Seasonal Changes in Nitrogen and Phosphorus Fractions and Autumn Retranslocation in Evergreen and Deciduous Taiga Trees

The concentrations and pool sizes of nitrogen— and phosphorus—containing chemical fractions were followed seasonally in leaves and young stems of four Alaskan tree species: larch (Larix laricina), black spruce (Picea mariana), birch (Betula papyrifera), and alder (Alnus crispa). We found no major differences in patterns of N and P distributions among major chemical fractions in nutritionally distinct deciduous trees. In the three diciduous species, all N and P fractions were highest in young leaves and declined in concentration through the season, first as concentration was diluted by increasing leaf biomass and later as organic N and P fractions were hydrolyzed and inorganic P and amino acid N were translocated out of leaves. The quantities of nucleic acids and phospholipids hydrolyzed in autumn were equivalent to 40—47% and 26—38%, respectively, of the total P retranslocated from leaves of deciduous species prior to abscission. Protein hydrolyzed and subsequently retranslocated as amino acids was equivalent to 82—91% of the N removed from senescing leaves. Leaching was much less important than retranslocation in removing nutrients from senescing leaves. In buds and stems, P was stored during winter primarily as phospholipid, nonhydrolyzable ester P, and nucleic acid P and was converted to inorganic P in spring. All species stored N primarily as protein and to lesser extent as nucleic acids; larch also stored some N in amino acids. In the evergreen spruce, N and P were stored in the same types of compounds as in deciduous species, but this nutrient storage occurred in leaves and involved no winter translocation to stems. Based on this study and a survey of the literature, we suggest that trees with low N and P status retranslocate a similar or smaller proportion of leaf N and P prior to leaf abscission than do trees with more favorable nutrients status. We conclude that tree have not adapted to nutrient stress through major changes in biochemical use of N and P. However, during the growing season there are important changes in allocation of N and P to different chemical fractions, associated with changing plant requirements.