關刀溪森林生態系干擾地土壤種子庫及小苗組成之研究

Abstract

關刀溪森林生態系之干擾包括地震崩塌、倒木及火燒等，本研究就關刀溪森林生態系內之崩塌地及倒木孔隙進行土壤種子庫及小苗監測。在種子庫調查方面，自2000年2月至2001年3月間每季由干擾地取回表土，進行發芽試驗，累計萌發602株小苗，已鑑定526株，分屬 28 科 48 種。陽性植物的種子普遍存在於干擾地的種子庫內。崩塌地及倒木孔隙均以懸鉤子屬植物較多。干擾後初期，原表土層之存留及新表土層的穩定度，會影響到土壤種子庫的密度。干擾地種子庫密度的消長與干擾地穩定度、種子萌發及干擾形成時間有關。 於干擾地設置樣區，自2000年1月至2001年6月間每二個月記錄樣區內小苗種類及數量。共記錄945株小苗，已鑑定出885株，分屬28科51種。小苗物種的分布在不同類型干擾地間有所差異。具較輕種子，可藉由風力傳播 (昭和草、青楓、木荷、白匏子) 或鳥類傳播 (山胡椒) 之陽性植物，其小苗在崩塌地均佔較大的比例。在樹冠層逐漸閉合之倒木孔隙，小苗組成以耐陰之林下灌木及藤本為主。干擾地之小苗密度與原表土層有無、新表土層穩定度以及干擾面積大小有關。推測崩塌地小苗密度消長及死亡率變化，則與原表土層之有無、崩塌地穩定度、種子萌發及小苗生長情形有關。倒木孔隙樣區之小苗密度呈季節變化，但小苗密度變動不大，成一暫時性的穩定狀態。 環境因子與小苗物種分布之相關檢定結果顯示，與小苗物種分布有關之環境因子有坡度、海拔高及含石率。陽性植物主要分布於坡度陡及含石率高之崩塌地。另各崩塌地之四類生活型 (喬木、灌木、藤本及草本) 小苗，數量上以喬木型植物種佔較大的比例，且喬木型植物之小苗死亡率較低並呈下降趨勢，推測陽性之喬木型植物之小苗較能適應崩塌後初期之生育地環境。 分析各干擾地樣區種子庫、小苗庫及上木 (DBH>1 cm) 等不同階段間之相似性指數，干擾地與非干擾地之種子庫組成間之相似性指數較高，顯示干擾後樹冠層開放，使陽性植物輕小之種子易飄散進入干擾地之種子庫，且陽性植物的種子會以持續性種子庫的方式，累積留存在林下種子庫內。上木組成、種子庫以及非干擾地小苗間相似性指數比較之結果顯示，上木組成與種子庫間之相似度低於上木組成與非干擾地小苗庫間之相似度。推測崩塌地之植群更新需藉由干擾區外之種實傳播，倒木孔隙及未受干擾植群之更新來源則為林下小苗庫。

The disturbances at Guandaushi forest ecosystem include earthquake-caused landslides, treefalls and fires. The composition and density of the soil seed bank, and the seedling emergence and morality have been monitored at landslide sites and treefall gaps. A total of 602 seedlings germinated from the collected soil samples. 526 seedlings belonging to 28 families and 48 species have been indentified. The seeds of shade-intolerant species were common in the soil of disturbed sites. The main components of the soil seed bank were Rubus species in disturbed sites. The results of seedling emergence indicated that the amount of remaining topsoil and the stability of newly formed topsoils influenced the density of soil seed bank. The fluctuation of the soil seed density was related to the stability of newly formed topsoil, the seed germination and also the time of disturbance. The composition and density of seedlings in the field plots were surveyed at 2 month intervals from January 2000 to June 2001. Totally 945 seedling were tagged. 885 seedlings belonging to 28 families and 51 species have been indentified. Most seedlings at landslide sites were shade-intolerant species with relatively small seeds that are easily to be dispersed by wind (Crassocephalum rabens, Acer serrulatus, Schima superb, Mallotus paniculatus ) and by birds ( Litsea cubeba ). The main components of the seedlings at treefall gaps were shade-tolerant species of shrub and vine. Seedling density was related to the existence of the original topsoil, the stability of new topsoil and the size of disturbed site. The fluctuation of density and morality of seedlings were also related to the existence of the original topsoil, the stability of new topsoils, the germination of seeds and the growth of seedlings. The fluctuation of the seedling density at treefall gaps was related to seasonal change. The result of DCA suggested that seedling distribution was correlated to slope, elevation and soil stoniness. Most shade-intolerant species were distributed at landslide sites with steep slope and high stoniness . Among the life forms (tree, shrub, vine and herb), seedlings of tree species were more abundant with lower morality. The above results implied that shade-intolerant tree species seem to be more adaptable to landslide sites. The similarity indices of soil seed bank between disturbed sites and adjacent understory were between 19.51% ~ 77.92%. The similarity indices between upper canopy species and understory seedlings were higher than those between upper canopy species and soil seed banks. Results in this study implied that the major source for the regeneration at landslide sites were the seeds of shade-intolerant species which were apart from the disturbed areas. The seedling bank, instead of the soil seed bank, would be the major source for gap regeneration and the recruitment of upper canopy species.