Site fertility affects seasonal carbon reserves in loblolly pine

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Abstract
Concentrations of nitrogen, phosphorus and total non-structural carbohydrates were examined in loblolly pine foliage from control stands and stands fertilized with sludge containing 450 kg N ha(-1). Needles from the fertilized sites had higher N concentrations regardless of needle age. Fertilization increased P concentration in needles relative to the control sites. Seasonal patterns of change in sugar concentrations were similar among sites for both current-year and one-year-old needles, the highest concentration (126 mg g(-1)) occurring in February. Sugar concentrations were lowest in June and were generally lower in current needles than year-old needles. Starch concentrations and the amount of starch per fascicle varied both seasonally and among sites. Fertilization increased starch reserves in current needles at the end of the growing season (7 mg g(-1) and 14 mg g(-1) for the control and fertilized sites, respectively), and increased the rate of starch mobilization during the initiation of needle growth. In the growing season, starch concentrations were lower in current foliage from high fertility sites than low fertility sites. Carbon reserves accumulated during the growing season in mature one-year-old needles from control sites indicating a lower sink strength for carbon under low nutrient conditions. There was a positive relationship between starch and nitrogen concentrations during the dormant season whereas the relationship was negative during the growing season. The magnitude and direction of change in starch reserves with the onset of growth following the dormant season may indicate nutrient limitation and potential growth response to fertilization.

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