Seasonal Photosynthesis in Fertilized and Nonfertilized Loblolly Pine
Net photosynthesis (Pn) of loblolly pine (Pinus taeda L.) foliage was monitored monthly in 14 yr old stands under near-ambient conditions over an entire year in upper and lower crowns and in both nonfertilized stands and stands receiving nutrient amendments for six consecutive years. Air temperature, humidity, vapor pressure deficit (VPD), photosynthetic photon flux density (PPFD), and plant water potential were monitored concurrently with Pn. Foliar nitrogen (N) concentration was also monitored. The effect of fertilization on Pn was inconsistent and generally not significant. Rates were consistently higher in the upper crown compared to the lower crown primarily due to variable light intensity. Multiple linear regression analysis shows that PPFD and VPD explain between 56% and 64% of the variability in foliar Pn, depending on the treatment. Little or no correlation between foliar N concentration and Pn was found, despite greater N concentrations in fertilized foliage, suggesting that fertilization does not enhance the photosynthetic capacity of loblolly pine foliage over the long term. Substantial amounts of carbon were fixed on measurement days during the winter season, even after freezing nights. Predicted light response curves indicate that foliar photosynthetic capacities are similar year-round, and gross primary productivity estimates (GPP) indicate that over 20% of the annual carbon fixation occurred during the nongrowing season.