Biophysical Processes and Primary Production 91 



CONTROL OF EVAPOTRANSPIRATION BY 

 PLANTS AND PLANT-WATER RELATIONS 



The control of water loss by the plants affects both surface energy 

 exchange processes and plant physiological processes. The maintenance 

 of turgor is essential for photosynthesis, respirative growth, and develop- 

 ment (Hsaio 1973). Various aspects of the plant water relations are af- 

 fected at temperatures well above those typical at the Barrow research 

 sites, including water absorption (Kuiper 1964), plant growth and 

 osmotic potential of the leaves (Kleinendorst and Brouwer 1970) because 

 of decreased root permeabihty. The relative water content and growth of 

 alpine and subalpine species were reduced by soil cooling, although net 

 photosynthesis and transpiration were unaffected (Anderson and 

 McNaughton 1973). Stomatal opening in temperate plants was inhibited 

 at leaf temperatures lower than 10 °C (Kuiper, cited by Ketellapper 1963, 

 Courtin and Mayo 1975). 



The flow of water was viewed in a simple model of the soil-plant- 

 atmosphere continuum (Stoner and Miller 1975, Ehleringer and Miller 

 1975, Miller et al. 1976). Water flows through the roots and stems to the 

 leaves because of a difference in the water potentials of the soil and 

 leaves, and is lost to the air because of a difference in the vapor densities 

 of the leaves and air. The flow of water through the plant is restricted by 

 the resistance of the root-soil system to water uptake and the resistances 

 of the leaves and leaf air boundary layer to water vapor diffusion. 

 Mosses were viewed similarly except that water uptake is mainly on the 

 surface of the moss, Hquid water is absorbed through the surface, and 

 water loss from the tissue is suppressed when a surface film of water is 

 present (Miller et al. 1978a, Stoner et al. 1978b). 



Water Relations of Vascular Plants 



The partitioning of absorbed energy into convection and evapora- 

 tion is controlled by stomatal closure, which occurs to prevent low water 

 contents and detrimental water potentials, either of which may be harm- 

 ful (Jarvis and Jarvis 1963). Leaf resistances increased abruptly with rel- 

 ative water contents below about 91% in Arctophila, 80 to 89% in Du- 

 pontia. Potent ilia hyparctica and Salix pulchra, and 72% in Eriophorum 

 angustifolium (Figure 3-15) (Stoner and Miller 1975). Species in other 

 regions are similar, i.e. between 80 and 89% for Caltha leptosepala and 

 Bistorta bistortoides (Ehleringer and Miller 1975), 85 and 90% for alpine 

 plants (Anderson and McNaughton 1973), and 90% for Populus tremula 



