Can deuterium tracing be used for reliably estimating water use of tropical trees and bamboo?

Reliable estimates of water use by trees and other woody plants are crucial for an improved understanding of plant physiology and for water resource management. Since the 1980s, the thermal dissipation probe (TDP) method has been widely applied in trees, proved to be fairly accurate but is challenging in remote areas. Also in the 1980s, the deuterium (D2O or deuterium oxide) tracing method was proposed, which so far has less often been applied. However, deuterium tracing requires less sophisticated equipment in the field and new analytical methods reduce costs and increase sample throughput. The objectives of this study were (i) to compare plant water use estimates of the TDP and D2O method and (ii) to determine whether the D2O method is appropriate for assessing absolute magnitudes of plant water use. The two methods were employed on five tropical tree species and a bamboo species growing in a reforestation stand in the Philippines and an agroforestry system in Indonesia. For bamboo, an increase in D2O values in neighbouring, non-labelled culms suggests that injected D2O was partly redistributed among culms, which would seriously limit the accurate estimation of water use for the target culm. For trees, water use estimates resulting from the D2O tracing method were proportional to the TDP results (r(2) = 0.85, P < 0.001), but absolute values were, on average, about seven times higher. This overestimation may be due to the assumptions underlying the D2O tracing method, such as the conservation of tracer mass, not being met. Further, it cannot be excluded that underestimation of water use by the TDP method contributed partly to the observed difference. However, when considering known sources of error, a large part of the observed difference remains unexplained. Based on our results, the use of the D2O tracing method cannot be recommended without further experimental testing if absolute values of whole-plant water use are a major goal. However, the D2O tracing method appears suitable for answering other questions, such as relative differences in water use among trees, water redistribution among neighbours and internal water transport and storage processes in plants.