- 學位論文
頻繁颱風擾動下的臺灣亞熱帶闊葉林樹木組成與動態
Tree Composition and Dynamics in a Frequently Typhoon-Disturbed Subtropical Broadleaved Forest, Taiwan
摘要
臺灣是一座森林島嶼，全島約有61%的面積為森林所覆蓋。臺灣同時也是一座長期不斷遭受颱風侵襲的島嶼。瞭解這個島嶼處在如此的擾動型態(disturbance regime)下如何能維持其森林植被，以及其組成樹種是否已發展出適應策略，是極為重要的生態課題。本論文以臺灣東北部福山地區一處長期頻繁受到颱風擾動的天然老熟常綠闊葉林為研究對象，進行樹木組成與動態的長期研究。利用此地建立的25公頃永久樣區所收集到的3次樹木普查資料(調查對象為胸高直徑達到1公分以上之樹木，每間隔5年調查一次，研究期間共10年)，進行資料分析。本研究的主要目的是要探索出這片森林與組成樹種之所以能在頻繁颱風擾動下維持存續(persistence)與共存(coexistence)的關鍵生態特性。 研究結果顯示，微地形因子對於福山森林植被的組成具有顯著的影響，形成明顯的梯度變化，而且有超過九成的樹種對於特定的微地形生育地具有指標能力。微地形因子對於樹木群落的死亡(mortality)、補充(recruitment)、生長(growth)情形也具有顯著的影響。相較之下，樹冠開闊度(canopy openness)只有和樹木補充情形相關，呈現出負向而且較為次要的效應。這些結果共同反映出此地的地形異質性造成樹種普遍具有生育地分化(habitat differentiation)的分布現象，也明顯影響著樹木動態，因此有助於維持不同樹種之間的共存。此外，相反於孔隙動態理論(gap dynamics theory)的預測，樹冠開闊度對於此地樹木動態只呈現次要影響，這也表示這片受到頻繁颱風擾動的森林可能主要是由其他不同於孔隙動態的生態過程與機制所影響。 本論文也發現到微地形因子與樹冠開闊度對於此地前三名最優勢種的樹木存活幾乎不具有影響，而鄰樹間的生物交互作用(biotic neighborhood interactions)也只呈現次要的效應。與優勢種樹木存活最為相關的，反而是多樹幹生長(multi-stemming)與樹木大小(tree size)這些屬於個體上的性狀特徵(trait)，無論是對於優勢種的小樹或大樹，均一致展現出顯著而且正向的效應。基於這些有助於提升樹木存活率的個體性狀，以及優勢種所表現出的較低死亡率，本研究推論福山森林的優勢種可能已發展出存續性生存策略(persistence strategy)，有助於適應本地週期性的颱風擾動，從而維持其優勢。此外，優勢種具備這些與樹形結構(architecture)、體型(stature)密切關聯的性狀有利於其存活，這同時也會有助於維持整體森林之結構穩定性，而這樣的過程在長期發展下也會形塑出較為抗風的森林形相。 綜上所述，本論文的研究結果可支持「臺灣天然林應具有適應策略以在頻繁颱風擾動下維持其存續」的假說。總體而言，此地頻繁的颱風擾動與地形異質性，可能會藉由多種生態過程與機制共同影響著福山森林樹木群落的結構、組成與動態。本研究同時也凸顯出森林植被長期生態研究的重要性。面對環境的不斷變遷，持續進行森林長期生態監測極為必需，唯有如此方能增進吾人對於臺灣森林的瞭解。
並列摘要
Taiwan is an island covered with forests (ca. 61% of the area). Taiwan is also an island constantly battered by typhoons. Under such a disturbance regime, how forests can maintain and whether the tree species have developed adaptive strategies in Taiwan are of great ecological importance. In this dissertation, I investigated tree composition and dynamics of a frequently typhoon-disturbed forest, by analyzing ten-year census data (with two five-year census intervals) of trees ≥ 1 cm in diameter in a 25-ha permanent plot established in an old-growth evergreen broadleaved forest in Fushan, northeastern Taiwan. The overall objective is to explore the key ecological characteristics of that forest and the tree species therein that contribute to their persistence and coexistence under frequent typhoon disturbances. The results showed that micro-topography had significant influences on vegetation in the Fushan forest, forming distinct compositional changes along the micro-topographic gradients. Over 90% of tree species were indicative of specific topographic microhabitats. Micro-topographic factors also significantly influenced the mortality, recruitment, and growth of the tree community, whereas canopy openness only had a minor and adverse effect on tree recruitment. These results indicate that the topographic heterogeneity leads to a pervasive habitat differentiation of tree species distributions and significantly influences the tree community dynamics, which may contribute to species coexistence in the forest. Besides, the minor role of canopy openness on tree dynamics contradicts with the predictions from the classic gap dynamics theory, suggesting different dynamic processes and mechanisms operating in that frequently typhoon-disturbed forest. This dissertation also found that for the survival of the three most-dominant tree species in the forest, both micro-topography and canopy openness had little effect on tree survival, and biotic neighborhood interactions only played a minor role. Instead, the individual traits such as multi-stemming and tree size significantly enhanced the survival of both large and small trees of the dominant species. Such survival-enhancing traits, coupling with the observed low tree mortality, indicate a persistence strategy for dominant species to adapt to recurrent typhoons and thus maintain their dominance in the forest. Furthermore, these stature and architectural traits associated with the higher survival rates of dominant species may also help to maintain the structural stability of the forest, and over time, shape the forest into a more wind-resistant physiognomy. The results of this dissertation support the hypothesis that situated in the major typhoon "hot spot" area, the natural forests of Taiwan should have adaptive mechanisms to persist through the recurrent disturbances. Overall, the frequent typhoon disturbance and topographic heterogeneity may co-influence the structure, composition, and dynamics of the tree community in the Fushan forest via multiple processes and mechanisms. This study also highlights the importance of long-term ecological research of forest vegetation. Under a changing environment, long-term forest monitoring is essential to our ecological understandings of the forests in Taiwan.
參考文獻
延伸閱讀
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