Effect of Sprouting and Corresponding Root Distribution of the Shrub Species Eurya Japonica on Slope Stability

After disturbance sprouts emerge from the ground, generating multi-stemmed plants as they grow, which represents a means of rapidly recovering above ground biomass. We hypothesised that the above- and below ground characteristics would differ depending on the number of stems in individual shrubs, which is likely to compensate for the slope stability of soil reinforcement by plant roots in forests. We focused on the dominant shrub species of abandoned secondary forests, Eurya japonica, and conducted an excavation of the entire root system. The aims of this study were to (i) determine the root system architecture of single- and multi-stemmed E. japonica, (ii) determine whether there were differences in the slope stability by soil reinforcement of roots between the single- and the multi-stemmed plants, and (iii) provide recommendations based on the study finding on how single- and multi-stemmed plants generated during the disturbance should be managed to improve the slope stability. Our results showed that an increase in the total basal stem area at 0.1 m above ground level (BA0.1) increased the above- (MA) and the below ground biomass (MB) with high fitting linear regression, regardless of whether the plants were single- or multi-stemmed. In contrast, the spread of roots, as indicated by the maximum lateral root spread (Rdis) and maximum rooting depth (Rdep), was related to the maximum stem diameter at 0.1 m above ground level (D0.1) with high fitting linear regression among the shoots in the plant. If the above ground shoots and below ground roots are considered as a continuous pipe and the multi-stemmed plants as an assemblage of single-stemmed plants, we can effectively explain the relationship between the total basal stem area or maximum diameter of each stem, and the below ground biomass, or the spread of the roots. There is less space available below ground than above ground for multi-stemmed trees. This is especially the case around the root stump, where the roots of each stem compete and inhibit growth. This can result in significant lower root-shoot ratios of 0.23 +/- 0.04 for multi-stemmed plants compared to 0.33 +/- 0.01 for singlestemmed plants. This imbalance suggests that multi-stemmed plants growing on forest slopes are more unstable and are more likely to fall. Soil reinforcement by roots did not significantly differ between single- and multistemmed plants. However, the displacement at the maximum soil reinforcement by roots was significantly greater in 83.3 +/- 21.7 cm for the multi-stemmed plants than in 40.0 +/- 18.2 cm for single-stemmed plants because multi-stemmed plants with competing roots resulted in fewer roots of less than 15 mm in diameter. This allowed the formation of a slip surface on the forest slopes because of the substantial displacement (bundle elongation) of roots during the pullout. Therefore, the resistance of multi-stemmed plants to shallow landslides on forest slopes was lower than that of single-stemmed plants. Traditional forest management, that is, reducing the number of stems of multi -stemmed plants, is effective in increasing sprouting growth and biomass production. It may also be effective in increasing forest slope stability at landscape levels over longer timescales.