78 



P. C. Miller et al. 



E 

 u 





10 



o 



20 - 



■ Stem base 

 Rhizome 

 D Root 



Thaw Depth 



899 g m" 

 15 Jun 



If 



570 g m' 

 3 Aug 



100 



2 623 g m^ 

 24 Aug 



FIGURE 3-6. Seasonal progression of the vertical distribu- 

 tion of belowground biomass as percentage of total in stem 

 bases, rhizomes, and roots at different soil depths. The total 

 belowground biomasses are given for each date. The dashed 

 line indicates the seasonal progression of the depth of thaw. 

 (After Dennis 1977, Dennis et al. 1978.) 



was composed of roots. Between 5 and 10 cm most of the biomass con- 

 sisted of roots, since stem bases did not occur below 5 cm and rhizomes 

 made up less than 1% of the biomass. Although roots were concentrated 

 in the upper 10 cm, they occurred to a depth of 25 cm. The relative pro- 

 portion of the different belowground parts at a given depth varied sea- 

 sonally. The percentage of the total belowground biomass that was stem 

 bases and rhizomes was lowest in early July, but by late August the per- 

 centage had increased to the early season levels. Thus most of the below- 

 ground biomass was in thawed soil early in the season, even though 

 the total soil volume available for exploitation by the plants remained 

 constrained by the underlying permafrost. Species differences in rooting 

 patterns are discussed in Chapter 5. 



INFLUENCE OF THE CANOPY ON THE 

 PHYSICAL ENVIRONMENT 



Interception and Absorption of Radiation 



A discussion of radiation in the tundra canopy must include several 

 wavelength bands, depending on the biological processes being consid- 

 ered. Radiation in the 300- to 700-nm band, "photosynthetically active 

 radiation" or PAR, provides energy for photosynthesis. Radiation 

 between 300 and 3000 nm (insolation) and 9000 and 1 1 ,000 nm (infrared) 

 provides energy that warms the plant above air temperature, and affects 

 the rates of metabolism, growth, and development. The canopy affects 

 the spectral composition of the radiation (Lemon 1963). Within the 



