Linking tree water use efficiency with calcium and precipitation.

Water use efficiency (WUE) is a key physiological trait in studying plant carbon and water relations. However, the determinants of WUE across a large geographical scale are not always clear, limiting our capacity to predict WUE in response to future global climate change. We propose that tree WUE is influenced by both calcium (Ca) availability and precipitation. Also, although it is well-known that transpiration is the major driving force for passive nutrient uptake, the linkage between these two processes has not been well-established. Because Ca uptake is an apoplastic and passive process that purely relied on transpiration, and there is no translocation once assimilated, we further developed a theoretical model to quantify the relationship between tree Ca accumulation and WUE using soil-to-plant calcium ratio (SCa/BCa) and tree WUE derived from δ13C. We tested our theoretical model and predicted relationships using three common tree species (Pinus tabuliformis, Quercus mongolica, and Platycladus orientalis) across their native habitats in Northern China spanning 2,300 km, and a controlled greenhouse experiment with soil Ca concentrations manipulated. We found that tree WUE was negatively related to precipitation of the growing season (GSP), and positively with soil Ca. Both a multiple regression model and a path analysis suggested a higher contribution of soil Ca to WUE than GSP. As predicted by our theoretical model, we found a positive relationship between WUE and SCa/BCa across their distribution ranges in all three tree species and in the controlled experiment for Pinus tabuliformis. This relationship suggests a tight coupling between water and Ca uptake, and the potential use of SCa/BCa to indicate WUE. A negative relationship between SCa/BCa and GSP also suggests a possible decrease in tree Ca accumulation efficiency in a drier future in Northern China.