A Foliar Geographic Study of Leaf Water Repellency as an Adaptation to Cloud Forest Environments

雾的持续性有助于热带山地云雾森林独特的生物地理和水文学。叶表面雾滴的存在可能会抑制光合作用的碳交换。此前的研究报告称，二氧化碳在水中的扩散速度大约是空气中的10000倍。叶片表面的水珠能够保持较高的光合作用速率。在雾的持续性较高的云雾森林环境中，增加叶片的拒水性可能是提高多云期光合作用效率的一种选择性策略。在地理学和水文学中，叶的特性，如叶的排水性，作为一种影响水文过程的机制的重要性，是一个未被探索的研究课题。关于叶表面的物理性质与叶表面上的雾或水滴之间相互作用的信息有助于开发云森林水文过程的增强模型，该模型研究穿透雨、树干径流和雾降水总量。叶表面高度排斥的物种可能会增加一个地点的穿透雨、树干径流和雾的降雨量，并可能导致云雾森林树冠下更大的水文输入。这项比较叶地理学研究将检查24种主要树种的叶拒水性能，这些树种分布在拉斯米纳斯山脉生物圈保护区的云雾林和危地马拉圣何塞·拉阿拉达社区附近的低地森林中。通过计算水滴与树叶表面的接触角来测量树叶的拒水性。接触角越大，表明水滴越球形，表面的拒水性越强。研究人员和两名学生将测量叶片的拒水性，并收集每个物种的叶片特征(叶面积、叶厚度、叶质量、比叶质量、比叶面积、叶片韧性、气孔密度、毛密度、附生叶的存在以及食草动物和病原菌损害的证据)的数据。该项目的目的是研究热带云雾林树种是否比低地热带树种具有更强的叶拒水能力。此外，该项目将确定有助于叶片拒水性的叶片特征。云雾森林内的水文过程知识可用于指导管理这些濒危生态系统的环境政策。在水文学研究中，确定不同生境和物种之间的叶片拒水性的空间格局是一个重要的缺失环节。随着对同一立地优势树种间和立地间树叶拒水性差异的更清楚地了解，可以建立更有意义的包含树叶表面变量的森林水文过程模型。此外，这项研究将为探索由于树叶特性变化而导致的土地利用变化的潜在水文影响提供基础。云雾森林的水文投入和对增加云雾森林水资源可获得性的水文过程的了解，对危地马拉干旱低地地区人民的生计至关重要。

Fog persistency contributes to the unique biogeography and hydrology of tropical montane cloud forests. The presence of fog droplets on leaf surfaces may inhibit photosynthetic carbon exchange. Previous studies have reported that carbon dioxide diffuses approximately 10,000 times slower in water than air. The beading of water on leaf surfaces enables the maintenance of high photosynthetic rates. In cloud forest environments with a high persistency of fog, increased leaf water repellency may be a selective strategy to promote photosynthetic efficiency during cloudy periods. The significance of leaf characteristics such as leaf water repellency as a mechanism that influences hydrological processes is an unexplored research topic in geography and hydrology. Information on the interaction between the physical properties of the leaf surface and fog or water droplets on the leaf surface is useful to develop enhanced models of cloud forest hydrological processes that investigate throughfall, stemflow, and fog precipitation totals. Species with highly repellent leaf surfaces may increase the quantities of throughfall, stemflow, and fog precipitation at a site and may result in greater hydrological inputs beneath the cloud forest canopy. This comparative foliar geography study will examine leaf water repellency of 24 dominant species in cloud forests of the Sierra de las Minas Biosphere Reserve and in lowland forests near the community of San Jose La Arada, Guatemala. Leaf water repellency is measured by calculating the contact angle between a water droplet and the leaf surface. A greater contact angle indicates a more spherical water droplet and a more water-repelling surface. The investigator and two students will measure leaf water repellency and will collect data on leaf characteristics (leaf area, leaf thickness, leaf mass, specific leaf mass, specific leaf area, leaf toughness, stomatal density, trichome density, presences of epiphylls, and evidence of herbivore and pathogen damage) of each species. The objective of this project is to examine whether tropical cloud forest species have a higher degree of leaf water repellency than lowland tropical species. Furthermore, this project will identify leaf characteristics that contribute to leaf water repellency. Knowledge of hydrological processes within cloud forests can be used to guide environmental policy on the management of these endangered ecosystems. Defining the spatial patterns of leaf water repellency between different habitats and species is an important missing link in hydrological research. With clearer understanding of the differences between leaf water repellency among dominant species at a site and between sites, more meaningful models of forest hydrology processes can be formulated that incorporate leaf surface variables. Furthermore, this study will provide a basis for exploring the potential hydrologic impact of land use changes that result from changes in leaf characteristics. The hydrological inputs in cloud forests and an understanding of the hydrological processes that increase water resource availability from the cloud forests are vital to the livelihood of people in the arid lowland regions of Guatemala.