Temporal-Spatial Variabilities and Influencing Factors of Terrestrial Wet-Dry States and Vegetation Development: A Case Study in Northeast China
von Lin Xue
Datum der mündl. Prüfung:2023-12-14
Erschienen:2024-08-05
Betreuer:Prof. Dr. Martin Kappas
Gutachter:Prof. Dr. Martin Kappas
Gutachter:Prof. Dr. Daniela Sauer
Zum Verlinken/Zitieren:
http://dx.doi.org/10.53846/goediss-10635
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Name:Xue_Wet-Dry States and Vegetation Development.pdf
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Zusammenfassung
Englisch
The number of hydroclimate disasters has increased dramatically since the late 20th century with profound changes in the environment. The Meteorological Drought Index facilitates long-term monitoring of wet-dry trends and soil moisture (SM) is critical for regulating eco-hydrological and meteorological processes. Evaluating the spatiotemporal variation of terrestrial dry-wet states and its influencing factors from different perspectives was expected to provide a scientific basis for land and water resource management and drought mitigation, yet such comprehensive assessments are lacking. Northeast China (NEC) is a typically sensitive area toward climate change. It has been regarded as a major ecological barrier for northern China and even Northeast Asia, which is important in regulating the regional climate. Therefore, this thesis used NEC as a case study to explore reliable long-term spatiotemporal variation characteristics and the influencing factors of wet-dry states from meteorological (Chapter 2) and SM perspectives (Chapter 3), respectively. Vegetation controls water exchange between the soil and the atmosphere and has faced dual challenges posed by hydroclimate and human activities. On one hand, vegetation growth is expected to become more water-constrained in the context of global warming. Understanding how vegetation responds to water availability results in larger concerns. On the other hand, large demands for food and economic development have led to the degradation of natural vegetation in NEC. Recognizing the importance of sustainable forest utilization, the Chinese central and local governments have implemented a series of ecological conservation projects since 1998. An in-depth understanding of how climate change and human activities affect vegetation at various spatial and temporal scales is essential for managing human activities and restoring vegetation. However, the factors dominating vegetation development and their contribution remain unclear and require quantitative assessment (Chapter 4). Here, the Standardized Precipitation Evapotranspiration Index (SPEI)-aggregated multi-time scales were calculated to comprehensively assess wet-dry states and trends in NEC from 1990 to 2018. The analysis revealed a general drying tendency, with 86.3% of the area exhibiting a drying trend. The percentage of annual drought-affected areas increased by 0.7%/a. Seasonal analysis showed that spring and winter exhibited wet trends in 71% and 84% of NEC, respectively. In contrast, 92–93% of NEC displayed dry tendencies in summer and autumn. The highest interannual drought severity occurred in May and June. The most significant drought impacts and trends were observed in shrub and grassland, sparsely vegetated land, and middle-temperate semiarid regions (M-semiarid). The findings indicate that the warmer the temperature zone, the more sensitive it is towards drought under the same hydrological conditions, resulting in higher drought-affected areas. Similarly, the drier the land, the higher the drought-affected area within the same temperature zone, with pronounced drought trends in spring and summer. Future projections indicate that 86% of NEC will experience wetting trends, while 17% will continue to dry. The ordinary least squares model was employed to investigate the factors (i.e., precipitation, land surface temperature (LST), wind speed, vegetation greenness, elevation, slope, and clay content) influencing the spatiotemporal variability of SM. Results indicate that environmental factors differentially influence SM across various terrains (plains, tablelands, hills, and mountains), ecosystems (croplands, forests, and grasslands), and soil water status (wet and dry). The correlations between SM and land surface temperature (LST), elevation, as well as vegetation greenness were most terrain- and ecosystem-dependent, while precipitation was the least. A significantly different effect of each environmental factor on SM was observed between plains and mountains. Transitioned from wet to dry status, the dependence of SM on precipitation increased in forests but decreased in grasslands, while the opposite held for wind speed. The transit increased the contribution of LST, elevation, and slope to SM while decreasing the contribution of vegetation greenness. Furthermore, the effect of clay content on SM shifted from positive to negative in mountains and forests as soil transitioned from wet to dry. From 2013 to 2017, SM experienced a sharp drop across all terrains and ecosystems. A partial correlation analysis was conducted to identify the drivers of soil drying. The analysis identified several key factors: reduced precipitation in all spatial domains except mountains, elevated LST in grasslands, increased wind speeds in plains, croplands, and grasslands, and a rise in vegetation greenness in plains, tablelands, and croplands. The spatiotemporal variability in vegetation response to water availability, across different land cover types, climate regions, and time scales, was evaluated based on the relationship between Normalized Difference Vegetation Index (NDVI) and SPEI. The effects of human activities and climate change on vegetation development were then quantitatively assessed using the residual analysis method. Our findings indicate that NDVI of grasslands, sparse vegetation, rain-fed crops, and built-up lands, as well as sub-humid and semi-arid areas (drylands), positively correlated with SPEI, and the correlations increased with time scales. Conversely, negative correlations were concentrated in humid areas or areas covered by forests and shrubs. In humid areas, vegetation water surplus weakens with warming, while in drylands, vegetation water constraints intensify. Furthermore, potential evapotranspiration had an overall negative effect on vegetation, and precipitation was a controlling factor for vegetation development in semi-arid areas. From 1990 to 2018, 53% of the total area in NEC showed a trend of improvement, primarily attributed to human activities (93%), especially through the implementation of ecological restoration projects since 1998. In areas of vegetation degradation, the relative contributions of human activities and climate change were 56% and 44%, respectively. In summary, our study deepens the understanding of terrestrial wet-dry states and trends, providing valuable perspectives for land and hydrological management across diverse terrains and ecosystems amidst global shifts toward wetter or drier patterns. The government should develop targeted drought mitigation strategies in accordance with the spatiotemporal variation characteristics of drought, with a particular focus on droughts in M-semiarid regions occurring in summer, especially in May and June. Measures to monitor and mitigate the adverse effects of elevated LST and wind speed on SM in plains, croplands, and grasslands under dry soil conditions should be strengthened. Attention should be paid to addressing land cracks in grasslands and soil aggregation issues in mountainous and forested areas during water replenishment efforts. We also recommend regulating tree and crop planting densities in plains, tablelands, and croplands to mitigate trends of soil drying. Additionally, we mapped the relative contributions and roles of climate change versus human activities in areas experiencing vegetation change, offering crucial insights for developing more detailed vegetation restoration and management strategies to cope with future climate changes.
Keywords: wet-dry states; drought; soil moisture; water availability; vegetation development; ecological projects; Northeast China
