| Water is usually considered as a key factor limiting the growth and survival of plants,especially in arid and semiarid region.Robinia pseudoacacia has been the main plantation tree species and widely cultivated in the semi-arid and sub-humid areas of the Loess Plateau.In order to clarify the adaptability strategy of Robinia pseudoacacia to different environments,it is important to study their eco-physiological characteristics,especially the transpiration characteristics in different climatic zones.The sap flow dynamics of Robinia pseudoacacia and the meteorological environmental factors were continuously monitored in a semi-arid(Yan'an)and a sub-humid(Yongshou)climatic area of the Loess Plateau during the growing season.The eco-physiological indexes such as leaf morphological index,stable carbon isotope abundance(?13C)and branch xylem water potential were measured monthly.The dynamic characteristics of transpiration and its response to environmental factors were analyzed.This should have great significance in revealing its adaptability and mechanism in different climatic zones and improving the management practices of the plantations.The main conclusions are as follows:(1)Sap flux density of Robinia pseudoacacia in Yan'an and Yongshou test plots showed the same diurnal variation trend and common diurnal patterns,but it is significantly higher in Yongshou test plot than that in Yan'an test plot,showing higher transpiration rate.As the season and leaf phenology progressed,the overall magnitude of sap flux density was relatively low at early stage(June),higher in the mid and late period(July-September),and rapidly declining in the later stage(October).Yongshou test plot also showed a high level of transpiration on the seasonal scale(2)Sap flux density of Robinia pseudoacacia has a significant positive correlation with solar radiation,VPD and air temperature,and a significant negative correlation with relative humidity in both climatic zones.The solar radiation and VPD are the main factors influencing sap flux density on the daily scale.(3)The transpiration characteristics of Robinia pseudoacacia in the two climatic zones are related to the differences in morphology and physiology.In the morphological features,both showed a good power function relationship between the DBH and the sapwood area,butthe power function coefficient in Yan'an test plot(0.67)was smaller than that in Yongshou test plot(0.90),coinciding the lower transpiration water consumption in Yan'an test plot.The specific leaf area of Robinia pseudoacacia in Yongshou test plot was 1.75-2.24 times of that in Yan'an test plot,which was consistent with its higher transpiration characteristics.The stomatal density of Robinia pseudoacacia in Yan'an was 1.14-1.54 times of the Yongshou test plot,which was consistent with its smaller transpiration characteristics.In the physiological characteristics,the mean xylem water potential in Yan'an is lower than that in Yongshou test plot,which was consistent with its lower soil water content.The leaf stable carbon isotope abundance(?13C)in the late growth season is significantly lower than that in the early growth season in the two climatic areas,showing that the water use efficiency of Robinia pseudoacacia decreased with the change of phenology.The leaf ?13C values of Robinia pseudoacacia in Yan'an test plot was about 5.1 ‰ higher than that of the Yongshou during the whole growing season,which indicated that the Robinia pseudoacacia of Yan'an could reduce the transpiration rate through the stomatal regulation and showed higher water use efficiency.The transpiration characteristics of Robinia pseudoacacia in the two climatic zones are related to the differences in morphology and physiology.The Robinia pseudoacacia in Yan'an tend to reduce transpiration,improve water use efficiency and enhance root water absorption to adapt to the arid environment. |
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