|The major findings of the studies of plant/insect interactions by Stiling and colleagues are that all insect species indigenous to scrub oak occur in both ambient and elevated atmospheric CO2 treatments; that there are fewer insects in the elevated CO2 chambers, perhaps because the lower nitrogen concentration of leaves in elevated CO2 are less palatable for insects, and that mortality of those who remain in the elevated CO2 chambers is increased (Stiling et al. 2002). Other findings include that mining and chewing species stimulate abscission of leaves, (Stiling et al. 2002) which may thereby alter ecosystem nutrient cycling. Although damage by herbivore populations declined under elevated CO2 (because there are fewer insects), total herbivore damage per capita of insect increased (because each insect consumes more leaves). No effects of elevated CO2 have been seen on secondary metabolites or on defense compounds (Hunter, unpublished). An important line of research is suggested by the idea that damaged leaves have not senesced prior to being dropped by the plant and that these leaves could alter nitrogen cycling.
Summary of findings.
The response of all levels of the ecosystem to elevated CO2, (i.e. photosynthesis, respiration, growth of shoots and roots, increased bio-geochemical processes) suggests there is a substantial amount of carbon sequestered or cycled through this ecosystem. Some fraction of the additional soil carbon created by decomposition of roots and rhizomes is exported from the site in shallow ground water. Other data indicate a shift from relatively labile to more recalcitrant carbon pools, which could mean that a long-term shift in decomposition can be expected in the future. Hydrologic studies are underway to determine if the export of dissolved carbon in soil water is the fate of the additional carbon. An increase in dissolved carbon also implies rising soil surface elevation and/or an increase in carbon density at the site. Some data from the wetland show that the marsh surface has increased in response to elevated CO2. This suggests that stimulation of carbon assimilation by rising atmospheric CO2 may allow coastal wetlands to keep pace with rising sea level caused by climate change.
The picture emerging from these long-term studies is that rising atmospheric CO2 will cause increased carbon cycling in the native plant communities through a variety of mechanisms all dependent on the responses of photosynthesis and transpiration. Although acclimation of photosynthesis has been repeatedly observed , this has not eliminated the impact of rising CO2 at the plant and ecosystem levels. The greatest effects of rising atmospheric CO2 result from interactions with environmental stress (Rasse et al. Submitted). The effect of the carbon subsidy was most dramatic during periods of stress from drought, high temperature, and high salinity.
Numerous important ecosystem functions are altered by rising atmospheric CO2 from canopy structure to soil water, palatability of foliage for herbivores, habitat for endangered species, and carbon export to the adjacent estuary. We are exploring the interaction with fire in the scrub oak ecosystem to determine whether an increase in the frequency of fire will lead to effects on diversity of bird populations (the Florida Scrub Jay is dependent on frequent fire for nesting sites free of predatory Raptores) and whether a change in fire frequency will result in an increase or decrease in ecosystem carbon. In the wetland ecosystem, studies are under way to determine the role of a change in the quality of organic compounds (labile or refractory) in carbon sequestration.