Abstract: We assessed the effects of moderate and heavy thinning intensities (30% and 45%) after 12 years on the structure, spatial distribution, and seedling regeneration of dominant canopy species, Pinus tabuliformisand Quercus liaotungensis, inP. tabulifomis plantations in the Huanglong Mountain, Shaanxi Province, northwest China. The thinning promoted a shift in curve ofdiameter class from positive distribution to negative one. The proportion of trees with large diameter (DBH≥20 cm) under moderate and heavy thinning intensities was higher than that under the control (nonthinning plots). Moderate and heavy thinning intensities significantly increased understory structure diversity. The diversity indices in the moderate and heavy thinning plots were 1.34 and 1.52 times as higher as those in the control, respectively. The populations of P. tabuliformiswere evenly distributed at the small scale (0-4.5, 0.5-4 m) under moderate and heavy thinning, but were aggregated at 2-4 m under the control, and were randomly distributed for all treatments at large scale. At 0-10 m scale, there was no difference in cumulative biomass between thinning treatment and control. Moderate thinning significantly increased the cumulative biomass at the scale of 15-25 m, with an increase of 29.55% at 25 m scale compared with the control. Heavy thinning increased the cumulative biomass at 25 m scale by 8.35%. There was no correlation between mature trees of P. tabuliformis and Q. liaotungensis at the entire spatial scale under heavy thinning. The age class of seedlings and saplings ofQ. liaotungensshowed an inverted “J” growth type under heavy thinning, suggesting that such an intensity would accelerate the transition from coniferous plantation to coniferous-broadleaved forest. In conclusion, thinning can optimize the structure of plantations, change the distribution pattern of biomass, and affect the succession of the community, which provides methodological and theoretical support for the directional cultivation of ecological plantations.

Key words: stable isotope, Phragmites australis, feeding habit, Helice tientsinensis, Yellow River Delta, food source