Effects of Summer Frost on the Gas Exchange of Field-Grown Pinus sylvestris L. Seedlings

The effect of night temperatures on the gas exchange of naturally established Scots pine (Pinus sylvestris L.) seedlings was investigated at a site with frequent summer frosts in northern Sweden. During the growing season, a reversible inhibition of light-saturated rates of CO2-assimilation was observed after nightfrosts. Stomatal conductance decreased or increased in parallel with changes in the CO2-assimilation rate. The inhibition of photosynthesis was not primarily the result of stomatal closure, since intercellular CO2-concentration increased after night frosts. Naturally regenerated Scots pine seedlings survived repeated frosts during the growing season and showed no visible damage although air temperatures as low as -8.2 degrees C were recorded during the period of shoot elongation.