Can Oxygen Isotopes in Tree Rings Be Used to Detect Stomatal Responses to Global Change?

Stomatal conductance (g_s) regulates CO₂ and water fluxes of plants. Although experiments have shown that g_s decreases with elevated CO₂, it is unclear how g_s is responding in situ to long-term exposures to rising CO₂ and a changing climate. Tree ring isotope analysis provides a unique method to assess tree ecophysiological responses to long-term exposures of slowly changing environmental conditions. In particular, it has been suggested that changes in g_s can potentially be inferred from tree ring stable oxygen isotope ratios (δ¹⁸O_trc). Several studies have indeed used δ¹⁸O_trc trends to conclude that g_s has not significantly changed from pre-industrial values. However, it remains unclear whether δ¹⁸O_trc is sufficiently sensitive to detect the magnitude of change in g_s expected due to CO₂ increases and climatic changes. Here, we evaluate the sensitivity of δ¹⁸O_trc trends to CO₂ and climate induced changes in g_s, and to VPD and temperature increases since the beginning of the 20th century, using current theoretical models. We find that temporal changes in g_s only significantly affect δ¹⁸O_trc trends when the Péclet effect is present, and then only in dry climates. In contrast to the weak effects of g_s on δ¹⁸O_trc trends, we find that temporal increases in VPD and temperature, independent of changes in g_s, have far greater contributions to δ¹⁸O_trc trends. Thus, this increasingly popular method should be used with caution, because it is highly challenging to unambiguously attribute trends in δ¹⁸O_trc to changes in g_s. Despite current limitations, we recommend how future studies can address these challenges in efforts to detect long-term g_s trends from tree ring records.