Ecological Function Service Value and Quantity of Fruit-Tree Economic Forests in the Semi-Arid Loess Hilly and Gully Region of Central Gansu—A Case Study of the Hulu River Basin

Abstract: The compensatory effect and deep impact of the fruit tree economic forest on water and soil conservation ecology in a semi-arid region are investigated by exploring the ecological service value and ecological functions of the fruit tree economic forest, and further by analyzing its functional effects on reducing water and soil loss, conserving water and soil, conserving the water source, improving environmental quality and maintaining biodiversity. This analysis provides a theoretical basis and support for coordinating the relationship between economic and social development, ecological protection, agriculture and forestry in the semi-arid area of the Loess Plateau; promoting the systematic management of mountain, water, forest, farmland, lake grass and sand; and promoting the ecological protection and high-quality development of the whole Hulu River Basin. According to the Standards for Evaluation of Forest Ecosystem Service Function (GB/T38582-2020), the forestry industry standard of the People's Republic of China, the current market method, shadow price method, opportunity cost method, Swedish Carbon Tax law and other methods were adopted. The main functions of the fruit-tree economic forest ecosystem and its eco-economic value in the Hulu River Basin in Pingliang City were quantitatively analyzed, and the existing measured data from domestic ecological stations were combined with quantitative analysis and qualitative evaluation. The calculations included the ecological service values of the fruit tree economic forest ecosystem in water conservation, soil conservation, carbon sequestration, oxygen production, nutrient accumulation, environment purification and biodiversity protection, and the dynamic change characteristics of the ecological function quantity corresponding to its value were systematically analyzed. (1) In the four developmental stages of the fruiting economic forest in the Hulu River Basin in Pingliang City, the ecological function service value showed an increasing trend. Among the stages, the total value contribution of the first stage (2005–2009) is 1.299×1010 yuan; the second stage (2010–2013) is 2.497×1010 yuan; the third stage (2014–2017) is 2.662×1010 yuan; and total value contribution of the fourth stage (2018–2020) is 2.774×1010 yuan. (2) In the composition of the ecological functional service value of the fruit tree economic forest, the value of water conservation is the highest, accounting for the largest proportion at 32.97% of the total value of ecosystem services. Therefore, it plays an important role in regulating the hydrological balance of the basin in the arid and semi-arid region of the Loess Plateau. The function value of the purifying environment is relatively small, accounting for only 0.19% of the total value, followed by the function value of species conservation, accounting for 5.42%. In order of service value, the ecological function values of water conservation, oxygen release, carbon sequestration, nutrient accumulation, soil conservation, fertility maintenance, species conservation and environment purification accounted for 32.97%, 25.94%, 11.63%, 11.34%, 6.37%, 6.14%, 5.42% and 0.19% of the total value, respectively. The conclusions of this study are basically consistent with other domestic studies. Compared with the annual output value of fruit trees in the same period, the total value of ecological services was 2.42 times of the annual output value of the fruit. Therefore, the fruit tree economic forest not only provides a large amount of fresh fruit products and creates tremendous economic and social benefits for the people, but it also provides a major increase in ecological service value, and the contribution of local GEP is greater than that of GDP. (3) After accounting, the water conservation amount of the fruit tree economic forest (2005–2020) in the Hulu River Basin in Pingliang City was 2.586×109 m3, with 2.135×109 t of soil fixation, 2.264×105 t of fertilizer retention, 9.568×106 t of carbon fixation, 2.562×107 t of oxygen production, 9.278×105 t of nutrient accumulation, and 1.137×105 t of environmental purification function (and within that function, the amount of sulfur dioxide absorbed is 94656.02 t, the amount of fluoride absorbed is 1793.82 t, the amount of nitrogen oxide absorbed is 6406.50 t, and the amount of dust fall is 10794.95 t), and the amount of negative ions provided is 1.564×1025, which greatly regulates and improves the ecological environmental quality of the region.