Date of Award

Fall 1994

Project Type

Dissertation

Program or Major

Natural Resources

Degree Name

Doctor of Philosophy

First Advisor

John Aber

Abstract

The patterns and controls of fine root (i.e., non-woody roots generally $\le$ 0.5 mm and 1 mm in diameter for deciduous and coniferous species, respectively) turnover and tissue chemistry were assessed along artificial and natural nitrogen availability gradients within and among forest ecosystems. Fine root turnover rate and chemical composition were generally strongly correlated with nitrate availability indices. A pilot study using a new $\sp{15}$N tracer technique suggested that turnover rates increased with nitrate availability in a red pine (Pinus resinosa Ait.) stand and indicated that this turnover assessment technique may avoid the major limitations which hinder existing methods. Assessments of fine root total non-structural carbohydrate concentrations suggested that turnover rates also increased with nitrate availability across temperate coniferous and deciduous forest ecosystems.

Surveys of fine root chemistry (i.e., carbon-fraction and nitrogen concentrations) indicated that nitrogen concentrations generally increased while lignin (average of 48.8 $\pm$ 3.0 percent for all species and sites) remained relatively constant along nitrate availability gradients within red pine and red spruce (Picea rubens Sarg.)-balsam fir (Abies balsamea (L.) Miller) ecosystems, as well as among temperate coniferous and deciduous forest types. As a result, fine root lignin:N was inversely correlated with nitrate availability within and among forest ecosystems.

The results of these studies combined with the findings of previous assessments in these sites indicate that fine root and foliar production, turnover (for the foliage of coniferous species), and potential decomposition (via increases in fine root nitrogen concentrations and decreases in foliar lignin concentrations) rates are positively correlated within and among forest types. In turn, this direct relationship suggests that both the fine root and foliar systems exhibit positive carbon and nutrient cycling feedbacks with nitrogen availability in forest ecosystems.

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