Photosynthesis and dark respiration in three alpine snow tussocks (Chionochloaspp.) under controlled environments

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Abstract
Rates of carbon dioxide exchange for individual tillers of three alpine species of snow tussock, Chionochloa rigida, C. macra, and C. oreophila, were measured under controlled conditions to study influences of several environmental factors. Although maximum net assimilation rates are uniformly very low (range 2.7–4.6 mg CO2 g−1 hr−1), the differences between species in optimum temperatures, responses to low and high temperatures, and adaptation to increasing soil moisture stress all reflect what is known of these species in their natural habitats. Tolerance of actively growing plants to sub-freezing temperatures and their maintenance of a positive carbon balance under such conditions, ranks these snow tussocks with several Northern Hemisphere alpine species in their ecophysiological adaptations to cold environments. The ability of at least one snow tussock species to achieve maximum net assimilation rates during a brief period above freezing is a phenomenon apparently so for unreported. It is suggested that the low-temperature tolerance of these species may be facilitated by a permanent internal moisture stress, as has been reported in a few other cold region species. The physiological strategy involving slow assimilation and growth rates, revealed for the ecologically successful alpine snow tussocks, supports the view that the evergreen habit coupled with low energy demands are an effective adaptation to cold windy environments.