NITROGEN ALLOCATION AND CARBON-ISOTOPE FRACTIONATION IN RELATION TO INTERCEPTED RADIATION AND POSITION IN A YOUNG PINUS-RADIATA D. DON TREE

The three dimensional distribution of intercepted radiation, intercell ular CO2 concentration (C-i) and late summer needle nitrogen (N) conce ntration were determined at the tips of all 54 branches in a 6.2-m-tal l Pinus radiata D, Don tree growing in a New Zealand plantation. Measu rements included above- and below-canopy irradiance, leaf stable carbo n isotopic composition (delta(13)C) and tree canopy architecture. The radiation absorption component of the model, MAESTRO, was tested on si te and then used to determine the branch tip distribution of intercept ed radiation. We hypothesized that in branch tip needles: (i) the allo cation of nitrogen and other nutrients would be closely associated wit h the distribution of intercepted radiation, reflecting carbon gain op timization theory, and (ii) C-i would predominantly reflect changes in photosynthetic rate (A) rather than stomatal conductance (g(s)), indi cating that the increase in A for a given increase in N concentration was larger than the corresponding increase in g(s). Needle nitrogen co ncentration was poorly related to intercepted radiation, regardless of the period over which the latter was calculated. At a given height, t here was a large azimuthal variation in intercepted radiation but N co ncentration was remarkably uniform around the tree canopy, There was, however, a linear and positive correspondence between N concentration and delta(13)C and needle height above ground (r(2) = 0.73 and 0.68, r espectively). The very strong linear correspondence between N concentr ation and C-i (r(2) = 0.71) was interpreted, using gas exchange measur ements, as supporting our second hypothesis. Recognizing the strong ap ical control in P. radiata and possible effects of leaf nitrogen stora ge in an evergreen species, we propose that the tree leader must have constituted a very strong carbon sink throughout the growing season, a nd that the proximity of branch tip needles to the leader affected the ir photosynthetic capacity and nutrient concentration, independent of intercepted radiation. This implies an integrated internal determinati on of resource allocation within the tree and challenges the current c onvention that resources are optimally distributed according to the pr ofile of intercepted radiation.