溪頭地區柳杉人工林降雨再分佈之研究

Abstract

為探討降雨經過樹冠層後的再分佈現象，本研究觀測臺大實驗林溪頭營林區第3林班柳杉(Cryptomeria japonica)人工林之幹流水、穿落水，配合溪頭氣象站的林外降雨資料，估算柳杉樹冠截留量，及分析樹冠結構對降雨再分佈過程中的穿落水、幹流水之影響。觀測期間為2012年2月至2013年2月，選定20 × 20 m2的試驗樣區2處，分別設置穿落水收集裝置（包括樣區1的貯水桶9組及樣區2的雨量計5組）；並於樣區1挑選樣木11株供設置幹流水收集裝置（包括貯水桶6組及雨量計5組）；另外，以浸水秤重法估算樣區內林下植物之截留量。 在2處樣區之穿落水觀測點，使用魚眼鏡頭拍攝柳杉樹冠影像後進行分析，樹冠孔隙率分析結果，樣區1介於20.08 ~26.10%，樣區2介於19.54% ~22.93%，而樣區1樹冠的LAI平均為2.66。其次，以無降雨間隔時間6小時以上為基準分割林外降雨事件，對每一場次的降雨事件分析穿落水、幹流水及樹冠截留百分比，統計觀測期間穿落水、幹流水及樹冠截留百分比各為林外降雨量的84.48%、2.15%和13.37%，平均每場降雨事件柳杉樹冠能截留3.78 mm，可視為柳杉截留飽和點。從觀測期間月降雨量較少（月降雨量<100 mm的2012年2、3、9、10月及2013年1、2月），和月降雨量較多（月降雨量>100 mm的2012年4、5、6、7、8、11、12月）的時段來看，降雨量多月份的穿落水、幹流水、樹冠截留百分比平均分別為89.99%、3.25%、6.76%，而降雨量較少的月份分別為77.58%、0.80%、6.76%，顯示穿落水及幹流水受降雨量多寡的影響，故以間隔15日或月為單位的觀測資料分析樹冠截留百分比時，在降雨量多的月份忽略幹流水會高估樹冠截留百分比。 另外，幹流量與柳杉之DBH無顯著相關，產生幹流水的降雨量門檻值為1.68 mm，因柳杉的樹皮具有0.18 mm的含水能力，其餘1.50 mm為枝葉吸收等其他因子造成。而林下植物平均每一場次降雨的最大截留量為0.25 mm，以2012年共計194場次的降雨事件估算，林下植物截留量占全年降雨量的1.53%。

To investigate the phenomenon of rainfall redistribution through forest canopy, this study measure the throughfall and stemflow in a Japanese cedar (Cryptomeria japonica) plantation at the third compartment in Xitou tract, NTU Experimental Forest, which be used to calculate the canopy interception with rainfall data of Xitou weather station. Meanwhile, canopy structure analysis was involved to understand that influence rainfall redistribution process. Study period was from February 2012 to February 2013. In study area, two 20 × 20 m2 experimental plots were selected and then set 14 throughfall collection devices (including 9 storage bucket type of plot 1 and 5 rain gauge type of plot 2) and stemflow collection devices for 11 selected sample trees in plot 1 (including 6 storage bucket type and 5 rain gauge type). Moreover, wetting method was used to estimate the amount of understory vegetation interception. Took the canopy image at each throughfall measured point in the two experimental plots by a fish-eye lens and executed image analysis. Gap fraction at each measured points of the plot 1 was from 20.08%~26.10% and plot 2 was from 19.54% to 22.93%, and the canopy LAI of plot 1 were averagely 2.65. Secondarily, more than 6 hours without rain occurred was criterion to separate rainfall events during study period. The percentage of throughfall, stemflow and canopy interception in each rainfall event were calculated. From the results, the percentage of throughfall, stemflow and interception for total rainfall of study period was 84.48%, 2.15% and 13.37%, respectively. The canopy inecterception capacity of Japanse cedar for each rainfall event was 3.78 mm, which could be considered as the maximum interception capacity. Moreover, higher rainfall months (monthly rainfall >100 mm, including 2012/2, 3, 9, 10 and 2013/1, 2) and lower rainfall months (monthly rainfall <100 mm, including 2012/4, 5, 6, 7, 8, 11, 12) were distinguished. In higher rainfall months, the average of throughfall, stemflow and canopy interception percentage was 89.99%, 3.25% and 6.76%, by contrast, in lower rainfall months was 77.58%, 0.80% and 21.62%. Which shows that throughfall and stemflow were obviously influenced by rainfall amount. From the above results, on the basis of 15 days or monthly observed data as to calculate canopy interception percentage, it would be overestimated if stemflow was ignored in higher rainfall month. Furthermore, stemflow has no significant correlation with Japanese cedar’s DBH. The rainfall threshold of stemflow generated was 1.68 mm. Because of the 0.18 mm bark water storage, the remaining 1.50 mm may be caused by other factors like foliage absorbed, etc. In addition, the maximum understory interception in each rainfall event was averagely 0.25 mm. In 2012, there were 194 rainfall events, the estimated understory interception percentage was 1.53% for annual rainfall.