Photosynthesis 129 



field data and vascular plant simulations suggest that such water stress 

 occurs infrequently and only when leaf temperatures and/or irradiances 

 are significantly higher than the mean. Dupontia may have allocated suf- 

 ficient resources to root absorptive tissue to meet the demands of the 

 evaporative leaf surfaces. 



The water vapor density gradient is one of the factors determining 

 the rate of water loss. The gradient is normally small because the air is 

 nearly saturated and the leaves are close to air temperature. Simulations, 

 in which air water vapor density was varied from -AQ^o to + 30% of am- 

 bient, indicate a slight sensitivity of -9% to +9% change in photosyn- 

 thesis to changes in the water vapor density gradient. This effect was 

 related to lower leaf water potentials and higher leaf resistances as the 

 water vapor density gradient increased. Transpiration losses also in- 

 creased as the gradient increased, to the maximum simulated, resulting in 

 an increase in water loss of 42*^0 in the upper part of the canopy, 26% in 

 the center, and 21 % at the bottom. The greater increase at the top of the 

 canopy was because the gradient of water vapor density from the air to 

 the leaf became more important than the higher leaf temperatures at the 

 bottom of the canopy. 



The slight increase in leaf resistance and the associated slight 

 decrease in photosynthesis when transpiration changes are large suggests 

 that root resistance in Dupontia is small relative to the water re- 

 quirements. Increasing root resistance by 50% results in less than a 6% 

 reduction in daily photosynthesis. The reduction is caused by a midday 

 decrease of -2.6 to -3.8 bars in leaf water potential at the top of the can- 

 opy and a decrease of -3.7 to -4.9 bars at the canopy bottom. However, 

 leaf water potentials increase to standard day values near solar midnight 

 as the plant regains its water deficit. These simulations strongly suggest 

 that Dupontia is sensitive to periods of high water demand but that it can 

 normally supply the amount of water required to keep stomates open. 

 Although this is in agreement with data for Carex from both Barrow and 

 Devon Island, it contrasts sharply with Dryas (Mayo et al. 1977), which 

 shows decreased rates of uptake, associated with low water potentials, at 

 high temperatures. 



Calliergon is more sensitive to a decrease in soil moisture levels than 

 are the other moss species analyzed. For Dicranum elongatum, Dicran- 

 um angustum and Pogonatum alpinum, simulations indicate a 25% 

 decrease in photosynthesis associated with a 10-bar decrease in soil water 

 potential (from to -10 bars). Under the same conditions, Calliergon 

 undergoes an 80% decrease in photosynthesis. Calliergon appears to be 

 much more dependent than the other species on a liquid water film and 

 on standing water to maintain a beneficial moisture status (Figure 4-16). 



Calliergon takes up water poorly from depth and displays low resis- 

 tances to water loss when compared to the other species. In the Dicranum 



