100 p. C. Miller et al. 



available water by reducing the vapor density gradient between the 

 mosses and the air but also tended to decrease the available water by in- 

 tercepting 30% of the summer precipitation. The significance of the 

 counteracting tendencies varied with the substrate water potential. Both 

 Calliergon and Dicranum showed similar seasonal courses of water con- 

 tent with and without the canopy and with different substrate water po- 

 tentials. With substrate water potentials of -1 bar both species had a 

 higher water content under the canopy than in full sun. The effect of the 

 canopy on plant water potential was much reduced at a substrate water 

 potential of -5 bars. During the late-summer dry period interception of 

 precipitation reduced the water reaching the surface, and moss in the full 

 sun had higher water contents than did moss under the canopy. The 

 water contents of Dicranum had the same pattern as Calliergon, 

 although the effect of the canopy in reducing evaporation was less be- 

 cause of the higher resistances of Dicranum to water loss. 



SUMMARY 



In summary, in 1972, a year of near normal temperatures and pre- 

 cipitation, calculated gross primary production above and below ground, 

 averaged for the whole Barrow region, was about 465 gdw m"% including 

 358 gdw m"^ for vascular plants, 106 gdw m"^ for mosses, and 1 gdw m'^ 

 for lichens. Net primary production was about 230 gdw m"^ including 

 162 gdw m"^ for vascular plants, 66 gdw m"^ for mosses and less than 1 

 gdw m"^ for lichens. The average net primary production above ground 

 was 108 gdw m"^ Belowground production was about 120 gdw m'^ 



In the Carex-Oncophorus meadow vegetation type, in which most 

 research was concentrated, gross primary productivity was 450 gdw m"^ 

 yr"', including 414 gdw m'^ yr"' for vascular plants and 36 gdw m"^ yr"' 

 for mosses. Net primary productivity was 209 gdw m"^ yr"', including 187 

 gdw m"^ yr"' for vascular plants and 22 gdw m"^ yr"' for mosses. The res- 

 piratory cost for maintaining the above- and belowground vascular bio- 

 mass was 170 gdw m'^ yr"' and the respiratory cost for growing new bio- 

 mass was 57 gdw m"^ yr"'. For mosses the maintenance and growth costs 

 were each 7 gdw m"^ yr"'. The gross primary productivity of vascular 

 plants was equivalent to a carbon dioxide incorporation of 609 g CO2 m"^ 

 yr-'. 



The plant canopy of the Carex-Oncophorus meadow interacts with 

 various biophysical factors to affect production and water loss. Several 

 features of the Carex-Oncophorus meadow canopy structure act to in- 

 crease plant temperatures, which are usually below optimum for physio- 

 logical processes in these tundra species. The steeply inclined leaves of the 

 grasses and sedges and the accumulated standing dead material increase 



