The effect of climate on the photosynthesis of Picea mariana at the subarctic tree line. 1. Field measurements

Field measurements of the diurnal rates of photosynthesis of Picea mariana, the dominant tree species at the subarctic tree line, were made during the summers of 1972 and 1973 at Schefferville, Quebec (latitude 55° N). All relevant plant physiological and environmental parameters were also monitored. Photosynthesis was measured with an open gas analysis system with temperature-controlled cuvettes. Maximum daily rates were 2.0–3.5 mg CO2 g−1 dry weight h−1. Daily totals were between 15 and 30 mg CO2 g−1 dry weight. Temperature was unimportant in affecting daily photosynthesis totals during the summer months. The photosynthesis vs. needle temperature curve had an optimum of 15C. Dark respiration rates were 0.2–0.4 mg CO2 g−1 dry weight h−1 at 15C. The photosynthesis vs. light intensity curve was saturated at 0.8 ly min−1 (1.0 μE cm−2 s−1 PhAR.). As a result, heavy cloud cover considerably reduced daily photosynthesis. No seasonal variations in photosynthesis over June, July, and August were observed. No differences in maximum rates occurred between the three experimental sites. Needle temperatures within the cuvettes were 2–4C above air temperature under full sunlight (1.2 ly min−1). Needle temperatures under natural conditions were up to 7C above Stevenson screen temperatures and fluctuated rapidly with changes in turbulence.