Foliar responses to fertilization identify nutrient limitation in loblolly pine

Postfertilization patterns of fascicle weight and foliar nutrient concentration can diagnose nutrient limitation and stem growth response of coniferous species that have one annual needle flush. We tested this approach on loblolly pine (Pinustaeda L.), a species with several annual growth flushes, by using only foliage from the first annual growth flush following fertilization. We analyzed the responses of foliage weight and nutrient concentrations to a 4 × 3 factorial application of nitrogen (0–336 kg/ha) and phosphorus (0–56 kg/ha), and used the results to predict growth limitation. Nitrogen fertilization increased fascicle weight and foliar nitrogen concentration in eight of the nine study sites, implying nitrogen limitation. Phosphorus fertilization increased fascicle weight and foliar phosphorus concentration in three sites, implying phosphorus limitation. Two- and 4-year diameter growth response data generally supported these predictions. Diameter growth responded to nitrogen or nitrogen plus phosphorus in all predicted sites, but the technique failed to identify phosphorus limitation in one site and nitrogen plus phosphorus limitation in another. Growth response predictions based on initial foliar nutrient concentrations or foliar responses to fertilization were equally accurate. The greater expense of using the foliar response method may not be justified in operational assessments of forest nutrition.