Autumnal changes in total nitrogen, salt‐extractable proteins and amino acids in leaves and adjacent bark of black alder, eastern cottonwood and white basswood

Autumnal changes in total nitrogen, salt‐extractable protein and amino acid concentrations in leaves and adjacent bark of black alder [ Ainus glutinosa (L.) Gaertn.], eastern cottonwood ( Populus deltoides Bartr. ex Marsh.) and white basswood ( Tilia heterophylla Vent.) were determined for trees growing on minespoils and a prairiederived loamy soil in central Illinois. The composition of free amino acids in foliage was also determined at peak concentration for each tree species during late senescence. Total nitrogen concentration in the leaves decreased slowly throughout most of the fall for all species. In the final stages of senescence, total leaf nitrogen concentrations were about halved in eastern cottonwood and white basswood but continued to decrease slowly in black alder. The concentration of salt‐extractable proteins in leaves of all species peaked early in the fall and then declined prior to leaf abscission. This decline coincided with an increase in the concentration of free amino acids in the leaves. The increase stabilized in both eastern poplar and white basswood but continued in black alder. Glutamine in black alder and eastern cottonwood, and asparagine in white basswood were the most abundant free amino acids at the time of peak concentration of total free amino acids in senescent leaves. Bark of trees of all species had higher nitrogen concentrations and higher proportions of salt‐extractable proteins to estimated total proteins after leaf senescence than during the preceding summer. Results indicate that autumnal fluxes in leaf and bark nitrogen fractions of alder can differ substantially from fluxes in other broadleaved winter‐deciduous trees in a way which suggests that alder does not effectively conserve leaf nitrogen through retranslocation to bark tissue.