溪頭天然闊葉林枯落物及其落葉氮、磷濃度之動態變化

Abstract

本研究以台大實驗林溪頭自然教育園區內的鳳凰山稜線、中坡天然闊葉林為調查對象，自2006年1月至2007年1月，每個月底收集枯落物一次，共收集13次。本研究探討稜線、中坡兩天然闊葉林的枯落物動態變化並分析主要樹種的落葉特性和落葉氮、磷濃度差異，藉此增加我們對台灣森林養分循環的了解。 稜線與中坡天然闊葉林的年枯落量分別為7602、5148 kg ha-1，稜線與中坡均以落葉為枯落物最重要的組成，分別佔枯落總量的62.5、68.6 %。枯落物和落葉均有明顯且相當一致的季節變化，稜線天然闊葉林的枯落物出現雙高峰，4月的高峰主要是因為林木換葉所導致，6、7月的高峰則是由林木換葉和枝條枯落共同引起；中坡天然闊葉林的枯落物僅在3月出現單一高峰，同樣是因為林木換葉所致。稜線天然闊葉林中，以長尾尖葉櫧 (Castanopsis cuspidata)的落葉最多，木荷(Schima superba)、香楠(Machilus zuihensis)次之，三個樹種的落葉佔總落葉量的41.9 %；中坡天然闊葉林中，以樟葉槭(Acer albopurpurascens)的落葉最多，瓊楠（Beilschmiedia erythrophloia）和大葉石櫟(Pasania kawakamii)次之，三個樹種的落葉佔總落葉量的46.2 %。 稜線與中坡天然闊葉林主要樹種間的落葉型式各異，稜線中6個主要樹種的落葉高峰在3 ~ 4月間，但是長尾尖葉櫧和木荷落葉高峰出現時間則明顯延後，其中又以木荷的落葉最為集中，黃杞(Engelhardia roxburghiana)的落葉歷時最久；中坡以樟葉槭與假長葉楠(Machilus japonica)的落葉高峰最早出現，瓊楠則最晚，其餘4個主要樹種的落葉高峰時間約在4 ~ 5月間，中坡主要樹種以樟葉槭的落葉最為集中，山香圓(Turpinia formosana)的落葉歷時最久。 稜線天然闊葉林主要樹種落葉的平均氮濃度為1.05 ~ 1.50 %，平均磷濃度為0.047 ~ 0.070 %，所有樹種中以黃杞落葉的氮、磷濃度最高，而紅楠(Machilus thunbergii)落葉的氮、磷濃度則最低；中坡天然闊葉林主要樹種落葉的平均氮濃度為1.10 ~ 1.70 %，平均磷濃度為0.066 ~ 0.095 %，最高的氮、磷濃度分別出現在大葉石櫟和杏葉石櫟(Lithocarpus amygdalifolius)的落葉中，而樟葉槭落葉的氮、磷濃度則是最低的。在兩天然闊葉林中，大部分樹種落葉養分並沒有明顯的季節變化趨勢，但是少數樹種在落葉高峰時，落葉的氮、磷濃度仍有偏低的趨勢。 本研究發現樹種間的落葉型式與落葉養分的確有差異，由於台灣天然闊葉林組成複雜，枯落物動態各異，若能真正掌握個別樹種的落葉型式與落葉養分變化，將能對枯落物動態和養分循環有更精確的了解。

From January 2006 to January 2007, we collected the litterfall monthly in two natural hardwood forests on the ridge (called “Top” hereafter) and middle slope (called “Middle” hereafter) of Fenghuang Mountain in the Experimental Forest, National Taiwan University. The objectives were to examine the litterfall dynamics and seasonal leaf fall patterns and concentrations of nitrogen and phosphorus of leaf fall in major tree species. The results showed that the annual litterfall was 7602 and 5148 kg ha-1 in the Top and Middle natural forests, respectively. The leaf fall were the most imporatnt component of litterfall in both forests. The leaf fall accounted for 62.5 % and 68.6 % of the annual litterfall in the Top and Middle forests, respectively. The amount of litterfall and leaf fall varied significantly among months in both forests. The seasonal pattern of litterfall was quite similar to that of leaf fall. In the Top forest there were two peaks of the litterfall. The first peak was caused by leaf exchange of some major species in April, and the second by leaf exchange of the other species as well as woody tissues broken off in July. In the Middle forest, there was only one peak of litterfall, which was caused by leaf exchange. The leaf fall of Castanopsis carlesii contributed the most (18.6 %) to the total leaf fall, followed by Schima superba (11.9 %) and Machilus zuihensis (11.3 %). The leaf fall of Acer albopurpurascens (25.1 %) contributed the most to the total leaf fall, followed by Beilschmiedia erythrophloia (11.2 %) and Pasania kawakamii (9.9 %). The major species evidently differed in the patterns of leaf shedding. In the Top forest the peaks of leaf fall in most major species occurred in March or April, earlier than the remaining two species, Castanopsis carlesii and Schima superba (June to August). Schima superba shed most of its leaves in the shortest period, and Engelhardia roxburghiana the longest. In the Middle forest the leaf fall of Acer albopurpurascens and Machilus japonica peaked the earliest (March) and Beilschmiedia erythrophloia the last (June). The leaf fall of the remaining major species peaked in April or May. Acer albopurpurascens shed most of its leaves in the shortest period, and Turpinia formosana the longest. Both nitrogen and phosphorus concentrations of leaf fall differed significantly among major species in two forests. In the Top forest the nitrogen concentrations of leaf fall in major species ranged between 1.05 and 1.50 % and phosphorus concentrations between 0.047 and 0.070 %. Nitrogen and phosphorus concentrations were highest in the leaf fall of Engelhardia roxburghiana and lowest in that of Machilus thunbergii. In the Middle forest the nitrogen concentrations of leaf fall in major species ranged between 1.10 and 1.70 % and phosphorus concentrations between 0.066 and 0.095 %. Nitrogen concentration was highest in the leaf fall of Pasania kawakamii and phosphorus concentration highest in that of Lithocarpus amygdalifolius. Both nitrogen and phosphorus concentrations were lowest in the leaf fall of Acer albopurpurascens. Nitrogen and phosphorus concentrations of leaf fall in most major species showed no evident seasonal patterns. However, nitrogen and phosphorus concentrations of leaf fall in some species tended to lower during the peaks of leaf fall. We found differences in the seasonal patterns of leaf shedding and nutrients of leaf fall among major species in both forests. With the great diversity in species compositions, natural hardwood forests in Taiwan also varied greatly in their litterfall dynamics. The investigation of leaf shedding patterns and leaf fall nutrients in individual species can help us understand the complexity of litterfall dynamics and nutrient cycling in forest ecosystems of Taiwan.