Growth of remaining branches in pruned Douglas-fir trees

1
Growth of remaining branches in pruned
Douglas-fir trees

• E.C. Turnblom and R.L. Collier

2
Growth of remaining branches in pruned
Douglas-fir trees

• Many variables enter into the decision whether or
  not to prune
• The usual reason is to enhance growth of clear
  wood volume after pruning but may involve
  intangibles such as aesthetics, understory growth
  enhancement for habitat enrichment, etc.
• Threats to value of the final clear wood crop can
  come from excessive epicormic branching (Collier
  and Turnblom 2000)

3
Growth of remaining branches in pruned
Douglas-fir trees

• If lower branches are removed by pruning, more
  resources might be available for those branches
  that remained above the pruning lift resulting in
  increased branch size there.
• Anecdotal observations indicated that the
  remaining branches on the pruned trees seemed
  larger than the branches on the unpruned trees.
• Paired tree study was set up to test if there are
  any differences in diameter or length of branches
  near the base of the live crown remaining after
  pruning

4
Data and Methods

• 18 Douglas-fir installations with 3 plots in each
  installation, 56 plots total
• Two plot configurations one with two ISPA/2 and
  one ISPA/4 plot, the other with two ISPA/4 plots
  and one ISPA/8 plot
• Plot densities range from 85 to 270 SPA
• There is one plot each of 20, 40, and 60 green
  crown removal intensity - every tree pruned
• Pruning triggered by attainment of 30-ft height

5
Data and Methods

• Pruned trees were selected from each pruned plot
  in a stratified random sample from the diameter
  distribution for measurement
• Unpruned counterparts were chosen purposively so
  that pre-pruning DBH was within 0.5 inch and
  height was within 3 feet of height of pruned tree
• Rely on the assumption that trees with similar
  DBH and height will have same branch diameter at
  same relative height on the bole - tenable given
  how branch profile models operate

6
Data and Methods

• Measured variables largest branch diameter
  (vertical and horizontal), largest branch length,
  bole diameter at crown base
• Simple differences between pruned and unpruned
  branch dimensions (dp - du) and (lp - lu) led to
  paired t-test analysis

7
Results
Difference (cm / m)
8
Results

• 20 live crown pruning intensity
• 78 sample pairs available for analysis
• Mean branch diameter difference was -0.02cm
  (-0.06in, p0.4024)
• Mean branch length difference was -0.342m
  (-1.12ft, p0.1923)
• New crown base is likely still in the region of
  the crown which is least productive, therefore
  residual branches do not undergo any appreciable
  increase in either internal or external resources

9
Results

• 40 live crown pruning intensity
• 103 sample pairs available for analysis
• Mean branch diameter difference was -0.04cm
  (-0.10in, p0.0935)
• Mean branch length difference was 0.178m (0.58ft,
  p0.3649)
• Crown actually removed apparently is still not
  enough to induce any significant difference

10
Results

• 60 live crown pruning intensity
• 77 sample pairs available for analysis
• Mean branch diameter difference was -0.06cm
  (-0.16in, p0.0687)
• Mean branch length difference was 0.9692 (3.18ft,
  p0.0002)
• Diameter response lagging temporally behind
  length response - diameter growth of the stem and
  branches is almost last on the allocation
  priority list, trailing behind maintenance
  respiration, fine root production, seed and other
  reproductive tissue production, and vertical and
  lateral shoot elongation

11
Conclusion

• Branches at the pruned crown base were not
  affected by either light or moderate pruning when
  compared to branches at the same height position
  on unpruned trees after four years
• severe pruning produces statistically significant
  differences between pruned and unpruned trees in
  terms of branch length but not branch diameter
• previous research demonstrated that with severe
  pruning there is also substantial risk of
  epicormic branching on the pruned portion of the
  bole that is likely to degrade the quality of the
  pruned log