祁连山青海云杉树干液流密度的优势度差异

以祁连山排露沟小流域青海云杉林为研究对象,选取有代表性的优势木、亚优势木、中等木和被压木各3—5株,2015年6月16日至10月14日应用热扩散技术对不同优势度青海云杉树干液流密度进行测定,并同步测定相关的林外气象因子。结果表明:(1)青海云杉液流密度呈昼高夜低趋势,晴天液流密度变化幅度较大,而阴雨天变化幅度较小。(2)晴天树木优势度越大,其液流在日内的启动越早,结束越晚,峰值也越大;优势木的平均液流密度为(0.0758±0.0475)m L cm~(-2)min~(-1),是亚优势木的1.5倍,是中等木和被压木的1.68倍。(3)青海云杉平均液流密度基本呈现6月份最大,其次是8月份,9、10月份明显减小,且优势木亚优势木中等木被压木。(4)相关性分析和逐步回归表明,青海云杉日均液流密度与太阳辐射强度、饱和水气压差和空气温度呈正相关关系,与空气相对湿度和降雨量呈负相关关系。影响优势木、亚优势木和中等木液流密度的主要气象因子是太阳辐射强度,被压木液流密度主要受空气相对湿度的影响。

Variation in sap flow density among levels of tree dominance in Picea crassifolia in the Qilian Mountains

Tree transpiration plays a determining role in water balance for forest stands and in water yield from forested catchments. In the present study, an experiment was conducted in the Pailugou watershed in the Qilian Mountains, in the arid region of Northwest China. In a 86-year-old Picea crassifolia forest stand, 3-5 trees from each of dominant, subdominant, intermediate and suppressed trees were chosen as sample trees. The sap flux density for these trees was measured using the thermal dissipation probe (TDP) method from June 16 to October 14, 2015. Furthermore, the related meteorological factors, including solar radiation (R_s), air temperature (T), air relative humidity (R_h), and precipitation (P) were simultaneously monitored by an automatic meteorological station in the outer forest. The results showed that: (1) Daily sap flow density was higher during the day than during the night. On sunny days, daily variation in sap flow density exhibited large amplitudes, whereas it exhibited little variation on cloudy and rainy days. (2) On sunny days, daily sap flow density began earlier in the morning, ended later at night, and exhibited a larger maximum of sap flow density as tree dominance class increased. The value of the mean sap flow density of dominant trees was (0.0758 ± 0.0475) mL cm^-2 min^-1, which was 1.5 times of that of subdominant trees and 1.68 times that of intermediate and suppressed trees. (3) Mean daily sap flow density for all dominant trees showed that the maximum values were achieved in June, with the next highest values in August, and decreasing values in September and October. In addition, sap flow density exhibited the following order: dominant > subdominant > intermediate > suppressed trees. (4) Correlation and stepwise regression analyses indicated that mean daily sap flow density was significantly correlated with solar radiation, vapor pressure deficit, and air temperature, and was negatively correlated with air relative humidity and precipitation. Solar radiation was the main meteorological factor influencing mean daily sap flow density of dominant, subdominant, and intermediate trees, whereas air relative humidity primarily influenced suppressed trees.