Photosynthesis 107 



are hydrolyzed and mobiHzed. Obviously, the dynamics of leaf photo- 

 synthesis will vary with plant growth forms as patterns of leaf develop- 

 ment and retention vary. In Dupontia exsertion is followed by an elonga- 

 tion period of 20 to 22 days, followed by a shorter period of 8 to 10 days 

 during which the growth rate is near zero. At the end of this period, the 

 leaf initiates senescence and in about 25 to 30 days it is dead. Thus, in 

 comparison with other growth forms where the mature phase may last 

 more than one growing season, Dupontia has a short period of maximal 

 photosynthesis (Johnson and Tieszen 1976). Carex and Eriophorum an- 

 gustifolium have somewhat longer mature periods than Dupontia, while 

 moss tissue may remain photosynthetically active for at least 3 years 

 (Collins and Oechel 1974). 



Photosynthetic activity of vascular plants does not occur beneath 

 the winter snow even though substantial carboxylation activity is present 

 (Tieszen 1974). Thus photosynthesis begins concurrently with growth, 

 which is initiated within one day of snowmelt. This has now been con- 

 firmed in the Arctic not only for graminoids but also for Dryas (Mayo et 

 al. 1977), which remains inactive until snowmelt. This is not unexpected 

 since the plant temperatures beneath late-winter snow may approximate 

 the permafrost temperature, thereby presenting a distinct contrast with 

 conditions that may occur in mid-latitude alpine areas. As mehwater per- 

 colates through the snowpack, however, temperatures abruptly approach 

 0°C (Tieszen 1974). 



Following snowmelt, leaf expansion and growth of Dupontia occur 

 rapidly and are accompanied by the development of photosynthetic com- 

 petence. The first leaf elongates and exserts some chlorophyllous tissue 

 produced the previous season. This tissue never becomes very active al- 

 though it does make a positive contribution to the carbon balance. By 

 about 19 June, however, the second and third leaves have elongated and 

 they are soon active (see Chapter 5). Although the sequential pattern of 

 photosynthesis is somewhat obscured by the short growing season, suc- 

 cessive leaves become more active as the season progresses. This general 

 ontogenetic leaf pattern is similar to that of other graminoids, and results 

 in a sequence of developing photosynthetic competence as leaves elon- 

 gate or enlarge, a period of maximal photosynthetic competence associ- 

 ated with maturation, and a subsequent decline in photosynthetic com- 

 petence as senescence develops (Johnson and Caldwell 1974, Johnson 

 and Tieszen 1976, Tieszen 1978b). 



In a short growing season a sequential leaf pattern seems costly since 

 it requires a large investment in synthetic and growth processes (see 

 Chapter 5). Although it does replace leaves at successively higher posi- 

 tions in the canopy in more favorable radiant flux (but less favorable 

 thermal) environments, this pattern must have other selective value, e.g. 

 as a mechanism for withstanding acute or chronic grazing pressures. 



