Date of Award

Spring 2000

Project Type

Dissertation

Program or Major

Natural Resources

Degree Name

Doctor of Philosophy

First Advisor

John D Aber

Abstract

This thesis stems from several ongoing efforts to characterize patterns of productivity and nitrogen cycling in northeastern US forests and to address the effects of nitrogen deposition, tropospheric ozone and rising atmospheric CO2. The work reported on involves two related projects; (1) an ecosystem model analysis that integrates physiological and biogeochemical processes with important environmental variables across the northeast region and (2) a field and remote sensing analysis that examines landscape-level patterns of forest biogeochemistry in the White Mountains of New Hampshire.

Chapter 1 presents a regional analysis of forest productivity using the PnET forest ecosystem model and discusses the relative importance of water, temperature and nitrogen on predicted spatial patterns. Chapter 2 integrates ozone effects on leaf-level carbon gain and describes interactions with canopy and stand-level processes. Using ambient ozone data from across the northeast region, the model predicted declines in annual forest production of between 3% and 16% and demonstrated an interaction with water availability whereby ozone damage declined during periods of drought. In chapter 3, a physiological response to CO2 is added to the model and applied with historical changes in N deposition and ozone. This analysis suggested that increased CO2 and N deposition have stimulated forest carbon uptake, but to different degrees following agriculture and timber harvesting. Further, the concurrent increases in ozone offset a large fraction of the predicted growth enhancement. This result is particularly relevant given the related spatial distributions of ozone and N deposition.

The final chapter presents a field study in the White Mountain National Forest that examines relationships between nitrogen cycling and foliar chemistry among forests of diverse history and composition. Across a wide range of conditions, foliar lignin:N ratios were correlated with soil C:N ratios, providing a means of assessing soil N status using hyperspectral remote sensing. Relationships between foliar chemistry and soil N transformations (mineralization and nitrification) were also observed, but these trends differed between historically disturbed versus undisturbed stands. Disturbed stands had significantly lower rates of mineralization and nitrification and higher soil C:N ratios than undisturbed stands, but these trends were not clearly reflected in stand-level foliar chemistry.

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