Seasonal Photosynthetic Activity in the Crown Compartments of European Ash (Fraxinus excelsior)

Abstract

Leaves facing different directions (north, south, east, and west) receive differing levels of illumination, resulting in spatial differences in photosynthesis PN in the crowns of mature trees. We measured diurnal trends in PN for a semi-solitary European ash (Fraxinus excelsior) over spring, summer, and autumn and compared these data with leaf biometric traits and leaf area distribution. The highest light-saturated PN (PNmax) was to the south and west, and the lowest to the north. Likewise, intrinsic water use efficiency, defined as the ratio (PN/gS) of photosynthetic rate (PN) and stomatal conductance (gS), was also lowest to the north. The thickest leaves were found on the northern face and the thinnest in the south, suggesting differences in leaf anatomy may have contributed to differences in PN. The greatest leaf area was recorded in the southern crown quadrant, which contributed more than 50% of the tree’s accumulated PN. Our research emphasises the importance of choosing representative leaves for gas exchange measurements. In-depth studies into the spatial distribution of leaves and their traits will be necessary for accurate upscaling of leaf-level photosynthesis to whole tree and canopy levels.