**4. Discussion**

Oasis is an important area that guarantees the normal life and production of residents in arid region. Its climate effects are vital important to the sustainable development of the oasis shelterbelts [30]. The stability of the microclimate was maintained and natural disasters were reduced by oasis shelterbelts [31]. Our results showed that under the influence of a large-scale oasis shelterbelts, air temperature, ground temperature and evaporation decreased 5.13% ~ 24.74%, 2.38% ~ 20.09% and 7.06% ~ 17.68%, respectively. However, relative humidity and precipitation increased 6.93% ~ 25.53% and 4.30% ~ 50.15%, respectively. The cold-humid effect of the atmosphere inside shelterbelt formed a cold-humid column. The warm-dry effect of the desert atmosphere outside shelterbelt caused a warm-dry air current. The interaction of the cold-humid effect and the warm-dry effect formed a local circulation. The warm air outside shelterbelt was transported to the sky above the shelterbelt, thus formed a stable thermal inversion layer. Therefore, the cold and humid air inside shelterbelt was maintained, which was beneficial to crop growth in oasis [32].

The microclimate of oasis shelterbelts was conducive to the overwintering of plants and kept them from the damage of high temperature in summer. Therefore, it played a vital role in plant growth, nutrient accumulation and quality improvement [33]. The relative humidity was increased 0.5% ~ 18.6%, whereas the evaporation was decreased 18.4 ~ 1282.8 mm by oasis shelterbelts in the northeastern edge of Ulan Buh Desert. This played a positive role in increasing soil moisture and inhibiting crop transpiration, thereby increasing crop yields and improving the soil quality in long time [33]. The oasis shelterbelts in the northeastern edge of Ulan Buh Desert also had the effect of cooling in summer and heat preservation in winter. It decreased air temperature in spring, summer and autumn, whereas increased air temperature in winter inside shelterbelt. This was similar to the microclimate effect of the shelterbelt in Heihe River Basin [34]. The air heat transfer was affected by the barrier of shelterbelt, caused the temperature dynamic lag between inside and outside shelterbelt [33]. The attraction and reflection of plants inside shelterbelt to the solar radiation energy reduced the solar radiation energy absorbed by the air. Meanwhile, the growth and transpiration of plants consumed a lot of heat energy. In addition, the shading effect of the shelterbelt also resulted in the cooling effect especially in spring, summer and autumn. On the other side, it had a heat preservation effect in winter [35]. During the growing season of crop, the blocking effect on airflow was enhanced by the shelterbelt in the leafy season. The wind speed was reduced by the weakening of turbulence exchange, whereas the wind speed was increased inside shelterbelt in the leafless season [35]. Saturated water vapor was formed when the temperature inside shelterbelt was lower than that outside shelterbelt. The canopy blocked the exchange of airflow between inside and outside shelterbelt. In addition, the water vapor diffusion from inside to outside shelterbelt was reduced by the decrease of wind speed, which resulted in a higher relative humidity inside than outside shelterbelt [36].

#### *Ecological Effects of Oasis Shelterbelts in Ulan Buh Desert DOI: http://dx.doi.org/10.5772/intechopen.98679*

Wind speed increased gradually with the increase of height inside and outside shelterbelt with the significant relationship of a power function. There were obvious differences in wind speed, wind direction, and eolian sediment flux inside and outside shelterbelt in the northeastern edge of Ulan Buh Desert. The results were consistent with the wind speed distribution characteristics of Minqin Oasis and Badain Jaran Desert on the oasis and desert-oasis region, which increased with the height from 0 to 50 m [37]. During the occurrence of sandstorms, wind speed profile and the migration process of sand and dust particles were affected by the type of underlying surface, thereby affecting the distribution of aeolian sediment flux [38–40]. For example, the relationships between the near-surface aeolian sediment flux and height were expressed as an exponential function in Tengger Desert [40–42], a power function in Minqin Oasis [38], whereas both functions in Taklimakan Desert [43]. Wind speed and the aeolian sediment flux through the shelterbelt were reduced significantly by oasis shelterbelts. The microclimate was improved by oasis shelterbelts, together with weakening turbulent exchange inside shelterbelt, and inhibiting vertical transportation of aeolian sediment. Therefore, the quality of aeolian sediment was less inside than outside shelterbelt. The dust is transported under the action of wind, and the sediment flux varies with height during the transport process.

The difference of sand driving wind speed inside and outside shelterbelt was different in wind speed. The frequency of strong sand driving wind exceeding 11 m·s−1 was decreased significantly inside than that outside shelterbelt in the northeastern edge of Ulan Buh Desert. However, the frequency of sand driving wind between with 5 m·s−1 - 7 m·s−1 and between 7 m·s−1 and 9 m·s−1 increased inside than outside shelterbelt. When sandstorm passed through the oasis shelterbelts, which acted as a tall rough element, and part of the airflow was lifted up, a relatively high speed free-stream was formed above the shelterbelt canopy. After crossing the shelterbelt, it formed a sinking airflow, which spread in all directions at a certain distance in the leeward zone. Another part of the airflow entered into the shelterbelt. Due to the block and friction of the trees, the airflow consumed a large amount of energy during dispersion, thus a low speed bound-stream was formed under the shelterbelt canopy. Therefore, the frequency of sand driving wind with high speed was decreased, whereas the frequency of sand driving wind with low wind speed was relatively increased inside oasis shelterbelts [34]. Because there was spatial and temporal heterogeneity in the shelterbelt distribution in a specific space with the form of grids [38], our methods on the shelterbelt scale cannot be applied to the regional scale [44]. Therefore, the comprehensive evaluation index system of the environmental benefits of shelterbelts needs to be studied urgently on regional scale. Moreover, it was suggested to focus on deducing the results from shelterbelt scale to multiple shelterbelts or even landscape scales in the future, using interpolation methods to establish models on the time scale [45], and integrating vegetation, soil, climate and other factors, in order to obtain simple and effective results.
