130 



L. L. Tieszen et al. 



e 



E 



X 



o 



5 



c 



01 



u 



0. 



75- 



50- 



25- 



^0 -10 -20 -30 



Soil Water Potential, bors 



FIGURE 4-16. Simulated annual net 

 photosynthesis at different soil water 

 potentials expressed in absolute terms 

 and as a percentage of maximum for 

 four arctic moss species: Pogonatum 

 alpinum (P. a), Calliergon sarmen- 

 tosum (C.S.), Dicranum elongatum 

 (D.eJ, and Dicranum angustum 

 (Dm.). (After Miller et al. 1978a.) 



species examined, the mat growth form plays an important role in water 

 retention by increasing the apparent air resistance to water loss and in 

 aiding water uptake by maintaining a nearly saturated environment at 

 the base of the photosynthetically active zone. In Pogonatum, xeromor- 

 phic adaptations of tissues that are deeply rooted in the substrate and are 

 efficient in translocating moisture are important in maintaining advan- 

 tageous moisture balances under xeric conditions (Miller et al. 1978a). 

 Calliergon 's susceptibility to xeric conditions is shown by the effect 

 of a hot season. At soil water potentials of -5 bars and less, Calliergon 

 shows net carbon dioxide loss in the hot climate. By comparison, Dicran- 

 um and two vascular plant species show little suppression by photosyn- 



