Vegetation plays a key role in the Earth system, regulating carbon, water, and energy exchanges between vegetation and atmosphere. Transpiration, photosynthesis, and respiration are the main processes that govern these exchanges and link vegetation to the climate (Bonan, 2008;Bonan et al., 1992Bonan et al., , 2003. Evapotranspiration (ET, with accompanying latent-heat release) and photosynthesis are tightly linked. Transpiration is the dominant component of ET, hence plants are the main conduit of water from the soil to the atmosphere. Plants control both transpiration and the flux of carbon dioxide (CO 2 ) into leaves by regulating the opening or closing of stomata.