Biophysical Processes and Primary Production 93 



1972, midday water potentials reached -28 bars in Dupontia, -25 in 

 Carex, and -13 in Poa arctica. But in the relatively wet, cold summer of 



1973, water potentials were above -15 bars (Stoner and Miller 1975). In 

 other studies water potentials of soft-leaved forbs were similar, above 

 -10 bars in Thlaspi alpestre (Rochow 1967) and above -11 bars in Caltha 

 leptosepala (Kuramoto and Bliss 1970), but those of evergreen dwarf 

 shrubs and trees were lower, -35 bars in D. integrifolia (Courtin and 

 Mayo 1975), -60 bars in D. integrifolia, -54 in Picea englemanii, -35 to 

 -54 in S. oppositifolia, -62 in spruce, and -54 in Diapensia lapponica 

 (Courtin 1968). 



Leaf resistances of arctic and alpine plants were also similar to those 

 of temperate zone plants (Miller et al. 1978b). Minimum leaf resistances 

 in the coastal tundra at Barrow were 1 to 3 s cm"'. Cuticular resistances 

 were 12 to 39 s cm"'. Leaf resistances decreased as temperatures rose to 

 15 °C. The light response curve for stomatal opening indicates that mini- 

 mum leaf resistances of 1 to 2 s cm"' were approached at 140 J m"^ s"' 

 (400 to 700 nm) in plants grown at 5 °C in the laboratory (Figure 4-4). In 

 the field open stomata occurred at 70 J m"^ s"' (400 to 700 nm). Ehler- 

 inger and Miller (1975) reported minimum leaf resistances at 140 to 210 J 

 m"^ s"' solar irradiance (300 to 3000 nm) in the alpine tundra on Niwot 

 Ridge for Caltha leptosepala and Bistorta bistortoides. Courtin and 

 Mayo (1975) reported high minimum resistances for Dryas integrifolia. 

 With reasonable conversions between incoming shortwave and photo- 

 synthetically active radiation, it can be demonstrated that tundra plants 

 at Barrow open stomates at lower light intensities than do alpine plants. 



The root-soil resistance to water uptake of an entire plant included 

 the root's permeability to water uptake, and the total root mass. Root re- 

 sistances measured in situ were 0.6 to 1.7x10* bar s cm"'. Root resis- 

 tances decreased in the order Dupontia, Carex and Eriophorum, which 

 had rooting depths of 15, 20 and 25 cm, respectively. Field measurements 

 on coastal tundra species and laboratory measurements on alpine species 

 indicated that root resistances were independent of root temperatures but 

 related to transpiration rates (Stoner and Miller 1975, Caldwell et al. 

 1978). Typha latifolia, from high elevations, showed no change in rela- 

 tive water content with changes in root temperature between 20° and 

 30 °C, while plants from low elevations showed decreases (McNaughton 

 et al. 1974). Taken as a whole, these data support the idea that species 

 from colder climates have lower root resistances at low temperature. 



Water Relations of Mosses 



Water relations of mosses differ from those of vascular plants, part- 

 ly because mosses show little control over tissue moisture status. Mosses 



