華中瘤足蕨及臺灣瘤足蕨在臺灣北部霧林帶分佈差異及其葉片生理生態特徵

Abstract

瘤足蕨屬(Plagiogyria Mett.)植物主要分佈於熱帶及亞熱帶山地霧林，臺灣有8個該屬物種，其中臺灣瘤足蕨(P. glauca (Blume) Mett.)及華中瘤足蕨(P. euphlebia (Kunze) Mett.)為霧林帶林下最常見的物種，且僅臺灣瘤足蕨葉背被有白色表角質層蠟質。前人研究指出兩者在臺灣山地霧林中的分佈數量似乎會隨著海拔高度和與稜線的距離而不同。本研究探討：(1)兩種瘤足蕨在塔曼山霧林帶中的分佈是否不同？如果不同，是什麼環境因子造成其分佈差異？又兩者的葉片生理生態特徵是否能解釋及反映其分佈差異？(2)臺灣瘤足蕨葉背蠟質可能有什麼功能？為了回答上述問題，我調查塔曼山兩種瘤足蕨在不同海拔高度(1780、1880、1980及2080公尺)棲地的植株覆蓋度、進行溫室環境因子(溫度和光強度)生長實驗並測量其葉片生理生態特徵，及觀察臺灣瘤足蕨的表角質層蠟質並探討其對葉片的影響。 野外調查結果發現：華中瘤足蕨在較低海拔棲地(1780公尺)數量較多，而臺灣瘤足蕨在高海拔(2080公尺)且林冠較開闊處的數量較多，兩物種數量有變化趨勢相反的現象。環境因子處理實驗結果顯示：相較於在15°C和20°C下，移植植株在高溫(25°C)下生長時，臺灣瘤足蕨的存活狀況及光合作用速率顯著降低，而華中瘤足蕨則沒有此現象；相較於在最大光環境為photosynthetic photon flux density (PPFD) = 100 μmol photon m-2s-1的溫室中生長時，在高光強度(最大PPFD = 300 μmol photon m-2s-1)下生長的移植植株，華中瘤足蕨的光合作用能力顯著下降，而臺灣瘤足蕨則未受影響，上述結果可以解釋兩者在海拔高度分佈的差異。相較於華中瘤足蕨，臺灣瘤足蕨的比葉面積(specific leaf area)較低、葉肉結構較緻密，並在光強度300 μmol photon m-2s-1生長時的葉綠素a/b比值顯著較高，顯示臺灣瘤足蕨具有能適應較低溫、光強度較高的葉片生理生態特徵。上述結果支持環境因子中的溫度及光強度是影響此兩種瘤足蕨分佈差異的假說，兩種蕨類葉片的生理生態特徵可以反映其分佈差異。 以掃描式電子顯微鏡觀察發現：臺灣瘤足蕨羽片遠軸面的表角質蠟質層由細絲狀的蠟質堆積而成，但並未覆蓋氣孔。表角質層蠟質經移除後會再重新生成。移除蠟質並不會改變臺灣瘤足蕨羽片的溫度也不會增加羽片對於低溫或高光的耐受度，但會減少羽片遠軸面對於可見光的反射度、增加羽片的蒸散作用速率和氣孔導度，並降低羽片的最大淨光合作用速率。結果支持臺灣瘤足蕨羽片遠軸面的表角質蠟質層可以降低其羽片的蒸散作用，減少水分喪失的假說。

Plagiogyria Mett. species are primarily distributed in tropical and subtropical cloud forests. Among the eight species reported in Taiwan, P. glauca (Blume) Mett. and P. euphlebia (Kunze) Mett. are the most common species in the understory of cloud forest zone, and P. glauca is the only species of the genus in Taiwan that bears white epicuticular wax on the abaxial surface of its pinnae. Previous studies have suggested that the two Plagiogyria species differ in distribution patterns in terms of elevation and distances of their habitats from ridges in Taiwanese cloud forests. This study has two objectives. To understand whether the two Plagiogyria species exhibit differential distributions within the cloud forest zone. If so, what are the main environmental factors that cause their differential distributions and can their leaf ecophysiological characters explain and/or reflect their distribution patterns. To investigate the possible functions of P. glauca’s epicuticular wax. To answer these questions, field surveys were conducted to estimate the coverage of the two Plagiogyria species in habitats at elevation of 1780, 1880, 1980 and 2080 m above sea level (a.s.l.) of Tamanshan in Northern Taiwan. Additionally, plants were grown in greenhouse with controlled abiotic environmental factors, including temperature and light availability, and their leaf ecophysiological characteristics were measured, and the function of P. glauca’s epicuticular wax was investigated. Results of field survey show that in comparison between the two species, P. euphlebia has higher coverage at habitats at lower elevation of 1780 m a.s.l., while P. glauca exhibits higher coverage in exposed habitats at the higher elevation of 2080 m a.s.l. Temperature and light intensity controlled experiments revealed that compared to grown under 15°C or 20°C, P. glauca grown under 25°C exhibited poorer growth and had significantly lower maximal photosynthetic rates (Amax) while P. euphlebia did not show such phenomena. In comparison to grown under maximal photosynthetic photon flux density (PPFD) of 100 μmol photon m-2s-1, P. euphlebia under PPFD of 300 μmol photon m-2s-1 exhibited significant reduction in Amax while P. glauca remained unaffected. Furthermore, compared to P. euphlebia, P. glauca showed lower specific leaf area, denser mesophyll structures, and an increased chlorophyll a/b ratio when grown under a high light intensity. These results suggest that P. glauca is better adapted to habitats of lower temperatures and higher light intensities. These results also support the hypothesis that temperature and light intensity are the primary environmental factors that drive the differential distributions of P. euphlebia and P. glauca. Also, the ecophysiological traits of pinnae of these two species reflect their distribution patterns. SEM showed that the epicuticular wax layer of P. glauca is composed of filiform wax which does not cover stomatal pores. The epicuticular wax seems to be regenerated after being removed. Wax removal experiments showed that the absence of epicuticular wax does not alter leaf temperature or the resistance of P. glauca’s pinnae to low temperature and high light stress. However, the removal of epicuticular wax significantly decreases the reflectance of the abaxial surface, increases the transpiration rate and stomatal conductance, while reduces Amax. The results support the hypothesis that the epicuticular wax can reduce the transpirational water loss of P. glauca.