調節幹流水對福山試驗林兩種附生植物影響之探討

Abstract

近年來全球降水模式改變，降水集中、乾旱延長，對植物的多樣性與生產力造成負面影響，當中附生植物因生長在樹冠層、缺乏接地根系，故可能為首當其衝的類群。過去部分學者認為，相較地生植物，附生植物面對突發性或延長的乾旱，可能會更加敏感、衝擊更大，因此認為可做為氣候變遷之早期指標類群。但也有部分研究認為，因為長期生活在水份動盪的微環境中，許多附生植物已發展出可協助面對乾旱的特徵，因此受到的影響可能未必較為嚴重。過去相關研究受限於器材的架設，多以觀察性實驗或溫室實驗等方法探討附生植物面臨缺水壓力的反應，但此兩種方法各有限制，因此本研究嘗試以自製調節供水裝置，首次於野外對附生植物進行調節供水來探討這個問題。實驗選在福山植物園的天然闊葉林進行，針對兩種常見附生植物—臺灣巢蕨(Asplenium nidus)與、垂葉書帶蕨(Haplopteris zosterifolia)，利用自製裝置調節幹流水量，減少25%與50%，為期十個月(2020年7月~2021年4月)。藉由測量生長狀況、葉片橫切構造、葉片營養元素及與比葉面積 (SLA)、葉片乾燥物質含量 (LDMC)、葉片與角質層厚度與δ13C等可作為缺水壓力指標的相關特徵，探討附生植物對於水分壓力的反應。實驗結果發現形態構造具有明顯差異的兩附生植物，在實驗前受到的水分壓力相當。減水處理對垂葉書帶蕨的葉片數與葉面積有較明顯的負面影響，但其餘特徵則無顯著差異，顯示葉片維持一定的光合作用與生長，代表葉片數的下降可能為水分調節與生長間的權衡結果，水分散失下降與營養重新分配，協助垂葉書帶蕨面對水分壓力。此外實驗期間較低的空氣水氣不飽和度 (VPD) 也可能協助緩和附生植物受到的水分壓力，空氣中有更多可利用的水氣，附生植物可能透過葉片吸收攔截水氣，緩解缺水壓力。葉片營養分析也發現，移除部分幹流水並未造成葉片營養含量下降。除了幹流水外，附生植物或許有其他獲得營養的管道。垂葉書帶蕨可能是透過攔截水氣從中獲取營養。在雲霧頻繁濕度高的福山試驗林，水氣可能是附生植物重要的水分與營養來源之一，在水氣充足的情況下可以緩解幹流水減少造成的壓力，未來若因氣候變遷導致霧氣減少，則可能會對附生植物造成更嚴重的衝擊。Climate Change has caused changes in precipitation pattern and longer and more severe drought, which has negative influences on the biodiversity and productivity of plants. Epiphytes are exposed to the ambient water fluctuation so that could be most sensitive to such changes compared to plants rooted in soil. Some researchers consider epiphytes as early indicator group of climate change, while the others suspect that epiphytes may not be sensitive to climate change. This is because through long-term adaptation epiphytes might have developed traits allowing them to grow in micro-environment with high fluctuation of water availability. Observational studies and common garden studies are commonly used approaches to study the response of epiphytes to changes in water availability. However, both approaches has major limitations. In this study, customary-made stemflow-manipulation devices were set up to explore how drought stress affect two epiphytes, Asplenium nidus and Haplopteris zosterifolia, in the Fushan Experimental Forest of northern Taiwan for ten months. Using the devices, three levels of stemflow reduction were created, 0% (Control), 25% and 50%. We examined the traits indicative of plant growth condition, number of leaves and leaf area, and drought stress, SLA, LDMC, thickness of leaves and cuticle and δ13C, and nutrients of leaves to explore the effects of the treatments. Although H. flexuosa does not have a large substrate which Asplenium nidus has, both epiphytes experienced similar water stress before the experiment. The removal of 25% and 50% stemflow did have more negative impact on leaf number and area for H. flexuosa than A. nidus. Despite the negative effect on leaf area, the treatment didn’t have impacts on other traits. The results indicated the possible trade-off between the photosynthesis and water regulation. Losing leaves relieved water deficit by decreasing total transpiration and as such maintained the growth of the rest of leaves. Frequent fog and high humidity of the Fushan Experimental Forest may also mitigate water stress of H. flexuosa. However, if the atmosphere become drier due to the climate change, it could negatively affect the epiphytes.