Sweetgum Half‐Sib Seed Source Response to Nitrogen and Phosphorus Fertilization in Mississippi

This field experiment was undertaken to determine the response to N and P of sweetgum ( Liquidambar styraciflua L.) under plantation culture, and the variation in response among seedlings derived from four half‐sib seed sources from alluvial and upland sites. Two‐year‐old seedlings were planted in a Mathiston silt loam (fine‐silty, siliceous, acid, thermic Aeric Fluvaquent) in 1972. In 1973, 0‐0‐0, 100‐0‐0, 100‐100‐0, and 200‐100‐0 kg/ha of N‐P‐K were broadcast. In 1975, the N treatments were repeated. Height, diameter at breast height (DBH), and foliar N were monitored through nine field growing seasons. There was no response to P. At 9 yr, the mean total height of the unfertilized half‐sibs was 7.1m, mean DBH was 7.8 cm, basal area was 5.0 m 2 /ha, stem volume outside bark was 15 m 3 /ha, and woody biomass was 8.1 Mg/ha. Increases from fertilization were 9, 14, 28, 39, and 40%, respectively. There was a half‐sib × fertility interaction with maximum response of three half‐sibs to 200 kg N/ha and one to 400 kg N/ha. The response of the two half‐sibs from alluvial seed sources was about double that of the two from upland seed sources. Foliar N concentrations of all treatments decreased during the 9 yr; however, foliar concentrations in fertilized trees were only slightly higher than in the unfertilized throughout. Although increases in annual increments due to fertilization were measurable for only 1 or 2 yr following fertilization, the resulting differences in volume and biomass were maintained in subsequent years.