溪頭地區柳杉人工林林內降雨及其水化學之研究

Abstract

本研究於臺灣大學實驗林溪頭營林區柳杉人工林進行穿落水、幹流等林內降雨觀測及推估樹冠截留量，並分析降雨、穿落水和幹流中的離子濃度與估算養分通量，觀測期間自2012年2月至2014年10月，每間隔約15日收集1次資料。扣除截留量小於0之資料，總降雨量為2,270.0 mm，穿落水量、幹流量及樹冠截留量分別為2,022.7 ± 48.0 mm、55.5 ± 1.9 mm、191.8 ± 4.1 mm，穿落水、幹流及樹冠截留百分比各為89.1%、2.5%及8.5%，由於研究對象柳杉林齡高、林分密度低，以及葉面積指數較低使其截留能力低。穿落水量(y)與降雨量(x)的線性迴歸式為y = 0.9664x - 4.0271，r2 = 0.99，顯示穿落水量隨降雨量增加而增加，二者間呈高度正相關，隨著降雨量增加，穿落水百分比大幅增加，當降雨量40 mm時穿落水百分比約80%，降雨量100 mm時穿落水百分比約90%，當降雨量達200 mm時穿落水百分比接近100%；幹流(y)與降雨量(x)的線性迴歸y = 0.0372x - 0.3111, r2 = 0.87。整體而言，幹流量隨降雨量增加而增加，但幹流百分比變動相當大，r2=0.37。發生幹流之降雨量門檻值為1.5 mm，截留飽和點約在3.9 mm左右。 降雨中濃度較高的陽離子使溪頭未有明顯酸雨情形（pH = 5.56）， Ca2+為降雨、穿落水和幹流中濃度最高的離子，穿落水中的陽離子除了K+以外，其它陽離子濃度和降雨均無顯著差異；陰離子則都較降雨顯著減少，顯示柳杉樹冠層有緩衝酸性沉降的效果，由於陽離子增多、陰離子減少，使穿落水的pH值較降雨顯著提高（pH = 6.24），雖然幹流的陽、陰離子濃度特徵和穿落水相似，但可能因柳杉釋放其他物質的影響，使柳杉幹流呈偏酸性（pH = 4.29）。本研究中的溪頭柳杉人工林由於樹冠具有緩衝酸性沉降的效果，使林地之酸性沉降量（SO42和NO3-）減少。

This study was conducted in the Cryptomeria japonica plantation located at Xitou tract, NTU Experimental Forest. The throughfall and stem flow were observed in the plantation and the canopy interception was calculated. Also, the ion gradients of precipitation, throughfall and stem flow were analyzed and thus calculated ion flux. This observation started from February 2012 to October 2014, data was collected about twice a month. The effective total rainfall was 2,270 ± 48.0 mm, throughfall was 2,022.7 ± 45.7 mm, stemflow was 55.5 ± 1.9 mm and interception was 191.8 ± 4.1 mm. The percentage of throughfall was 89.1%, stemflow was 2.5% and interception was 8.5%. The capacity of interception was low due to the low stem density of old stand and low LAI in study area. The linear regression equation between throughfall (y) and rainfall (x) was y = 0.9664x - 4.0271, r2 = 0.99, which indicated the amount of throughfall was highly positively related to rainfall. The percentage of throughfall also increased as rainfall increased, it was about 80% when the rainfall was 40 mm, 90% when the rainfall was 100 mm and close to 100% as the rainfall was 200 mm. The linear regression equation between stemflow (y) and rainfall (x) was y = 0.0372x - 0.3111, r2 = 0.87. The percentage of stemflow was also positively related to rainfall, however, the correlation was weaker (r2 = 0.37). The threshold of stemflow was 1.5 mm, and the saturation point of interception was about 3.9 mm. Higher gradient of cation led to no events of acidic rain in Xitou (pH = 5.56). Ca2+ was the most ion gradient in throughfall, stemflow and rainfall, and there were no significant difference in gradient of cation between throughfall and rainfall except K+. The gradient of anion in throughfall was significantly lower than that in rainfall, showing the effect on canopy of Cryptomeria japonica to buffer acid deposition. Because of more cation and less anion, the pH of throughfall was significantly higher than rainfall (pH = 6.24). Although the characters of ion gradient was similar to throughfall, the pH of stemflow was lower (pH = 4.29), which may be affected by other substance released from tree. Because of the canopy of Cryptomeria japonica in this study had capacity to buffer acid deposition, the acidic depositions (SO42- and NO3-) was fewer.