Potted Pinus montana seedlings, age 4 years, transplanted on adjacent east and north facing slopes 25 m apart at alpine timberline (2,020 m a.s.l.) were measured for net photosynthesis and transpiration under ambient conditions using climatised Koch-Siemens cuvettes. Concurrent recordings were made of air temperature, atmospheric water vapour pressure deficit and illuminance at each site.
On a typical summers day the northern aspect averaged 9% less light, 1.8° C cooler air temperatures and 25% lower v.p.d. levels than the eastern aspect. The order of these differences was found to increase in the autumn. Net photosynthetic rates of seedlings on the northern aspect were on average 28% lower than the rates of seedlings on the warmer eastern aspect. Differences in transpiration rates were even greater with north slope seedlings averaging rates 42% lower than east slope seedlings.
Maximum CO₂ uptake rate per hour of east slope seedlings was 3.2 mg CO₂ g^-1 d.w. h^-1 but average rates when light was not limiting were around 2.0 mg CO₂ g^-1 d.w. h^-1. Corresponding values for the north slope seedlings were 3.0 mg CO₂ and 1.8 mg CO₂ g^-1 d.w. h^-1 respectively.
Light intensities below 10 klx, when photosynthesis was strongly limited by light, totalled 48% of available daylight hours on the east slope and more than 50% on the north slope.
Net photosynthesis was largely unaffected by air temperature between 10° C and the recorded maximum at either site (24° C east, 20° C north) and there was no apparent response to v.p.d. at levels up to 10 mbar. However the consistently higher net photosynthesis of east slope seedlings under all combinations of weather conditions indicated a possible acclimatisation of seedlings at each site.