Relationships between nutrient status, carbon assimilation and water use efficiency in Pinus radiata (D. Don) needles.

The productivity of Pinus radiata D. Don plantations can be increased on many soils by N and P fertilization. Foliar analyses are usually taken as a guide for nutrient management, but there is little information about the relationship of foliar nutrient status to foliar physiology. This paper provides such information from N and P fertilization experiments. The concentrations and contents of N and P in the needles of P. radiata change because of accumulation during needle growth, retranslocation during shoot growth, and replenishment from current uptake during periods of little shoot growth. Up to 60% P and 50% N may be retranslocated in a single growing season from needles less than one year old. Three years after fertilizer application to a 10-year-old thinned P. radiata stand, net carbon assimilation rate per unit leaf area was in the order NP>P>N. There were positive associations between foliar P and assimilation at full sunlight, diffusive conductance and quantum yield. Foliar N was negatively associated with these variables. Higher concentrations of P and N were both accompanied by higher instantaneous water use efficiency by the foliage. Estimated trends in whole tree assimilation were similar to those in aboveground biomass accumulation. The interplay between the dynamic nature of nutrients in the foliage and their influence on the physiology of needles is discussed.