Research Papers
Carbon Fluxes on North American Rangelands

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Abstract

Rangelands account for almost half of the earth’s land surface and may play an important role in the global carbon (C) cycle. We studied net ecosystem exchange (NEE) of C on eight North American rangeland sites over a 6-yr period. Management practices and disturbance regimes can influence NEE; for consistency, we compared ungrazed and undisturbed rangelands including four Great Plains sites from Texas to North Dakota, two Southwestern hot desert sites in New Mexico and Arizona, and two Northwestern sagebrush steppe sites in Idaho and Oregon. We used the Bowen ratio-energy balance system for continuous measurements of energy, water vapor, and carbon dioxide (CO2) fluxes at each study site during the measurement period (1996 to 2001 for most sites). Data were processed and screened using standardized procedures, which facilitated across-location comparisons. Although almost any site could be either a sink or source for C depending on yearly weather patterns, five of the eight native rangelands typically were sinks for atmospheric CO2 during the study period. Both sagebrush steppe sites were sinks and three of four Great Plains grasslands were sinks, but the two Southwest hot desert sites were sources of C on an annual basis. Most rangelands were characterized by short periods of high C uptake (2 mo to 3 mo) and long periods of C balance or small respiratory losses of C. Weather patterns during the measurement period strongly influenced conclusions about NEE on any given rangeland site. Droughts tended to limit periods of high C uptake and thus cause even the most productive sites to become sources of C on an annual basis. Our results show that native rangelands are a potentially important terrestrial sink for atmospheric CO2, and maintaining the period of active C uptake will be critical if we are to manage rangelands for C sequestration.

Resumen

Los pastizales nativos constituyen casi la mitad de la superficie terrestre y pueden desempeñar un papel importante en el ciclo global del carbón (C). El objetivo de esta investigación fue estudiar el intercambio neto de carbono dentro del ecosistema (NEE) en ocho sitios de pastizales de Norteamérica durante un período de seis años. Las prácticas de manejo y grados de disturbio pueden influenciar el NEE, pero para consistencia, se compararon pastizales con y sin pastoreo. Se usaron cuatro sitios de las Grandes Planicies desde Texas a Dakota del Norte, dos sitios del desierto al sudoeste de New México y Arizona y dos sitios del noroeste del desierto de arbustivas en Idaho y Oregon. Se utilizo el sistema proporción-energía de Bowen para las medidas continuas de energía, de vapor de agua y de los flujos del CO2 en cada sitio (1996 a 2001 para la mayoría de los sitios). Se examinaron los datos usando procedimientos estandarizados que facilitaron comparaciones entre sitios. Aunque casi cualquier sitio podría actuar como reservorio o fuente de C dependiendo de los patrones anuales. Cinco de los ocho pastizales nativos típicamente demandaron CO2 atmosférico durante el período del estudio. Ambos sitios del desierto arbustivo y tres de cuatro sitios de las Grandes Planicies demandaron CO2. En contraste, los dos sitios secos del desierto del sudoeste fueron fuentes del carbón anualmente. La mayoría de los pastizales se caracterizaron por períodos cortos de alta absorción de C (2 a 3 meses) y largos periodos del balance de C o bien pequeñas pérdidas por respiración. Las condiciones ambientales durante el período de evaluación influenciaron fuertemente las conclusiones sobre NEE en todos los sitios evaluados. Las sequías limitaron períodos de alta absorción de C y la productividad de los sitios para convertirse anualmente en fuentes de C. Estos resultados demostraron que los pastizales nativos son potencialmente un reservorio terrestre para el CO2 atmosférico y el mantenimiento del período de absorción de C activo será crítico si se manejan los pastizales para el secuestro de C.

Section snippets

INTRODUCTION

There is considerable interest in potential effects of increasing atmospheric carbon dioxide (CO2) on global climate, and consequently in maximizing terrestrial and oceanic sinks for carbon (C) as a means of ameliorating negative impacts. Attempts to balance known sources and sinks of the global C cycle often result in a “missing sink” of 1.3 Gt C · yr−1 to 1.4 Gt C · yr−1 (Woodwell et al. 1998). Carbon sequestration by terrestrial ecosystems may account for a portion of the missing C, although

Study Sites

Eight individual sites were used in this study. Specific location, vegetation type, dominant species, and general site characteristics are presented in Tables 1 and 2. The sites were selected to represent native rangeland in the area. All sites were considered to be in good ecological condition (based on species composition) and were not managed (grazed by livestock, burned, or fertilized) during the course of the study. The one exception was the tallgrass prairie in Temple, Texas, which was

RESULTS AND DISCUSSION

Five of the eight rangeland sites that we studied typically functioned as sinks for atmospheric CO2 during the study period, but sink strength varied among biomes and years (Table 6). The sagebrush steppe sites in the northwestern United States were surprisingly strong sinks for C during the study period (Fig. 1; Table 6). Grassland sites on the US Great Plains were variable, with most sites either weak or moderate sinks for atmospheric CO2 (Figs. 2 and 3; Table 6). In the sagebrush steppe,

MANAGEMENT IMPLICATIONS

Although we did not test the impact of management practices on rangeland NEE, there are some important points to be gleaned from the annual NEE patterns. Because NEE and vegetation productivity are linked, these patterns have implications for managing rangelands. Most of the sites exhibited rapid CO2 uptake for a short period (2 mo to 4 mo) and a longer period of low CO2 loss (where ecosystem respiration exceeded photosynthesis). Drought reduced the period and magnitude of positive NEE.

ACKNOWLEDGMENTS

The authors wish to thank the many individuals who provided technical and administrative support to this project. We also thank Joel Brown, Wayne Polley, several anonymous reviewers, and Associate Editor Robert Nowak for comments that improved the manuscript.

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