The ecophysiological research into carbon assimilation is mainly concerned with how carbon assimilation in evergreen conifers is modified by fluctuating external conditions. It involves both the descriptive study of photosynthetic responses to ambient conditions and the causal analysis of physiological mechanisms. Many abiotic environmental factors such as radiation, temperature, water and mineral nutrients can result in stress. In the field, several of these stress factors interact with each other and result in complex physiological responses.

Responses to low air and soil temperatures

Evergreen conifers of the boreal forest have pronounced seasonal variations in their rates of carbon-dioxide assimilation, which generally decrease during autumn and early winter, depending on the severity and frequency of night frosts. Winter inhibition of photosynthesis involves inhibition of photochemical and enzymatic reactions at the chloroplast level, which depresses the photosynthetic potential of the plant. In addition to low and subfreezing temperatures, winter inhibition of photosynthesis is affected by the light regime to which the needles are exposed, indicating the importance of excess light (photoinhibition). Winter inhibition of photosynthesis is also influenced by nitrogen availability, increases of which appear to decrease sensitivity to photoinhibition. The main recovery from winter inhibition coincides with rising temperatures during spring and early summer. However, low daytime air temperatures and freezing nights can adversely affect the recovery process. Furthermore, frozen soils and low root-zone temperatures may influence the recovery of photosynthesis during spring.

There are large variations in the extent and duration of soil frost within the boreal forest, depending on climatic factors, soil characteristics and the vegetation cover. Soil frost severely restricts or prevents the uptake of water by roots. In addition, sub-optimal root-zone temperatures affect the movement of water through soil and roots by increasing its viscosity and decreasing the permeability of roots to water. Low root-zone temperatures also decrease the metabolism and growth of roots. Thus, frozen soils and low root-zone temperatures may induce water deficits in the above-ground parts of evergreen conifers, especially when the evaporative demand is high during spring. However, severe water deficits in needles can be minimized by stomatal closure, which greatly reduces stomatal conductance to water vapour.

To evaluate the effects of soil frost and low soil temperatures on gas exchange of Scots pine, the extent and duration of soil frost, as well as the onset of soil warming, were manipulated in the field. The results showed that prolonged exposure to soil temperatures slightly above 0 °C during spring severely restricted the recovery of the light-saturated rate of net photosynthesis. Inhibition of net photosynthesis by low soil temperatures was related to both stomatal closure and effects on the biochemistry of photosynthesis, the relative importance of which appeared to vary during spring and early summer. Because silvicultural practices can have pronounced effects on the extent and duration of soil frost, further knowledge about the effects of low root-zone temperatures on carbon assimilation in evergreen conifers may be valuable in practical forestry. Such knowledge is also needed for predicting the consequences of expected increases in temperature in boreal forests due to global warming.

 

Strand, M., Lundmark, T., Söderbergh, I. and Mellander, P.-E. . Impacts  of seasonal air and soil temperatures on photosynthesis in Scots pine trees. Tree Physiology 22: 839-847.

 

Lundmark, T., Bergh, J., Strand, M. and Koppel, A. . Seasonal variation of maximum photochemical efficiency in boreal Norway spruce stands. Trees 13: 63-67.

 

Strand, M. . Effect of mineral nutrient content on oxygen exchange and chlorophyll a fluorescence in needles of Norway spruce. Tree Physiology 17: 221-230.

 

Strand, M. and Lundmark, T. . Recovery of photosynthesis in 1-year-old needles of unfertilized and fertilized Norway spruce [Picea abies (L.) Karst.] during spring. Tree Physiology 15: 151-158.

 

Strand, M. . Inhibition of photosynthesis in current-year needles of unfertilized and fertilized Norway spruce [Picea abies (L.) Karst.] during autumn and early winter. Trees 9: 332-340.