Desertification of the semi-arid steppe of Mongolia is advancing very rapidly, motivating afforestation efforts. The “Green Belt” joint project (Government of Mongolia and Republic of Korea), which aims to mitigate soil degradation and develop agroforestry activities through the planting of a forest shelterbelt, is one such response. In these plantations, tree growth has been supported by different watering regimes (no watering, 2, 4, and 8 L h−1) and by two types of soil fertilization (NPK and Compost). The present paper analyses the effect of these techniques on soil chemistry and root biomass partitioning of Populus sibirica (Horth ex Tausch) and Ulmus pumila (L.) tree species. In July 2019, at the plantation site in Lun Soum, Tuv province (Mongolia), six trees were excavated by hand in each treatment, the root system was divided into taproot and five diameter classes (0–2; 2–5; 5–10; 10–20; > 20 mm), and the biomass was measured. Soil organic matter, macronutrients, and pH were also measured. The addition of fertilizers in the long-term did not enhance the soil chemical properties. The build-up of root biomass in both species correlated positively with increasing levels of the watering, while the application of fertilizers led to root growth suppression. For most of the root classes and both species, an irrigation level of 4 L h−1 was sufficient to yield the highest biomass and could be recommended for afforesting the semi-arid steppe of Mongolia. The root biomass of P. sibirica was more dependent on the watering regimes and of U. pumila was more negatively influenced by the application of fertilizers, indicating that U. pumila, due to the its lower water need, could be suitable for afforesting semi-arid environments. Our experiments suggest that afforestation practices in the semi-arid steppe of Mongolia should be supported by a prior analysis of plants' needs, soil type, dose, and type of fertilizers to be applied. Knowledge of the root response to the supporting techniques is necessary for choosing the best one for the plantation and, thus, to develop a sustainable and successful strategy to restore these degraded lands.

Long-term studies on plant response mechanisms to different irrigation regimes will provide a better understanding of the survivability and establishment of plant communities in a desert environment. Thus, across 10 years, we regularly investigated the effects of the rainfall (control), rainfall + 4 L h−1, rainfall + 8 L h−1, and rainfall + 12 L h−1 irrigation regimes on the growth and leaf morpho-physiology of Tamarix ramosissima Ledeb., Ulmus pumila L., Elaeagnus moorcroftii Wall. ex Schltdl., and Hippophae rhamnoides L. to suggest an optimal irrigation regime for each woody species for effective and sustainable afforestation in Mongolia. We measured the root collar diameter (RCD), annual height growth, survivability, leaf area (LA), specific leaf area (SLA), leaf biomass (LB), total chlorophyll concentration, and predawn (ψp) and midday (ψm) leaf water potentials across the treatments and species. Results showed that trees grown at 12 L h−1 grew taller per year and generally resulted in a higher SLA, but generally resulted in a lower survival rate compared with those in the other treatments in all species. Total chlorophyll content was higher in trees grown under 4 and/or 8 L h−1, particularly for T. ramosissima and E. moorcroftii. Lastly, leaf water potentials were found more negative for trees subjected to 4 L h−1, especially in T. ramosissima and U. pumila, but still resulted in a higher survival rate and LB compared with 12 L h−1. H. rhamnoides showed higher survivability at 8 and/or 12 L h−1 than at 4 L h−1. Therefore, we suggest 4 L h−1 to be the optimal irrigation regime for irrigating T. ramosissima, U. pumila and E. moorcroftii, and 8 and/or 12 L h−1 for H. rhamnoides. Our findings are relevant to ensuring the sustainability of afforestation programs in arid and semiarid landscapes in Mongolia.

Although afforestation is one of the best solutions for combating desertification, it requires intensive management. To minimize management efforts in plantations in arid area, it is crucial to select suitable tree species, i.e. those with a high adaptability to xeric environments. In this study, we investigated key tree traits related to drought adaptation in Populus sibirica Hort. ex Tausch and Ulmus pumila L., planted in the Korea-Mongolia Green Belt Plantation in Lun soum, Mongolia. We found that U. pumila had a smaller leaf size and smaller specific leaf area (SLA) than P. sibirica. In addition, the water use efficiency (WUE) of U. pumila (carbon isotope-based) was significantly higher than that of P. sibirica. Although biomass production was $2.5 times higher in P. sibirica than in U. pumila due to the differences in leaf area, there were no statistically significant differences in the photosynthetic characteristics on an area basis. These results indicate that U. pumila has a higher adaptive potential to dry conditions than P. sibirica due to its small leaf size, low SLA, and high WUE.