Biophysical Processes and Primary Production 97 



The low irradiance, low temperature, and low evapotranspiration 

 helped maintain the turgidity of moss tissue. Pogonatum had favorable 

 moisture levels throughout some summers, 150% to 200^^0 water con- 

 tent, which were sufficient to maintain photosynthesis at near optimal 

 levels (Oechel and Collins 1976, Sveinbjornsson 1979). Calliergon oc- 

 curred primarily in troughs and wet meadow areas and was often sub- 

 merged or inundated with water early in the year, resuhing in water con- 

 tents greater than 1400%. As the season progressed, water levels dropped 

 and water contents declined to 400% (Figure 3-17). Even at this level, 

 photosynthesis was at near-maximum rates. 



Locally, however, tissue moisture relations affected species distribu- 

 tion and survival. The moisture regimes of rims of low-centered polygons 

 appeared to restrict the growth of Calliergon in those areas, and the wet 

 nature of many Calliergon habitats would depress photosynthesis in 

 Pogonatum if it occurred there (Oechel and Collins 1976). 



Local populations of all species, and especially of Calliergon, 

 became desiccated at various times during the summer. Populations on 

 sides and tops of high-centered polygons drained free of water early in 

 the summer and became desiccated. Individuals growing on the sides of 

 polygon troughs that were water-filled early in the season were especially 

 prone to desiccation. Also, periods of drought had a major influence on 

 bryophyte growth and survival. An exceptionally warm, dry period of 

 several weeks in 1972 resulted in the death of numerous individuals of 

 Pogonatum and Calliergon. Damage was delayed and less extensive in 

 Pogonatum than in Calliergon. At the end of the drought period, large 

 areas of Calliergon mats recovered by initiating new shoots from existing 

 material (Oechel and Collins 1976). Because of water uptake through the 

 deep stems of Pogonatum, this species can maintain turgidity in the 

 absence of standing water or precipitation for much longer periods than 

 the other species examined (Oechel, unpubl.). 



SIMULATION OF PLANT-WATER RELATIONS 



The dynamics of vascular plant-water relations were simulated for 

 the June- August period in 1970-73 (Stoner and Miller 1975). Simulations 

 for 1973, which was cold and wet relative to the long-term average, indi- 

 cated that leaf resistances of Arctophila and Potent ilia decreased in the 

 early morning, then increased in midmorning to a plateau which contin- 

 ued through the day. Dupontia resistances increased to a maximum at 

 midday and decreased in the afternoon. Salix and Eriophorum showed 

 slight increases in leaf resistance at midday, while Carex leaf resistances 

 were low throughout the day. The increase in leaf resistances in the mid- 

 dle of the day reflected the sensitivities of leaf resistance and leaf water 



