Abstract: This study was conducted in a typical mollisols area in Northeast China. Soils from the layers of 0-0.2 m, 0.2-0.4 m, and 0.4-0.6 m were stripped, mixed, and then refilled in the same soil layer to simulate lightly, moderately, and severely degraded mollisols. We investigated root morphological characteristics and growth dynamics of Larix gmelinii and Populus simonii × P. nigra (P. xiaohei) under different degrees of mollisols degradation from May to October in 2023 and 2024 by using in-situ minirhizotron observations. The results showed that root length density, root surface area density, and root volume density of L. gmelinii decreased with increasing mollisols degradation. Its root fractal dimension showed stage-dependent adjustments in response to degradation, with the highest values in lightly degraded mollisols in 2023, which was 15.3% and 14.0% higher than in moderately and severely degraded mollisols, respectively. In 2024, the highest value was found in moderately degraded mollisols, 9.6% and 1.5% higher than in lightly and severely degraded mollisols, respectively. For P. xiaohei, root length density, root surface area density, and root volume density were higher in lightly and moderately degraded mollisols than in severely degraded mollisols. Root fractal dimension of P. xiaohei was highest in lightly degraded mollisols, 6.2%-14.5% higher than in moderately and severely degraded mollisols, respectively, which decreased with increasing mollisols degradation. L. gmelinii exhibited significant increases in medium and thick roots, with root length increments of 27.4%-56.0% for medium roots and 40.2%-44.7% for thick roots, with higher increments in moderately and severely degraded mollisols. P. xiaohei exhibited significant increases in fine roots across all degradation degrees, with a root length increment of 23.5%-63.9%, and the increment decreased with increa-sing mollisols degradation. In conclusion, L. gmelinii exhibited a root adaptation strategy characterized by root thickening and architectural adjustments in response to increased mollisols degradation stress, which was more consistent with a “resistance-type” root strategy, making it more suitable for ecological restoration in degraded mollisols areas. In contrast, P. xiaohei increased resource acquisition efficiency through rapid fine root growth, demonstrating an “avoidance-type” root adaptation strategy, and was more suitable for restoration and reconstruction in lightly degraded mollisols.

Key words: degraded mollisols, root morphology, root fractal dimension, root adaptation strategy