半干旱黄土丘陵区河北杨和油松生长季树干液流特征

为探究半干旱黄土区河北杨Populus hopeiensis和油松Pinus tabulaeformis树干液流密度变化特征，利用Granier热扩散探针监测陕西省延安市吴起县河北杨和油松人工林生长季（6-10月）树干液流密度的变化过程，并同步测定气象因子、林下土壤含水量和树木茎周长变化。结果表明:①2种树种树干液流密度日变化表现为“昼高夜低”单峰或双峰型曲线，油松比河北杨晚1 h左右到夜间水平。河北杨各月日均液流量大小为7月（3.69 kg·d-1）> 6月（2.90 kg·d-1）> 8月（2.63 kg·d-1）> 9月（1.47 kg·d-1）> 10月（1.16 kg·d-1），油松为7月（5.74 kg·d-1）> 6月（3.30 kg·d-1）> 8月（3.29 kg·d-1）> 10月（2.06 kg·d-1）> 9月（1.76 kg·d-1）。2种树种晴天树干液流密度变化幅度较大，而雨天变化幅度较平缓。②观测期间，2种树种均存在夜间液流活动，主要发生在0:00-6:00，在1:00点开始逐渐上升。河北杨和油松夜间液流量占全天总液流量最大比例分别为23.8%和16.6%，观测期间平均比例分别为12.1%和10.0%。③相关分析表明:河北杨液流密度与太阳辐射和气温呈极显著正相关（P < 0.01），与风速显著正相关（P < 0.05），与相对湿度极显著负相关（P < 0.01），油松液流密度与太阳辐射、风速、气温和土壤含水量呈极显著正相关（P < 0.01），与相对湿度和气压显著负相关。2种树种液流密度日变化与茎周长日变化呈显著负相关关系，液流活动能够指示树干茎周长白天收缩和夜间膨胀的日变化。

Growing season sap flow of Populus hopeiensis and Pinus tabulaeformis in the semi-arid Loess Plateau,China

To provide a scientific basis for water resources carrying capacity and tree species selection, Granier's thermal dissipation probe method was used to measure sap flow density of Populus hopeiensis and Pinus tabulaeformis during the growing season in Wuqi County, Yan'an City, Shaanxi Province. Climatic factors, soil moisture, and stem perimeter were simultaneously measured. The dynamic characteristics of sap flow and its response to environmental factors were analyzed. Results showed that (1) The daily variation of sap flow density for Populus hopeiensis and Pinus tabulaeformis demonstrated a typical wide single-peak or double-peak curve with high values in the daytime and low values in the nighttime. The appearance of valleys was about one hour earlier in Populus hopeiensis than in Pinus tabulaeformis. The mean daily sap flow for both species varied from June to October (in kg·d-1):for Populus hopeiensis with July (3.69) > June (2.90) > August (2.63) > September (1.47) > October (1.16) and for Pinus tabulaeformis with July (5.74) > June (3.30) > August (3.29) > October (2.06) > September (1.76). On sunny days, daily variation in sap flow density was exhibited on rainy days. (2) During the study period, nocturnal sap flow was observed with both species experiencing a progressive increase from 1:00 to dawn on most sampled nights. Nocturnal sap flow was related to individual water losses in these two species being up to 16.6%-23.8% of the daily sap flow. During the whole monitoring season, the total nocturnal sap flow for Populus hopeiensis was 12.1% of the total transpiration and Pinus tabulaeformis was 10.0%. (3) For Populus hopeiensis, sap flow density was positively related with solar radiation, air temperature, and wind speed, but negatively related with relative humidity. For Pinus tabulaeformis, sap flow density was positively related with solar radiation, air temperature, wind speed, and soil moisture but negatively related with relative humidity and air pressure. Sap flow density for the monitored trees was also negatively related with perimeter variation. Also, diurnal stem shrinkage and swelling for both species were indicated by sap flow density.