The effect of elevated atmospheric CO2 concentration (Ca) on the aboveground biomass of three oak species, Quercus myrtifolia, Q. geminata, and Q. chapmanii, was estimated nondestructively using allometric relationships between stem diameter and aboveground biomass after four years of experimental treatment in a naturally fire‐regenerated scrub‐oak ecosystem. After burning a stand of scrub‐oak vegetation, re‐growing plants were exposed to either current ambient (379 µL L−1 CO2) or elevated (704 µL L−1 CO2) Ca in 16 open‐top chambers over a four‐year period, and measurements of stem diameter were carried out annually on all oak shoots within each chamber. Elevated Ca significantly increased aboveground biomass, expressed either per unit ground area or per shoot; elevated Ca had no effect on shoot density. The relative effect of elevated Ca on aboveground biomass increased each year of the study from 44% (May 96–Jan 97), to 55% (Jan 97–Jan 98), 66% (Jan 98–Jan 99), and 75% (Jan 99–Jan 00). The effect of elevated Ca was species specific: elevated Ca significantly increased aboveground biomass of the dominant species, Q. myrtifolia, and tended to increase aboveground biomass of Q. chapmanii, but had no effect on aboveground biomass of the subdominant, Q. geminata. These results show that rising atmospheric CO2 has the potential to stimulate aboveground biomass production in ecosystems dominated by woody species, and that species‐specific growth responses could, in the long term, alter the composition of the scrub‐oak community.

No abstract

Leaf conductance often decreases in response to elevated atmospheric CO 2 concentration (C a ) potentially leading to changes in hydrology. We describe the hydrological responses of Florida scrub oak to elevated C a during an eight-month period two years after C a manipulation began. Whole-chamber gas exchange measurements revealed a consistent reduction in evapotranspiration in response to elevated C a , despite an increase in leaf area index (LAI). Elevated C a also increased surface soil water content, but xylem water deuterium measurements show that the dominant oaks in this system take up most of their water from the water table (which occurs at a depth of 1.5±3 m), suggesting that the water savings in elevated C a in this system are primarily manifested as reduced water uptake at depth. Extrapolating these results to larger areas requires considering a number of processes that operate on scales beyond these accessible in this field experiment. Nevertheless, these results demonstrate the potential for reduced evapotranspiration and associated changes in hydrology in ecosystems dominated by woody vegetation in response to elevated C a .

No abstract

Hurricane disturbances have profound impacts on ecosystem structure and function, yet their effects on ecosystem CO 2 exchange have not been reported. In September 2004, our research site on a fire-regenerated scrub-oak ecosystem in central Florida was struck by Hurricane Frances with sustained winds of 113 km h À1 and wind gusts as high as 152 km h À1 . We quantified the hurricane damage on this ecosystem resulting from defoliation: we measured net ecosystem CO 2 exchange, the damage and recovery of leaf area, and determined whether growth in elevated carbon dioxide concentration in the atmosphere (C a ) altered this disturbance. The hurricane decreased leaf area index (LAI) by 21%, which was equal to 60% of seasonal variation in canopy growth during the previous 3 years, but stem damage was negligible. The reduction in LAI led to a 22% decline in gross primary production (GPP) and a 25% decline in ecosystem respiration (R e ). The compensatory declines in GPP and R e resulted in no significant change in net ecosystem production (NEP). Refoliation began within a month after the hurricane, although this period was out of phase with the regular foliation period, and recovered 20% of the defoliation loss within 2.5 months. Full recovery of LAI, ecosystem CO 2 assimilation, and ecosystem respiration did not occur until the next growing season. Plants exposed to elevated C a did not sustain greater damage, nor did they recover faster than plants grown under ambient C a . Thus, our results indicate that hurricanes capable of causing significant defoliation with negligible damage to stems have negligible effects on NEP under current or future CO 2 -enriched environment.