The Ecophysiology of Surface Cryptogams from Alpine Tundra and Semi-Arid Grassland of Southwestern Alberta

Abstract

<p>The seasonal response patterns of net photosynthesis and respiration (and nitrogenase activity in Nostoc) are described within a multivariate framework of temperature, moisture and light for the alpine and grassland crustaceous lichens Rhizocarpon superficiale and Caloplaca trachyphylla respectively and for the grassland surface cyanophyte Nostoc commune. These physiological responses are discussed in context of each species' boundary layer environment, with particular emphasis placed on interactions between environmental constancy and adoption of acclimation strategies.</p> <p>For R. superficiale the high frequency of thermal fluctuations experienced by hydrated thalli, sometimes on an hourly basis, precluded any strategy of seasonal acclimation. Instead, photosynthetic rates exhibited a broad temperature response, remaining near 1 mg CO₂ h⁻¹g⁻¹ from 1 up to 21°C, with no changes evident between seasonal responses. In marked contrast C. trachyphylla shows a distinct winter/summer pattern of photosynthetic acclimation. In winter months rates are optimal near 7°C, while in summer the temperature optima of net photosynthesis shift to 21°C. These changes correlate well with predictable seasonal microclimate events, particularly those associated with winter Chinook snowmelt periods. A third pattern of response was seen in N. commune, where no seasonal changes in response patterns were evident and both nitrogenase activity and net photosynthesis were maximal near 35°C. This response pattern allows maximum carbon gain and nitrogen fixation during spring and summer periods following precipitation events, while its more sheltered aspect reduces the importance of winter snowmelt periods.</p>