Abstract:
Key message: Analysis of sap flux density during drought suggests that the large sapwood and rooting volumes of larger trees provide a buffer against drying soil. Abstract: The southern conifer Agathis australis is amongst the largest and longest-lived trees in the world. We measured sap flux densities (Fd) in kauri trees with a DBH range of 20–176 cm to explore differences in responses of trees of different sizes to seasonal conditions and summer drought. Fd was consistently higher in larger trees than smaller trees. Peak Fd was 20 and 8 g m−2 s−1 for trees of diameters of 176 and 20 cm, respectively, during the wet summer. Multiple regression analysis revealed photosynthetically active radiation (PAR) and vapour pressure deficit (D) were the main drivers of Fd. During drought, larger trees were more responsive to D whilst smaller trees were more responsive to soil drying. Our largest tree had a sapwood area of 3,600 cm2. Preliminary analysis suggests stem water storage provides a buffer against drying soil in larger trees. Furthermore, Fd of smaller trees had higher R2 values for soil moisture at 30 and 60 cm depth than soil moisture at 10 cm depth (R2 = 0.68–0.97 and 0.55–0.67, respectively) suggesting that deeper soil moisture is more important for these trees. Larger trees did not show a relationship between Fd and soil moisture, suggesting they were accessing soil water deeper than 60 cm. These results suggest that larger trees may be better prepared for increasing frequency and intensity of summer droughts due to deeper roots and/or larger stem water storage capacity.
