Study On The Regulation Effects Of Subalpine Forests On Hydrological Process In Western Sichuan Based On The Stable Isotopes

The impact of forests on hydrological processes is one of the important ecological functions, and the relationship between forests and water is a hot spot of forest hydrology and ecology researches. In study of the impact of forest vegetation on hydrological processes, stable hydrogen isotope in water molecules is an ideal tracer. This technique can be used to clarify the process and mechanisms in hydrological cycle processes and can overcome shortcomings of traditional methods, and its values comprehensively reflect capability of vegetation and soil to intercept rainfall, and ultimately are used to quantitatify hydrological cycle processes. The subalpine forests in Wolong Nature Reserve, Sichuan, are located in the source region of the Minjiang River, an important branch of Yangtze River tributaries, and are part of the main forest ecosystem of the Yangtze River upper reaches, which plays an important role in mainteining regional micro–climate, conserving water, as well as soil and water. Previous studies on hydrological process of subalpine forest ecosystem in western Sichuan, featured with more qualitative and less quantitative. Although there have been a few of studies of surface water in the forest, soil water, plant water and evaporation, studies on the water cycle process control mechanism of different forest ecosystems is still missing.In this paper, we studied quantitatively the isotopic compositions of precipitation, canopy penetration water, soil water, streams, plant water and evaporation by means of hydrogen and oxygen stable isotope technique, in three different types of subalpine forests in Wolong Nature Reserve. Meanwhile the relationship between water bodies was investigated to explore the water use patterns of dominant plants and clarify the regulating fuction on hydrological processes.The main purpose of this study is to reveal the regulation mechanism of subalpine forest vegetation on the regional stormwater runoff and to provide the scientific basis for water resources protection and rational utilization.The results of this study showed that:(1) In different types of subalpine forests in Wolong Nature Reserve, Sichuan, the hydrogen isotope values of soil water were significantly different. Multiple comparison showed that: soil waterδD values in non–forest land were highly significantly different from that in the bamboo–fir forest, rhododendron–fir forest and oak forest (p <0.01); soil waterδD values of bamboo–fir forest and rhododendron–fir forest were not significantly different (p = 0.104> 0.05). Within 6 days after a rain pulse of 11.6 mm, the precipitation contributed the most to litter layer of subalpine forest (59.53% ~ 98.11%), however the least to 40 ~ 60 cm deep soil (11.83% ~ 41.74%). Soil water in different forest types was recharged by precipitation with significantly different rates, showing the obviously regulateing functions of subalpine vegetation structure and soil structure on the migration of precipitation in the soil layers.(2) The hydrogen and oxygen isotopeδvalues of stem (xylem) water of the dominant plants in Wolong subalpine forest types varied between precipitation and 60 ~ 80cm deep soil water, which were the main water source of the dominant plants. With an 11.6mm rainfall, Abies. faxoniana had the lowest utilization of the precipitation, 6.15% ~ 31.38%, Betula. utilis was 42.89% ~ 78.89%, and Bashania. fangiana had the highest utilization of the precipitation, 50.10% ~ 90.85%, in a bamboo– fir forest. In comparison, Abies. faxoniana had 12.51% ~ 36.14%, Betula. utilis had 35.96% ~ 81.10%, and Rhododendron. faberi had 44.95% ~ 87.26% utilization of the precipitation in the rhododendrom– fir forest; Quercus. aquifolioides had 58.93% ~ 94.11%, Salix. cupularis had 7.45% ~ 49.58% utilization of the precipitation in the oak forest. The deep–rooted plant, A. faxoniana, was less dependent on rainfall, more dependent on deep soil water, while shallow–rooted plants, B. fangiana, had higher dependence on rainfall, with R. faberi in the between. Q. aquifolioides was more dependent on rainfall than S. cupularis.(3) Hydrogen and oxygen isotopes of evaporation water collected from subalpine forests in Wolong Nature Reserve varied to different range in different forests, and there was a good linear correlation between the hydrogen and oxygen isotopes. The stable isotopes value of evaporation water in non-forest land was less negtive than that in forests, suggesting that the fractionation of non–forest land was stronger than forests. Different forests in Wolong Nature Reserve all played some regulatory role in water evaporation.(4) TheδD(δ(18)~O) of stream water in subalpine forests (bamboo– fir forest and oak forests) was considerable stability, with bamboo– fir forest stream waterδD(δ(18)~O) ranged from–88.814‰~–95.559‰and–11.257‰~–13.076‰; oak forests stream waterδD(δ(18)~O) ranged from–47.129‰~–80.264‰and–9.415‰~–11.121‰, respectively. The precipitationδD(δ(18)~O) ranged from–66.806‰~–156.168‰and–8.556‰~–17.731‰. The stream waterδD(δ(18)~O) of subalpine forests in Wolong Nature Reserve was hardly affected by rain. Variation in the stream (S1)δD(δ(18)~O) of the bamboo– fir forest was smaller than the stream (S2)δD(δ(18)~O) of the oak forest, indicating that impact of precipitation on S1 was even smaller. The impact of rain on stream waterδD(δ(18)~O) was related to the amount of precipitation. When the precipitation was 0～10 mm, there was small effect on stream water by the precipitation, which occured on the 4th day after the rain event. The effct occurred in day 2～3 if the precipitation amount was up to 10～20 mm. And stream waterδD(δ(18)~O) value became more negative or less following that of rainfall, the situation would appear on the day of rain or 1 day later, when precipitation amount was 20～30mm. All above results suggested that the forests of Wolong subalpine could regulate the stream water significantly.