Tree water transport mediating the changing environmental conditions to tree physiological processes

Abstract

Tree vascular tissues connect resource availability to tree physiological processes and growth. The xylem transports water from the soil up to the canopy of even 100-metre tall trees, whereas phloem transport connects the photosynthesis in leaves and the tree metabolic processes, including growth and tree defences against insect and pathogen attacks. Water deficit results in the closing of leaf stomata and decreasing photosynthetic production, as water and carbon dioxide are exchanged through the stomata between the leaf and ambient air. Phloem transport is driven by turgor pressure gradients generated by the interplay of phloem osmotic concentration and xylem water potential. Trees have adapted to local environmental conditions and they adjust to fast environmental changes with physiological responses. This thesis investigates tree physiological responses in vascular tissues, such as osmolality, water potential and stomatal conductance, to environmental conditions in two conifers: Scots pine and Norway spruce. Seasonality in soil temperature and soil water content affect soil-to-leaf hydraulic conductance, and stomatal conductance is connected to these seasonal patterns in water transport. Soil environment is thus mediated to tree functionality through tree water transport. This thesis also supports Münch’s theory that it is plausible to explain phloem transport in conifers in field conditions with osmotic gradients and gravity. Xylem water potential reflects to osmotic potential and turgor pressure of the inner bark by modifying tissue solute and water content. The turgor gradients hence seem to determine daily and seasonal carbon allocation patterns according to water availability. Pathogenic infections may introduce more rapid changes in tree hydraulic conductance through a decrease in xylem sap surface tension and xylem conductivity during massive invasions of bark beetles that vector blue-stain fungi such as Endoconidiophora polonica. These pest attacks weaken tree vitality and may also increase tree vulnerability to hydraulic failure in the xylem.