Seasonal variability in the effect of elevated CO2 on ecosystem leaf area index in a scrub-oak ecosystem

Authors


Graham J. Hymus, Smithsonian Environmental Research Center, Mail Code DYN-2, Kennedy Space Center, FL, 32899, USA, fax + 321 861 6996, e-mail: sico2@titan02.ksc.nasa.gov

Abstract

We report effects of elevated atmospheric CO2 concentration (Ca) on leaf area index (LAI) of a Florida scrub-oak ecosystem, which had regenerated after fire for between three and five years in open-top chambers (OTCs) and was yet to reach canopy closure. LAI was measured using four nondestructive methods, calibrated and tested in experiments performed in calibration plots near the OTCs. The four methods were: PAR transmission through the canopy, normalized difference vegetation index (NDVI), hemispherical photography, and allometric relationships between plant stem diameter and plant leaf area. Calibration experiments showed: (1) Leaf area index could be accurately determined from either PAR transmission through the canopy or hemispherical photography. For LAI determined from PAR transmission through the canopy, ecosystem light extinction coefficient (k) varied with season and was best described as a function of PAR transmission through the canopy. (2) A negative exponential function described the relationship between NDVI and LAI; (3) Allometric relationships overestimated LAI. Throughout the two years of this study, LAI was always higher in elevated Ca, rising from, 20% during winter, to 55% during summer. This seasonality was driven by a more rapid development of leaf area during the spring and a relatively greater loss of leaf area during the winter, in elevated Ca. For this scrub-oak ecosystem prior to canopy closure, increased leaf area was an indirect mechanism by which ecosystem C uptake and canopy N content were increased in elevated Ca. In addition, increased LAI decreased potential reductions in canopy transpiration from decreases in stomatal conductance in elevated Ca. These findings have important implications for biogeochemical cycles of C, N and H2O in woody ecosystems regenerating from disturbance in elevated Ca.

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