Seasonal water uptake pattern in Agathis australis (kauri), a large and long-lived Southern Hemisphere conifer, using water stable isotopes

Understanding seasonal water uptake depth (WUD) in trees is critical for assessing trees’ physiological responses to seasonal variability in microclimatic conditions and soil water availability. While broadleaf and conifer species in the Northern Hemisphere have been widely studied, studies of seasonal changes in WUD of large and long-lived evergreen conifers in the Southern Hemisphere are rare.

We investigated the effect of season and tree size on the water uptake pattern of Agathis australis (kauri), a large and long-lived endemic conifer, over an 18-month period. Our study site, a remnant kauri-dominated forest, is located in West Auckland, northern New Zealand. We collected stem cores from seven kauri trees (n = 3 < 50 cm diameter, n = 4 > 100 cm diameter) and soil samples underneath each of the seven trees (organic layer (OL), 0-10 cm, 10-20 cm, 20-30 cm, 30-50 cm, 50-70 cm, 70+ cm) across six seasons (austral spring 23,  austral summer 23-24, austral autumn 24, austral winter 24, austral spring 24, and austral summer 24-25). Water from soil and stem cores was extracted using cryogenic vacuum extraction. We measured δ²H and δ¹⁸O in all samples and used a Bayesian mixing model (MixSIAR) to determine WUD.

Our preliminary results show that across season and tree size, kauri obtained a larger proportion of water from the shallow layer (OL to 30 cm depth; ~ 60%) compared to ~ 40% sourced from layers below 30 cm. There was greater reliance on water from the shallow layer (up to 75%) during austral summer 23-24 and 24-25. We did not observe strong differences in WUD between small and large trees across our study seasons. These insights advance ecohydrological research on Southern Hemisphere evergreen conifers and highlight the importance of understanding species-specific response to microclimatic conditions and changing water availability.