4. Discussion

Several previous studies have referred to O'Hara (1991) and explained that the removal of up to one-third, or 33% of the live crown did not noticeably reduce diameter growth (Robbins, 2000; Clark & Matheny, 2010; Kirby, 2016; Rais et al. 2020; Suchocka et al., 2021). Nevertheless, there remained the question of how the limit value of 33% was obtained by calculation.
  The present study applied segmented regression to the data of O'Hara (1991) and revealed that in the whole data set, an abrupt change toward a decrease in diameter growth response was detected at 25% of the live crown removed. This value was a little smaller than the limit value of 33% estimated by O'Hara (1991) and cited by the above-mentioned studies.
  O'Hara (1991) mentioned the effect of different pretreatments on tree growth, but did not take it into consideration when evaluating the crown removal limit. However, the present results revealed that the different pretreatments produced different relationships between pruning severity and growth response diameter.
  In the group of fully crowned and open-grown, diameter growth response continuously decreased with increasing pruning severity with no significant abrupt change. Therefore, it seemed difficult to determine the crown removal limit. However, in the group of 70-90% live crown, diameter growth response did not significantly decrease up to the break point (53% crown removed) and then abruptly decreased. Consequently, the difference in pretreatments was found to have a great impact on the relationships between pruning severity and growth response diameter.

5. Conclusion

This is probably the first study that evaluated the effect of pruning severity on tree growth by change point analysis using segmented regression. The advantage of segmented regression is that it enables us to analyze continuous data as they are. In other words, segmented regression does not treat continuous predictors such as pruning severity as categorical predictors. Therefore, the present study with segmented regression was able to analyze the relationship between pruning severity and growth response diameter to estimate the limit value up to which pruning can be done without serious growth losses. The results will recommend the use of change point analysis in future studies on pruning severity.

Acknowledgements

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The author confirms sole responsibility for the following: study conception and design, data collection, analysis and interpretation of results, and manuscript preparation.

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