© 2000 Heron Publishing—Victoria, Canada
Effect of pruning on nitrogen dynamics within crowns of Pinus radiata
M. F. Proe (1), D. J. Mead (2) and D. Byrne (2)
1. Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, U.K. (m.proe@mluri.sari.ac.uk) / 2. Plant Science Department, P.O. Box 84, Lincoln University, Canterbury, New Zealand / Received August 20, 1999
Summary
Stem injection of 15N-labeled ammonium sulfate was used to determine effects of pruning on canopy nitrogen dynamics in open-grown Pinus radiata D. Don in New Zealand. Trees were planted in July 1990 and the isotope introduced in December 1994. Tree crowns were divided
into three zones: base section, from which branches of pruned trees were removed; mid section, between the pruned zone and
the height of the trees at the start of the year in which they were pruned; and top section, which grew predominantly after
the isotope was applied. Pruning removed 32% of the green crown length, representing 75% of foliage biomass. Needles were
sampled from each region of the crown until July 1996. Branch growth was used to predict foliage biomass for each sampling
occasion.
Approximately 45% of the applied isotope was recovered from needles sampled in December 1994 (1 week after application and
immediately before pruning), two-thirds of which occurred in needles in the base section. Thereafter, changes in isotope content
of needles in the base section of unpruned trees largely reflected foliage biomass fluctuations and dilution of the isotope
by continued uptake from the unlabeled soil nitrogen pool. Recovery of isotope in needles from the mid-crown section increased
by 58 and 86% from December 1994 to July 1995 in control and pruned trees, respectively. Within this crown section, there
was evidence of isotope translocation from old to new needles, with both isotope dilution and efflux observed in the needle
cohorts that had been present at the time the isotope was applied. Therefore, isotope dynamics did not reflect the dynamics
of the total nitrogen pool in the mid-crown section.
By July 1996, a small proportion of the applied isotope was recovered from the new foliage formed in the top section of the
crown. Within the top section, isotope dynamics closely matched total nitrogen fluxes. Pruning the lower crown did not affect
nitrogen dynamics elsewhere in the crown for the following 18 months.
Keywords:
foliage biomass, internal cycling, isotope content, 15N-labeling, stem injection.
