Differences in the response sensitivity of stomatal index to atmospheric CO2 among four genera of Cupressaceae conifers

Background and Aims

The inverse relationship between stomatal density (SD: number of stomata per mm² leaf area) and atmospheric concentration of CO₂ (CO₂]) permits the use of plants as proxies of palaeo-atmospheric CO₂. Many stomatal reconstructions of palaeo-CO₂] are based upon multiple fossil species. However, it is unclear how plants respond to CO₂] across genus, family or ecotype in terms of SD or stomatal index (SI: ratio of stomata to epidermal cells). This study analysed the stomatal numbers of conifers from the ancient family Cupressaceae, in order to examine the nature of the SI–CO₂] relationship, and potential implications for stomatal reconstructions of palaeo-CO₂].

Methods

Stomatal frequency measurements were taken from historical herbarium specimens of Athrotaxis cupressoides, Tetraclinis articulata and four Callitris species, and live A. cupressoides grown under CO₂-enrichment (370, 470, 570 and 670 p.p.m. CO₂).

Key Results

T. articulata, C. columnaris and C. rhomboidea displayed significant reductions in SI with rising CO₂]; by contrast, A. cupressoides, C. preissii and C. oblonga show no response in SI. However, A. cupressoides does reduce SI to increases in CO₂] above current ambient (approx. 380 p.p.m. CO₂). This dataset suggests that a shared consistent SI–CO₂] relationship is not apparent across the genus Callitris.

Conclusions

The present findings suggest that it is not possible to generalize how conifer species respond to fluctuations in CO₂] based upon taxonomic relatedness or habitat. This apparent lack of a consistent response, in conjunction with high variability in SI, indicates that reconstructions of absolute palaeo-CO₂] based at the genus level, or upon multiple species for discrete intervals of time are not as reliable as those based on a single or multiple temporally overlapping species.