Controls on interspecific variation in tree-ring oxygen isotopes among three co-dominant species in a temperate forest

Background/Question/Methods

Although a number of studies have reported significant interspecific variations in stable oxygen isotope compositions of tree ring cellulose among co-occurring tree species, there is a lack of comprehensive understanding of the underlying mechanism behind such variations, potentially limiting the predictive ability of the currently widely used Roden-Ehleringer model.

We investigated interspecific variation in tree-ring oxygen isotopes among pitch pine, black oak and chestnut oak, three co-dominant species in a temperate forest located in the New Jersey Pine Barrens. For two consecutive growing seasons, we collected microclimatic (canopy leaf temperature and RH), physiological (gas exchange), and oxygen isotopic (leaf and stem water, atmospheric water vapor, phloem sap carbohydrates, tree ring cellulose) data for each of the tree species. 

Results/Conclusions

We found that pitch pine had the highest oxygen isotope ratios, averaged at 31.6 ‰ for the two growing seasons. Significant differences were also found between chestnut and black oaks, with chestnut oak (28.7 ‰) being 1.1 ‰ higher than black oak (27.6 ‰). By relating tree-ring oxygen isotope data to the various microclimatic, physiological and isotopic data that we collected, we were able to both identify and rule out the causes for the observed among-species isotope differences. In summary, we ruled out the microclimatic difference as the cause because when GPP averaged across the growing seasons, both canopy leaf temperature and RH show little variation among the three species. We observed more enriched source water isotope signals in chestnut oak than in black oak, largely explaining the difference in tree ring signature found between these two species. However, source water differences could not explain isotope ratio differences between pitch pine and the oak species. Rather, as was revealed by our model simulation study, it was the difference in the importance of the leaf level Peclet effect, that accounted for the pine and oak difference in tree ring isotope ratios.