Relationship between temperature and the seasonal course of photosynthesis in Scots pine at northern timberline and in southern boreal zone

In earlier studies the seasonal dynamics of photosynthetic capacity in northern conifers has been explained as a slow response to the ambient temperature. We tested this concept with Scots pine (Pinus sylvestris L.). We analysed the seasonal dynamics of photosynthetic efficiency in Scots pine at the timberline in Finnish Lapland, and in a southern boreal forest in Southern Finland. The relationship between the daily photosynthetic efficiency and leaf temperature history was determined from continuous measurements of shoot CO₂ exchange. The shoot CO₂ exchange and photosynthetic efficiency showed similar seasonal patterns in the northern and in the southern locations, following daily mean temperature with a delay. The relationship between the temperature history and photosynthetic efficiency appeared to be near sigmoidal both in the northern and in the southern trees. The relationship was also consistent from year-to-year, thus the seasonal course of photosynthetic efficiency can be predicted accurately from the ambient temperature using a sigmoidal relationship. A rapid decrease of photosynthetic efficiency was observed when daytime temperature dropped below zero or frost had occurred in the previous night. The difference in the rate of acclimation of photosynthetic efficiency between the north and the south was small.