5. Discussion

In this study, we constructed a directed and weighted complex network, naming the oasis transition network, which was used to express the transition patterns between different oasis types, identify the key land type and quantify the oasis structural stability. Compared with traditional transition analysis, the complex network described the dynamic process and the influences of different oasis types in the oasis system. Between centrality was used to identify the key land type, which was explained the possibility the land type being the transitional type. The findings of the oasis transition network indicated that the oasis transitions were increasingly complicated from 1990 to 2020. Grassland was the key oasis type in the oasis transition process, and the importance decreased during 1990 and 2020. Bare land had the highest betweenness centrality in 2015-2020 and cropland ranked third among all the land types, indicating the important roles and the changes of key land type in the transition network. According to the relevant report and statistic year book published by the beau of statistic in Turpan and Hami region, through the thirteenth ‘five-year’ plan, greening construction projects like returning farmland to grassland, afforestation, combating and controlling desertification and three north protective forests have gained significant effectiveness (Lyu et al. , 2020). During that period, grassland was not the most important transitional land type due to the efforts of planting and reclamation of the bare land.
Notably, in the oasis transition network, the land types with low proportion of coverage also contacted with other land types frequently. For example, shrubland expressed high degree values and was one of the principal components in the oasis transition network and the interactions with bare land dominated during the sub-periods of 1995-2000, 2000-2005, 2005-2010, 2010-2015 and 2015-2020. According to the local statistic yearbook, a large proportion of shrubland were planted artificially to combat wind and sand at the edge of the oasis and against land degradation in the inner oasis, like shuttle and camel thorn; the area of shrubland showed a distinct increasing trend in 2015-2020. We were inspired those human forces dominated in the oasis transition process for most sub-periods.
Oasis structural stability was quantified using average path length in the oasis transition network, which was a crucial index to express the oasis sustainability in dryland. So far, few studies had investigated the structural stability of the entire oasis system in dryland. In this study, we applied the average path length to reveal the difficulty of the interactions among the oasis types. With the results of more unstable oasis structural stability, we concluded that there were easier transitions among different land types recent years than before. Therefore, we suggest that more attentions on key land types (e.g. grassland) should be paid for the dryland sustainable development.
Even though the correlations between oasis and precipitation were insignificant, water resource was one of the most important restraint factors in dryland development (Yang et al. , 2021). The surface water in Tuha Basin is obviously shrinking recent years partly because of the increasing temperature and intense evaporation (Rodell et al. , 2018; Wang et al. , 2023) in Tuha Basin. Additionally, water resources were over-exploited and underground water level had continuously dropped for agricultural irrigation and mining (Hu et al. , 2019; Wang et al. , 2022; Cheng et al. , 2023). A more adaptive strategy for regulating cropland areas and water resource allocation is needed. From the driving factors analysis, population played a key role in oasis expansion, for example, to meet the requirements of the fast-growing population, the urbanization including artificial reclamation and construction speeded up, making built-up a most transfer-in type in the oasis transition network. Human factors including policy factors, economic factor and cultural factors were all important in the maintaining of the entire oasis system and land optimization in drylands. Therefore, further survey and analysis for the oasis driving should be deep considered.