Radial Growth and Increased Water-Use Efficiency for Ponderosa Pine Trees in Three Regions in the Western United States

Abstract

We examined changes in and relationships between radial growth and intrinsic water-use efficiency (iWUE) of ponderosa pine (Pinus ponderosa) trees, climate, and atmospheric CO₂ in the western United States since the mid-nineteenth century. We developed tree-ring chronologies for eight sites in three climate regions and used carbon isotope data to calculate pentadal values of iWUE. We examined relationships among radial growth, climate, iWUE, and CO₂ via correlation and regression analyses. Significant upward trends in iWUE occurred at all sites, and despite an absence of climate changes that would favor growth, upward radial growth trends occurred at five sites. Our findings suggest that increased iWUE associated with rising CO₂ can positively impact tree growth rates in the western United States and are thus an evolving component of forest ecosystem processes.

Examinamos los cambios entre crecimiento radial y la eficiencia intrínseca del uso del agua (iWUE, sigla en inglés) del pino ponderosa (Pinus ponderosa), y las relaciones con el clima y el CO₂ presente en la atmósfera del occidente de los Estados Unidos, desde mediados del siglo XIX. Desarrollamos cronologías basadas en los anillos del tronco de los árboles para ocho sitios de tres regiones climáticas y utilizamos datos de isótopos de carbono para calcular los valores pentadales de la iWUE. También, examinamos las relaciones entre crecimiento radial, clima, iWUE y CO₂, por medio de análisis de correlación y regresión. En todos los sitios se detectaron tendencias significativas de aumento de la iWUE y a pesar de la inexistencia de cambios climáticos que hubiesen favorecido el crecimiento, en cinco sitios se registraron tendencias de aumento en el crecimiento radial. Nuestros hallazgos sugieren que el incremento de la iWUE asociado con elevación del CO₂ pueden impactar positivamente las tasas de crecimiento de los árboles en la región occidental de EE.UU. y son, por lo tanto, un componente en evolución de los procesos que ocurren en el ecosistema de bosque.

Key Words:

• atmospheric CO₂
• climate
• drought
• intrinsic water-use efficiency
• Pinus ponderosa

关键词:

• 大气二氧化碳
• 气候干旱
• 内在水分利用效率
• 黄松

Palabras clave:

• CO₂ atmosférico
• clima
• sequía
• eficiencia intrínseca del uso del agua
• Pinus ponderosa

Acknowledgments

PETER T. SOULÉ is Professor in the Department of Geography and Planning at Appalachian State University, Boone, NC 28608. E-mail: [email protected]. His primary research focus deals with understanding the linkages between changing climate and atmospheric conditions on forest ecosystems.

PAUL A. KNAPP is Professor in the Department of Geography at the University of North Carolina Greensboro, Greensboro, NC 27412. E-mail: [email protected]. His research interests focus on climatology, biogeography, and tree-ring science projects in the American West.

Notes

*This project was funded by the USDA NRI Competitive Grant Program, Plant Adaptations to the Environment #2005-35100-15226, and Faculty Research Grants from both Appalachian State University and the University of North Carolina at Greensboro. The Environmental Isotope Laboratory at the Laboratory of Tree-Ring Research, University of Arizona, conducted the isotopic analyses for use under the guidance of Dr. Steve Leavitt. Steve Shelly, USDA Forest Service Region 1 RNA coordinator, helped us identify study sites and helped us in the field. Also assisting in field and laboratory work were Justin Maxwell, Jason Ortegren, Ian Snider, William Tyminski, and Philip White. We thank all of these individuals and the two anonymous reviewers of the article.