水蚀风蚀交错区河北杨树干液流密度特征及其对环境因子的响应

目的研究黄土高原水蚀风蚀交错区乡土树种河北杨的树干液流密度变化特征及其与环境因子的关系，为该地区水资源承载力研究和树种选择提供理论依据。方法利用Granier热扩散探针法和自动气象监测系统对陕北吴起县河北杨树干液流密度和周围气象条件、土壤含水量等进行连续测定分析。结果河北杨树干液流密度日变化呈宽幅单峰形，液流密度峰值提前于太阳辐射强度峰值约2.5h，提前于水汽压差(VPD)峰值约4h。夜间液流活动主要发生在后半夜00:00—06:00，晴天液流密度变化幅度小，雨天变化幅度大；灌水当天干基液流密度峰值出现的时间明显提前，峰值大小比灌水前提高66.66%，冠基处峰值比灌水前提高73.62%，灌水处理后实验组连续两日液流密度均值比灌水前减少2.21%，而对照组减少21.89%，晴天条件下，树干液流密度与太阳辐射、VPD、气温和风速极显著正相关(P < 0.01)，与土壤含水量显著正相关(P < 0.05)，与相对湿度极显著负相关(P < 0.01)。降雨天树干液流密度与太阳辐射、气温和风速极显著正相关(P < 0.01)，与VPD显著正相关(P < 0.05)；与对照组比较，灌水之后液流密度与气象因子的相关系数绝对值没有明显增加；河北杨树干储存水的日动态总体表现为上午释放和下午补充，并存在两次较明显的释放-补充周期。结论河北杨树干液流密度主要受太阳辐射强度、VPD、气温、相对湿度、风速和土壤含水量的影响，液流密度峰值时间与太阳辐射强度、VPD、气温等气象因子存在时滞，土壤水分的增加可缩短液流密度与气象因子峰值的时间差，上午储存水的持续释放是导致树干液流密度与气象因子时滞效应的重要原因。 

Dynamics of sap flow density of Populus hopeiensis and its responses to environmental variables in the water-wind erosion crisscross region on the Loess Plateau

ObjectiveWe aimed to analyze the differences and characteristics of sap flow of Populus hopeiensis among the different environment conditions in the water-wind erosion crisscross region on the Loess Plateau, and to clarify the relationship between sap flow density and environmental factors. This study provides a scientific basis for the water resource carrying capacity and tree species selection.MethodThe Granier's thermal dissipation probe method and a micrometeorological monitoring system were used to measure the sap flow density of Populus hopeiensis during growing season in Wuqi County, Shaanxi Province of northwestern China.ResultThe results showed that the diurnal process of sap flow density in P. hopeiensis showed a broad unimodal change and the maximum sap flow density reached about 2.5 hours before the maximum of solar radiation, and about 4 hours before the maximum of vapor pressure deficit (VPD). During the studying period, the nocturnal sap flow was noticeable in 00:00—06:00 and only in very few occasions sap flow was totally stopped. On sunny days, daily variation in sap flow density exhibited little amplitudes, whereas it exhibited large variation on rainy days. The peak value appearing time of sap flow density of trunk base was early apparently after being watered, and the peak sap flow density of trunk base and crown base increased by 73.62% and 66.66%, respectively after being watered. The results showed that the peak value in the sap flow density was greatly influenced by the soil moisture. And the mean daily sap flow density of EG (experimental group: watering treatment) and control check(CK) decreased by 2.21% and 21.89%, respectively after being watered. It showed that artificial irrigation can supplement the soil water deficit in the forest land in a short time, and restore the normal transpiration water consumption pattern of the tree. With respect to P. hopeiensis, sap flow density on sunny day was positively correlated with each of solar radiation, VPD, air temperature, wind speed (P < 0.01) and soil volumetric moisture content(P < 0.05), but negatively correlated with relative humidity(P < 0.01). On rainy day, sap flow density was positively correlated with solar radiation, air temperature, wind speed (P < 0.01) and VPD(P < 0.05). Compared with CK, the correlation coefficients of sap flow density of EG and meteorological factors were not higher.ConclusionOur findings suggest that the meteorological factors and soil moisture had important impacts on sap flow density and the time lags of sap flow density to environmental variables were related with the discharge of stem water storage. Watering treatment can shorten the time lags of sap flow and meteorological factors.