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theses

Trace element (TE) pollution of soil is a pervasive global problem which affects both human health and ecosystem function. However there is a lack of mechanistic understanding in the ways TE effects on individual organisms ultimately alter ecosystem function. The goal of this dissertation was to explore the effects of TE contamination on primary productivity in a hardwood forest which spontaneously established in an urban brownfield. Given the age of the site, the study first compared a set of measurements made on soil data collected at the site over the course of 20 years. This analysis revealed that pseudo-total concentrations of copper, lead, and zinc in the soil remained fairly stable in this time period. However between 2005 and 2015 concentrations of arsenic and chromium increased. Next, the study measured photosynthesis rates and other related leaf level biophysical parameters over the course of two growing seasons in Betula populifolia which were growing in plots with low or high TE concentrations (trees were at least 10 years old). The maximum carboxylation rate and electron transport rate of trees growing in high TE plots was significantly lower than those in low TE plots during July 2014 and May 2015. TE alone was not a significant predictor of photosynthesis parameters. These findings suggest TE effects on photosynthesis apparatus in these trees may transient and seasonal in nature and that photosynthesis is fairly robust along the gradient of TE contamination at the research site. In the third study, leaf area index (LAI) measured over the course of seven years was compared between two low and two high TE plots within the study site. In the first three years of LAI measurements, one high TE plot consistently had the highest LAI while the second high TE plot had the lowest LAI. The LAI results suggest that other factors such as soil nutrient availability, facilitative mycorrhizal interactions, stand age and plot history may also be important drivers of canopy productivity in addition to TE stress. These studies, taken together with other research conducted at the site, highlight the challenge of developing a mechanistic understanding of TE impact on hardwood primary productivity. TE may play a more important role earlier in assemblage development by acting as an abiotic filter on species establishment, though more work is needed to confirm this hypothesis. These findings also demonstrate the potential of such ecosystems to function in spite of severe abiotic stress.

ETD_8011

Ph.D.

Includes bibliographical references

by Allyson Salisbury

doi:10.7282/T35T3PC8

ETD doctoral

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