Background/Question/Methods Ongoing debate questions the role of carbon (C) assimilation by photosynthesis (C source) versus direct constraints on cambial cell development (C sink) in determining tree secondary growth response to environmental fluctuations. However, disentangling the effect of environmental cues on C source and sink has proven uneasy because both processes essentially respond similarly to typical environmental fluctuations under natural conditions. By transiently modifying global atmospheric conditions, in particular the ratio of diffuse to direct sunlight reaching tree canopies, stratospheric volcanic eruptions nevertheless constitute opportune large-scale experiments, where diffuse light is expected to fertilize the C source independently from C sink activity. Here we use global tree ring records to test the hypothesis that enhanced C source activity resulting from diffuse light fertilization following large volcanic eruptions in turn yields enhanced C sink activity. Tree ring records include c. 5000 tree ring width and density chronologies extracted from the International Tree-Ring Data Bank as well 425 chronologies of tree-ring based estimates of intrinsic water use efficiency (iWUE), which were retrieved from a recently published database. Compiled chronologies cover the period 1000 AD to present and represent boreal, temperate and seasonally dry forests. The date of past large volcanic eruptions was retrieved using an index of volcanic sulfate aerosol optical depth covering the same time period and was subsequently used to perform a superposed epoch analysis of the different tree ring records. Results/Conclusions Our results show widespread increase in iWUE during the 2-4 years following large volcanic events, whereas tree ring width and density display 8-10 years long negative anomalies overall, which tends to fade or become positive under hotter and drier conditions. These results suggest that transient diffuse light ratio increase following eruptive events during the last millennium did yield a short-term enhancement of C assimilation but had a limited effect on C sink activity, which more likely responded to longer term temperature reduction, contrary to the formulated hypothesis. Our study provides a new line of evidence supporting a limited coupling between tree C source and sink activities at the global scale. Implications include potentially limited fertilizing effect of elevated carbon dioxide atmospheric concentration on terrestrial C sink.
