Age-related changes in foliar morphology and physiology in red spruce and their influence on declining photosynthetic rates and productivity with tree age

The contribution of changes in meristem behavior to age-related decline in forest productivity is poorly understood. We studied age-related trends in needle morphology and gas exchange in a population of red spruce (Picea rub ens Sarg.) growing in a multi-cohort stand where trees ranged from first-ye ar germinants to trees over 150 years old, as well as in grafted scions fro m these trees. In the field study, age-related trends in foliar morphology were determined in six cohorts ranging in age from 2 to 120 years, and diff erences in gas exchange characteristics were compared between 60- and 120-y ear age classes. In a common-rootstock study, scions from trees representin g 20-, 60-, and 120-year cohorts were grafted onto juvenile rootstock and m aintained for three growing seasons, after which morphological and physiolo gical foliar attributes were evaluated. The field study revealed significant age-related trends in foliar morpholog y, including decreasing specific leaf area, and increasing needle width, pr ojected area, and width/length ratio. Similar trends were apparent in folia ge from the grafted scions. Both in situ foliage and shoots of grafted scio ns from the oldest cohort showed significantly lower photosynthetic rates t han their counterparts from younger trees; however, differences in stomatal conductance and internal CO2 concentrations were not significant. These re sults suggest that: (1) foliage of red spruce exhibits age-related trends i n both morphology and physiology; (2) age-related decreases in photosynthet ic rates contribute to declining productivity in old red spruce; (3) declin es in photosynthetic rates result from nonstomatal limitations; and (4) age -related changes in morphology and physiology are inherent in meristems and persist for at least 3 years in scions grafted to juvenile rootstock.