Can the Pinus sylvestris var. mongolica sand-fixing forest develop sustainably in a semi-arid region?

Abstract. Desertification is a global environmental and societal concern at present, and China is one of the countries that face the most severe damage of desertification. China’s so-called Three North shelterbelt Program (3NSP) has produced a vast area of lined forest in the semi-arid regions with the purpose of battling desertification. Such a wind-breaking and sand-fixing forest has successfully slowed down the incursion of desert. However, the vast artificial forestry consumes a large amount of water resources, which profoundly affect the fragile ecological environment in the semi-arid regions. In turn, a large amount of water loss also causes a great number of vegetation deaths or defects. To understand the water balance and sustainable development of artificial forest in semi-arid region, this study uses the 30-year-old lined Pinus sylvestris var. mongolica sand-fixing forest in the eastern part of Mu Us Sandy land in Northwestern China as an example. Specifically, this investigation studies the redistribution of water in soil under existing precipitation conditions, so as to evaluate whether the rain-feed forestry can develop sustainably or not. Rain gauge, newly designed lysimeter and soil moisture sensor are used to monitor precipitation, deep soil recharge (DSR) and soil water content, resulting in an accurate estimation of annual moisture distribution of the rain-feed Pinus sylvestris var. mongolica. The study shows that there are two obvious moisture recharge processes in an annual base for the Pinus sylvestris var. mongolica forest soil in Mu Us Sandy land: 1) the snow melted water infiltration-recharge process in the spring, and 2) the precipitation-recharge process in the summer. The recharge depth of the first process is 160 cm. The second process results in DSR (referring to recharge that can reach a depth more than 200 cm and may eventually replenish the groundwater reservoir). The DSR of 2016–2018 is 1.4 mm, 0.2 mm, 1.2 mm, respectively. To reach the recharge depths of 20 cm, 40 cm, 80 cm, 120 cm, 160 cm, and 200 cm, the corresponding precipitation intensities have to be 2.6 mm/d, 3.2 mm/d, 3.4 mm/d, 8.2 mm/d, 8.2 mm/d, and 13.2 mm/d, respectively. The annual evaporation amount in the Mu Us Sandyland Pinus sylvestris var. mongolica forest is 426.96 mm in 2016, 324.6 mm in 2017, 416.253 mm in 2018. This study concludes that under the current precipitation conditions, very small but observable DSR happened, thus the groundwater system underneath the forest may be replenished, meaning that the artificial Pinus forestry can probably develop sustainably. This study confirms that developing limited amount forestry in semi-arid regions is likely in a sustainable fashion. The widely variable annual precipitation in semi-arid areas may affect this conclusion and should be investigated in the future.