134 



L. L. Tieszen et al. 



high 



c 

 o 



o 



3 

 O 



o 



3 

 C 

 C 

 < 



low 



With 

 Canopy 



low 



high 



FIGURE 4-19. A diagram- 

 matic summary of the effect 

 on moss production of the 

 vascular canopy along a 

 moisture gradient. (After 

 Stoner et al. 1978b. f 



Available Water 



bon dioxide uptake due to the lowered irradiance and temperature may 

 be more than compensated for by the increased photosynthesis because 

 of the improved tissue moisture status. 



The result of an interplay between the environment in the vascular 

 canopy and the moss-water relation is a changing relationship of moss 

 production and canopy cover along a moisture gradient (Figure 4-19). 

 The simulations indicate that at high levels of available moisture, there is 

 no water limitation, and mosses are most productive in the absence of a 

 vascular plant canopy under full ambient radiation. Production is 

 limited by light, and plant resistances to gas exchange are important. At 

 moderate moisture availability, a vascular plant canopy increases moss 

 production by decreasing evaporation by more than the amount lost 

 through interception. At low levels of available moisture, mosses are 

 most productive in the absence of a canopy. In this situation, a canopy 

 intercepts much of the precipitation available and mosses are seldom hy- 

 drated. Under these conditions, brief periods of light precipitation are 

 not effective in penetrating the canopy to hydrate the moss tissue (Stoner 

 et al. 1978b). These results should be viewed as speculative because of 

 uncertainties in the interception model and in the water uptake relation- 

 ships in mosses. The water relationships of mosses are poorly understood 

 compared with the level of understanding of moss photosynthesis. 



Grazing 



Grazing is an important biotic interaction that affects the photosyn- 

 thetic rates and patterns described above. In the absence of a peak in the 

 lemming population, the grazing pattern in the coastal tundra at Barrow 

 results in the removal of some vegetation, usually as tiller units or shoots, 

 near the moss surface. Thus, photosynthetic tissues- at various stages of 

 photosynthetic competency are removed and consequently seasonal pro- 

 duction is reduced. Heavy grazing, especially in late winter or early 

 spring, removes the canopy, including standing dead and live material. 

 Thus, the microenvironment surrounding the photosynthetic leaves 



