Eco-hydrological Processes In Two Typical Natural Forests And Artificial Forests In The Loess Hilly Region Of China

The Loess Hilly region,located in the middle of the Loess Plateau,is suffering serious soil erosion with its fragile ecosystem.In recent decades,several ecological projects,such as“Grain for Green Project”and“Natural Forest Protection Project”have been widely carried out and have produced some ecological benefits.It was reported that both artificial plantation and natural forests played important roles in increasing vegetation coverage,reducing water loss and soil erosion and improving regional ecological environment.However,with the increase of vegetation restoration years,the negative effects of plantations on the regional hydrological and ecological environment are gradually prominent,such as the soil desiccation and the premature declination of artificial plantations.This may be due to the different regulatory mechanism of vegetation types on eco-hydrological processes.In present study,based on lots of field investigation,collections and measurements,the eco-hydrological functions and processes of two typical forest communities in this region,known as oak（Quercus liaotungensis）and black locust（Robinia pseudoacacia）,were explored.We hope this work could provide scientific basis for the comprehensive evaluation of the ecological service functions of different forest communities and the research on the eco-hydrological effects of forest.The main results are as follows:（1）The climate of Yan’an showed a warming and drying trend.From 1956 to2015,precipitation in this region showed a downward trend at a rate of 4.45mm/10-year,and both the average and the lowest temperature increased at a rate of0.29℃/10-year and 0.44℃/10-year,respectively.Oak land can reduce the average daily maximum temperature of the hottest month by about 1.57℃,and increase the temperature of the under-forest at the beginning of the growing season by about0.93℃.Furthermore,the relative air humidity beneath the Q.liaotungensis canopy increased by about 7.39%during the growing season.All of these showed its significant effects in humidifying and heat preservation.The solar radiation beneath Q.liaotungensis and R.pseudoacacia forest canopy accounted for 24.77%and 29.23%of the solar radiation outside the forest,respectively.The peak of solar radiation under forest occurred in April,earlier by two months than outside the forest.（2）The canopy interception of oak land was larger than that of black locust land,while the net precipitation input（throughfall and stemflow）was less.A total of 43effective rainfall redistribution events were monitored during the study period（July to October in 2016 and 2017,respectively）.The average throughfall rates of oak land（OL）and black locust land（BL）were 76.39%and 82.66%,the average stem flow rates were 4.52%and 5.56%,and the canopy interception rates were 19.09%and11.78%,respectively.Precipitation amount was the key factor determining precipitation redistributions,showing a significant linear correlation with throughfall and stemflow（p<0.01）as well as an exponential function with canopy interception（p<0.01）.The surface runoff coefficients of oak land and black locust land were 7.01%and 30.55%,respectively.Both the two forests communities could significantly reduce the frequency and magnitude of surface runoff.The effects of oak land in reducing runoff were better than black locust land.There was a significant power function relationship between surface runoff and precipitation amounts（p<0.01）.（3）The infiltration capacity of oak land is higher than that of black locust land,and the rainfall infiltration patterns along the soil profile（0-100 cm）of the two forest communities are also different.During the study period（April to October in 2016 and2017,respectively）,the soil precipitation infiltration replenishment of oak land and black locust land was 434.39 mm and 575.35 mm,respectively,and the corresponding precipitation infiltration replenishment coefficients were 65.74%and 49.63%,respectively.The precipitation infiltration amounts in deep layer（15-100 cm）of oak land forest were significantly higher than that of black locust land（p<0.05）,which accounting for the great differences in precipitation infiltration replenishment between the two forest communities.No significant decrease trends along the profile of the absolute cumulative increase in soil moisture（ASWI）and the ratio of ASWI between adjacent soil layers（RSWI）were observed in the oak land,while those decreased significantly with increasing soil depth in black locust land.The maximum precipitation infiltration rate(R_max)and average infiltration rate(R_mean)of the oak land on surface soil（0-15 cm）were significantly higher than that of black locust land（p<0.05）,which was conducive for the infiltration of more precipitation entering the soil and recharging deep soil layers.The lag time of soil moisture response to precipitation increased with soil depth,and it was significantly higher in black locust land than that of oak land（p<0.05）.There was a significant linear correlation between precipitation infiltration replenishment amounts and precipitation（p<0.01）.Oak land showed higher infiltration capacity than black locust land,especially in deep soil layers.（4）There were significant differences in the response of soil water change to precipitation between oak land and black locust land forest land.Comparing the soil water content under different hydrological years and vegetation types,it was found that the soil water content of natural forest land represented by oak land and shrubs was significantly higher than that of black locust land and abandoned farmland（p<0.05）.Precipitation was the dominant factor for soil water content and distribution patterns in the shallow（0-100 cm）and middle（100-200 cm）soil layers,while the deep soil moisture（200-300 cm）was significantly determined by vegetation types（p<0.01）.The oak land showed larger intra-annual changes in soil moisture（ΔSWS_t）and a steeperΔSWS_t-precipitation slope value,indicating its more effective regulation in soil moisture utilization than black locust land.Compared with abandoned farmland（ΔSWS_v）,soil water deficits in oak land and shrub land mainly occurred in shallow soil layers,while severe soil water deficits occurred in both shallow and deep soil layers in black locust land.In the 0-300 cm profile,theΔSWS_v of oak land and shrubs showed positive correlations with precipitation（p<0.01）,while that of black locust had a negative correlation with precipitation（p<0.01）.In summary,the main eco-hydrological processes based on precipitation in natural forests represented by Quercus Liaotungensis and artificial forests represented by Robinia Pseudoacacia in the loess hilly regions were stydied systematically.The results showed that Quercus Liaodungensis natural forests was better than Robinia pseudoacacia artificial forests in terms of improving microclimate,rainfall infiltration,runoff reduction and response of soil moisture to precipitation,which was more conducive to the reconstruction of ecosystem and sustainable development of vegetation restoration.