Water relations of the southern conifer Agathis australis in a throughfall exclusion experiment

The southern conifer Agathis australis is among the largest (by volume) and longest-lived trees in the world. They are confined to the north of New Zealand's north island where droughts will become more frequent and severe under future climatic conditions. To explore the impact of drought on water relations of A. australis we installed a throughfall exclusion experiment at the Huapai Scientific Reserve, West Auckland. We measured sap flow density (F_d) in both control and drought trees during spring and summer seasons at the base of the stem. We used relative sap flow density (F_d-R) to offset the original sap flow difference between individuals. We then calculated time lags and whole tree hydraulic conductance in both control and drought trees. F_d-R was higher during January 2019, which had the highest stomatal conductance and the shortest time lag (0.5-1.5 h) between canopy and base. The smallest daily F_d-R was observed in drought trees during March 2019, which had the lowest water potential and had the driest soil (17% at 20 cm and 33% at 60 cm). We observed lower F_d-R values and lower whole-tree hydraulic conductance (K_h) in drought trees compared to control trees suggesting that drought significantly affected the water relations even though pre-dawn leaf water potential (Ψ_pd) and stomatal conductance (g_s) were not consistently lower in drought trees across seasons.