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Tree Nitrogen Physiology

by Nancy Falxa-Raymond

Fall 2010

Nitrogen deposition has been increasing throughout the world due mostly to fossil fuel emissions and agricultural fertilizer use.  These increases are expected to alter forest system dynamics through changes in plant physiology and productivity. In a system with chronic nitrogen deposition, biological retention of nitrogen becomes ineffective and net primary productivity decreases while nitrate leaching increases substantially. Excess nitrate in streams can have long-lasting effects on water quality and human health.

For my master’s thesis, I am studying nutrient cycling and associated physiological changes in several native tree species in response to the loss of oak trees and urbanization, two types of disturbance that affect soil nitrogen dynamics and nitrogen availability in forest ecosystems. In Black Rock Forest’s Future of Oak Forests research (see “Summer Research,” Fall 2010, p. 1), soil nitrate and ammonium levels have becone higher in the girdled stands than in the control plots. Previous studies have also found increases in nitrogen mineralization and nitrification in urban oak stands relative to rural stands.

I am measuring nitrogen content, nitrogen isotope ratios (to identify the source of nitrogen in a plant), and the activity of a key leaf enzyme, nitrate reductase (to determine how much nitrate the plant is taking up) in young black birch trees (Betula lenta) throughout the growing season. I expect to find higher nitrogen content and increased enzyme activity in the oak-girdled plots than in the control plots, demonstrating the ability of black birch to utilize the increased amounts of nitrate available in the soil.

 I am also collaborating with the New York City Department of Parks and Recreation, a Consortium member, to study the nitrogen use of several native tree species in five urban parks that have been reforested as part of the MillionTreesNYC initiative. I expect to find higher nitrogen content and enzyme activity in the trees at the urban sites than at Black Rock Forest.  I will explore whether some species are better able to take advantage of the available nitrate through increased enzyme activity.

Modified nitrogen use by regenerating trees may have cascading, ecosystem-level effects on an entire forest. I hope to elucidate the physiological response of trees to changes in nitrogen cycling. This will have implications for carbon sequestration, water quality, and species composition, all of concern for urban and rural ecosystem management.n

Nancy Falxa-Raymond is an MA candidate in Columbia University’s Department of Ecology, Evolution, and Environmental Biology.