Internal nutrient recycling through retranslocation (resorption) is important for meeting the nutrient demands of new tissue production in trees. We conducted a comparative study of nutrient retranslocation from leaves of five tree species from three genera grown in plantation forests for commercial or environmental purposes in southern Australia--Acacia mearnsii De Wild., Eucalyptus globulus Labill., E. fraxinoides H. Deane & Maiden, E. grandis W. Hill ex Maiden and Pinus radiata D. Don. Significant amounts of nitrogen, phosphorus and potassium were retranslocated during three phases of leaf life. In the first phase, retranslocation occurred from young leaves beginning 6 months after leaf initiation, even when leaves were physiologically most active. In the second phase, retranslocation occurred from mature green leaves during their second year, and in the third phase, retranslocation occurred during senescence before leaf fall. Nutrient retranslocation occurred mainly in response to new shoot production. The pattern of retranslocation was remarkably similar in the leaves of all study species (and in the phyllodes of Casuarina glauca Sieber ex Spreng.), despite their diverse genetics, leaf forms and growth rates. There was no net retranslocation of calcium in any of the species. The amounts of nutrients at the start of each pre-retranslocation phase had a strong positive relationship with the amounts subsequently retranslocated, and all species fitted a common relationship. The percentage reduction in concentration or content (retranslocation efficiency) at a particular growth phase is subject to many variables, even within a species, and is therefore not a meaningful measure of interspecific variation. It is proposed that the pattern of retranslocation and its governing factors are similar among species in the absence of interspecies competition for growth and crown structure which occurs in mixed species stands.

Phosphorus (P) is often a limiting factor of forest growth but our knowledge of the processes governing P availability in forest soils is rather limited. In the present work, we combined a isotopic dilution method with extraction methods to evaluate the P status in Pinus pinaster plantation forests on highly P-deficient soils. Total, organic, and inorganic P, dissolved and diffusive P, i.e. ionic P species that can be transferred from the solid phase to the soil solution due a gradient of concentration, were determined to a soil depth of 120 cm in a gradient of 18 forest sites (seven humid sites, five mesic sites, and six dry sites). Our objective was to assess the potential contribution of organic and inorganic P to plant available P. Based on results and our original assumptions, we observed that the contribution of organic P fractions (mineralization of soil organic P) to P availability related to the contribution of inorganic P fractions (diffusive P for durations up to 1 year) was predominant in litter, less important in top soil horizons, and negligible at depths below 30 cm. This was partly due to a decreasing proportion of organic P and an increasing proportion of diffusive P with soil depth. Owing to a very low amount of diffusive P in the top soils in dry sites, the relative contribution of organic P was actually higher in these sites than in humid and mesic sites, despite a lower overall organic P fraction. The combination of extraction and isotopic dilution methods in our study shed new light on P status in this forest range. In particular, these methods enable assessment of both the size of the pools and their dynamic fractions.

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We measured patterns of change in concentrations and contents of nitrogen, phosphorus, potassium, magnesium and calcium in fully expanded leaves of young Eucalyptus globulus (Labill.) trees growing in a plantation in southeastern Australia, over a 12-month period beginning at the onset of spring. There was significant net retranslocation of mobile nutrients on a seasonal basis from green leaves, coinciding with continued growth and production of foliage. There was a close positive relationship between initial nutrient content (N, P and K) of the leaf and amount retranslocated, and a tight coupling between N and P retranslocated from leaves. Net retranslocation was significantly correlated with basal area growth increments. Artificial shading of leaves resulted in senescence and reduction in leaf mass. It also induced retranslocation of N, P and K from leaves of different ages and from different position in the canopy. Although the mechanisms underlying the effects of shading intensity on these changes were not elucidated, shading provided an experimental tool for studying retranslocation. Comparison of the results with published data for Pinus radiata (D. Don) grown in the same environment indicated a similarity between the species in patterns of change in foliar nutrient contents and in factors governing foliar nutrient retranslocation, giving rise to unifying principles.