Effects of prolonged drought on the anatomy of sun and shade needles in young Norway spruce trees

Abstract

Predicted increases in the frequency and duration of drought are expected to negatively affect tree vitality, but we know little about how water shortage will influence needle anatomy and thereby the trees’ photosynthetic and hydraulic capacity. In this study, we evaluated anatomical changes in sun and shade needles of 20‐year‐old Norway spruce trees exposed to artificial drought stress. Canopy position was found to be important for needle structure, as sun needles had significantly higher values than shade needles for all anatomical traits (i.e., cross‐sectional needle area, number of tracheids in needle, needle hydraulic conductivity, and tracheid lumen area), except proportion of xylem area per cross‐sectional needle area. In sun needles, drought reduced all trait values by 10–40%, whereas in shade needles, only tracheid maximum diameter was reduced by drought. Due to the relatively weaker response of shade needles than sun needles in drought‐stressed trees, the difference between the two needle types was reduced by 25% in the drought‐stressed trees compared to the control trees. The observed changes in needle anatomy provide new understanding of how Norway spruce adapts to drought stress and may improve predictions of how forests will respond to global climate change.

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

“…This study follows up previous studies in the same experimental stand (see Gebauer et al 2011Gebauer et al , 2015 and we sampled needles from the same trees used in these studies. Further details about the study site and experimental design can be found in Gebauer et al (2011).…”

“…However, very few studies have focused on the structural response of Norway spruce needles to drought, even though needles are the key organ for photosynthesis and transpiration. One of the few existing studies on Norway spruce needles found that drought reduced needle tracheid dimensions, needle cross-sectional area, and needle xylem area (Gebauer et al 2015). In another study, drought was found to increase the area of the needle vascular cylinder (Kivimäenpää et al 2003), but Gebauer et al (2015) observed negligible changes in this trait.…”

“…A few studies have described the interacting effects of drought and light intensity on seedling growth (Holmgren 2000, Aranda et al 2005. However, to our knowledge, studies of interacting effects of drought and light intensity (i.e., canopy position) are limited to our recent study on needle xylem/phloem area, needle cross-sectional area and tracheid dimensions in Norway spruce (Gebauer et al 2015). Increased knowledge about needle development in response to interacting effects of drought and canopy position is crucial to understand forest productivity and for ecosystem modelling (Niinemets & Kull 1995).…”

“…Photosynthetically active radiation (PAR) displays steep vertical gradients within Norway spruce crowns (Kučera et al 2002), and this favors the differentiation of typical sun and shade adapted needles (Spunda et al 1998). Relative to shade adapted needles, sun needles are generally shorter (Kubínová et al 2018) and have higher density (Niinemets & Kull 1995), more tracheids (Gebauer et al 2015), larger xylem area (Gebauer et al 2015), higher leaf mass per area (Niinemets & Kull 1995), greater surface area of mesophyll (Lhotáková et al 2012) and rounder crosssections (Kubínová et al 2018). Under natural conditions drought episodes often increase light availability inside the canopy, because drought-stressed trees often shed some foliage or whole branches die off.…”

Within‐crown plasticity in leaf traits among the tallest conifers

“…Within‐crown variation in water and light availability induces leaf‐level responses in conifers that reflect a combination of biophysical constraints on leaf growth and functional traits supporting both water stress tolerance and photosynthetic efficiency (Niinemets and Kull, ,; Niinemets, ; Cescatti and Zorer, ; Ishii et al., ; Ambrose et al., ; Gebauer et al., , , ). In all tall conifers studied to date, leaf length decreases while leaf mass per projected area (LMA) increases with height.…”