A Physiological Examination of the Age-related Decline in Photosynthesis in Picea rubens

Numerous conifer species undergo predictable age-related changes in productivity, photosynthesis and foliar morphology and anatomy. While these phenomena have been demonstrated for many species, the physiological mechanisms controlling them are not well understood. In order to better understand this issue, we examined four possible controls of the age-related decline in photosynthesis in red spruce: stomatal limitation, a decline in investment into photosynthetic capacity, nutrient limitations and a demand-side decline in sink: source relations. We investigated these age-related trends in physiology for juvenile, mid-age and old (mean age ~13, ~54 and ~128 years old) red spruce trees in a multi-cohort stand in Maine. In order to examine stomatal limitations, we examined the diurnal trends in gas exchange parameters, whole tree conductivity and stable carbon isotopes. Photosynthetic capacity parameters (Vcmax and Jmax) and the amount of chlorophyll were examined to evaluate investment in photosynthetic capacity. Amounts of biologically important foliar nutrients (N, P and K) were analyzed to rule out any age-related nutrient deficiencies. Finally, the amount of total non-structural carbohydrates (NSC) were examined in order to better understand the supply and demand of photosynthetic end-product for each age class. All gas exchange parameters were measured in situ, on fully-expanded, current year foliage from the top 1/3 of the canopy using a LI-6400 gas exchange system. The transition from juvenile (sub-canopy) to mid-age (emergent) trees is controlled by increased stomatal limitation and decreased photosynthetic capacity. This was demonstrated by decreases in diurnal trends of gas exchange parameters, photosynthetic capacity parameters (Vcmax and Jmax) and chlorophyll between juvenile and older (mid-age and old) trees. Juvenile trees appear to be operating under a “go for broke strategy” to adapt to intraspecific and interspecific competition. The nutrient limitation hypothesis does not hold true, as there are no age-related difference in foliar nutrient quantities. The transition from mid-age to old trees is marked by a change from source (supply) control to sink (demand) control. Analysis of NSC demonstrates a build-up of photosynthate in old trees, indicating that older, established trees may be using a more conservative life-strategy. As a result, we conclude that the decrease in productivity in old P. rubens is not due to decreased photosynthesis, but to a decreased demand for new carbon compounds. Chapter 2 attempts to distinguish between maturational influences and those due to environmental factors, factors external to the meristem by minimizing the confounding factors of tree size and complexity. Growth and gas exchange measurements were performed on reciprocal grafts in the summer of 2004 using field-grown mid-age and old rootstock and potted juvenile rootstock. Results indicate that factors external to the meristem appear to be the principle influence for age-related changes in photosynthesis and physiology in red spruce