Attribution-Underdetermined Signals in Long-Term delta-15N Chronologies: A Multi-Pathway Diagnostic Using TRIDENT

Long-term declines in tree-ring and foliar delta-15N in boreal forests have been attributed to progressive nitrogen limitation (PNL) under rising atmospheric CO2. However, delta-15N integrates multiple mechanistically distinct pathways, including (1) CO2-driven nitrogen constraint migration via progressive nitrogen limitation, (2) deposition isotope forcing resulting from changes in the isotopic composition of atmospheric nitrogen inputs, and (3) photosynthetic acclimation without true nitrogen scarcity. This technical note applies the TRIDENT diagnostic framework to formalise the attribution structure underlying long-term delta-15N chronologies. TRIDENT evaluates boundary-driven system claims through four…