Intra-annual nutrient flux in Pinus taeda. 
Intra-annual nutrient (nitrogen, phosphorus, potassium, calcium and magnesium) flux was quantified for Pinus taeda L. at a nutrient-poor, well-drained sandy site in Scotland County, NC, USA where a 2 × 2 factorial of irrigation and nutrition was applied in four replications in a 10-year-old stand with 1200 stems ha(-1).  Treatments were applied with the goal of providing optimum nutrition (no nutritional deficiencies) and water availability.  Component (foliage, branch, stem and root) nutrient content was estimated monthly for 2 years using nutrient concentration and phenology assessments combined with destructive harvests.  Positive flux values indicated nutrient accumulation in the trees while negative values indicated nutrient loss from the trees.  Fertilization significantly increased nitrogen, phosphorus, potassium, calcium and magnesium flux 140%, on average, over non-fertilized.  Irrigation significantly increased calcium flux 28% while there was no significant irrigation effect on nitrogen, phosphorus, potassium or magnesium.  Maximum nutrient fluxes (kg ha(-1) day(-1)) for non-fertilized and fertilized stands were 0.36 and 1.05 for nitrogen, 0.042 and 0.095 for phosphorus, 0.13 and 0.51 for potassium, 0.27 and 0.42 for calcium, and 0.04 and 0.12 for magnesium, respectively.  Maximum flux was coincident with ephemeral tissue (foliage and fine root) development and likely would be higher in stands with more foliage than those observed in this study (projected leaf area indices were 1.5 and 3.0 for the non-fertilized and fertilized stands).  Minimum nutrient fluxes (kg ha(-1) day(-1)) for non-fertilized and fertilized stands were -0.18 and -0.42 for nitrogen, -0.029 and -0.070 for phosphorus, -0.05 and -0.18 for potassium, -0.04 and -0.05 for calcium, and -0.02 and -0.03 for magnesium, respectively.  Minimum fluxes were typically observed in the dormant season and were linked to foliage senescence and branch death.  Foliage and branch component nutrient contents were out of phase for nitrogen, phosphorus, potassium and magnesium, indicating nutrient retranslocation and storage in branches prior to foliage development and after foliage senescence.  In contrast to current operational fertilizer programs which often target winter application these data suggest the best application times would be during foliage development.