Large volcanic eruptions elucidate source vs sink limitations to tree growth

Forest productivity projections remain highly uncertain, notably because underpinning physiological controls are delicate to disentangle. Whereas photosynthesis (carbon source) has been commonly assumed to drive tree growth, growing evidence show that direct limitations to cambial activity (sink limitation) represent a substantial control of tree growth. It nevertheless remains unclear to which extent source and sink limitations interact to determine tree growth because these processes mostly respond to the same environmental cues. Radiation is a notable exception, but its annual variations are typically small and covary with multiples cofactors in natural settings. Large volcanic eruptions, which have been suggested to enhance forest photosynthesis globally through diffuse light fertilization, provide a unique opportunity to retrospectively isolate source and sink activities. Here, we use a multi-proxy dataset of tree-ring records distributed over the extra-tropical Northern Hemisphere to investigate the effect of eruptions on tree photosynthesis and growth. Dual tree-ring isotope records (¹³C and ¹⁸O) denoted a widespread 2–4 years increase of photosynthesis following eruptions, likely as a result of diffuse light fertilization. We found evidence that enhanced photosynthesis transiently drove ring width, but the latter further exhibited an independent decadal anomaly. Our results provide empirical evidence of essentially decoupled photosynthesis and tree growth response to large volcanic eruptions, hence suggesting widespread sink limitation to tree growth over the Northern Hemisphere.