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• Vegetation types significantly affect soil wetting depth and velocity. • Preferential flow enhances on the wetting front propagation. • Vegetation and soil properties are the main factors influencing the wetting front. Understanding the mechanisms governing the infiltration of precipitation into soil is crucial in eco-hydrological processes. However, the effect of vegetation types on the wetting front depth and velocity is poorly understood. Here, we analyzed 1234 infiltration events based on a large-scale long-term in-situ soil moisture monitoring network in arid mountainous area of northwest China. Our results show that the proportion of preferential flow was the largest in shrub (52.38 %), followed by alpine meadow (36.55 %), grassland (11.51 %), and barren (0.70 %). The wetting front velocity was consistent with the order of the proportion of preferential flow, with values of 11.42, 4.96, 2.32, and 1.16 cm/h, respectively. The mean velocity of preferential flow events was 2.05 times (0.06–71 times) higher in the shallow soil layer and 3.86 times (0.3–68 times) higher in the deep soil layer compared to matrix flow events. The wetting front depth was shallowest in alpine meadow (14.31 cm), followed by barren (15.70 cm), grassland (18.95 cm), and shrub (39.81 cm). Moreover, the wetting front depth and velocity reach their peak values in summer, primarily influenced by precipitation. Random Forests analysis results demonstrate that preferential flow is the primary factors influencing the profile wetting front depth, with control factors varying across different soil depths, soil water characteristic curve in shallow soil layers, and vegetation in deep soil layers, respectively. Meanwhile, soil organic carbon emerged as the most important factor impacting wetting front velocity. These findings contribute to a deeper understanding of infiltration processes in arid mountainous areas and offer a theoretical foundation for refining and enhancing mountain hydrological models. [ABSTRACT FROM AUTHOR]

• The CO 2 fluxes in winter were three times higher than in other seasons. • Glacier meltwater altered the diel pattern of CO 2 fluxes. • The total CO 2 fluxes from the headwaters were 39.57 (30.04–50.21) Gg C yr−1, ignoring the diurnal variation overestimated the CO 2 fluxes. • The CO 2 fluxes from 1st and 2nd order streams accounted for 77% of the headwaters. Rivers are important sources of the carbon dioxide (CO 2) released into the atmosphere; however, research on CO 2 emission from riverine headwater regions is sparse, particularly from rivers in the Tibetan Plateau (TP) region, which has large area of glaciers and permafrost. We conducted a three-year (2020–2022) observational study of CO 2 flux rates from the riverine headwater region of the Qilian Mountains (QLMs) to determine diel and seasonal CO 2 variations and flux rates. Our results revealed that the annual average CO 2 emission was 0.45 (0.03–1.60) kg CO 2 m−2 yr−1, with the highest flux rates observed in winter [0.87 (0.08–2.67) μmol CO 2 m−2 s−1], which was approximately three times higher than flux rates in other seasons. Glacier meltwater altered the diel pattern of riverine CO 2 flux rates by diluting CO 2 and dissolved inorganic carbon. Meanwhile, CO 2 release from rivers in the permafrost region was dictated by stream order, with a linear decrease as stream order increased. Considering diel variations, the total CO 2 fluxes from the headwater regions of the QLMs were approximately 32.70 (25.00–41.28) Gg C/yr, representing 76 % of the pre-calibration total CO 2 fluxes ignoring diel variations. This study provides essential insights into CO 2 release from headwaters, which have substantial implications for understanding CO 2 outgassing. [ABSTRACT FROM AUTHOR]

• We monitored sap flow for 16 spruces and 14 junipers at 4 arid to semi-arid sites. • Sap flow is almost completely controlled by VPD except for spruce in the arid site. • Increasing site aridity drives a shift towards more anisohydric stomatal behavior. • Spruce exhibits stronger stomatal regulation of transpiration than juniper. • Transpiration may increase first and then decrease from semi-arid to arid forests. Climate change is altering regional aridity and species composition in dryland ecosystems. Understanding the effects of long-term increasing aridity and species-specific stomatal behaviors on transpiration is therefore important for water-resource forecasting. To assess the effects of site aridity levels and species on stand transpiration rate (E s), we monitored sap flux density (J s) and relevant environmental parameters for 16 isohydric Picea crassifolia (spruce) and 14 anisohydric Juniperus przewalskii (juniper) trees over three growing seasons at four arid to semi-arid high-elevation sites on the Tibetan Plateau. Our results show that E s was about 5–9 times higher in semi-arid sites than in arid sites, and about 6–9 times higher in spruce than in juniper. Spruce exhibited stronger stomatal regulation of transpiration than juniper. Soil water supply strongly promoted J s only for spruce in the arid environment (R2 = 0.23), while at the other sites, J s was mostly controlled by atmospheric vapor pressure deficit (VPD) (R2 > 0.82). However, increasing site aridity greatly reduced the sensitivities of both J s and canopy conductance to VPD , especially for juniper. We expect that in the transition from semi-arid to arid alpine forests, E s will initially rise due to increasing VPD. However, in the long term, there may be a stronger decline in E s since both the sensitivity of J s to VPD and the stand sapwood area will decrease. These findings could be used to reduce the impacts of climate change on water resources in high-elevation drylands. [ABSTRACT FROM AUTHOR]

Highlights • Temporal stability of soil water storage (SWS) in high-elevation forests are analyzed. • Temporal changes and spatial variation of SWS decreased with increasing soil depth. • One time-stable location (TSL) can be representative of the mean SWS. • These TSLs be with average LAI or relatively low canopy interception loss locations. • Temporal stability of SWS was mainly affected by soil and vegetation properties. Abstract Understanding soil water storage (SWS) dynamics in soil profiles is important for hydrological modeling and restoration of vegetation in semi-arid areas. Using the temporal stability method, we aimed to investigate the temporal stability of SWS at 0–40, 40–80, and 80–120 cm depth, and to identify representative sites for reliable estimates of the mean SWS in the permanent forest plot in the Qilian Mountains of China. Further, we wanted to identify correlations between temporal stability of SWS and soil, topography, and properties of the vegetation. Soil water content at soil depths 0–120 (for locations 1–40) and 0–70 cm (for locations 41–60) were measured using time-domain reflectometry (TDR) on 52 dates from 2016 to 2017. Results revealed that time-averaged mean SWS for the three layers differed significantly (P < 0.05), and the temporal changes and spatial variations of the mean SWS all decreased with increasing soil depth. Based on either the Spearman correlation coefficient or the standard deviation of relative difference (SDRD) index, the temporal stability of SWSs within the soil profiles under different soil layers were strong, and the number of time-stable locations increased with increasing soil depth, indicating that the SWS intended to be more temporally stable in deeper soil. One time-stable site can be representative of the mean SWS for multiple soil layers for the whole plot, and can accurately estimate the mean SWS for the three soil layers (R2 > 0.86, P < 0.001). Moreover, these time-stable locations should be those having average LAI, SBD or relatively low canopy interception loss compared to the corresponding field means. Soil bulk density, canopy interception rate, and aboveground biomass of mosses can significantly (P < 0.05) affect the stability of SWSs in this high-elevation forests. Such effects, however, differed among the different soil layers due to different conditions of soil characteristics, distribution of root biomass, and freeze–thawing processes. These results suggest that the influence of vegetation properties of the temporal stability of SWS should attract more attention and that both the relative importance of and interactions among different determining factors is helpful for better understanding the mechanistic determinants of SWS temporal stability in these areas. [ABSTRACT FROM AUTHOR]

• The intensity of flash drought in the Qilian Mountains increased from 1982 to 2018. • The response of NPP to flash drought was highly sensitive in the western part and eastern edge of the Qilian Mountains. • The intensified flash drought could enhance the coupling between NPP and temperature/VPD. Understanding the impact of high-altitude flash droughts on vegetation net primary productivity (NPP) is crucially important for ecosystem management. Here, we investigated the response of NPP to flash drought with a case of the Qilian Mountains (QLMs). Our results indicated that the intensity of flash drought in the QLMs increased significantly during 1982–2018, and NPP was highly sensitive to flash drought, with 64.3 % (124,245 km2) of the region showing highly negative responses. The partial correlation analysis between NPP and meteorological factors in terms of different drought intensities showed that intensified flash drought enhanced the coupling between NPP and temperature/VPD. In the short-duration flash drought events with positive temperature anomalies, temperature increase will promote the accumulation of NPP; however, the carbon uptake of plants was significantly compromised by VPD in the flash drought events with negative temperature anomalies. Our study may provide valuable information for drought monitoring and comprehensive ecosystem management. [ABSTRACT FROM AUTHOR]

• Dynamics of grassland ecosystems vulnerability to climate change were evaluated over the Qilian Mountains. • Patterns of change in grassland ecosystems vulnerability under different environmental gradients were explored. • Spatial distributions of grassland ecosystems vulnerability were mainly determined by exposure. • Climate, elevation, and slope gradients together influenced the degree of grassland ecosystems vulnerability. With climate warming and frequent extreme weather events, the structure and function of grassland ecosystems have been significantly altered. The Qilian Mountains are a wet island, serving as an important ecological barrier in China and central Asia. It is highly sensitive and prone to climate change, yet the vulnerability of grasslands to climate variability in this region remains unclear. In this study, based a NDVI dataset and meteorological data, we evaluated the relative vulnerability of grassland ecosystems to short-term climate variability by combining three indices: exposure, sensitivity, and resilience, and explored its relationship with water use efficiency (WUE). The results show that the spatial distribution patterns of grassland vulnerability in the Qilian Mountains from 2000 to 2018 were largely determined by exposure, and sensitivity is positively correlated with exposure and negatively correlated with resilience. The vulnerability of alpine meadow and alpine steppe is higher than that of temperate typical steppe and temperate desert steppe. Climate and topography gradients together affect the degree of grassland vulnerability. Grasslands with higher vulnerability are mainly found at medium elevation (3200–4000 m) area, where the topography is flat. The northern grasslands are primarily influenced by temperature, while the western and southern parts are mainly affected by precipitation. Alpine meadow and alpine steppe are more vulnerable to temperature and temperate desert steppe is more vulnerable to changes in precipitation. The higher the vulnerability, the lower the WUE of the grasslands. This study provides a reference for assessing the impact of climate change on grassland ecosystems and developing grassland vegetation management strategies. [ABSTRACT FROM AUTHOR]

Summary: Knowledge of soil moisture is critical to understanding many of the hydrological processes that are of interest in soil hydrology, meteorology, and ecology research. In the present study, we used rainfall and soil moisture data from 2003 to 2008 measured at grassland and meadow sites in the Qilian Mountains of northwestern China to analyze the response of soil moisture to rainfall event size during the growing season. The responses of soil moisture at the grassland and meadow sites to rainfall event size were similar, although total rainfall, the frequency of large rainfall events, and the vegetation types were different. Soil moisture at depths of 20 and 40cm increased significantly after rainfall events larger than 15 and 20mm, respectively, but the magnitude of these changes varied in response to differences in the duration of the dry interval preceding the rain. Soil moisture at depths from 60 to 80cm in the grassland increased obviously with rainfall events larger than 40mm or after two consecutive large rainfall events, but was not significantly correlated with rainfall event size in the meadow. Soil moisture at depths from 120 to 160cm did not change significantly during the growing season at either site. The soil water storage at depths from 20 to 80cm at both sites increased obviously after rainfall events larger than 20mm. The present results suggest that large rainfall events (>20mm) play a key role in increasing soil water storage in the grassland and meadow ecosystems of these semi-arid mountains. [ABSTRACT FROM AUTHOR]

• Moisture recycling indicated water resource renewal capacity. • Transitional characteristics of moisture recycling are evident. • Moisture recycling contributed 22% to precipitation. • Good ecological conditions enhance moisture recycling. Moisture recycling, an important component of regional precipitation, is a key indicator of a region's ability to renew its water resources. Quantifying moisture recycling allows for a comprehensive understanding of the spatial and temporal structure of the regional water balance. Therefore, an observation and sampling network for precipitation, soil samples, plant stems, and surface water was established in the Qilian Mountains, a transition zone between the Tibetan Plateau and an arid region. Approximately 2749 samples were collected and analyzed for their isotopic compositions. Based on the Craig–Gordon model and the three-end-member mixing model, the evaporation, transpiration, and advection contributed 8%, 14%, and 78% to precipitation, respectively, and the total contribution to local moisture recycling was equivalent to 71 mm of precipitation. The evaporation fraction tended to decrease from the central Qilian Mountains to the surrounding areas, whereas the transpiration fraction exhibited a decreasing trend from south to north in the south-central Qilian Mountains. If all of the moisture recycling precipitation is produced in the flow, the water production is approximately 13 billion m3, which is approximately 87% of the average total annual runoff from the Qilian Mountains and twice the average total annual runoff of the three major inland river basins in the Hexi corridor. Additionally, altitude, topography, vegetation conditions, and meteorological factors influence local moisture recycling. The results showed that moisture recycling precipitation in cold regions is crucial to ecological stability and water resource supply. This study demonstrates that good ecological conditions enhance moisture recycling, improve the renewal capacity of water resources, and support the stability and sustainable use of regional water resources. Therefore, ecological protection remains an important support for sustainable development in the Qilian Mountains. [ABSTRACT FROM AUTHOR]