Within-crown acclimation of leaf-level physiological and morphological parameters in young loblolly pine stands

Abstract

Whether the plasticity of foliage within the crown is driven by light or height is still not well known, especially for small trees. The present study was conducted in two loblolly pine (Pinus taeda L.) stands growing on two sites, during the third year after planting. The following traits were investigated in the lower and upper crowns of the same individuals twice per season (spring, summer, and autumn): photosynthesis (Asat), stomatal conductance (gs), intrinsic water use efficiency (WUEint), nitrogen concentration (N), specific leaf area (SLA), photosynthetic nitrogen use efficiency (PNUE), fascicle diameter, and needles per fascicle. The trees were open-grown at the beginning of the study; thus, any vertical gradient of the traits was not assumed to be driven by light. Overall, Asat, gs, PNUE, and SLA tended to be higher in the lower than in the upper crown, whereas N, fascicle diameter, and needles per fascicle were higher in the upper than in the lower crown. However, although all traits were affected by crown position, the magnitude of this effect differed by season. The fact that crown position affected most of the traits in spring and summer, but not in autumn, suggest that height might be the main driver of foliage acclimation. However, considering that crown position affected SLA in autumn but not the photosynthetic rate or N concentration, it is possible that foliar acclimation of these traits was mainly driven by light in this season.