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In this study, we assessed the functional and architectural traits in the coarse roots of Ulmus pumila trees, which are used for afforesting the semi-arid steppe of Mongolia. Tree growth was supported by different watering regimes (no watering, 2, 4, and 8 L h−1) and by two types of soil fertilization (NPK and compost). In July, 2019, for each of these treatments six trees, outplanted in 2011 as 2-year-old seedlings from a container nursery, were randomly selected, excavated by hand, and digitized. The build-up of root length correlated positively with increasing levels of watering for both soil depths analyzed. The application of fertilizers led to root growth suppression resulting in a general reduction of root length in a lowered rooting depth. When root system characteristics were analyzed in relation to wind direction, unfertilized trees showed higher root diameter values in both soil layers of leeward quadrants, likely a response to mechanical forces to improve stability. On the contrary, fertilized trees did not show differences in root diameter among the different quadrants underscoring a strong reduction in root plasticity with a lack of morpho-architectural response to the mechanical forces generated by the two prevailing winds. Finally, the root branching density, another important trait for fast dissipation of mechanical forces, was significantly reduced by the fertilization, independently of the quadrants and watering regime. Our results suggest that knowledge of the root response to the afforestation techniques applied in the semi-arid steppe of Mongolia is a necessary step for revealing the susceptibility of this forest shelterbelt to the exacerbating environmental conditions caused by climate change and, thus, to the development of a sustainable and successful strategy to restore degraded lands.

This study used seedlings of Quercus robur L. and Pinus nigra Arn. to investigate the respanse of root systems to nutrient supply and roat density in the sail. Variation in nutrient supply was tested by using two different concentrations of nitrate and phosphate. The influence of root density was tested by spacing seedlings to provide a density of 100, 67 and 25 plants m-2. At the end of the treatments the two species presented two different root topologies which apparently each derive from a proper ontogenic speciticity. Nevertheless, the data clearly indicate that competition for nutrients and root density have affected both root system topologies by inducing a considerable modification in root traits such as total fOot length and number of apices. These modifications were different when measured in first-or second-order root laterals. This study reveals that the response of these two species to heterogeneity in soil resource distribution and root competition is regulated by root turnover. A better understanding of the response of roots to environmental signals improves our knowledge of the interaction between plants and the soil ecosystem in forests.