Age-related trends in red spruce foliar plasticity in relation to declining productivity
Michael S. Greenwood (1, 2), Margaret H. Ward (1, 3), Michael E. Day (1), Stephanie L. Adams (1, 4) and Barbara J. Bond (5)
1. School of Forest Resources, 5755 Nutting Hall, University of Maine, Orono, ME 04469-5755, USA / 2. Corresponding author () / 3. Present address: Institute for Ecosystem Studies, Box AB, Millbrook, NY 12545-0129, USA / 4. Natural Resources and Earth Systems Science Program, University of New Hampshire, Durham, NH 03824, USA / 5. Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA / Received May 31, 2007; accepted July 18, 2007; published online December 3, 2007
Summary
Phenotypic plasticity in needle morphology with increasing tree size and age was investigated by comparing four age classes
of red spruce (Picea rubens Sarg.) ranging from juvenile (3–12 years old) to mature (over 100 years old). With increase in tree age there were significant
increases in leaf mass per unit area (LMA), mesophyll and vascular bundle area as a percentage of total needle cross-sectional
area, and stomatal density. Within the vascular bundle, both xylem cross-sectional area and tracheid lumen area increased
significantly, whereas air space as a percentage of total cross-sectional area decreased. These morphological changes were
associated with a significant decrease in photosynthetic capacity and stomatal conductance, and an increase in 13C enrichment. Although both photosynthetic capacity and whole-tree conductance decreased significantly between age classes
3 and 12 years, they did not differ between age classes 53 and 127 years, even though needle 13C enrichment was significantly greater in the 127-year age class. Thus there appear to be compensatory mechanisms that maintain
photosynthetic capacity as trees increase in size and vascular complexity, which in red spruce and other species, may affect
leaf hydraulic conductance. Although increased LMA may contribute to reduced photosynthetic capacity in red spruce, similar
relationships are not seen in other conifers.
