College of Life and Environmental Sciences: Biosciences, University of Exeter, Geoffrey Pope Building, Stocker Road, Exeter, UK.
Ann Bot. 2012 Apr;109(5):1019-26. doi: 10.1093/aob/mcs012. Epub 2012 Feb 14.
Hymenophyllaceae (filmy ferns) are typically plants of shady, constantly moist habitats. They attain greatest species diversity and biomass in humid tropical montane forests and temperate hyperoceanic climates. This paper presents ecophysiological data bearing on their worldwide ecological niche space and its limits.
Chlorophyll fluorescence was used to monitor recovery in desiccation experiments, and for measurements of 95 % saturating irradiance [photosynthetic photon flux density (PPFD(95 %))] of photosynthetic electron flow and other parameters, in the New Zealand Hymenophyllum sanguinolentum, and three species each of Hymenophyllum and Trichomanes from forests in Trinidad and Venezuela.
Hymenophyllum sanguinolentum was comparable in desiccation tolerance and light responses with the European species. The more common species in the two tropical forests showed PPFD(95 %) >100 µmol m(-2) s(-1), and withstood moderate desiccation (-40 MPa) for several days. The four most shade-adapted species had PPFD(95 %) ≤51 µmol m(-2) s(-1), and were sensitive to even mild and brief desiccation (-22 MPa for 3 d).
Light and desiccation responses of filmy ferns can be seen as an integrated package. At low light and windspeed in humid forests, net radiation and saturation deficit are low, and diffusion resistance high. Water loss is slow and can be supported by modest conduction from the sub-stratum. With higher irradiance, selection pressure for desiccation tolerance increases progressively. With low light and high humidity, the filmy fern pattern of adaptation is probably optimal, and the vascular plant leaf with mesophyll and stomata offers no advantage in light capture, water economy or CO(2) uptake. Trade-offs between light adaptation and desiccation tolerance, and between stem conduction and water absorption through the leaf surface, underlie adaptive radiation and niche differentiation of species within the family. Hymenophyllaceae are a rare example of an evolutionary shift of adaptive strategy from typical vascular plant adaptation to the poikilohydry most typical of bryophytes.
水龙骨科（膜蕨）通常是生长在阴暗、持续潮湿环境中的植物。它们在潮湿的热带山地森林和温带大洋性气候中拥有最多的物种多样性和生物量。本文提出了与它们的全球生态位空间及其极限有关的生理生态数据。
使用叶绿素荧光监测脱水实验中的恢复情况，并测量新西兰水龙骨和来自特立尼达和委内瑞拉森林的三种水龙骨和三种 Trichomanes 物种的光合电子流的 95%饱和辐照度[光合光子通量密度（PPFD(95%)）]和其他参数。
水龙骨在耐旱性和光响应方面与欧洲物种相当。在两个热带森林中更常见的物种显示出 PPFD(95%)>100 µmol m(-2) s(-1)，并能承受数天的中度脱水（-40 MPa）。四个最适应阴凉的物种的 PPFD(95%)≤51 µmol m(-2) s(-1)，对轻度和短暂的脱水（3 天内-22 MPa）敏感。
膜蕨的光和脱水响应可以被看作是一个综合的整体。在潮湿森林中低光照和风速下，净辐射和饱和亏缺低，扩散阻力高。水分流失缓慢，可通过从基质适度传导来支撑。随着辐照度的增加，对耐旱性的选择压力逐渐增加。在低光照和高湿度下，膜蕨的适应模式可能是最佳的，而具有叶肉和气孔的维管植物叶片在光捕获、水分利用或 CO2 摄取方面没有优势。光适应和耐旱性之间、茎传导和通过叶片表面吸水之间的权衡，是该科内物种的适应性辐射和生态位分化的基础。水龙骨科是一个罕见的例子，它代表了一种从典型的维管植物适应向典型的苔藓植物的变水性的进化策略转变。
