Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)




K.A. Kershaw


The multivariate examination of nitrogenase activity in two populations of the lichen Peltigera canina is described. Only a small increase in activity was observed with increasing radiant flux density and a marked but relatively constant temperature optimum was noted for both varieties that was independent of all other variables. However, very large rate changes were observed for all light levels and temperatures throughout the year. The influence of snowmelt on subsequent rates of nitrogenase activity is also described. While temperature was intimately involved in the velocity of the recovvery of nitrogenase activity after snowmelt, the prime requirement appeared to be one of light. Lichens are believed to be extremely resistant to high temperature stress when dessicated. A reexamination of this concept for several temperature and northern populations of lichens in the genera Peltigera and Stereocaulon indicated that some air-dry lichen thalli can be extremely sensitive to even moderate levels of heat stress whereas others exhibited a considerable degree of heat resistance. These differential levels of thermal resistance correlated exactly with the ecology of these populations. Thermal sensitvity was identified as an important influence in the succession of plant species following forest fire and it is suggested that thermal stress may be one of the dominant parameters in the ecology of lichens. Lichens are also believed to be extremely resistant to high light levels when in the air-dry state. A reexamination of this concept identified light stress as another ecological factor of considerable importance to some lichens. A seasonal adjustment of net photosynthetic capacity is described for the woodland population of Peltigera canina. Changes in the maximal rates of net photosynthesis and respiration correlated strongly with concurrent changes in the leaf canopy. The seasonal adjustment of net photosynthetic rates is considered to be essential to the success of the organism which must adapt to a continually changing light environment.

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