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

雾的持久性有助于热带山地云雾林独特的生物地理和水文。叶片表面雾滴的存在可能抑制光合作用下的碳交换。此前的研究报告称，二氧化碳在水中的扩散速度比在空气中慢约1万倍。水在叶片表面的串珠可以维持较高的光合速率。在雾霾持续时间长的云雾林环境中，增加叶片的疏水性可能是在多云时期提高光合效率的一种选择性策略。在地理和水文学研究中，叶片的疏水性等特性对水文过程的影响是一个尚未探索的研究课题。叶片表面的物理性质与叶片表面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，以研究穿透、茎流和雾降水总量。具有高度排斥叶表面的物种可能会增加一个地点的透雨、茎流和雾降水的数量，并可能导致云雾森林冠层下更大的水文输入。这项比较叶片地理学研究将研究塞拉德拉斯米纳斯生物圈保护区云雾林和危地马拉圣何塞拉阿拉达社区附近低地森林中24种优势物种的叶片驱水性。叶片的疏水性是通过计算水滴与叶片表面的接触角来测量的。接触角越大，水滴越球形，表面越疏水。研究者和两名学生将测量每个物种的叶片抗水性，并收集叶片特征数据（叶面积、叶厚度、叶质量、比叶质量、比叶面积、叶韧性、气孔密度、毛状体密度、外生基的存在以及草食和病原体损害的证据）。该项目的目的是研究热带云雾林物种是否比低地热带物种具有更高程度的叶片拒水性。此外，该项目将确定有助于叶片抗水性的叶片特性。云雾林中水文过程的知识可用于指导管理这些濒危生态系统的环境政策。确定不同生境和物种间叶片拒水的空间格局是水文研究中一个重要的缺失环节。随着对同一立地优势种间和不同立地间叶片拒水性差异的进一步了解，可以建立包含叶片表面变量的更有意义的森林水文过程模型。此外，该研究将为探索由叶片特征变化引起的土地利用变化的潜在水文影响提供基础。云雾林的水文输入和对增加云雾林水资源可用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.