|dc.description.abstract||One-year-old (C+1) and current (C) cohorts, labelled with a ¹⁵N tracer, were studied on second order branch systems of four trees of five-year-old Pinus radiata D. Don. Concurrent with this, individual one-year-old (C+1) and current (C) fascicle nitrogen (N) and ¹⁵N dynamics were also studied on the second order branches. The objective was to improve our understanding of internal cycling of N within second order branches, used for standard foliage analysis, within different canopy positions over a growing season.
Branches were destructively sampled from two canopy levels on the north side of the four trees. Individual C+1 fascicles were sampled from set positions along second order branches at each sampling, including new current fascicles (C). The trees were classed as blocks and the canopy level as a split-plot. The experiment was carried out between August 1995 and June 1996 over one complete growing season at the Lincoln University agroforestry experiment (43°38'S, 172°30' E) Canterbury, New Zealand. The climate is temperate and has a mean annual rainfall of 695mm which is evenly distributed and the soil is a Templeton silt loam, a moderate to free draining fertile soil.
Mass of C+1 fascicles over all positions decreased 7-12% over the experimental period. The C-base and C-top fascicles increased in dry weight mass over the growing season to 63 and 64 mg respectively but never attained the same mass as the C+1 fascicles. The second order branch system mass of the C+1 foliage accounted for over 70%, or 19.3g, of the branch system in August 1995. By June 1996 the C+1 and C cohorts accounted for 32% and 38% respectively of the total mass of the branch system. C+1 wood and bark components had increased their mass by June 1996 by 136% and 16% respectively.
C+1 fascicle N and ¹⁵N concentration declined from August 1995 until March 1996, however, from March 1996 to June 1996 there was a small increase in concentration again in all fascicle positions. C fascicles fluctuated in concentration from December 1995 to June 1996. Branch system foliage N concentrations for both the C+1 and C cohorts exhibited periods of increases and decreases. However, at the final sampling in June 1996, the C+1 cohort N concentration was 9.0% less than the concentration of the C cohort which was 1.66% N. Other components were considerably lower, apart from the bud component, in N concentration with the C+1 wood the lowest at 0.237% N at June 1996. ¹⁵N atom% in initial components at August 1995 had decreased from 0.571% at August 1995 to 0.506% in June 1996, an 11% decrease.
N and ¹⁵N content of the three C+1 fascicles decreased 8-17% and 32-41% respectively from August 1995 to June 1996. From March 1996 to June 1996 there was an increase in all fascicle positions. Both the new C fascicle positions increased their N and ¹⁵N content at each sampling. The branch system total N content nearly doubled over the experimental period from 359 mg to 666 mg, while excess ¹⁵N content was relatively constant from December 1995 to March 1996. A 20% reduction in N content between December 1995 and March 1996 was evident for the C+1 cohort indicating possible stress due to seasonal drought. The N content in the C cohort increased over the experimental period following growth patterns. There was evidence of late N and ¹⁵N content, 8% and 30% respectively, withdrawal from the C cohort from March 1996 to June 1996. The increase in excess ¹⁵N in the C cohort indicated retranslocation from the older components from within the whole tree.
The net losses of N content from the C+1 cohort did not meet the N needs of the expanding C cohort. Excess ¹⁵N content loss from the C+1 cohort exceeded the accrual of excess ¹⁵N content in the C cohort. The branch system was deemed not be independent for N use but showed autonomous behaviour for the excess ¹⁵N pool within the branch system. The variability in excess ¹⁵N and atom% data may indicate isotope fractionation within the trees at the experimental site.||en