Depth of soil water uptake by tropical rainforest trees during dry periods: does tree dimension matter?
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Though the root biomass of tropical rainforest trees is concentrated in the upper soil layers, soil water uptake by deep roots has been shown to contribute to tree transpiration. A precise evaluation of the relationship between tree dimensions and depth of water uptake would be useful in tree-based modelling approaches designed to anticipate the response of tropical rainforest ecosystems to future changes in environmental conditions. We used an innovative dual-isotope labelling approach (deuterium in surface soil and oxygen at 120-cm depth) coupled with a modelling approach to investigate the role of tree dimensions in soil water uptake in a tropical rainforest exposed to seasonal drought. We studied 65 trees of varying diameter and height and with a wide range of predawn leaf water potential (Ψpd) values. We confirmed that about half of the studied trees relied on soil water below 100-cm depth during dry periods. Ψpd was negatively correlated with depth of water extraction and can be taken as a rough proxy of this depth. Some trees showed considerable plasticity in their depth of water uptake, exhibiting an efficient adaptive strategy for water and nutrient resource acquisition. We did not find a strong relationship between tree dimensions and depth of water uptake. While tall trees preferentially extract water from layers below 100-cm depth, shorter trees show broad variations in mean depth of water uptake. This precludes the use of tree dimensions to parameterize functional models.
KeywordsDeuterium Oxygen Soil water Tropical rainforest Root
We are grateful to J. Cazal, J.-Y. Goret, F. Kwasie, A. Etienne and B. Leudet for their field assistance in collecting leaves and soil samples. We thank P. Petronelli and J. Engel for the taxonomic identifications and C. Hossann for the water extraction work. We thank the handling editor and two anonymous reviewers for their comments on a previous version of this manuscript. This study was part of the GUYAFLUX project funded by the French Ministry of Research, INRA, and the CNES, in the framework of the PO-Feder Région Guyane, and was partially funded by an Investissement d’Avenir grant of the ANR (CEBA: ANR-10-LABEX-0025).
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