Foliar structural and physiological responses of young Picea abies trees to elevated environmental humidity

Climate change affects the physiology and growth of trees, as well as the functioning, health and productivity of forest ecosystems. This also concerns forests dominated by Norway spruce (Picea abies (L.) Karst.), one of the most important tree species in Europe, in both economic and ecological aspects. In the southern part of its range, the growth of spruce trees will be primarily constrained by limited soil water and high atmospheric vapor pressure deficit in the future. However, there is little information on how the trees will respond to increasing precipitation and the concomitant rise in environmental humidity, predicted for high latitudes. In this article, we evaluated the effects of increasing environmental humidity (soil vs atmospheric humidity) on needle morphology, water status and gas exchange in trees in a northern climate, aiming to understand the reasons for the growth decline in response to elevated air humidity revealed in our previous studies. A 15% increase in summer precipitation and the associated change in soil moisture do not considerably affect trees' performance in northern mesic conditions. Growing in increased air humidity (relative humidity elevated by ~5%) did not influence photosynthetic capacity, but it increased the light compensation point of photosynthesis (19.7 → 31.4 μmol m-2 s-1) and dark respiration rate (-0.216 → -0.297 μmol m-2 s-1) of the spruce trees. In light of climate change, both responses reduce photosynthetic output, increasing respiratory losses and reducing the growth potential of trees.