Ecological Studies, Vol. 166 Analysis and Synthesis

Edited by

I.T. Baldwin, Jena, Germany M.M. Caldwell, Logan, USA G. Heldmaier, Marburg, Germany O.L. Lange, Wiirzburg, Germany H.A. Mooney, Stanford, USA E.-D. Schulze, Jena, Germany U. Sommer, Kie1, Germany

Ecological Studies

Volumes published since 1996 are listed at the end of this book.

Springer Science+Business Media, LLC

Paul J. Hanson Stan D. Wullschleger Editors

North American Temperate Deciduous Forest Responses to Changing Precipitation Regimes

Foreword by Jerry W. Elwood

With 129 Figures, 4 in Full Color

, Springer

Paul J. Hanson Environmental Sciences Division Oak Ridge National Laboratory Bethel Valley Road, Building 1059 P.O. Box 2008 Oak Ridge, TN 37831-6422 USA hansonpj@oml.gov

Stan D. Wullschleger Environmental Sciences Division Oak Ridge National Laboratory Bethel Valley Road, Building 1059 P.O. Box 2008 Oak Ridge, TN 37831-6422 USA wullschlegsd@oml.gov

Cover illustration: Designed by LeJean Hardin of the Oak Ridge National Laboratory's Creative Media Services Division, Oak Ridge, Tennessee.

Library of Congress Cataloging-in-Publication Data

North America temperate deciduous forest responses to changing precipitation regimes / editors, Paul 1. Hanson, Stan D. Wullschleger.

p. cm. - (Ecological studies ; 166) Includes bibliographical references (p. ). ISBN 978-1-4612-6506-1 ISBN 978-1-4613-0021-2 (eBook) DOI 10.1007/978-1-4613-0021-2

1. Rain and rainfall-North America. 2. Forest ecology-North America. 3. Precipitation variability-North America. 4. Climatic changes-North America. I. Hanson, Paul J. (Paul James) 11. Wullschleger, Stan D. III. Ecological studies ; v. 166.

SD390.7.R34N672003 577.3'097-dc21

ISSN 0070-8356 ISBN 978-1-4612-6506-1 Printed on acid-free paper.

© 2003 Springer Science+Business Media New Y ork Originally published by Springer-Verlag New York, Inc. in 2003 Softcover reprint of the hardcover I st edition 2003

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Foreword

Ecologists have long been active in conducting observational studies and exper­imental research aimed at better understanding the response of terrestrial ecosys­tems to changes in their chemical and physical environment, including changes in climate. Early studies that focused on primary and secondary response of plants to air pollution gave way to studies that dealt with acidic precipitation, that in turn gave way to studies on pressing concerns of global change. Understanding the effects of increases in atmospheric CO2 concentration, global warming, and regional changes in precipitation offers a unique set of challenges for the scientific community.

The interagency United States Global Change Research Program (USGCRP), established in 1989 and authorized by Congress in the Global Change Research Act of 1990, has been an advocate for research in these and other areas. A goal of the USGCRP is to understand and predict the consequences of global environmental change in terrestrial ecosystems. As a participating agency in the USGCRP, the Department of Energy (DOE) shares this goal through our Program for Ecosystem Research (PER) which seeks to improve the scientific basis for predicting and assessing effects of environmental changes on terrestrial ecosystems and their com­ponent organisms and processes. Specific goals of PER include understanding (1) how organisms within terrestrial ecosystems respond to environmental change, (2) how these responses are controlled, (3) how the responses and their underlying control mechanisms can be detected and measured, and (4) how responses of or-

v

vi Foreword

ganisms can be integrated across organizational levels from the results of studies of specific research organisms to ecosystems.

Given these many objectives, the DOE Office of Biological and Environmental Research in the Office of Science began funding the Throughfall Displacement Experiment (IDE) in 1992. The results were expected to provide an improved mechanistic understanding of how upland mixed deciduous forests respond to altered precipitation amount. The 26 chapters of this book demonstrate that many of our expectations have been fulfilled. They represent an important source of information regarding the resiliency and/or vulnerability of our nation's decidu­ous forests to long-term (10 years to date) altered precipitation amount. Results from the TDE are important not only from the perspective of understanding the many mechanisms by which temperate deciduous forests respond to changes in the amount of precipitation, but also provide new information and insights needed for predicting and assessing whole forest responses to possible future changes. Such is the opportunity afforded by long-term studies like the TDE that are con­ducted at ecosystem spatial scales. It remains a challenge to incorporate the data and conclusions reported herein into future national assessments of climate change impacts, and make them available for discussion within the policy arena. Comprehensive and long-term data, from the IDE and similar studies, present the scientific communities with a unique opportunity to judge the efficacy of ecosystem models that are ultimately useful for forecasting effects of possible future environmental changes on the structure and functioning of ecosystems.

Finally, operation of the IDE since 1992 has required a long-term commitment of resources, and periodic peer-review has been required to support this sustained research. The goal has been to operate such studies, with continuing input and guidance from the scientific community, so that potentially important, but slow­to-develop, ecosystem responses have time to express themselves and be detected. As the IDE progresses towards its completion over the next several years, some conclusions will be strengthened but other insights may need to be modified or abandoned in favor of alternative explanations. Such is the scientific process and such is the return that can only come from dedicated investments in long-term studies.

Jerry W. Elwood Director, Climate Change Research Division

Office of Biological and Environmental Research Office of Science

U.S. Department of Energy

Preface

The current volume is in many respects the next issue in a series of books by Springer-Verlag that summarize fundamental research on functional processes operating in eastern deciduous forested ecosystems. Analysis of Temperate Forest Ecosystems (Reichle 1970) provided a broad description of temperate forest eco­systems and their nutrient and hydrologic cycles. That book resulted mainly from research on the Eastern Deciduous Forest Biome as a part of the International Biological Programme. Four books, Biogeochemistry of a Forested Ecosystem (Likens et al. 1977), Pattern and Process in a Forested Ecosystem (Borman and Likens 1979), Forest Hydrology and Ecology at Coweeta (Swank and Crossley 1988), and Analysis of Biogeochemical Cycling Processes in Walker Branch Watershed (Johnson and Van Hook 1989), include long-term studies of watershed­scale processes, including forest-management perspectives. Atmospheric Depo­sition and Forest Nutrient Cycling (Johnson and Lindberg 1992), motivated by the acid rain issue of the 1980s, includes thorough descriptions of nutrient cycling processes and ozone exposure in a variety of forested ecosystems including south­ern and northern hardwood sites. The present book was based on a need to better understand deciduous forested ecosystem responses to potential climatic condi­tions of the future. We hope that this work, when combined with its predecessor volumes, will provide important information on mechanisms of eastern deciduous forest plant and ecosystem responses to altered precipitation patterns, and that this will enhance assessments of potential impacts of environmental change on forested ecosystems.

vii

viii Preface

Data from the Walker Branch Throughfall Displacement Experiment (TDE) detailed in this volume has been archived by the Carbon Dioxide Information and Analysis Center (CDIAC) and can be found at the following web address: http:// cdiac.esd.oml.gov/epubs/ndp/ndp078a/ndp078a.html. The archive includes data for site characterization, environment conditions, soil-water content, growth, sea­sonal phenology, plant physiology, soil respiration and decomposition, and nu­trient cycling necessary for future development and testing of ecosystem models of upland oak deciduous forests.

References

Paul J. Hanson Stan D. Wullschleger

Bonnan FH, Likens GE (1979) Pattern and process in a forested ecosystem. Springer-Verlag, New York. Likens GE, Bonnann FH, Pierce RS, Eaton JS, Johnson NM (1977) Biogeochemistry of a Forested Ecosystem.

Springer-Verlag, New York. Johnson DW, Lindberg SE Editors (1992) Atmospheric deposition and forest nutrient cycling. Springer-Verlag, New

York. Johnson DW, Van Hook RJ Editors (1989) Analysis of biogeochemical cycling processes in Walker Branch Water­

shed. Springer-Verlag, New York. Reichle DE Editor (1970) Analysis of temperate forest ecosystems. Springer-Verlag, New York. Swank WT, Crossley DA Jr. Editors(1988) Forest hydrology and ecology at Coweeta. Springer-Verlag, New York.

Acknowledgments

All research on the Walker Branch Watershed Throughfall Displacement Exper­iment (TDE) was sponsored in full or in part by the Office of Science, Biological and Environmental Research (BER) Program, U.S. Department of Energy (DOE) as a part of the Program for Ecosystem Research. Work conducted by Oak Ridge National Laboratory personnel was conducted under contract No. DE-AC05 OOOR22725 with UT-Battelle, LLC. The TDE is on the Oak Ridge National Environmental Research Park.

Many people contributed to the completion of this book. Frederick M. O'Hara, Jr., did an excellent job as technical editor of the volume. Not only did he carefully edit each chapter, but in some cases he suggested major revisions of sections. The authors sincerely appreciate his knowledge of science, his humor, and his tenacity in performing the many tasks needed to bring this book to pUblication.

We thank the following students, faculty participants, and co-workers for par­ticipating in the long-term collection of data used in several chapters of this book: Philip Allen, Steve Armes, James E. Arnett III, Stephanie Bohlman, Bridgette Boudreaux, Mogan Castner, Jason Fults, Kristin Harter, Michael Hayes, David Holt, Joshua Johnson, Sabina Madsen, Howard Marotto, Tracy Misek, Mark Scannell, Kevin Smith, James B. Stringfellow, Todd Tabberer, and Karen Voiles.

The authors of Chapter 5 thank Donald E. Todd for the extensive effort in­volved in sample collection and Ramie V. Wilkerson for excellent technical support.

ix

x Acknowledgments

Work on Chapter 7 was supported by The University of Tennessee Agricultural Experiment Station and by the DOE's BER, National Institute for Global Envi­ronmental Change (NIGEC), Southeast Regional Center under Cooperative Agreement No. DE-FC03-90ER6101O. Ann Stodola, Craig Green, Jenny Moore, Xiangrong Duan, Brendon Johnson, Dr. Arnold Saxton, Dr. Richard M. Evans, and Joshua Olinick were instrumental in accomplishing the work.

Support for G. Michael Gebre in the development of Chapter 8 was provided by an appointment to the Oak Ridge National Laboratory (ORNL) Postdoctoral Research Associates Program administered jointly by ORNL and the Oak Ridge Institute for Science and Education. The authors of Chapter 8 thank Donald E. Todd and Paul J. Hanson for the extensive effort involved in sample collection and Ramie V. Wilkerson for excellent technical support.

The authors of Chapter 9 thank Paul J. Hanson, Michael Huston, Donald E. Todd, and Scott Dolvin for assistance in obtaining samples from the TDE site and for assistance with sample processing. We thank the staff at the Biology Station of Chamela, Universidad Nacional Aut6noma de Mexico for assistance with our work at Chamela. Sandy McLaughlin provided assistance with the tree core sectioning.

The authors of Chapter 13 thank Patrice Cole, Caroline De Van, Leigh Thomas, and Don Todd who helped collect, process, and plant seeds and assisted in the construction and maintenance of the seed-planting experiment. Paul Hanson en­couraged our initiation of this project on the TDE, assisted in the development of our soil-water monitoring protocol, loaned us TDR probes and temperature dataloggers, and collated and summarized the temperature dataset. Travis Belote improved earlier drafts of the manuscript. Research on Chapter 13 was supported in part by the Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville.

The authors of Chapter 14 thank the following individuals for their facilitation of the collection of field data: D. Boldt, Danim Moe, Nilong R. Parikh, Jorge Quiles, Shawnette Warner, and Donald E. Todd.

The authors of Chapter 16 acknowledge the cooperation of the Public Power Institute of the Tennessee Valley Authority with Oak Ridge National Laboratory. They also thank Emily Childs, Brian Hale, Alan Mays, Jennifer Martin, Anita Rose, Larry Shelton, Brian Sunderland, Erich Webber, and numerous students for assistance in the collection and extraction of root samples and in the photograph­ing and analyzing of minirhizotron data. Thanks are also due to E.G. O'Neill, Ron Hendricks, and Kurt Pregitzer for supplying minirhizotron equipment, assis­tance, and advice.

The authors of Chapter 18 thank Kenny Peavy, Larry Wilson, and Sue Car­stensen for help in field sampling and/or laboratory analyses. George Cotsonis provided statistical advice and assistance in analyzing the data, and Chris Mowry assisted with the graphical presentation of the final results. Funding was provided by DOE's BER, NIGEC, Southeast Regional Center under Cooperative Agree­ment No. DE-FC03-90ER6101O.

Acknowledgments Xl

The authors of Chapter 19 thank Dr. James A Brenneman and James J. Wet­teroff, Jr. for their work in the field and laboratory, and Dr. Theodor D. Leininger as well for valuable contributions to study planning and preparation of an earlier manuscript. Work described in Chapter 19 was partially supported by the USDA Forest Service, Missouri Department of Conservation, Missouri Ozark Forest Ecosystem Project, and the Missouri Agricultural Experiment Station (Project PSSL 0112).

The author of Chapter 20 thanks the Associated Colleges of the Midwest for making possible his stay at ORNL as part of the resident faculty for the Oak Ridge Science Semester. Dr. Michael Draney identified problematic specimens of linyphiids and Dr. William Ehmann provided valuable input on an early version of this chapter.

Chapter 23 was sponsored in part by the Nevada Agricultural Experiment Sta­tion, publication number 52031032.

Work on Chapter 24 was also sponsored by the US. Environmental Protection Agency's National Center for Ecological Research (NCER) STAR program grant number R825 157-01 to the University of Tennessee.

Work associated with Chapter 26 was supported by the DOE's BER, NIGEC Southeast Regional Center. Dan Tinker, Jerry Winslow, and Sharon Stewart are thanked for assistance in developing the BGC ++ model.

Paul J. Hanson Stan D. Wullschleger

Foreword, by Jerry W. Elwood Preface Acknowledgments Contributors

Section 1. Introduction

1. Introduction

Contents

Paul J. Hanson and Frederick M. O'Hara, Jr.

V

Vll

ix xvii

3

2. Walker Branch ThroughfaU Displacement Experiment 8 Paul J. Hanson, Michael A. Huston, and Donald E. Todd

Section 2. Carbon-Cycle Processes

3. Deciduous Hardwood Photosynthesis: Species Differences, Temporal Patterns, and Responses to Soil-Water Deficits 35 Kell B. Wilson and Paul J. Hanson

4. Aboveground Autotrophic Respiration 48 Nelson T. Edwards and Paul J. Hanson

xiii

xiv Contents

5. Dormant-Season Nonstructural Carbohydrate Storage 67 Timothy J. Tschaplinski and Paul J. Hanson

Section 3. Water-Cycle Processes

6. Sensitivity of Sapling and Mature-Tree Water Use to Altered Precipitation Regimes Stan D. Wullschleger and Paul J. Hanson

7. Stomatal Behavior of Forest Trees in Relation to

87

Hydraulic, Chemical, and Environmental Factors 100 Robert M. Auge

8. Leaf Water Potential, Osmotic Potential, and Solute Accumulation of Several Hardwood Species as Affected by Manipulation of Throughfall Precipitation in an Upland Quercus Forest 121 Timothy J. Tschaplinski and G. Michael Gebre

9. 180 and 13C in Leaf Litter Versus Tree-Ring Cellulose as Proxy Isotopic Indicators of Climate Change 140 Lee W. Cooper and Corina Solis

Section 4. Decomposition and Soil Carbon Turnover

10. Soil Respiration and Litter Decomposition 163 Paul J. Hanson, Elizabeth G. O'Neill, M. Lala S. Chambers, Jeffery S. Riggs, 1. Devereaux Joslin, and Mark H. Wolfe

11. Soil Carbon Turnover 190 Julia B. Gaudinski and Susan E. Trumbore

12. Rates of Coarse-Wood Decomposition Donald E. Todd and Paul J. Hanson

Section 5. Plant Growth and Mortality

13. Tree Seedling Recruitment in a Temperate Deciduous Forest: Interactive Effects of Soil Moisture, Light, and

210

Slope Position 217 Jake F. Weltzin and Philip B. Allen

14. Response of Understory Tree Seedling Populations to Spatiotemporal Variation in Soil Moisture 227 Michael A. Huston, Milena Holmgren, and Michelle B. Kreh

Contents xv

15. Tree and Sapling Growth and Mortality 255 Paul J. Hanson, Donald E. Todd, Darrell C. West, Nelson T. Edwards, M. Lynn Tharp, and William A. Simpson, Jr.

16. Fine Root Growth Response 274 J. Devereux Joslin and Mark H. Wolfe

17. Canopy Production 303 Paul J. Hanson, Donald E. Todd, and J. Devereux Joslin

Section 6. Response of Other Organisms

18. Foliar Chemistry and Herbivory 319 Donald J. Shure, Peter D. Mooreside, Rebekah E. Chapman, and Allan D. Wilson

19. Opportunistically Pathogenic Root Rot Fungi: Armillaria Species 337 Johann N. Bruhn and Jeanne D. Mihail

20. The Influence of Precipitation Change on Spiders as Top Predators in the Detrital Community 347 Kenneth L. Cramer

Section 7. Forest Stand-Level Syntheses

21. Forest Water Use and the Influence of Precipitation Change 363 Stan D. Wullschleger, Paul J. Hanson, and Donald E. Todd

22. Estimating the Net Primary and Net Ecosystem Production of a Southeastern Upland Quercus Forest from an 8-Year Biometric Record 378 Paul J. Hanson, Nelson T. Edwards, Timothy J. Tschaplinski, Stan D. Wullschleger, and J. Devereux Joslin

23. Nutrient Availability and Cycling 396 Dale W. Johnson, Paul J. Hanson, and Donald E. Todd

Section 8. Extrapolations

24. Long-Term Forest Dynamics and Tree Growth at the TDE Site on Walker Branch Watershed 417 Michael A. Huston, Donald E. Todd, and D. Gregory Barlar

xvi Contents

25. Simulated Patterns of Forest Succession and Productivity as a Consequence of Altered Precipitation 433 Stan D. Wullschleger, Carla A. Gunderson, M. Lynn Tharp, Darrell C. West, and Wilfred M. Post

26. Regional Implications of the Throughfall Displacement Experiment on Forest Productivity 447 E. Raymond Hunt, Jr.

Appendix. List of Scientific and Common Species Names 461

Index 465

Philip B. Allen

Robert M. Auge

D. Gregory Barlar

Johann N. Bruhn

Contributors

Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996-1610, USA. E-mail: pallen 1 @utk.edu

Department of Plant Sciences and Landscape Systems, University of Tennessee, Knoxville, TN 37901-1071, USA. E-mail: auge@utk.edu

Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, USA.

Department of Plant Microbiology and Pathology, University of Missouri­Columbia, Columbia, MO 65211, USA. E-mail: BruhnJ@missouri.edu

xvii

xviii

M. Lala Chambers

Rebekah E. Chapman

Lee W. Cooper

Kenneth L. Cramer

Nelson T. Edwards

Jerry W. Elwood

Julia B. Gaudinski

G. Michael Gebre

Carla A. Gunderson

Contributors

Idaho National Engineering and Environmental Laboratory, Ecological and Cultural Resources Group, Idaho Falls, ID 83415-2213, USA. E-mail: CHAMML@inel.gov

Institute of Ecology, University of Georgia, Athens, GA 30602, USA. E-mail: rebekahc@arches.uga.edu

Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, USA. E-mail: lcooperl @utk.edu

Department of Biology, Monmouth College, Monmouth, IL 61462, USA. E-mail: kenc@monm.edu

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. E-mail: edwardsnt@ omI.gov

Environmental Sciences Division, ER-74, U.S. Department of Energy, Germantown, MD 20874, USA. E-mail: Jerry.Elwood@science.doe.gov

Department of Integrative Biology, University of California Berkeley, Berkeley, CA 94720, USA. E-mail: gaudinsk@socrates.berkeley.edu

Environmetal Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

Environmetal Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422, USA. E-mail: gundersonca@ornl.gov

Paul J. Hanson

Milena Holmgren

E. Raymond Hunt, Jr.

Michael A. Huston

Dale W. Johnson

J. Devereux Joslin

Michelle B. Kreh

Jeanne D. Mihail

Contributors xix

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422, USA. E-mail: hansonpj@ornl.gov

Forest Ecology and Forest Management Group, Department of Environmental Sciences, Wageningen University, PO Box 342, 6700 AH Wageningen, The Netherlands. E-mail: Milena.Holmgren@ btbo.bosb. wau.nl

USDA Agricultural Research Service Hydrology and Remote Sensing Laboratory, Building 007, Room 104, BARC-West, 10300 Baltimore Ave., Beltsville, MD 20705, USA. E-mail: erhunt@hydrolab.arsusda.gov

Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, USA. E-mail: hustonma@ornl.gov.

Environmental and Resource Sciences, Fleischmann Ag Building, MS 370, University of Nevada, Reno; Reno, NV 89506, USA. E-mail: dwj@um.edu

Belowground Forest Research, 112 Newcrest Lane, Oak Ridge, TN 37830, USA, and, Public Power Institute, Tennessee Valley Authority, LAB lC-N, Pine St., Norris, TN 37828-0920, USA. E-mail: jdjoslin@esper.com

Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, USA.

Department of Plant Microbiology and Pathology, University of Missouri­Columbia, Columbia, MO 65211, USA.

xx

Peter D. Mooreside

Frederick M. O'Hara, Jr.

Elizabeth G. O'Neill

Wilfred M. Post

Jeffery S. Riggs

Donald J. Shure

William A. Simpson, Jr.

Corina Solis

M. Lynn Tharp

Contributors

The Nature Conservancy, Fort Indiantown Gap Office, Environmental Unit, Building 11-19, Annville, PA 17003, USA. E-mail: pmooreside@ tnc.org

Consultant in Technical Communication, P.O. Box 4273, Oak Ridge, TN 37831, USA. E-mail: fohara@fohara.com

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. E-mail: postwrniii@ oml.gov

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. E-mail: riggsjs@ oml.gov

Biology Department, Emory University, Atlanta, GA 30322, USA. E-mail: dshure@biology.emory.edu

Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. E-mail: simpsonwajrl@ oml.gov

Instituo de Ffsica, Universidad Nacional Autonoma de Mexico, A.P. 20634, Mexico, D.E 01000. E-mail: corina@ fenix.ifiscacu.unam.ffiX

Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. E-mail: tharprnl@oml.gov

Donald E. Todd

Susan E. Trumbore

Timothy J. Tschaplinski

Jake F. Weltzin

Darrell C. West

Allan D. Wilson

Kell B. Wilson

Mark H. Wolfe

Stan D. Wullschleger

Contributors xxi

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. E-mail: toddejr@ ornl.gov

Department of Earth System Science, University of California-Irvine, Irvine, CA 92697, USA. E-mail: setrumbo@ uci.edu

Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. E-mail: tschaplinstj@ornl.gov

Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996-1610, USA. E-mail: jweltzin@utk.edu

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

Biology Department, Emory University, Atlanta, GA 30322, USA.

NOAA, Atmospheric Turbulence and Diffusion Division, Oak Ridge, TN 37831-2456, USA. E-mail: wilson@ atdd.noaa.gov

Tennessee Valley Authority, Norris, TN 37828, USA. E-mail: mhwolfe@tva.gov

Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6422, USA. E-mail: wullschlegsd@ornl.gov.