Changes in Water Chemistry of Precipitation Passing through Three Forests of the Guandaushi Forest Ecosystem

Abstract

The study objectives were to compare the changes of water chemistry of bulk precipitation, throughfall and stemflow for 3 types of forest stands, China-fir (Cunninghamia lanceolata) plantation, secondary hardwood stand, and natural hardwood stand in Guandaushi Forest, in central Taiwan from January 1996 to December 1998. Annual bulk precipitation had an average pH of 4.05 and stemflow of China-fir plantation was little more acidic than bulk precipitation, but throughfall of China-fir plantation almost maintain the same pH as in bulk precipitation. The pH values in throughfall and stemflow of secondary hardwood and natural hardwood were significantly higher than in bulk precipitation. It indicated that canopies and stems of hardwoods showed a high capacity to buffer the acidity of precipitation by accompanied with leaching and washing large amounts base cations. The conductivity in throughfall and stemflow of three stands were higher than in bulk precipitation. Most ion concentrations in throughfall and stemflow also showed higher values than in bulk precipitation except for H, F, Na and HCO3 in stemflow of China-fir plantation. The enrichment factors were from 1.50 to 64.49 in different ions, especially K, Ca, Mg, and HCO3 substantially increased in throughfall and stemflow of secondary and natural hardwood. Under three stands, H had a negative net canopy exchange (NCE) attributable to acid neutralization in the canopies and stems, especially in secondary and natural hardwood. K, Ca, and Mg had a positive NCE due to canopy and stem leaching and washing, especially K. A comparison of NCE of inorganic anions in three stands showed that Cl, NO3-N, and SO4-S had higher positive values in China-fir plantation than in secondary and natural hardwoods, while HCO3 had a highest positive value in natural hardwood. Regression analysis of single-event data showed that the most important factor regulating the event-to-event variation in net throughfall fluxes was the amount of precipitation, suggesting that canopy exchange (leaching and uptake) regulated the net throughfall fluxes for most ions. Duration of the interevent dry periods (dry deposition) was highly siginificant in most base cation fluxes for throughfall, especially in China-fir plantation, suggesting a higher scavenging efficiency of the needles for dry deposition. The mean DOC concentration in bulk precipitation was 4.7 mg/L. However the rain passing through the tree canopies and barks as throughfall and stemflow, the mean concentrations were 7.0 and 30.8, 9.9 and 10.0, and 8.3 and 7.2 mg/L in China-fir plantation, secondary hardwood, and natural hardwood, respectively. Significant correlation between DOC and charge balance deficit was observed, indicating that organic ions would contribute to the charge balance in the three stands. Hydrophilic and hydrophobic acids were the dominant fractions in bulk precipitation, throughfall and stemflow of three stands, accounting for ~ 90% of total DOC. However, differences in DOC fractions were found between bulk precipitation and throughfall or stemflow. In bulk precipitation, hydrophilic DOC were the dominant fraction but hydrophobic DOC were rich in throughfall and stemflow, indicating that these DOC fractions were altered when bulk precipitation were through canopies and stems. The elemental compositions of both humic polymers (MW>1000) and FAs (MW>1000), which were isolated from the water samples of throughfall and stemflow, were in the ranges typical for humic substances from other origin. Fourier transform infrared (FTIR) pectra also were close to the characteristics of reported humic substances. The results showed that the humic polymers (MW>1000) and FAs (MW>1000) in the water samples of throughfall and stemflow, which were above ground and soil mediation free environments, and in canopies and stems, may be a new pathway of formation as well as humification of humic substances. The occurrence and function of the humic substances in the water samples from various origins in forest ecosystem thus can not be overlooked.

本試驗的目的主要在探討雨水及比較雨水流經不同林分後水質之變化。樣區設於惠蓀林場關刀溪支流之一小集水區，在此集水區內除收集林外雨外，並於相鄰的杉木林、次生闊葉樹林及天然闊葉樹林三種不同林分內收集穿落水和幹流水，以比較雨水及雨水流經此三種不同林分後水化學的變化。資料採用時間為1996年1月至1998年12月。 三年試驗期間，林外雨的pH平均為4.05，杉木林幹流水的pH比林外雨低，穿落水則略高，而天然林和次生林則不論穿落水或幹流水皆比林外雨顯著高出甚多，顯示闊葉樹林對林外雨具有較高之中和能力，但卻會增加除H外之其他鹽基陽離子（Na, K, Ca, Mg）的濃度及輸入量。當雨水通過林冠及樹幹到達林地後，電導度值明顯增加，尤其是杉木林的幹流水增加最多。此三種林分的穿落水及幹流水的無機離子濃度除H, F, Na及杉木林幹流水的HCO3外，皆顯著增加，其增多係數（enrichment）由1.50到65.49不等，其中以K, Ca, Mg及HCO3增加較為明顯。 若以淨林冠交換量（net canopy exchange, NCE）而言，H在三種林分皆有被林冠保留現象，其保留情形以次生林及天然林較杉木林明顯。K, Ca, Mg皆是釋出，尤其是K的釋出為最大量。陰離子方面，Cl, NO3-N及SO4-S釋出都是以杉木林最多，次生林次之，天然林最少。HCO3的釋出則剛好相反。 以穿落水淨輸入量當應變數，雨量、降雨間隔日數（乾沉降）、林外雨氫離子濃度和林外雨成份離子濃度當作自變數，以每場雨的資料推導迴歸方程式，分析的結果顯示與各成分離子輸入量顯著關係最高的是雨量，因此，雨量是影響雨水穿過此三種林分冠層後，離子輸入量的最重要因子。降雨間隔日數和大部分穿落水陽離子的淨輸入量，也都存有高度相關，尤其是杉木林，此結果證明杉木林對乾沉降的攔截能力。 林外雨之可溶性有機碳（dissolved organic carbon, DOC）平均濃度為4.7 mg/L，而杉木林、次生林及天然林的穿落水濃度分別為7.0, 9.9及8.3 mg/L，幹流水則分別提高到30.8, 10.0及7.2 mg/L，以杉木林增加最為顯著，顯示雨水流經此三種不同林分冠層後，DOC和上述的養分離子一樣，濃度皆增加。DOC的測定結果與水樣陰離子缺乏所作的迴歸分析，顯示陰離子的缺乏主要來自DOC。 DOC的特性方面，林外雨以親水性酸佔大部分，但穿落水及幹流水則以疏水性酸佔大部分。表示雨水經過冠層後，所含的有機酸成分已有明顯改變。另由羧酸濃度、元素分析百分比及FTIR圖譜等之DOC的特性分析顯示，雨水及通過林冠之穿落水或幹流水皆有腐植化現象，惟林外雨腐植化程度低，但三種林分的穿落水及幹流水已有明顯腐植化現象，證實腐植化物質的形成及腐植化過程在林分冠層或樹體中亦進行著。