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.
