Self-thinning processes of Mongolian pine (Pinus sylvestris var. mongolica) plantations under different initial planting densities in a semi-arid sandy region of Northeast China

Abstract

Mongolian pine (Pinus sylvestris var. mongolica), the most important afforestation species in semi-arid sandy lands, is generally planted at high initial densities to ensure survival. As stands develop, increasing competition for limited water and nutrients drives growth decline and mortality, leading to density regulation through self-thinning. However, how initial density shapes long-term self-thinning trajectories remains unclear, limiting effective density management. Using a 42-year long-term dataset covering a broad initial density gradient (1,111–10,000 trees ha⁻¹), we characterized self-thinning trajectories. Results showed that stand density declined, and mortality ranged from 42 to 79% over the 42 years. Individual tree growth parameters (mean diameter at breast height, stem volume, and basal area) generally decreased with density, although the significance and strength varied with stand age. Stand basal area and stem volume were positively related to density at ages 14 and 22. Then, we parameterized the self-thinning law using four analytical approaches: Ordinary Least Squares (OLS), Reduced Major Axis (RMA), Quantile Regression (QR), and Stochastic Frontier Analysis (SFA), among which OLS and SFA produced the highest goodness-of-fit and yielded nearly identical estimates. Due to its strong explanatory performance and practical applicability for estimating stand carrying capacity, OLS was selected as the primary modeling approach. The estimated self-thinning slope was slightly lower than the theoretical slope derived from the Yoda’s − 3/2 law, potentially reflecting a shift from asymmetric light competition toward more symmetric competition for water resources under drought stress. Considering that traditional self-thinning does not explicitly account for below-ground competition, we integrated the estimated slope with our published root-to-crown projected area ratio (R/C, ~ 2.0) for Mongolian pine plantations. This yielded age-specific management densities of 1,057–1,651, 652–812, 324–377, 227–267, and 194–232 trees ha⁻¹ for 10, 20, 30, 40, and 45 years, respectively. These findings provide a quantitative basis for density regulation to sustain long-term stand stability in water-limited plantations.

Keywords

Mongolian pine plantations / Growth process / Self-thinning / Management density

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Funding

National Natural Science Foundation of China(32192435)

Liaoning Provincial Science and Technology Major Project(2023JH1/10400001)

RIGHTS & PERMISSIONS

Northeast Forestry University