Leaf senescence of Quercus myrtifolia as affected by long‐term CO 2 enrichment in its native environment

Abstract

Summary The long‐term effects of elevated (ambient plus 350 μmol mol −1 ) atmospheric CO 2 concentration (C a ) on the leaf senescence of Quercus myrtifolia Willd was studied in a scrub‐oak community during the transition from autumn (December 1997) to spring (April 1998). Plants were grown in large open‐top chambers at the Smithsonian CO 2 Research Site, Merritt Island Wildlife Refuge, Cape Canaveral, Florida. Chlorophyll (a + b) concentration, Rubisco activity and N concentration decreased by 75%, 82%, and 52%, respectively, from December (1997) to April (1998) in the leaves grown at ambient C a . In contrast, the leaves of plants grown at elevated C a showed no significant decrease in chlorophyll (a + b) concentration or Rubisco activity, and only a 25% reduction in nitrogen. These results indicate that leaf senescence was delayed during this period at elevated C a . Delayed leaf senescence in elevated C a had important consequences for leaf photosynthesis. In elevated C a the net photosynthetic rate of leaves that flushed in Spring 1997 (last year's leaves) and were 13 months old was not different from fully‐expanded leaves that flushed in 1998, and were approximately 1 month old (current year's leaves). In ambient C a the net photosynthetic rate of last year's leaves was 54% lower than for current year's leaves. When leaves were fully senesced, nitrogen concentration decreased to about 40% of the concentration in non‐senesced leaves, in both CO 2 treatments. In April, net photosynthesis was 97% greater in leaves grown in elevated C a than in those grown at ambient. During the period when elevated C a delayed leaf senescence, more leaves operating at higher photosynthetic rate would allow the ecosystem dominated by Q. myrtifolia to gain more carbon at elevated C a than at ambient C a .

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