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Abstract

Northern Mongolia marks the southern edge of Siberia’s boreal forest, where it transitions into the arid Central Asian steppes. This semi-arid region is highly susceptible to climate change, land-use intensification, and environmental degradation. Mongolia’s boreal forests, primarily found in the north, have been severely diminished by logging and wildfires: Of the ~14.2 million ha, ~13% has been affected by fires. Sugnugur Valley, situated north of Ulaanbaatar, lies within a transitional belt of steppe, boreal forest, and the alpine tundra of the Khentii Mountains. This mountain range extends from Siberia into Mongolia, reaching elevations of ~2800 m a.s.l. Its western slopes feed into the Selenge–Baikal drainage system, making it an important regional freshwater source. The area has an intensely continental semi-arid climate. Between 1980 and 2022, the average annual precipitation was <330 mm. However, this increased to 350 mm during 1980–2001 and declined to 300 mm during 2001–2022. Nearly 90% of rainfall occurs during summer (May to October). Over 1980–2022, the annual mean air temperature was roughly –2.7 °C, with an upward trend of ~0.04 °C/year. In winter (October–March), the mean temperature reached around –14 °C, showing a comparable increase of ~0.06 °C/year. These changes in climate, including decreased precipitation and increased temperature in early spring, can greatly affect wildfire behavior. Sugnugur Valley supports numerous crucial hydro-environmental processes characteristic of Mongolia’s hydrological regimes, such as snow storage, permafrost, and forest cover. Although parts of the area exhibit near-pristine boreal and alpine ecosystems, human impacts, particularly wildfires, have increased. Since 2000, recurrent wildfires have hindered natural regeneration and destroyed seed banks. Beyond altering land cover, these fires also affect local hydrological dynamics. One key objective of this study was to explore the temporal evolution of watershed forests under various wildfire regimes. Initial findings from monitoring transects across Sugnugur Valley showed that forest fires strongly determined headwater function, in turn diminishing water availability in the arid steppe lowlands of the Khentii Mountains. Repeated wildfires retarded forest recovery and promoted the dominance of fast-growing species, thereby decreasing biodiversity. The loss of the organic surface layer increased the soil absorption of solar radiation, increasing soil temperatures. These remote forests are crucial for local communities that depend on wood for heating and cooking, pine nuts for income, and forested areas for grazing. Nevertheless, the degree of human influence, especially wildfires, remains poorly documented, limiting the development of effective conservation measures. By combining field observations with the TRAnspiration and INterception (TRAIN) hydrological model, this thesis aimed to increase regional data availability and analyze how wildfires shape local water regimes through their effects on forests. The outcomes of this study fill an important knowledge gap in northern Mongolia, where little is known of the hydrological impacts of wildfires.