Format: ORAL

Wuu Kuang Soh1, Charilaos Yiotis2, Michelle Murray3, Sarah Pene4, Alivereti Naikatini5, Johan A. Elkink6, Joseph D. White7, Marika Tuiwawa5 and Jennifer C. McElwain3

1 National Botanic Gardens (OPW), Glasnevin, Dublin 9, Ireland. 2 Department of Biological Applications and Technology, University of Ioannina, 45110 Ioannina, Greece 3 Department of Botany, School of Natural Sciences, Trinity College Dublin, Dublin 1, Ireland. 4 School of Geography, Earth Science and Environment, University of the South Pacific, Laucala Campus, Suva, Fiji 5 South Pacific Regional Herbarium, University of the South Pacific, Laucala Campus, Suva, Fiji 6 School of Politics and International Relations, University College Dublin, Belfield, Ireland 7 Department of Biology, Baylor University, Waco, TX 76798, USA

To date, the effects of decadal scale rising atmospheric CO2 concentration on plant gas-exchange characteristics in the tropics are largely unknown. This research provides a rare insight on the little-known long-term impact of rising atmospheric CO2 on plant water-use and conductance of tree species in Fiji. Our study focuses on five tree species from the South Pacific Islands, a region that is particularly vulnerable to the impact of global warming. This is the first known study on the long-term physiological response of native plants to climate change in the region. Studying this long-term trend using herbarium records is challenging due to trait intraspecific variation. We assessed the impact of atmospheric CO2 concentration rise of about 95 ppm (19272015) on intrinsic water-use efficiency (iWUE) and theoretical maximum stomatal conductance of five tropical tree species in Fiji using isotopic composition and stomatal traits of herbarium leaves. Empirical results were compared with simulated values using models that uniquely incorporate intraspecific variation of empirical maximum stomatal conductance responses and specimen- or species-specific parameterization. While the magnitude of iWUE and maximum stomatal conductance response is species-specific ranging from strong to negligible, our findings indicate that the impact of stomata on determining the direction of iWUE response becomes apparent in situations where there is a consistent trend in photosynthesis. Generally, a stronger increase in iWUE was accompanied by a stronger decline in maximum stomatal conductance. This study demonstrates that incorporation of adaptations in maximum stomatal conductance, via changing size and/or density of stomata, in simulation is necessary for assessing an individual species iWUE response to changing atmospheric CO2 concentration.