Carbon-based secondary and structural compounds in Mediterranean shrubs growing near a natural CO₂ spring

We studied carbon-based secondary and structural compounds (CBSSCs) in Myrtus communis, Erica arborea, and Juniperus communis co-occuring in a natural CO₂ spring site and in a nearby control site in a Mediterranean environment. Leaf concentrations of phenolics and CBSSCs, such as lignin, cellulose, and hemicellulose, total nonstructural carbohydrates (TNCs), and lipids were measured monthly (phenolics) and every two months (the other compounds) throughout a year. There was a slight seasonal trend towards maximum concentrations of most of these CBSSCs during autumn-winter and minimum values during the spring season, particularly in Myrtus communis. For most of the CBSSCs and species, there were no consistent or significant patterns in response to the elevated [CO₂] (c. 700 μmol mol^-1) of the spring site. These results were not due to a dilution effect by increased structural or nonstructural carbon. Therefore, in contrast to many experimental studies of CO₂ enrichment, mainly conducted for short periods, there were no greater concentrations of phenolics, and, as in many of these studies, there were neither greater concentrations of the other CBSSCs. These results do not agree with the predictions of the carbon source-sink hypotheses. Possible causes of this disagreement are discussed. These causes include the complex heterogeneous environmental conditions and the variability of resource availabilities in the field, photosynthetic down-regulation, and/or the homeostatic and evolutionary nature of organisms. These results suggest evolutionary adaptive responses to changes in CO₂. They also suggest caution in attributing increased CBSSC concentrations to elevated [CO₂] at long-term scale in natural conditions, and therefore in their implications for plant-herbivore interactions and for decomposition.