Investigating the growth periodicity, stable carbon isotope trend and climate reconstruction potential of 'Akoko (Euphorbia olowaluana ), a native Hawaiian C4 tree on Mauna Kea, Hawai'i, using tree ring analyses

Tree ring patterns provide one of the best records of historical climate variability. I evaluated growth increment periodicity and the stable isotope ratios of carbon in two woody plant species using the C3- and C4-photosynthetic pathway. The investigated species, Mamane (Sophora chrysophylla , C3) and `Akoko (Euphorbia olowaluana, C4), are small endemic Hawai`ian trees sampled from a rather dry, high elevation habitat on the ridge between Mauna Loa and Mauna Kea on the island of Hawai`i, USA. A relatively strong correlation in ring patterns was found within the `Akoko and the Mamane individuals as well as with ring-width patterns from a nearby population of introduced Deodar Cedar (Cedrus odorata) trees that serve as a reference. This is evidence that the C4-plant `Akoko may form annual growth rings. In addition to being the first demonstration of annual growth rings in a C4 plant, our findings have important implications for future climate change research in Hawai`i. Unlike plants with a C3-photosynthetic pathway, C4 plants do not show strong bias against 13C during the photosynthetic fixation of CO2. Thus, `Akoko may provide a record of past atmospheric CO2 concentration that can be compared with, and possibly supplement, the well-known Keeling curve produced by the nearby Mauna Loa Atmospheric Observatory. Regression analysis indicates a significant relationship between `Akoko delta13C averages and atmospheric delta13C values. Furthermore, time series of tree ring data from both species provide long-term information on the response of C3 and C4-plants to increasing atmospheric CO2 concentrations and climate change. Trends in delta13C (intrinsic water-use efficiency) of the two species show similar responses in that both demonstrate an increase in iWUE over time and with increased CO2atm. 'Akoko and Mamane iWUE curves are different however, in that the 'Akoko (C4) curve is non-linear and a significant increase could only be observed post 1975, while the Mamane curve shows a distinct linearly increasing trend throughout the observation period.