| Summary: | 碩士 === 國立臺灣大學 === 森林環境暨資源學研究所 === 96 === As a major nutrient sink in a forest, knowing the canopy foliar nutrient components will enhance our understandings to the nutrient uptakes and utilizations of trees. To understand the within canopy nutrient allocation patterns and the factors influencing the patterns of Taiwan yellow false cypress (Chamaecyparis obtusa var. formosana), we selected three Taiwan yellow false cypress stands in the Chilanshan area, northeastern Taiwan. The three stands represented an undisturbed old-growth stand, an old-growth stand subjected to salvaging operations, and a naturally regenerated second-growth stand. Six trees were selected in each of the two old-growth stands, whereas 12 trees were selected in the second-growth for analysis. Six foliar samples were collected per tree from six positions of a canopy every month since January, 2006 to December, 2007. The odd month samples were then analyzed for concentrations of total carbon (TC), total nitrogen (TN), potassium (K), calcium (Ca), and magnesium (Mg). Soil properties were analyzed from soil samples collected in November, 2007.
The study showed that the canopy nutrient dynamics of Taiwan yellow false cypress were influenced by seasons, site fertility, and canopy positions. As a whole, there were significant differences of foliar N, Ca and Mg concentrations among the three stands, which were affected by site nutrition conditions. With respect to the seasonal dynamics, foliar C and N increased with the onset of the growing season, and were translocated (likely to the reproductive organs) in the late growing season. In late fall, foliar N was retranslocated from the senescent leaves to the younger ones. There was a strong dilution effect of foliar K in the growing season and a leaching effect in the typhoon season. In contrast, no strong seasonal trend was detected for the foliar Mg and Ca concentrations. In all three stands, optimizing carbon production could be used to explain the within canopy nutrient allocation patterns. In general, the foliar N concentration decreased with increasing vertical canopy position in the old-growth stands, which were relatively sparse. In contrast, due to a high stand density of the second-growth stand, foliar in the outer-top canopy position had the highest N concentration. Because K and N were both major nutrients related to photosynthetic process, their within canopy spatial patterns were almost identical in all three stands. The foliar Ca and Mg concentrations were higher at the inner than at the outer canopy positions in all three stands. The foliar Mg concentration decreased with increasing vertical position, as the top canopy position usually has a large amount of younger foliage.
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