Factors Influencing Net Primary Production in Red Spruce
Management of forest resources, even when practiced at the scale of ecosystems, is implicitly based on managing photosynthetic acquisition of carbon by trees. This thesis examines several aspects of carbon balance at the scale of individual trees, using red spruce (Picea rubens Sarg.) as a model species. Age-related declines in net primary productivity have been described for many species, including red spruce. A potential explanation is that ontogenetic changes in meristems lead to lower
... ynthetic rates, which was confirmed here for red spruce. A grafting study used scions from juvenile, 6Oy, and 120~ trees to demonstrate that age-related morphological and physiological trends were retained in the grafts after 3 growing seasons, providing evidence that those age-related trends are inherent in meristems. In contrast with the stomatal limitations to gas exchange indicated by research on conifers in western North America, gas exchange results from both field populations and grafted scions suggested that age-related declines in photosynthetic rates are caused by other factors in red spruce. Indirect evidence hints at feedback limitations to photosynthesis from reduced sink strength for carbohydrates in old trees. A further study evaluated the effects of temperature (T) and leaf-to-air vapor pressure deficit (VFD) on net photosynthesis in sapling red spruce. VPD responses were investigated in a field population and were found to have significant influence on stomatal conductance and photosynthesis above a threshold of 2 kPa. As VPD and T are highly correlated, a subsequent study employed growth chambers to separate their influences. Those results demonstrated that red spruce has a substantially broader T range for maximum photosynthetic rates than has been previously reported, and the effects of T and VPD are additive. These findings have implications for understanding the response of red spruce to silvicultural treatments and climate change. The relative importance of potential environmental stresses and their influence on in situ photosynthesis were tested using data collected from micrometeorological stations in three contrasting silvicultural systems and a seasonally integrative carbon gain model, VPD proved to be the most significant limiting factor, followed by T. Integrated photosynthethic carbon gain was roughly equal in clearcut and shelterwood systems, but was light-limited in selection stands.