Biophysical Processes and Primary Production 



99 







- -20 



FIGURE 3-19. Seasonal courses of midday leaf water 

 potentials for Carex aquatilis (C.aJ, Dupontia fisheri 

 (DfJ, Arctophila fulva (A.f), Eriophorum angusti- 

 folium (E.a.), Potentilla hyparctica (P.h.) and Salix 

 pulchra (S.p.) in 1972 and 1973. Water potentials at 

 which leaf resistances are three times the minimum (A) 

 and infinite (B) are given. (After St oner and Miller 

 1975.) 



Measurements of photosynthesis in the field (Tieszen 1975, 1978b) 

 indicated a midday depression in carbon dioxide uptake, which was relat- 

 ed to a slight stomatal closure (Tieszen 1978b). Thus water stress may 

 limit vascular plant production at Barrow by increasing stomatal resis- 

 tance and decreasing photosynthesis. The role of water in limiting pro- 

 duction via its effect on growth has not been studied. The species, in de- 

 creasing order of their sensitivity to water stress, were Arctophila, Du- 

 pontia, Carex, Salix, Potentilla and Eriophorum. 



The water relations of the mosses Dicranum and Calliergon were 

 simulated in the open or under a vascular plant canopy along a substrate 

 moisture gradient (Stoner et al. 1978b). The canopy tended to increase 



