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, p=0.4024) • Mean branch length difference was -0.342m (-1.12ft, p=0.1923) • 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, p=0.0935) • Mean branch length difference was 0.178m (0.58ft, p=0.3649) • 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, p=0.0687) • Mean branch length difference was 0.9692 (3.18ft, p=0.0002) • 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