Date of Award

Spring 2013

Project Type

Dissertation

Program or Major

Earth and Environmental Sciences

Degree Name

Doctor of Philosophy

First Advisor

Erik A Hobbie

Abstract

Understanding the interactions between global change, human and natural disturbances, and other factors on biogeochemical processes in forests is necessary to ensure the sustainability of forest management. Here I report the results of several investigations into nutrient acquisition processes in the forests of New Hampshire. I begin with a meta-analysis of fertilization studies showing that phosphorus (P) and calcium (Ca) as well as nitrogen (N) may limit primary production in deciduous forests of the region. Because these limiting nutrients are all removed from the ecosystem when trees are harvested, I compared nutrient budgets under a range of harvesting scenarios with a variety of soil nutrient stocks across a range of forest stands. I found that depletion of even long-term P and Ca soil stocks may occur over only a few rotations if intensive harvesting occurs in inappropriate stands.

Key to successfully managing such budgets is a better understanding of the processes by which trees access limiting nutrients in primary minerals such as apatite. I conducted a greenhouse and field study examining the potential for lead isotope ratios and rare earth elements to serve as tracers of apatite weathering by mycorrhizal fungi in soils. In the greenhouse experiment, both of these tracers showed clear effects of biological systems (mycorrhizal and non-mycorrhizal birch seedlings) on the weathering rates of trace minerals including apatite. However, there were no clear trends in these tracers when examined in mycorrhizal sporocarps collected from forest stands that we hypothesized would differ in allocation to mycorrhizal weathering.

If weathering can balance harvest losses of P and Ca, and N deposition continues its recent decline, N availability may constrain future productivity. I developed a novel tracer experiment intended to confirm earlier reports of the uptake of organic N compounds in temperate forests, which are richer in inorganic N than systems where this process is clearly demonstrated. I found low (≤16%) but significant contributions of organic compounds to the N nutrition of trees across a range of New Hampshire forest types. This research adds to our understanding of how forest ecosystems will respond to management and global change over the long term.

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