Nutrient retranslocation in temperate conifers.

Controversy exists about the relationship between retranslocation from leaves and uptake from soil in the mineral nutrition of trees. Evidence is presented to show that uncertainty has arisen partly because the significance of retranslocation from young leaves has been overlooked. Retranslocated nutrients are important for the production of new tissues at all stages of development from the seedling to the mature tree. Retranslocation is not, as has been thought, primarily associated with senescence and late stages in stand development. The key variables that determine the amount and rate of retranslocation in trees are the rates of nutrient uptake and growth. The amount and rate of retranslocation increase as tree growth rate increases, indicating that soil and environmental factors that promote growth also promote nutrient retranslocation. Retranslocation efficiency is increased by high soil fertility and rapid nutrient uptake and growth. Knowledge of the relationships between nutrient uptake and nutrient retranslocation should facilitate the development of simple models of whole-tree growth. Because nutrient retranslocation is driven by shoot growth rather than by nutrient supply in the soil, there is continual competition among shoots for internal nutrient reserves. This has implications for models describing shoot growth and crown development. Research on gas exchange processes in the crown should pay greater attention to the highly dynamic nature of nutrients in foliage.