Effects of mild drought on the morphology of sun and shade needles in 20-year-old Norway spruce trees

Abstract: Several studies have looked at how individual environmental factors influence needle morphology in conifer trees, but interacting effects between drought and canopyposition have received little attention. In this study, we characterized morphological responses to experimentally induced drought stress in sun exposed and shaded current-year Norway spruce needles. In the drought plot trees were suffering mild drought stress, with an average soil water potential at 50 cm depth of -0.4 MPa. In general, morphological needle traits had greater values in sun needles in the upper canopythan in shaded needles in the lower canopy. Needle morphology 15 months after the onset of drought was determined by canopy position, as only sunneedle morphology was affected by drought. Thus, canopyposition was a stronger morphogenicfactor determining needle structure than was water availability. The largest influence of mild drought was observed for needle length, projected needle area and total needle area, which all were reduced by ~27% relative to control trees. Needle thickness and needle width showed contrasting sensitivity to drought, as drought only affected needle thickness (10% reduction). Needle dry mass, leaf mass per area and needle density were not affected 15 months after the onset of mild drought. Our results highlight the importance of considering canopyposition as well as water availability when comparing needle structure or function between conifer species. More knowledge about how different canopyparts of Norway spruce adapt to drought is important to understand forest productivityunder changing environmental conditions.