| Summary: | 碩士 === 國立臺灣大學 === 森林環境暨資源學研究所 === 106 === 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.
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