Forest conversion in tropical montane landscapes is widespread and has potentially large implications for both biological and physical processes. Understanding the ecohydrologic processes that affect water can help efforts to predict the downstream effects of parcel-scale land use change. Differences in evapotranspiration between trees and grasses in humid, low wind environments are understudied, however. We analyze predictions of the Penman-Monteith model of potential evapotranspiration (PET) based on hourly meteorological inputs and direct measurements of stomatal resistance for leeward Hawai'i Island. While evapotranspiration is very low in all of these forest and pasture ecosystems, modeled PET from pasture is higher than is PET from forest. The balance between aerodynamically and stomatally controlled evapotranspiration differs significantly between the two vegetation types in such a way that the weighted sum of the two components yields lower overall PET at the forest sites. The interaction of aerodynamic and stomatal control on PET, in conjunction with tropical meteorology characterized by low wind speeds and low vapor pressure deficit (VPD) causes this unexpected phenomenon. Vegetation structure plays an important role: evapotranspiration from forest is increased considerably by contributions from the understory, while the shorter the stature of pasture grass, the higher its rate of PET. In tropical regions that do not experience water stress, grassland has the potential to transport as much or more water vapor to the atmosphere than does forest.
Bibliographical noteFunding Information:
The authors are most grateful for support from the Center for Conservation Biology, The Koret Foundation, The Moore Family Foundation, Peter and Helen Bing, The Winslow Foundation, The Natural Capital Project, an IPER Rudolf Research Fellowship, and McGee Stanford School of Earth Sciences Research Funds, as well as a National Science Foundation Graduate Research Fellowship, The Lucille and David Packard Stanford Graduate Fellowship, and The William C. and Jeanne M. Landreth IPER Student Fellowship (for support of KAB). Greg Asner, Joe Berry, T. Ka’eo Duarte, Paul Ehrlich, Dale Fergerstrom, Chris Field, Milton Garces and the UH Infrasound Lab, Rebecca Goldman, Josh Goldstein, Liba Goldstein, Aurora Kagawa, Kevan Moffett, Hal Mooney, Heather Tallis, Peter Vitousek, and Stacie Wolny provided help in the field and on campus. Kamehameha Schools, Kealakekua Ranch, and Palani Ranch graciously allowed us to build weather stations on their property. Jody Fergerstrom provided invaluable field assistance.
- Aerodynamic resistance
- Land use change
- Stomatal resistance
- Wet canopy evaporation