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Carbon flux in arid and semiarid area shrublands, especially in old-growth shrub ecosystems, has been rarely studied using eddy covariance techniques. In this study, eddy covariance measurements over a 100-year old-growth chamise-dominated chaparral shrub ecosystem were conducted for 7 years from 1996 to 2003. A carbon sink, from -96 to -155 g C m(-2) yr(-1), was determined under normal weather conditions, while a weak sink of -18 g C m(-2) yr(-1) and a strong source of 207 g C m(-2) yr(-1) were observed as a consequence of a severe drought. The annual sink strength of carbon in the 7-year measurement period was -52 g C m(-2) yr(-1). The results from our study indicate that, in contrast to previous thought, the old-growth chaparral shrub ecosystem can be a significant sink of carbon under normal weather conditions and, therefore, be an important component of the global carbon budget.