the highest, among which Betula platyphylla, Populus davidiana and Quercus mongolica were about 6.0−6.5 times as high as that of Pinus koraiensis and Fraxinus mandshurica and Tilia amurensis were about 2.6 and 2.7 times as high as that of Pinus koraiensis. (6) The niches overlap between Pinus koraiensis and Betula platyphylla, Populus davidiana, Quercus mongolica, Fraxinus mandshurica were small, which were 0.502, 0.426, 0.628 and 0.374 respectively, while the niche overlap between Tilia amurensis and Pinus koraiensis was as high as 0.903.

  Conclusion  According to the niche of underground resources, Pinus koraiensis and Betula platyphylla, Populus davidiana, Quercus mongolica, Fraxinus mandshurica and Tilia amurensis are all optimized mixed combinations, and the separation of niche of underground resources between Pinus koraiensis and broadleaved tree species are of great significance to their long-term coexistence and the sustained, stable and high yield of mixed communities.