Our 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. The ecophysiological approach also takes into account the structural and functional features of evergreen conifers in their responses to the environment. This is of great importance for their capacity to acclimate to prevailing environmental conditions. 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 plants* photosynthetic potential. In addition to low and subfreezing temperatures, winter inhibition is affected by the light regime to which the needles are exposed, indicating the importance of excess light in the photoinhibition of photosynthesis. 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 occurs in response to rising temperatures during spring and early summer. However, nocturnal frosts during the spring 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. Obviously, soil frost severely restricts or prevents the uptake ofwater by roots, but even above-zero, sub-optimal root-zone temperatures affect the movement of water through soil and roots by increasing its viscosity and the root resistance to its flow. Furthermore, low root-zone temperatures directly affect the metabolism and growth of roots. 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. Severe water deficits in needles can be minimized by stomatal closure, which greatly reduces stomatal conductance to water vapour. Furthermore, inevitable minor losses of water from the needles can be replaced by water stored in unfrozen stems, especially in larger trees. However, it is clear that low stomatal conductance represents a variable limitation to carbon-dioxide assimilation in evergreen conifers during the spring, depending on the extent and duration of soil frost and low root-zone temperatures. In addition to stomatal limitations, non-stomatal constraints to carbon-dioxide assimilation may occur. Acclimation to the extent and duration of soil frost and low root-zone temperatures may involve alterations affecting the resistance of roots to water flow and changes in the relative allocation of carbon between above-ground and below-ground plant parts.
The goal of our research is to better understand how soil frost and sub-optimal root-zone temperatures affect carbon-dioxide assimilation in evergreen conifers, especially during spring. Because silvicultural practices can have pronounced effects on the extent and duration of soil frost, deeper knowledge about the effects of low root-zone temperatures may be very 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. The research is being carried out as part of the ”Forest Management and Wood Products” programme at Vindeln Experimental Forests, SLU.
Selected publications:
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.
Strand, M. & 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. () Effect of mineral nutrient content on oxygen exchange and chlorophyll a fluorescence in needles of Norway spruce. Tree Physiology 17, 221-230.
Lundmark, T., Bergh, J., Strand, M. & Koppel, A. () Seasonal variation of maximum photochemical efficiency in boreal Norway spruce stands. Trees 13, 63-67.
Westin, J., Sundblad, L.-G., Strand M. & Hällgren, J.-E. () Apical mitotic activity and growth in clones of Norway spruce in relation to cold hardiness. Canadian Journal of Forest Research 29, 40-46.
