116 L. L. Tieszen et al. 



and occasional summers with warm, dry periods (e.g. 1972) can result in 

 the desiccation of mosses. However, these conditions are relatively un- 

 common. As indicated in the previous chapter, the vascular plants main- 

 tain low leaf resistances and generally high water potentials. Although 

 occasional midday stomatal closure is seen, water is generally not 

 limiting to carbon dioxide uptake in these Carex-Oncophorus meadow 

 forms. 



Diurnal and Seasonal Patterns 

 of Carbon Dioxide Exchange 



The daily and seasonal trends of photosynthesis integrate the re- 

 sponse patterns of the plant to environmental variables with the plant's 

 seasonal ontogenetic pattern. The early season pattern is characterized 

 by positive rates of photosynthesis throughout the day in both vascular 

 plants and mosses. (Oechel and Collins 1973, Tieszen 1975, Oechel and 

 Sveinbjbrnsson 1978). However, at the time of snowmelt the foliage area 

 index is near zero for vascular plants. Immediately after snowmelt vascu- 

 lar tissues develop photosynthetic competency, elongate, and begin to 

 produce an intercepting canopy (Tieszen 1975, Dennis et al. 1978, Ties- 

 zen 1978b). Mosses, on the other hand, have tissues that may overwinter 

 1 or 2 times (Collins and Oechel 1974) and can continue photosynthesiz- 

 ing as soon as they become snow-free, although at reduced rates. The 

 relatively large moss biomass at the beginning of the season can result in 

 high rates at a time when the vascular canopy is just developing (Miller et 

 al. 1976, Oechel and Sveinbjbrnsson 1978). 



In all vascular species the highest values for carbon dioxide uptake 

 on a leaf basis occur early in the season. As the season progresses (Figure 

 4-7), maximum values become lower, and towards the end of August 

 they are near 5 mg CO2 dm"^ hr'' for all species. Earlier in the season 

 periods with similar radiation and temperature produce substantially 

 higher rates of photosynthesis. The response patterns for mosses (Figure 

 4-8) show some interesting differences from the patterns displayed by 

 vascular plants, including more negative "night" values and depressed 

 early-season rates in some species. 



Integrating hourly values for a 24-hour period provides an estimate 

 of daily net carbon dioxide uptake by plants. Daily totals are generally 

 high for all vascular species, even early in the summer. Absolute amounts 

 vary somewhat, depending upon specific light and temperature combina- 

 tions, and are greatest on days of high solar irradiance. Carbon dioxide 

 incorporation correlates more highly with daily totals of radiation than 

 with temperature. Photosynthetic efficiencies for the entire season are 

 about 1% for the graminoids and 1.7970 for Salix (Table 4-5). An extrap- 



