Biophysical Processes and Primary Production 95 



resistances of the moss canopy were low when expressed on an areal 

 basis. Air resistances were similar for C sarmentosum and Dicranum 

 elcngatum and ranged from 0.9 to 1.0 s cm"' respectively at a wind speed 

 of 4.5 m s"'. P. alpinum had an air resistance of only 0.3 s cm"' at 4.5 m 

 s"', presumably as a result of the very rough canopy (Alpert and Oechel, 

 unpubl.). 



All moss species showed little tissue resistance to water loss (less 

 than 0.5 s cm'') at the field water contents commonly observed (Figures 

 3-16 and 3-17). As tissue desiccated C. sarmentosum increased resistance 

 to water loss only slightly. At the moisture compensation point for pho- 

 tosynthesis (75% w.c), C sarmentosum showed a leaf resistance of only 

 0.84 s cm"'. P. alpinum, on the other hand, had a higher resistance of 

 greater than 3.0 s cm"' at its moisture compensation point for photosyn- 

 thesis (60% w.c). At a water content of 200%, the resistance in 

 Pogonatum was the lowest of all species, 0.3 s cm"'. However, resistance 

 increased to 3.2 s cm"' at 75% water content. The low resistance at high 

 water contents of Pogonatum alpinum may partially be the result of the 

 photosynthetic lamellae present on the leaf surface. As desiccation pro- 

 ceeds, the rolling of the tissue margins over the photosynthetic lamellae 

 and the appression of the leaf tissue against the stem increased the leaf 

 resistance. The more responsive nature of resistance to drought in Pogo- 

 natum and Polytrichum species is presumably an important adaptation 

 to the more xeric sites where they often occur. 



The colony growth form has been found to increase the rate of water 

 uptake over that achieved by individual shoots. Pogonatum represents 

 the less dense turf growth form, and water uptake rates were dictated pri- 

 marily by the response of single shoots. Calliergon, on the other hand, 

 develops a carpet growth form with high shoot density. In Calliergon sar- 

 mentosum water is held on the tissue surface and the colony form is 

 much more important in controlling water uptake and loss than in Pogo- 

 natum (Gimingham and Smith 1971). However, it appears that appreci- 

 able water can be taken up via the tissue bases on both species. These 

 rates of water uptake vary considerably. Pogonatum, for example, had a 

 low rate of uptake, as shown by the length of time required to recover 

 50% of the total water content (WCso). The rate of water uptake from 

 air-dried status to WCso was 0.24 g H2O gdw"' min"' when the bases were 

 immersed in water to a depth of 2 mm and 0.01 g H2O gdw"' min"' when 

 the apexes were immersed to 2 mm. Carpets of Calliergon took up water 

 much more rapidly and to a larger extent. Water was taken up via the 

 apexes at a rate of 4.05 g H2O gdw"' min"' and WC50 was reached in 1.2 

 minutes. Water uptake via the bases was faster at a rate of 12.9 g H2O 

 gdw"' min"' with WCso achieved in only 0.3 minute (calculated from 

 Gimingham and Smith 1971). The rates of water uptake for these two 

 moss species approximate the range of extremes found at Barrow. 



