Effects of Several Microclimatic Factors and Nutrients on Net Carbon Dioxide Exchange in Cladonia Alpestris (L.) RABH. in the Subarctic

Abstract

Acclimation in net photosynthetic response patterns to changing conditions of temperature and light was found during the growing season in Cladonia alpestris in the subarctic at Schefferville, Quebec. However, adjustment of the photosynthetic response surface to temperature was found to be incomplete. Changes in the temperature optima for photosynthesis observed may be a natural consequence of seasonal changes in photosynthetic rates, and not a direct adaptation. There was no seasonal change in the moisture sensitivity of photosynthesis. However, seasonal trends in the water content at saturation and in the drying rate of the thallus conferred additional drought resistance in midsummer. The photosynthetic rate and drying rate reflected the light and temperature conditions experienced during the month preceding measurement, not the conditions immediately prior to measurement. No seasonal change was found in the magnitude of temperature sensitivity of dark respiration rates during the summer. Dark carbon dioxide flux in the middle and bottom portions of the thallus were lower than those in the top, photosynthetically active portion. Maximum photosynthetic rates of 2.15 mg CO₂ • g dry wt⁻¹ • h⁻¹ were found in the photosynthetically active section of the thallus.

Climatic factors were found to be more important than edaphic factors in limiting net carbon dioxide exchange in Cladonia alpestris. The addition of nutrients to the lichen did not appear to affect the nutrient concentration, chlorophyll content, net photosynthetic rates, or dark respiration rates of the thallus. The total chlorophyll content of the top section of the thallus decreased during the growing season and the chlorophyll a:b ratio increased.