Abstract:

The Loess Plateau has scarce rainfall and insufficient water supply, leading to a general water limitation for plant growth. Thermal dissipation probe (TDP) was used to monitor the dynamic changes of sap flow density for Platycladus orientalis and Caragana korshinskii in the growing season (from May to October) of 2022 in the Longtan watershed of the Loess Plateau. Meteorological factors and soil water content were synchronously measured. The characteristics of water consumption capacity and their responses to meteorological factors of the main tree species in the region were analyzed, aiming to explore the water use pattern and drought adaptability of different vegetation. Results showed that 1) during the growing season, the daily variation of sap flow density for Platycladus orientalis was unimodal, with the peak ranging from 0.448 to 0.537 g∙m⁻²∙h⁻¹. The daily variation of sap flow density for Caragana korshinskii was bimodal, with 2 peaks ranging from 0.0276 to 0.0393 g∙m⁻²∙h⁻¹ and 0.0315 to 0.0436 g∙m⁻²∙h⁻¹, respectively. 2) Solar radiation was the dominating environmental factor affecting the sap flow density of Platycladus orientalis (r²= 0.773, P=0.036), followed by vapor pressure deficit (r²=0.320, P=0.041). Caragana korshinskii was mainly affected by vapor pressure deficit (r²=0.678, P=0.011), followed by relative humidity (r²=0.564, P=0.044). 3) During different periods of the growing season, the sap flow density after rainfall events was significantly higher than that before rainfall of both species. The sap flow density could be simulated by transpiration variable, an integrated index calculated by solar radiation and vapor pressure deficit. Sap flow density of both species before and after rainfall events showed a positive exponential relationship with transpiration variables. According to different hydraulic conductance model coefficients (fitting parameter b value) before and after rainfall events, Caragana korshinskii showed more sensitivity to rainfall than Platycladus orientalis, while Platycladus orientalis had stronger water adaptability. Based on comprehensive results, it is evident that the response of sap flow density for Caragana korshinskii and Platycladus orientalis to the environment is different. Additionally, Caragana korshinskii could be considered as a rainfall-sensitive species, while Platycladus orientalis could be considered as a rainfall-insensitive species. The findings of this study can serve as scientific references for local afforestation tree species selection and forest water resource management.

Key words: Platycladus orientalis, Caragana korshinskii, sap flow density, water use, environmental parameters, Loess Plateau

摘要：

黄土高原降雨稀少，水分供应不足，植物生长用水普遍受限。通过热扩散式探针（TDP）于2022年生长季（5－10月）对黄土高原龙滩流域侧柏（Platycladus orientalis）和柠条（Caragana korshinskii）的树干液流密度进行连续观测，同步监测气象因子和土壤含水量。对该地区主要树种的蒸腾耗水特征及其环境响应机制进行分析，旨在解析不同植被的用水规律与干旱适应能力。结果表明，1）生长季内，侧柏液流密度日变化为单峰型，液流峰值分布在0.448－0.537 g∙m⁻²∙h⁻¹。柠条液流密度日变化为双峰型，2个液流峰值分布在0.0276－0.0393 g∙m⁻²∙h⁻¹和0.0315－0.0436 g∙m⁻²∙h⁻¹。2）侧柏液流密度主要受太阳辐射影响（r²=0.773，P=0.036），其次为饱和水汽压差（r²=0.320，P=0.041）。柠条液流密度主要受饱和水汽压差影响（r²=0.678，P=0.011），其次为相对湿度（r²=0.564，P=0.044）。3）在生长季内的不同时期，降雨后侧柏和柠条液流密度均显著高于降雨前。可利用能同时反映太阳辐射和饱和水汽压差的蒸腾变量模拟液流密度变化，降雨前后的侧柏、柠条液流密度均与蒸腾变量呈正向指数关系。通过对比降雨前后植被水力导度（拟合参数b值）发现：与侧柏相比，柠条液流密度受降雨影响更强烈，而侧柏的水分适应能力较强。综合研究结果表明：侧柏、柠条液流密度对环境的响应存在差异，柠条是降雨敏感型植物，侧柏是降雨不敏感型植物。研究结果可为当地造林树种选择及森林水资源管理提供科学依据。

关键词: 侧柏, 柠条, 液流密度, 水分利用, 环境因素, 黄土高原