Seasonal water relations of four co-occurring species in a bushland fragment in Western Australia

RN Pires, P Drake, P Poot and EJ Veneklaas

School of Plant Biology, Faculty of Science, The University of Western Australia, 35 Stirling Highway, Crawley, 6009, WA, Australia

Future climate change may drastically impact forest ecosystems due to increases in the dry season length and frequency/severity of drought and heat wave events. Tree mortality events, changes in vegetation structure and species distribution are likely to be associated with climate-driven extreme events. In this study, we investigated physiological responses and water relations of four species from distinct functional groups in two sites within a bushland fragment, differing in water-table depth and tree density, from Nov 2015 to May 2016. Gas exchange and water potential measurements indicate that the species (Allocasuarina fraseriana, Banksia menziesii, Corymbia calophylla, Eucalyptus marginata) exhibit distinct physiological responses during the dry season, especially where the groundwater is deeper in the soil profile. All species showed reduced photosynthesis, stomatal conductance and transpiration rates as well as declines in leaf water potentials (predawn/midday), between Dec 2015 and Feb 2016, with recovery due to rain in the following months (April and May 2016). Moreover, B. menziesii and C. calophylla maintain higher leaf water potentials, even during the dry season, whereas A. fraseriana and E. marginata showed lower values, demonstrating contrasting strategies, isohydric and anisohydric, respectively. These responses indicate that the isohydric species have a tighter control of their physiological activity when facing environmental pressures. Higher temperatures during the drier months (Dec to Feb) can increase/interact with the effects of reduced water availability and impose greater stress to the vegetation. The implications of the species strategies, higher temperatures and its impact on the physiology and water control of the study species will be discussed.