The conifer New Zealand kauri (Agathis australis) has enormous water storage capacity that trees can utilize when soil moisture declines. Previous work shows A. australis have tight stomatal control during both wet and dry summers, indicating that A. australis are drought super-avoiders. Over extended droughts, however, it is unclear how long trees can maintain these water savings at the leaf level over prioritizing carbon uptake or maintaining hydraulic integrity as they tap into their stored water. We know A. australis are susceptible to xylem embolism, so small decreases in xylem water potential could lead to loss of hydraulic conductance. A throughfall exclusion (TFE) experiment has been developed in a mature A. australis stand in Auckland, NZ focused on applying long-term drought treatment to individual mature trees. This study aims to address uncertainties around kauri water relations under extended soil moisture-deficit conditions. The current work was undertaken over summer-2018/19. Diurnal canopy analyses of drought and control trees were made on four days across the Austral summer (in October, December, January, and March) to measure the dynamics of stem water fluxes and leaf-level water-use indicators such as stomatal conductance (gs) and leaf water potentials (Ψl). Leaf traits such as relative water content (RWC) and turgor loss point (ΨTLP) were measured in the laboratory.
Results/Conclusions
Preliminary results from suggest that canopy processes were decoupled from declining soil moisture availability, drought treatment trees had only slightly lower gs rates relative to control trees across the summer season but rates did vary throughout the season, peaking in January. The strongest environmental driver of stomatal behaviour varied across the season between VPD and PAR and was not always consistent between treatment cohorts, which reinforces water savings at the leaf level. There also was not a significant difference in other leaf scale measurements between treatment and control, nor did time-of-season show any clear trend by these measures. There was, however, a significant drought effect for stem core RWC for December, January, and March. Sap flow rates of measurement days with control trees showed consistently higher peaks (between 28.5 – 61.7% higher) and also displayed a clear seasonal trend, peaking in January. The discrepancy that occurs when reduced sap flow rates in droughted trees do not translate to increased xylem tension or transpiration could be indicating that A. australis continue to reduce transpirational water losses in favour of excision and can access stem water storage in severe droughts.