Coupling between tree water use and calcium accumulation

<p>Although it is well-known that transpiration is the major driving force for plant nutrient uptake/transport, the coupling between these two processes has not been well-established. Calcium (Ca) is a good candidate to evaluate the coupling between transpiration (water use) and nutrient accumulation as Ca uptake is a passive process without relocation once incorporated, thus avoiding the confounding effects of active regulation and translocation. Here we developed a novel theoretical equation to use soil-to-plant calcium ratio (<i>S</i>_Ca/<i>B</i>_Ca) to predict<b> </b>plant water use efficiency (WUE) derived from δ¹³C. We tested this relationship with two conifer and one angiosperm tree species across their native habitats in China and a controlled greenhouse experiment with soil Ca concentrations manipulated. A linear relationship between WUE and <i>S</i>_Ca/<i>B</i>_Ca was found in all three species across their natural distribution ranges and in the greenhouse experiment. <i>S</i>_Ca/<i>B</i>_Ca was also strongly related to the precipitation of the growing season, and weakly with the mean annual precipitation. Our results suggest a strong linkage between water and Ca, and that <i>S</i>_Ca/<i>B</i>_Ca can be an index for assessing plant WUE. The close relationship between WUE and <i>S</i>_Ca/<i>B</i>_Caalso provides a model system to study the coupling between plant water use and passive nutrient uptake, and the response of this coupling to environmental change.</p>