Date of Award


Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)


Forest Resources


Michael S. Greenwood

Second Committee Member

Michael E. Day

Third Committee Member

Christa R. Schwintzer


Many species of trees undergo predictable age-related changes in foliar morphology and physiology. Age-related declines in photosynthetic rates, which may lead to decreases in productivity, have been described for numerous species. However, the physiological basis for these phenomena is unclear, as are linkages between agerelated trends in morphology/anatomy and physiology. Photosynthetic capacity in red spruce (Picea rubens Sarg.) may result from increased mesophyll resistance to CO2 uptake in older trees. Additional studies with other species imply that the foliage of older trees may have a lower ratio of photosynthetic to non-photosynthetic tissue and a larger proportion of xylem to leaf area to compensate for increased water stress. To better understand these linkages, we investigated the age-related trends in foliar anatomy for juvenile, mid-age, and old (mean ages 3-, ~12, ~53, and ~127-yearold) red spruce trees growing in a multi-cohort stand in Maine. In addition, a series of reciprocal grafts between age classes were made in order to distinguish between the influences of internal aging factors and external or environmental factors while minimizing the confounding effects of tree size and complexity. For natural foliage, six trees of each age class were randomly selected and photosynthesis measurements were taken on three shoots per tree. One needle from each shoot was collected and sectioned for anatomical observation. Needle cross-sectional area, width, height, perimeter, vascular bundle area, xylem and phloem area, and tracheid lumen diameters were described for all age classes. Internal air space was determined by a displacement technique for three separate shoots from each tree. Needles from older red spruce were wider and more massive. Relative to juvenile trees, needle cross-sectional area was roughly 2x greater for mid-age and old trees, and the vascular bundle cross-sectional area was approximately 3x larger. Although massbased photosynthetic rates declined with tree age, the proportion of photosynthetically active mesophyll tissue to non-photosynthetic tissue, such as vascular tissue and resin canals, increased from juvenile to old trees. However, the proportion of internal air space in needles decreased significantly with increasing age, with approximately a 20% decrease from juvenile to old trees. The combined influences of cross-sectional area and the smaller proportion of internal needle air space for old trees, indicating a more compact mesophyll layer, may contribute to increased gas exchange resistance in older trees. Also, a greater proportion of cross-sectional xylem area to perimeter (a surrogate for leaf area) was found in old trees and could be a mechanism for hydraulic compensation. The grafting study showed that both external and intrinsic genetic factors appeared to be influencing age-related changes in needle anatomy with increasing age in red spruce. While needle width increased with age regardless of rootstock age, photosynthetic rates were a function of the rootstock used. Many traits appeared to be regulated by a combination of extrinsic and intrinsic factors.