棲蘭山區臺灣扁柏老熟林及次生林枯落物養分動態

Abstract

枯落物是森林冠層之營養回歸土壤的重要過程之一，因此本研究以棲蘭山區臺灣扁柏(Chamaecyparis obtusa var. formosana)老熟林及天然更新次生林為對象，於2005至2007年調查枯落物量動態及落葉養分季節變化。在枯落物收集期間天然下種更新林樣區的年總枯落物量都高於老熟林。臺灣扁柏葉佔枯物物量的比例在各林分及各年間均為最大，約在79~87.8%間。 枯落物量及養分濃度季節變化，採用廣義加法模式(Generalized Additive Models, GAMs)分析月累積雨量、月均溫、林分、年分及月分等因子的效應。 研究期間枯落物量的季節變化方面，老熟林與次生林枯落物的變動趨勢大致相同，高峰出現在冬季(11~1月)、春季(4月)及夏季(8~10月)。 影響本研究枯落物量年度間及季節性變化最重要的因素為颱風，兩林分在2007年10月出現一致性的高峰期，原因為10月分颱風擾動而導致大量的枯落物被強風、大雨吹(打)落。另一較小的高峰兩林分兩年都出現在春季(4月)，原因為林木之生長季節，會使大量的老葉被新葉取代而掉落。 兩個林分落葉氮濃度自冬季進入生長季時有下降的趨勢，而高峰皆出現在夏季。碳濃度在老熟林自冬季到隔年的夏季有下降的趨勢，而天然更新次生林春季到夏季無明顯變化趨勢。鉀濃度在兩林分出現在高峰的時間不同，在老熟林為2006年7月，更新林則為2005年10月。鎂濃度在春季時較低，在冬季則又逐漸回升。 影響落葉中元素濃度變化除了養分再移轉作用造成氮元素下降外；鉀元素受到雨水淋洗作用的影響顯著；鈣元素由於會累積在較老的植體中因此濃度高低和落葉是否為老葉有關；鎂元素兩年6~9月季節上變動不一致，但在冬季大致上仍與枯落物高峰出現時間一致。 養分再吸收效率各元素季節變動各有不同。各月老熟林葉部氮再吸收效率皆高於次生林，可能和土壤中全氮量有關；季節變動部分出現在春季及秋末至冬季，受到新葉開始生長及老葉掉落前再吸收作用影響。葉部鉀再吸收效率則主要受雨量影響。鎂的再吸收效率則以春季至夏季時較高。

The objective of this study was to compare litterfall production and the leaf litter nutrient dynamics in two Taiwan yellow false cypress (Chamaecyparis obtusa var. formosana) stands. Litterfalls were collected monthly in two stands (second-growth stand and old-growth stand) in Chilanshan area in Taiwan during 2005-2007. Total annual litterfalls in the second-growth stand were higher than that those in the old-growth stand. In both stands, Taiwan yellow false cypress leaf litter comprised about 79~88% of the total annual production. The effects of monthly precipitation, mean temperature, stand status, and temporal trends on the litterfall production and nutrient concentration dynamics were analyzed by using generalized additive models (GAMs). Monthly litterfall in both types of stands showed a similar seasonal trend with peaks in the winters, the springs, and the summer. The most important factor influencing the litterfall productions was typhoons. Both stands showed a peak amount in October 2007, due to a typhoon. The smaller peaks in springs in both stands were due to leaf replacements. Leaf-litter nitrogen concentration in both stands showed a decreasing trend from late winter to early spring, and then peaked in the summer. Leaf-litter carbon concentration in the old-growth stand showed a downward trend from winter to summer, whereas in the second-growth stand no significant trend was detected. Potassium concentration in the two stands showed different patterns. The concentration peaked in July in the old-growth stand, whereas in the second-growth stand the peak occurred in October. In both stands, magnesium concentrations were lower during the growing seasons and returned to a higher level in the winter. Nutrient resorption efficiency (NRE) showed different patterns between the stands as well as among the nutrients. The nitrogen NRE of old-growth stand was higher than that of the second-growth stand, which was likely due to the total available nitrogen in the soil. The NRE of potassium was strongly affected by the amount of precipitation. The higher the precipitation, the lower the NRE, likely due to leaching. The NRE of magnesium was higher during the growing seasons.