Document: MIC-3-35-9

Long-term maturational changes in foliar morphology and physiology in red spruce in relation to declining net primary productivity with tree age.

DAY, M.E.*, M.S.GREENWOOD and A.S.WHITE

University of Maine, Orono, Maine 04469 USA ¹

Abstract:
The contribution of genetically-determined maturation to age-related decline in forest productivity is poorly understood. While changes in morphological and physiological attributes associated with transition from juvenile to reproductively-mature phases have been described for many woody species, few studies have examined maturational changes that occur after the onset of reproductive development. We studied needle morphology and gas exchange in a population of red spruce (Picea rubens (Sarg.)) growing in a multicohort stand where ages ranged from germinants to over 150 years. Both sun- and shade-adapted foliage were collected from cohorts with mean ages of 2, 5, 10, 20, 45, and 120 years, and foliar morphology was evaluated by image analysis. Differences in gas exchange characteristics were compared among 20, 45, and 120 year age-classes in a common-rootstock experiment, where grafting upper crown scions onto 3-year-old rootstock removed the influence of tree size. Grafted trees were maintained for three growing seasons in a common environment before measurement. Both sun- and shade-adapted foliage sampled from in situ trees showed rapid increase in mean needle size between 2 and 20 years of age, followed by a more gradual increase. Grafted scions showed a similar trend in needle size with age. Similar trends were observed for both shade and sun foliage. Grafted scions showed decline with age-class in MNA and needle packing, but not SLM. Stomatal conductance in full sunlight declined from a mean of 158 mmol m^-2 s^-1 in 20-year-old scions to 119 and 45 mmol m^-2 s^-1 in 45- and 120-year-old scions, respectively. Mean maximum photosynthetic rates decreased slightly (nonsignificant) between 20- and 45-year-old scions, but declined by about 50% between younger and 120-year-old scions. Apparent quantum efficiency and dark respiration were also significantly lower in the oldest age-class. These results suggest that morphological and physiological changes continue long after the onset of reproductive maturity in red spruce. Additionally, the common-rootstock study implies that productivity in older red spruce may be limited, at least in part, by developmental changes that influence gas exchange and, therefore, may have a genetic basis.

Keywords: maturation, age-related decline in NPP, red spruce, gas exchange, foliar morphology

This abstract is being presented at: 9:30 AM in session:
Oral Session #1: Plant Carbon Allocation.