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In northeastern Japan,an area of high precipitation and mountains,beech (Fagus creanata Blume),larch (Larix kaempferi Lamb.),cedar (Cryptomeria japonica D.Don) and black locust (Robinia pseudoacacia L.) were evaluated for N resorption and N isotope fractionation in preand post-abscission leaves in comparison to green leaves.The highest leaf N concentration in summer corresponded to the N-fixing black locust,followed in decreasing order by the deciduous beech and larch and evergreen cedar.On the other hand,the lowest N resorption efficiency corresponded to black locust and the highest to beech,in increasing order by larch and cedar.All tree species returned significant amounts of N before leaf abscission;however,N isotope fractionation during leaf N resorption was only found for beech,with a depleted N isotope value from green to pre-abscission leaf.The most N,however,was resorbed from pre-abscission to post-abscission.This result may indicate that δ15N fractionation took place during N transformation processes,such as protein hydrolysis,when the concentration of free amino acids increased sharply.The difference in the type of amino acid produced by each species could have influenced the N isotope ratio in beech but not in the other tree species.The results of this study showed that it is possible to infer the type and timing of processes relevant to N resorption by analyzing leaf δ15N variation during senescence.