Hydraulic and stomatal adjustment of Norway spruce trees to environmental stress

A study of how the water conducting systems of 30-50-year-old Norway spruce (Picea abies (L.) Karst.) trees growing at three sites adjust to shade and waterlogging indicated that water relations characteristics varied with the life histories of the trees. Xylem was more efficient at conducting water and stomata were more sensitive to atmospheric evaporative demand in trees subjected to favorable growth conditions (control trees) than in trees growing in shade or waterlogged conditions. At the same soil water availability, shade-grown trees suffered more severely from water deficit than control trees. Under conditions of high atmospheric vapor pressure deficit, foliage of shade-grown trees exhibited low water potentials, as a result of low hydraulic conductance of the vascular system and inefficient stomatal control. Because of the increased internal resistance to water flow, more negative leaf water potentials (Psi(x)) must be reached to provide an adequate water supply to the foliage. It is concluded that dynamic water stress is one of the main causes of the continuing growth retardation in suppressed Norway spruce trees after their release from the overstory. Trees growing in waterlogged soil (bog-grown trees) were characterized by weak stomatal control, resulting in large water losses from the foliage. Although bog-grown trees exhibited uneconomical water use, they possessed mechanisms (e.g., osmotic adjustment) that allowed leaves to tolerate low Psi(x) while stomata remained open. Under conditions of sufficient soil water availability and moderate atmospheric vapor pressure deficit, soil-to-leaf conductance was highest in bog-grown trees (1.45 +/- 0.06 mmol m(-2) s(-1) MPa(-1)), followed by control and shade-grown trees (1.04 +/- 0.04 and 0.77 +/- 0.05 mmol m(-2) s(-1) MPa(-1), respectively). The lowest soil-to-leaf conductance (0.45 +/- 0.04 mmol m(-2) s(-1) MPa(-1)) was recorded in control trees at high atmospheric evaporative demand, and was probably caused by tracheid cavitation.