贺兰山油松林冠层的辐射能量截获及传输利用特征

Characteristics of Radiation Interception,Transmission and Utilization in the Canopy of Pinus tabulaeformis Forest in Helan Mountain

油松作为我国北方地区典型的森林植被类型,研究辐射能量在其冠层中的分配、收支及传输,是构建森林冠层辐射传输模型和生态过程模型的理论基础,对阐明森林植被群落中能量流动和物质循环机理具有重要意义。基于贺兰山生态站油松林通量塔定位观测获取的上、下行长短波辐射、光合有效辐射(photosynthetically active radiation,PAR)以及气象数据,分析2021年4—9月不同天气和季节条件下油松林冠层的辐射能量截获及传输特征。结果表明:(1)晴天条件下太阳总辐射的日变化表现为光滑的“单峰”曲线,但多云条件下则呈现不规则的“多峰”曲线,且减弱48.3%的太阳总辐射能量到达冠层顶部。(2)太阳总辐射以下行短波辐射为主,晴天的上行短波辐射日变化虽也呈“单峰”形态,但仅占下行短波辐射的8.1%,上行和下行长波辐射随昼夜交替略有波动。(3)太阳短波辐射的季节动态特征表明,上行和下行短波辐射在6月最高,而上行和下行长波辐射在7月最高。(4)在冠层上方(25 m处)呈规则“单峰”形态的PAR日动态曲线,经过冠层截获和传输作用后,在冠层下方(7 m处)变为不规则曲线;PAR的透过率为32.2%~53.9%,透过率在6月达到最高。(5)4-9月生长季短波反射率为7.8%~8.8%,而地表长波辐射收支比为0.73~0.80。结果显示,贺兰山油松林冠层对辐射能量有明显的截获作用,其辐射传输在不同天气条件和不同季节表现出较大的差异。

Pinus tabulaeformis is a typical species in the forest of northern China.The researches on the distribution,budget and transmission of radiant energy of P.tabulaeformis canopy are the theoretical basis for the building radiation transmission model and the ecological process model of the forest canopy,which are crucial for exploring the mechanism of energy flow and material circulation in forest communities.Based on the upwelling and downwelling of longwave and shortwave radiation,photosynthetically active radiation(PAR)and meteorological data obtained from the flux tower over the P.tabulaeformis communities in Helan Mountain forest ecosystem research station,the radiative energy interception and transmission characteristics of the P.tabulaeformis forest canopy under various weather and season conditions over the period of April to September 2021 were analyzed.The results showed that:(1)Under sunny condition,the diurnal variation of total solar radiation showed a smooth curve with a single peak,but an irregular curve with multiple peaks under cloudy condition,and the total solar radiation energy decreased by 48.3%to reach the top of the canopy.(2)The total solar radiation reaching the top of the canopy was dominated by the downwelling shortwave radiation.Although the diurnal variation of upwelling shortwave radiation showed a single peak curve under sunny condition,it only accounted for 8.1%of downwelling shortwave radiation.The longwave radiation of upwelling and downwelling fluctuated slightly with the alternation of day and night.(3)The seasonal dynamic characteristics of solar shortwave radiation revealed that both upwelling and downwelling shortwave radiation were the highest in June,while the highest upwelling and downwelling longwave radiation appeared in July.(4)The diurnal variation of PAR was a regular single peak curve above the canopy(25 m),but became irregular below the canopy(7 m)after interception and transmission by the canopy.The transmissivity of PAR ranged from 32.2%to 53.9%,and the highest value appeared in June.(5)The shortwave reflectance was over 7.8%~8.8%during the growing season from April to September,while the ratio of incoming and outgoing longwave radiation of surface ranged from 0.73 to 0.80.The above results showed that the P.tabulaeformis canopy in Helan Mountain had a large radiation interception effect,and the transmission of radiation through the canopy changed significantly in different weather conditions and seasons.