Stem water storage of New Zealand kauri (Agathis australis)

Abstract

The southern conifer Agathis australis (kauri) is one of the largest trees in the world. Due to their large sapwood volume, we expect that some transpired water originates from storage. The discharge of stored water can buffer daily fluctuations of xylem tension and decrease the risk of hydraulic failure, providing hydraulic safety in times of low soil moisture. To examine the amount and diurnal patterns of stored water use, we measured sap flow at the base and the top of the stem simultaneously. The withdrawal of stored water was calculated by comparing integrated flows. We also installed point dendrometers at the same positions and on smaller branches within the canopy. Time lags of flows and radius changes were investigated by cross correlation analysis. On average 10 to 13% of transpiration was withdrawn from stem water storage during well-watered conditions. We observed time lags of up to 60 min between the base and top of the stem and up to 90 min between base and smaller branches of the spreading crown, indicating that branches also serve as water storage compartments. Analysis of radius changes over five months showed evidence for stem water storage maintaining hydraulic integrity under dryer conditions, as the stem of one tree showed greater relative depletion/shrinkage than the top of the stem or the branches.