Ecophysiological Explanations For Spatial And Temporal Variations In Autumnal Coloration Within The Canopies Of Ornamental Red Maple (Acer Rubrum ‘Armstrong’) And Freeman Maple (Acer X. Freemanii) Trees

Some urban red maples (Freeman maples, Acer x freemanii) turn red at the top of the canopy before the bottom, while others turn simultaneously all over (A. rubrum ‘Armstrong’). We compared leaves from the top and bottom of the canopy to see if coloration differences arise from environmental or physiological differences, or both. Mid-summer chlorophyll content in Freeman maples was higher in lower leaves, and in fall, anthocyanins accumulated earlier and to greater amounts in upper leaves, whereas no such differences occurred in Armstrong maples. Photosynthetic rates at saturating PAR and nitrogen contents were higher in lower leaves of Freeman maples but did not differ in Armstrong maples. Earlier and greater accumulation of anthocyanins in upper leaves correlated with their lower nitrogen content, a factor known to elevate leaf anthocyanin content. Lower nitrogen in upper leaves could be the proximate driver for early anthocyanin synthesis in upper leaves, but we do not know the ecophysiological reasons for why this occurs in Freeman, and not in Armstrong maples. The results of this study provide insight into intra-canopy variation in leaf ecophysiology of open-grown trees in an urban environment.