| Soil water content(SWC)and soil temperature(ST)are key variables influencing almost all ecosystem processes and functions which play a decisive role in the ecological effects of vegetation construction.Currently,very little is known of the functional relationship between surface vegetation cover(involving mostly scrub species)and SWC dynamics in drylands.Understanding how vegetation cover in drylands affects local-to-regional SWC and ST dynamics and associated water and temperature balances is of immense importance because of the abundance of afforestation projects worldwide,including in the drylands of China.The possible effects of surface vegetation on SWC and ST and their feedback on microclimate remain uncertain as the climate warms and vegetation cover increases due to the lack of in-situ and long-term environmental records,especially for arid and semiarid regions of the world.To address this knowledge gap,we conducted a two-year study in the Mu Us Desert of northwest China.In this study,the typical scrub communities of Hedysarum leave and Artemisia ordosica in Mu Us Sandy Land were selected as the research object.Dynamic changes in SWC and ST in scrub communities with different vegetation cover were analyzed,and the interaction mechanism and influencing factors between vegetation cover change and SWC and ST were revealed carried out by means of in-situ monitoring,laboratory and field tests and numerical simulation.Wavelet coherency analysis was used to reveal the correlation between SWC and ST and meteorological factors and between SWC and ST at different scales over time series.Finally,the water balance characteristics of scrub communities with different vegetation cover were analyzed by combining theory and numerical simulation.In this study,we explore the potential for SWC deficit mitigation,while maintaining ecological benefits and providing a reference for improving future restoration systems in drylands.The main conclusions are as follows:(1)SWC in both shallow and deep subsoil(0?100 and 100?180 cm,respectively)decreased with a linear increase in shrub cover.High shrub cover had significant impact on SWC by causing evapotranspiration to increase,effective rainfall to decrease,and dried soil layers to form more frequently.Dried soil layers act as a physical barrier to the uptake and recharging of soil water.Long-term soil water deficits with low initial soil water storage and relative extractable soil water tended to limit transpiration and caused total evapotranspiration to decline with elevated shrub cover.Leaving patches of bare land in the design of surface plant-cover mosaics may facilitate the recharging and retention of soil water in the restored areas of the Mu Us Sandy Land during and after rainfall.Surface vegetation cover should ideally involve native shrub species to mitigate excessive depletion of soil water and to avoid formation of dried soil layers,and striking a balance between vegetation restoration and soil water.(2)Increases in vegetation cover led to uniform-thaw-period cooling,uniform-freeze-period warming and net annual cooling.Also,variations in ST were shown to correlate in an exponential manner with vegetation cover during the uniform thaw and freeze periods.A two-equation ST model showed that prediction of seasonal variations in daily mean ST closely matched corresponding field observations,producing an R~2of 0.91.Collectively,model parameters summarize the trends in field-based observations of ST resulting from variances in external and internal soil-biophysical processes,including those associated with vegetation cover.These can lead to a multitude of direct and indirect changes in site biophysical conditions,impacting local ecosystems and their associated services.Uniform-freeze-period warming has the potential to benefit soil-dwelling insects overwinter survival,whereas net annual cooling can decrease litter decomposition rates and enhance development of on-the-ground litter mass.(3)The SWC and ST time series was mainly affected by precipitation air temperature respectively and were controlled by vegetation cover.With the increase of soil depth,the wavelet coherence decreases at small scale.With the increase of vegetation cover,the wavelet coherence decreases at small,medium and large scales.The correlation between SWC and ST decreased with the increase of soil depth and vegetation cove at small,medium and large scales.Positive and negative correlations were detected between SWC,ST and meteorological factors and between SWC and ST using wavelet coherency analysis,which showed great potential in identifying the scale-specific dependence of SWC and ST on controls at various depths throughout the soil profile.(4)A Hydrus-1D model was constructed based on the soil texture data of each soil layer in the first 0-180 cm soil complex in the study area and the hydraulic parameter data of van Genuchten-Mualem to simulate the coupled water and heat transport of soil at each observation site.A set of coupled water and heat transport parameters suitable for the study area is obtained,after model correction and verification.The simulated values of soil water and temperature in the bare land,medium and high cover observation fields are in general in good agreement with the observed values,and the simulation errors are acceptable and can be used to calculate the water balance.The deep drainage term is considered in the model,which makes up for the disadvantage that deep drainage cannot be calculated in the empirical formula.The water balance simulated by the model quantified the input and output of soil water on the basis of the empirical formula,and further revealed the interaction mechanism between vegetation cover change and soil water balance,indicating that vegetation plays a leading role in this process. |
