Sap flow of the southern conifer, Agathis australis during wet and dry summers
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Analysis of sap flux density during drought suggests that the large sapwood and rooting volumes of larger trees provide a buffer against drying soil.
The southern conifer Agathis australis is amongst the largest and longest-lived trees in the world. We measured sap flux densities (F d) 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. F d was consistently higher in larger trees than smaller trees. Peak F d 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 F d. 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, F d of smaller trees had higher R 2 values for soil moisture at 30 and 60 cm depth than soil moisture at 10 cm depth (R 2 = 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 F d 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.
KeywordsSap flow Plant water storage New Zealand Kauri Large trees Soil moisture
Author contribution statement
CM, MC and LS designed the experiments. MC designed and constructed the sap flow sensors. CM and SW did the statistical analysis and produced the figures with AV. All authors contributed to fieldwork and manuscript preparation.
We thank the following students and interns for assistance in the field: Chris Goodwin, Malani Sundaram, Andrew Wheeler, Tristan Webb, Roland Lafaele-Pereira. We acknowledge technical support from Colin Monk, David Wackrow, Brendan Hall, David Jenkinson and Colin Yong. Thanks Ian and Angela Knightbridge for hosting our weather station on their land and Freddie Hjelm and his team of climbers for climbing the trees. This project was supported by a grant from the Royal Society of New Zealand’s Marsden Fund (UOA1207) to CM and a Faculty Research Development Fund Grant from the University of Auckland to LS and CM.
Conflict of interest
The authors declare that they have no conflict of interest.
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