Adaptability Of Water Transfer In SPAC System To Arid Environment In Robinia Pseudoacacia Along The Precipitation Gradient On The Loess Plateau

The Chinese Loess Plateau is reputed to have the most severe soil erosion in the world.Afforestation practices have been implemented since the 1950s to reduce severe soil erosion and to improve environmental quality.Robinia pseudoacacia L.?black locust?was widely chosen as a pioneer afforestation species due to its fast growth and high tolerance to drought and poor soil fertility.However,the soil moisture was found to decline quickly after afforestation with R.pseudoacacia due to its high water consumption,and there has widely existed a dry soil layer in the deep soil across the Loess Plateau.R.pseudoacacia trees would suffer more frequent and severe drought stress after the formation of a dry soil layer,which could aggravate the drought-induced growth decline and increase the risk of mortality in R.pseudoacacia stands,presenting a serious threat to sustainable plant growth and benefits for soil and water conservation in this area.The long-term adjustments of plant morphological and structural characteristics in response to drought stress could efficiently regulate the water transfer in soil-plant-atmosphere continuum?SPAC?,which is essential for maintaining plant water balance and improving the safety of plant water transport,thus enhancing the survivability of plants.Therefore,assessing the adaptability of SPAC water transfer to arid environment along the precipitation gradient of Loess Plateau should be helpful for guiding vegetation restoration and promoting sustainability of plantation forests on the Loess Plateau.In this study,we examined the variation in leaf and individual level traits at regional scale by the field survey along the precipitation gradient on the Loess Plateau.We further characterized the temporal changes of plant traits with simulated different precipitation conditions using three levels of water supply in a controlled growth chamber.We designed a vertical split-root experiment with two soil types,and verified the applicability of plant supply-demand hydraulic model?Sperry model?in simulating plant water transport when there existed a dry soil layer,and we further analyzed the effect of vertical heterogeneity of root-zone soil moisture on the plant water transport safety with the Sperry model.On this basis,we modified the routines of SPAC water transfer in the Biome Bio Geochemistry model?Biome-BGC?with the Sperry model,and assessed its predictability with the field observations.We evaluated the drought-induced mortality risk for R.pseudoacacia plantations at the regional scale by simulating the tempo-spatial patterns of plant hydraulic traits with the modified model,and examined the effect of soil desiccation on drought-induced mortality risk in R.pseudoacacia trees.The main conclusions of this study were as follows:?1?There existed strong correlation between climatic factors and soil properties along the precipitation gradient on the Loess Plateau.The humidity index correlated highly with other climatic factors and soil properties,which could well represent the regional environmental variation.Each R.pseudoacacia stand experienced severe soil desiccation across the precipitation gradient.The R.pseudoacacia trees exhibited similar leaf level traits and rooting patterns at different sites,while whole-plant leaf area and fine root area decreased linearly with deacreasing humidity index,presenting as an efficient way to reduce plant water consumption and adapt to different water conditions.?2?According to the controlled growth chamber experiment,increasing transpiration with R.pseudoacacia growth leaded to a decline in soil moisture under each precipitation conditions.As drought progressed,leaves exhibited a coordinated increase in vein and stomatal densities,higher drought tolerance by increasing cell wall elasticity,and higher water storage capacitance.After 60 days,leaf traits were similar among the treatments while whole-plant leaf area decreased considerably with decreasing water supply.The results were in line with the spatial pattern of plant traits along the precipitation gradient study,and supported the association between temporal hierarchy and organizational levels.The controlled growth chamber experiment could well reflect the long-term interactions between plant traits and soil moisture under field conditions.?3?The Sperry model was capable of simulating the dynamics of midday leaf transpiration rate,predawn leaf water potential and midday leaf water potential with lower root-zone staying drought and upper root-zone experiencing drought-rewatered-drought process in both soil types.According to the numerical simulation,with decreasing upper root-zone soil moisture(SMC_up),the critical leaf transpiration rate at hydraulic failure(E_crit),actual transpiration rate?E?and hydraulic safety margin?SME?remained steady firstly and then dropped rapidly when SMC_up was lower than the thresholds in both soil types.The variation in percentage loss of whole-plant hydraulic conductance?PLK?with SMC_up exhibited an opposite trend.The curves of E_crit,E,SME with SMC_up showed descending trends while the curve of PLK with SMC_up went up with decreasing soil moisture content in lower root-zone.With increasing vapor pressure deficit?VPD?,the threshold of SMCup-E curve increased and was more sensitive to the decrease in lower root-zone soil moisture.SME generally decreased with increasing VPD,and the reduction was larger under high soil moisture content in both upper and lower root-zones.However,the sensitive of SME to variation in upper and lower root-zone soil moisture was lower under higher VPD.In Huangmian soil,the indicators exhibited similar variation patterns with Lou soil,while the SMC_up thresholds were lower and the indicators changed more rapidly than Lou soil when SMC_up decreased below the threshold.?4?The modified model significantly improved the predictive precision for simulating canopy transpiration and soil moisture,and captured the dynamics of leaf water potential and the decreasing trend in leaf area index with decreasing precipitation along the precipitation gradient.According to the long-term dynamics of plant hydraulic traits at each site,annual average percentage loss of whole-plant hydraulic conductance?APLK?showed strong temporal variation due to climatic variability,which was positively correlated with annual potential evapotranspiration and the aridity index.Along the precipitation gradient,the maximum APLK increased linearly with decreasing mean annual precipitation?MAP?and could exceed 60%at sites with MAP<446.1 mm.The sustainable growth of R.pseudoacacia plantations at these sites would face a severe threat.Soil desiccation increased the sensitivity of plant hydraulic safety to precipitation variability considerably,and the effect was more significant in areas with lower MAP.