Growth of northern deciduous trees under increasing atmospheric humidity: possible mechanisms behind the growth retardation.

Increasing atmospheric humidity-a climate trend predicted for northern Europe-will reduce water flux through vegetation. Diminished transpirational water flux impacts various physiological processes, causing growth decline in deciduous trees. We propose, based on the results obtained from the long-term free air humidity manipulation experiment, concurrent mechanisms to explain the growth deceleration due to increases in relative air humidity. Reduced atmospheric evaporative demand diminishes nutrient uptake and leads to lower leaf nutritional status and to an unbalanced foliar phosphorus/nitrogen ratio (P:N), resulting in a decline in leaf photosynthetic capacity. Elevated relative humidity induces readjustment of foliar metabolism: disturbed N metabolism, accumulation of starch and changes in secondary metabolite contents probably impair both photosynthetic performance and growth. Increased carbohydrate content in the leaves suggests that sink strength of trees is reduced under elevated humidity. As a consequence of the stress, foliar development is hindered, observed at individual leaf or whole-tree foliage levels, lowering production potential of trees proportionally to their foliar area. Larger investments in stem xylem in relation to foliage cause an increase in the ratio of non-photosynthetic to photosynthetic tissues, leading to larger maintenance respiration costs determined by the volume of parenchymatous tissue. An increase in the proportion of living parenchyma cells in relation to dead xylem elements in sapwood additionally enhances respiration costs. Disproportionate changes in hydraulic versus stomatal conductance become a critical factor in the case of weather extremes, which limit canopy conductance and may induce dysfunction of the hydraulic system. Increasing environmental humidity creates favourable conditions for development of pathogens, increasing frequency of fungal damage. [ABSTRACT FROM AUTHOR]