Spatial And Temporal Variability Of Soil Water Content And The Influencing Factors In Two Typical Forest Communities In Loess Hilly Region

Loess hilly region suffers the most severe soil and water loss in Loess Plateau and therefore it is also the key area of vegetation restoration.As the major direct water resource for local vegetation communities,soil water is one of the most important components of ecosystem hydrologic cycle in this region.The dynamic characteristics and the response patterns to climatic factors of soil water on different time scales are crucial to function and stability of the ecosystem.In this paper,two typical communities consisting of a natural secondary forest formed by the native species oak(Quercus liaotungensis)and an artificial black locust(Robinia psedoacacia)forest,which was most widely planted in this region,were chosen as research objects.The interannual soil water dynamic characteristics were identified by heat maps,possibility density distribution based on multi-year(2008–2021)multiple points and multiple frequencies soil water data(0–300cm).The influence of precipitation on soil water was investigated for different layers on month scale,and the seasonal dynamic rhythm was also analyzed for both communities.Combining synchronous climatic factors,different patterns for both communities before and after rainy season were quantified as well as the shifting thresholds between patterns were calculated by the least significant difference method,correlation matrixes,stepwise regression,and the operator receiver curves.Thresholds between different patterns were calculated and validated by the receiver operator curves.And the water potential gradients of soil water consuming were developed to elucidate regulatory mechanism of soil water based on dynamics of atmosphere water potential of canopy and forest inside and soil water potential.The main conclusions are as follows:(1)The soil water dynamic characteristics for both communities were similar on interannual scales,which was mainly influenced by precipitation.The main budget of soil water for both communities occurred in soil layers of 0–100 cm.Heavy precipitation events that could recharge soil water below 160 cm had low frequency.Compared to years with higher precipitation amount and uniformly distribution,in years with lower precipitation amount but with extreme precipitation events did the soil water replenishment happen more often in deep layer.(2)The seasonal soil water content dynamics of 0–100cm for both communities were similar,which were performed as “decline-replenish-decline again-replenish again”.The soil water content of upper layer responded rapidly to the precipitation while soil under 100 cm lagged obviously.And the replenishment of soil water would be kept for 9 months after extreme precipitation in deep layers.The soil water content monthly mean values of deep soil layers for the oak community converged to a constant while the deep soil water content monthly mean values of deep soil layers for the black locust community had decreasing tendency,which suggested that the black locust community had a more significant consumption on soil water of deep soil layer.(3)Distinct patterns were found in soil water consumption for both communities depending on soil water conditions before and after rainy season.These patterns were reducing the dependency on soil water with water condition improving.Soil water was the main limitation of community evapotranspiration.But the oak community had adaptability to significantly respond to evapotranspiration drivers like solar radiation and heat flux while the black locust community would keep consuming soil water to permanent wilting point in continuous dry events.Under ameliorated water condition,strong response of evapotranspiration to climatic drivers were found in both communities.(4)The soil water storage thresholds(0–100cm)that soil water consumption patterns shifted from water limitation to evapotranspiration energy driving limitation for the oak and the black locust communities before rainy season were 105.75 mm and 120.44 mm,respectively,and the thresholds for both communities after rainy season were 137.93 mm and150.70 mm,respectively.It had high accuracy and practicability to simulate soil water storage thresholds of soil water consumption patterns shifting through the operator receiver curves.(5)The mechanism of different soil water consumption patterns shifts was nonlinear variation of water potential gradients as driving force with soil water declining.Low soil water potential was the main limitation of water potential gradient increment when soil water condition was dry.And when soil water content exceeded the thresholds,the limitation of soil water potential on soil water potential gradients was substantially lift and the water potential gradients would increase hugely with atmosphere water potential decreasing.(6)The atmosphere water potential difference between canopy and forest inside could be the indictor that whether a community was suffering drought stress.It would happen when community soil water content was below the threshold that the absolute values of canopy water potential were much lower than the absolute values of inside water potential.And the absolute values of canopy water potential would always higher than the absolute values of inside water potential when community soil water content was above the threshold.This paper specified the characteristics of soil water distribution and dynamics for both typical forest communities on interannual and seasonal scales,identified two patterns of soil water consumption,developed a method to simulate the soil water threshold of soil water consumption pattern shifting,and illustrated the mechanism of different soil water consumption patterns by water potential gradient.The study enhanced the comprehension of the relation between vegetation evapotranspiration and soil water resource in this region as well as provided theoretical foundation of vegetation recovery and forest community management.