棲蘭山區亞熱帶雲霧林台灣扁柏冠層養分之動態

Abstract

植物葉部是森林養分主要的匯積處，藉由冠層葉部養分組成，有助於瞭解植物對養分的吸收利用，及反映生立木之營養狀況。為暸解各養分於台灣扁柏(Chamaecyparis obtusa var. formosana)林木冠層空間內的分配樣式與其影響因子，本試驗於台灣東北部之棲蘭山區選定三個林分進行研究，分別為未經干擾之老熟林、經林地枯倒木整理之老熟林及天然次生林，其中兩老熟林分別擁有6株樣木，而次生林則包含12株樣木。本研究將樣木冠層分為6個不同位置，自2006年1月至2007年12月逐月採取冠層各空間位置之葉部樣本，並取其奇數月分進行葉部全碳、全氮、鉀、鈣及鎂濃度之分析；另於2007年11月採集土壤樣本以進行土壤基本性質分析。結果顯示台灣扁柏冠層葉部養分受到季節、生育地土壤肥沃度以及不同的冠層空間等因子之影響，而具有不同的動態形式。整體而言，不同程度之林地擾動使土壤環境有異，進而影響冠層氮、鈣及鎂於林分間之差異。季節動態方面：碳、氮素隨生長季開始增加，且兩者於生長季後期均有再移轉之現象(轉移對象可能是繁殖體，如雌毬果)；休眠時期唯碳素持續累積而氮素則伴隨枯老葉凋零進行養分再移轉至新鮮葉片中。鉀於生長季時隨大量新鮮組織建構而受稀釋，且於颱風季節具有明顯的淋洗效應。相對來說，鎂及鈣之季節效應較為微弱。空間分布方面顯見老熟林及次生林分之冠層已達最佳碳分配。各養分方面：老熟林受光適應及冠層結構較稀疏之影響，隨冠層垂直高度位置越高而氮濃度降低；反之，次生林因林分密度高的影響，顯見頂層-外幅達氮濃度最高的表現。鉀與氮於光合作用同等重要，故於林分間的分配樣式近乎一致。三個林分之鈣與鎂濃度均以樹冠內側高於外幅，且因冠層位置高度越高具有較大量的新鮮葉片之影響，鎂濃度因此隨冠層位置越高而遞減。

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.