臺灣北部低海拔人工林與次生林之鳥類群聚比較

Abstract

全世界天然林面積快速減少，取而代之的是整齊劃一的人工林。在臺灣，人工林佔20 %的森林總面積，天然林所佔面積最大，但超過一半的天然林是次生林。一般認為，次生林比人工林能容納更多的生物多樣性。但全球針對上述理由的相關研究，並沒有一致的結論，而國內的相關研究並不多。本研究以物種多樣性高的鳥類為對象，於臺灣北部低海拔山區，選擇12組鄰近、環境類似、年齡近似的人工林與次生林，以圓圈法調查連續兩年(非繁殖季與繁殖季)的鳥類相，並調查鳥類之棲地資源利用，來比較兩林相環境中，鳥類種數、密度與棲地資源利用是否有差異。結果顯示，有45種鳥種在人工林與次生林的密度並沒有顯著差異，只有五色鳥(Megalaima oorti)、灰喉山椒鳥(Pericrocotus solaris)、以及赤腹山雀(Parus varius)在次生林密度顯著高於人工林。以鳥類棲地資源利用的結果，把穩定出現的38種鳥種分成十個同功群。其中，只有冠層蟲食同功群以及撲獲蟲食同功群密度在次生林顯著較高外，其他同功群密度在兩種林相間都沒有顯著差異。相似的森林垂直枝葉結構以及林下層次覆蓋度和枝葉量，可能是造成人工林與次生林鳥類群聚類似的主要原因。部分棲地因子雖然在這二種林相間呈顯著差異，但對鳥類活動的影響，可能需要整合各項因子來討論。棲地因子的差異太小，可能對鳥類群聚沒有明顯的影響。而且鳥類在利用棲地時，對棲地的要求並不一定很嚴苛，鑲嵌的地景結構也可以增加人工林的鳥類多樣性。若能經由適當的育林及撫育作業，使人工林的林木組成及垂直結構更加多樣紛歧，將可達到較佳的生物多樣性保育效果。

Natural forests have been diminishing rapidly worldwide, often replacing by monocultural plantations, which take about 20% of the forested areas of Taiwan. Natural forests make up the biggest proportion of forests of Taiwan, but more than half of them are secondary forests. Usually, people consider that secondary forests contain higher biodiversity than plantations. Nevertheless, there has been a lack of concrete evidence for this assumption and few studies have been conducted in Taiwan to examine this assumption. I compared birds, a taxonomically diverse group, of plantations and secondary forests in the low-elevation areas of Northern Taiwan. Twelve sets of paired sampling stations were established from neighboring plantations and secondary forests that have same development time and environmental condition. I used circular-plot method and direct focal observations of foraging behaviors to investigate the bird density, species richness, habitat selection, and resource utilization in the forests. The 45 bird species showed non-significant differences in population densities between plantations and secondary forests, except that Muller’s Barbet (Megalaima oorti), Grey-chinned Minivet (Pericrocotus solaris), and Varied Tit (Parus varius) had significantly higher densities in secondary forests. The 38 bird species were grouped into 10 ecological guilds based on their resource utilization patterns of habitats. The guild density of stations did not differ between plantations and secondary forests, excepting canopy insectivores and hawk insectivores. Similar forest foliage structure and coverage of understory layers may be the key reasons that the bird communities were similar between plantations and secondary forests. Although some habitat variables significantly differed between plantations and secondary forests, the differences in habitat structure might not be enough to significantly affect the composition of bird communities. Besides, birds might have weak discrimination in selecting these habitats; mosaic structure of plantations and secondary forests might increase the avian diversity in plantations. I conclude that, with appropriate silvicultural practices that create more heterogeneous floristic composition and vertical foliage structure, plantations can achieve greater contribution in conserving biodiversity.