Cellulose δ18O is an index of leaf-to-air vapor pressure difference (VPD) in tropical plants.

Cellulose in plants contains oxygen that derives in most cases from precipitation. Because the stable oxygen isotope composition, δ¹⁸O, of precipitation is associated with environmental conditions, cellulose δ¹⁸O should be as well. However, plant physiological models using δ¹⁸O suggest that cellulose δ¹⁸O is influenced by a complex mix of both climatic and physiological drivers. This influence complicates the interpretation of cellulose δ¹⁸O values in a paleo-context. Here, we combined empirical data analyses with mechanistic model simulations to i) quantify the impacts that the primary climatic drivers humidity (e_a) and air temperature (T_air) have on cellulose δ¹⁸O values in different tropical ecosystems and ii) determine which environmental signal is dominating cellulose δ¹⁸O values. Our results revealed that e_a and T_air equally influence cellulose δ¹⁸O values and that distinguishing which of these factors dominates the δ¹⁸O values of cellulose cannot be accomplished in the absence of additional environmental information. However, the individual impacts of e_a and T_air on the δ¹⁸O values of cellulose can be integrated into a single index of plant-experienced atmospheric vapor demand: the leaf-to-air vapor pressure difference (VPD). We found a robust relationship between VPD and cellulose δ¹⁸O values in both empirical and modeled data in all ecosystems that we investigated. Our analysis revealed therefore that δ¹⁸O values in plant cellulose can be used as a proxy for VPD in tropical ecosystems. As VPD is an essential variable that determines the biogeochemical dynamics of ecosystems, our study has applications in ecological-, climate-, or forensic-sciences. [ABSTRACT FROM AUTHOR]