不同因子對於冠層緩衝雨水酸性及穿落水水質之影響

Abstract

本研究目的為利用雨水與穿落水的水化學變化以評估冠層在不同因子條件下 (樹種、季節及葉候時期) 樹冠層對於雨水酸性的緩衝中和能力差異，並推估其緩衝機制。在福山試驗林中選定四種優勢常綠闊葉樹種 (青葉楠、白校欑、三斗石櫟、楊梅) 及兩種針葉樹種 (油杉、台灣杉) 之獨立樹冠層下方進行穿落水與雨水一整年的取樣觀測。結果顯示福山全年雨水的平均pH值為4.72，不同樹種下方之穿落水pH值皆顯著高於雨水，各樹種冠層對於雨水pH值提升程度依次為青葉楠 (pH=5.84)，白校欑 (pH=5.53)、台灣杉 (pH=5.22)、油杉 (pH=5.20)、三斗石櫟 (pH=5.13)及楊梅 (pH=5.09)。雨水和穿落水pH值夏季顯著高於冬季，而冠層在季節間有相同的H+留置效率。雨水中離子經過六種樹種冠層後，濃度增加最多的為K+，其次為Ca2+及Mg2+等鹽基陽離子，且對於人為汙染物NO3-、SO42-等致酸物質無顯著淨留置現象，雨水中H+由冠層大量置換淋溶鹽基陽離子 (K+、Mg2+、Ca2+)為本研究中冠層緩衝雨水酸性的主要機制，而緩衝機制樹種間及季節間仍略有差異。不同葉候時期，闊葉樹冠層葉部對於NH4+、NO3-和SO42-的交互作用 (留置、淋溶) 亦不同，葉片主要在新葉期及老葉期對NO3-和SO42-有留置作用。

The objectives of this study were to compare the influences of canopies of six tree species on the acidity and chemistry of precipitation and throughfall and their seasonal variation at the Fushan Experimental Forest. The six species included four hardwood species (Machilus zuihoensis, Castanopsis carlesii, Lithocarpus hancei, Myrica rubra) and two softwood species (Keteleeria davidiana, Taiwania cryptomerioides). Results showed that the average pH of precipitation was 4.72 indicating that precipitation at the Fushan Experimental Forest was still rather acidic. The pH values in throughfall of all the six species were significant higher than precipitation: 5.84 for Machilus zuihoensis, 5.53 for Castanopsis carlesii, 5.22 for Taiwania cryptomerioides, 5.20 for Keteleeria davidiana, 5.13 for Lithocarpus hancei, and 5.09 for Myrica rubra. The pH of precipitation and throughfall in summer were both significantly higher than that in the winter. Similar retention ratio of H+ were observed between seasons. The rest of the measured ions all increased from after passing through the tree canopies in all species, and the increase in concentration was greatest for K+, followed by increases in Ca2+ and Mg2+. Unlike previous studies, no significant NO3-, SO42- net canopy retention were observed in any of the six species canopy. These results suggested that canopy ion exchange between H+ and cations (K+, Mg2+, Ca2+) was important in buffering the acidity of the precipitation. There were different net canopy interaction (uptake or leaching) of NH4+, NO3- and SO42- between different leaf phenological phases of the hardwood tree species, with net NO3- and SO42- retention in the leaf emergence and senescence periods.