Towards establishing a new environmental archive – Annual growth periodicity, stable carbon isotope variability and reconstruction potential of 'Akoko (Euphorbia olowaluana), a native Hawaiian tree with C4 photosynthetic pathway

Abstract

Tree ring patterns provide one of the best records of pre-instrumental environmental and climate variability. To date, tree ring chronologies were explored from woody plant species with C3 photosynthetic pathway, only. For the first time, we have studied wood growth periodicity and stable carbon isotope ratios of tree ring cellulose of a tree species with C4 photosynthesis and compared these data to those of a C3 tree species from the same habitat. The investigated species, Māmane (Sophora chrysophylla, C3) and 'Akoko (Euphorbia olowaluana, C4), are small endemic Hawaiian trees sampled from a rather dry, high elevation site 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 Māmane individuals as well as with ring-width patterns from a nearby population of introduced Deodar cedar (Cedrus deodara) trees that serve as a reference. This correlation 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 discrimination against 13C during the photosynthetic fixation of CO2. Thus, 'Akoko may provide a record of past atmospheric CO2 concentration (CO2 atm) 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 δ13C averages and atmospheric δ13C 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 δ13C (intrinsic water-use efficiency, iWUE) of the two species show similar responses in that both demonstrate an increase in iWUE over time and with increased CO2 atm. 'Akoko and Māmane 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 Māmane curve shows a distinct linearly increasing trend throughout the observation period.