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Article

Effects of Girdling Intensity, Pruning Season and Thinning on Tree Growth, Crown Vigor and Wound Recovery in Japanese Larch

by
Michinari Matsushita
1,*,†,
Hiroki Nishikawa
2,† and
Akira Tamura
1,†
1
Forest Tree Breeding Center, Forestry and Forest Products Research Institute, 3809-1 Ishi, Juo, Hitachi 319-1301, Ibaraki, Japan
2
Fujiyoshida Examination Garden, Yamanashi Forest Research Institute, 1-18-2 Shinnishihara, Fuji-Yoshida 403-0017, Yamanashi, Japan
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2022, 13(3), 449; https://doi.org/10.3390/f13030449
Submission received: 22 December 2021 / Revised: 28 February 2022 / Accepted: 9 March 2022 / Published: 12 March 2022
(This article belongs to the Section Forest Ecology and Management)
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Abstract

:
To ensure sustainable forestry, it is important to establish efficient management procedures for seed orchards. We evaluated the effects of thinning, pruning season and girdling intensity on tree growth and vigor in an old Larix kaempferi seed orchard. Even after four years, tree size (crown width and tree height) increased, resulting in light intensity of an un-thinned class decreasing. Thinning counteracted the decrease in light intensity associated with tree growth, but it had no effect on promoting tree growth. Tree crown status was scored based on vigor and health. No significant difference in crown vigor was observed between unpruned and winter-pruned trees, but the summer-pruning class showed significantly lower vigor. Five years after girdling, trees with low and moderate girdling levels maintained high crown vigor, similar to intact trees, while the crown vigor of trees subjected to severe girdling was significantly lower. This is the first study to quantitatively model trends in remaining girdling depth over time, and to show that the half decay period was ca. 2–3 years. Our findings suggested that management procedures that combine moderate-level girdling, winter pruning and thinning to maintain rPPFD at 50% are well-suited to balancing tree vigor and reproduction in seed orchards.

1. Introduction

Japanese larch (Larix kaempferi (Lamb.) Carr.) often exhibits superior growth compared to other larch species, and the species has been widely used in breeding programs in many countries [1,2]. Hybrids between L. kaempferi and other larches have been used commercially in Europe [2], North America [3] and Asia [4,5]. In Japan, breeding programs for L. kaempferi were initiated in the 1950s, and more than 500 first-generation plus trees were selected [6]. Selection of second-generation trees started in the 2010s, and this is ongoing [6,7]. Reforestation using larches has increased over the last decade, and L. kaempferi is now the second most important forestry species in Japanese plantations [8]. Owing to the relatively higher growth rates and carbon fixation ability of this species [6,7], demand for improved seeds and seedlings from orchards is increasing [9].
Many seed orchards of first-generation plus trees in Japan are now more than 50 years old, but they still play an important role in seed supply as the seed orchards of second-generation plus trees are currently too young to produce enough seeds. In addition to the trees in seed orchards being too tall to be managed efficiently, the high density of old stems and branches in these old orchards means that they are too dark to produce sufficient quantities of good seeds. To resolve this issue, several treatments have been employed in orchard management; e.g., thinning [10,11,12,13,14] and girdling [15,16,17,18,19,20,21].
Thinning is a useful treatment method for improving the light environment in forests and orchards, as it artificially modifies the stand structure by opening spaces for the remaining trees. Previous studies that examined the effect of increasing light intensity found that thinning promotes reproduction [10,22,23,24,25]. In larch orchards, cone production is related positively to increases in light intensity [10,11,12,13,14]. However, after thinning, the remaining trees grow more and develop larger crowns [26], resulting in the gaps between crowns often being filled after thinning [27]. Conversely, the light intensity in un-thinned classes is typically lower. Obtaining quantitative estimates of changes in light intensity is therefore important.
Top pruning (topping) is a management technique that involves the removal of tree tops or dense upright branches, allowing trees to receive more light [28,29,30]. By reducing tree height, top pruning improves the ease and safety of seed collection in orchards [29,30]. However, pruning can affect tree growth and vigor negatively, as heavy pruning of fresh shoots reduces the photosynthetic capacity of the canopy [31,32]. It is therefore recommended that pruning be conducted in winter before carbohydrate reserves are depleted due to leaf flush [33].
Girdling (i.e., the removal of bark and cambium tissue) has been used extensively to control resource allocation [15,16,17,18,19,20,21] by interrupting phloem transport [34]. When the main stem (trunk) of a tree is girdled, the translocation of assimilates to the belowground parts is interrupted, resulting in a super-abundance of assimilates above the girdle [35,36]. Girdling is commonly used to promote plant reproduction [37] and to improve fruit yield [37,38]. Of the various cultural methods, girdling is one of the most effective ways to stimulate reproduction in conifers [15,16]. A common method involves removing a strip of bark from only half of the circumference of the trunk and a second half strip from the opposite side. However, there is often a trade-off between stimulating reproduction in this way and minimizing the detrimental effects of girdling; for example, while a high girdling intensity effectively disrupts short-term phloem transport, it may also damage tree health and decrease long-term reproductive output [39]. Further, because wounds take a long time to heal, severe girdling may be discouraged [40]. Therefore, the question arises as to how much girdling is required for a tree to survive and recover over the longer term. However, relatively few quantitative studies have been undertaken to assess the long-term effects of girdling severity to date.
It is expected that interventions such as increasing light intensity via thinning, pruning in different seasons and differences in girdling intensity have a marked influence on the vigor and growth characteristics of orchard trees. To ensure sustainable forestry, establishing an efficient management regime for seed orchards is necessary. We therefore quantitatively evaluated the effects of thinning, pruning and girdling on the growth and vigor (healthiness) of trees in an old L. kaempferi seed orchard. Moreover, we modeled time series trends in girdling wound depth and estimated the half decay period.

2. Materials and Methods

2.1. Study Site

The study site was the Fujisan Seed Orchard (35.42° N, 138.74° E; 1320–1350 m a.s.l.), which is managed by Yamanashi Prefecture. The orchard is located on the northeastern slope of Mount Fuji. The soil is composed primarily of weathered volcanic sediments derived from Mount Fuji. The mean annual temperature and precipitation were 10.6 °C and 1568 mm, respectively. The orchard, which was established between 1960 and 1962, is divided into several blocks. In this study, we used two large adjacent blocks (Blocks A and B; total area ca. 3 ha), which contained 44 L. kaempferi clones selected mainly from forests in Yamanashi and Nagano prefectures. The initial density before thinning was ca. 223.8 trees/ha in 2017. The following combinations of girdling, pruning and thinning treatments were studied: girdling was conducted in both Blocks A and B; thinning was conducted in the whole of Block B and half of Block A (i.e., setting both intact and thinning areas); top pruning was conducted in Block B. The details of the treatments are described in detail below.

2.2. Girdling

Girdling is a procedure that involves removing a 2 cm wide semilunar ring of bark and cambium at breast height on the main trunk using a knife. We randomly subjected 28, 36 and 27 trees to girdling intensity levels of 1, 2 and 3, respectively, and the remaining 66 trees were left intact. Four girdling levels were employed (Appendix A): one semilunar ring of bark and cambium was removed (level 1); two semilunar rings of bark and cambium were removed, with each ring oriented in opposite directions (level 2); three semilunar rings of bark and cambium with the same orientation were removed (level 3); and no girdling was performed (level 0; i.e., intact). Girdling was conducted in May 2016 in Block A and in May 2018 in Block B.

2.3. Thinning

To improve light environment, strip thinning was conducted in a north–south direction. The thinning intensity was 50%, and trees belonging to one half of the planting rows within blocks were cut, while trees belonging to the other half of the rows were not. In Block B, all of the area was subjected to thinning, and the trees in the half of the rows were cut in 2017. Block A was divided into two sub-blocks; one sub-block was subjected to conduct thinning, while the other sub-block was kept intact. The branches of remaining trees are valuable for seed harvesting, so thinning manipulation was conducted very carefully.

2.4. Top Pruning

In young seed orchards, terminal leader shoots (i.e., the young apical parts of the main trunk) are typically pruned to a height of 4 m. In old seed orchards, where the tree canopy has a high density of old stems and upright branches, the top nodes (i.e., the points on the trunk from which main branches originate) were pruned to a height of 6–8 m, usually during winter (i.e., before bud flush and shoot elongation). In February 2019, this top-pruning process was performed on half of the trees in Block B (hereafter, ‘winter pruning’ area). In the remaining half of Block B, top pruning was performed in June 2019 when the fresh leaves opened and current-year shoots fully elongated (‘summer pruning’ area).

2.5. Light Intensity

To investigate the effect of thinning on changes in the light environment, the photosynthetic photon flux density (PPFD) was used as an indicator of light intensity in this study. Measurements were conducted in Block A on cloudy days in July of 2017 and2021 using LI-250 light meters (LI-COR, Lincoln, Dearborn, MI, USA). The measurements were performed four times (in four directions) around the crown of each tree at the approximate midpoint of the height of the crown (5–6 m). The mean relative PPFD (rPPFD) was calculated for each planting position, as follows: rPPFD = PPFD above the crown of each tree / PPFD above the forest canopy (i.e., open sky).

2.6. Tree Size and Healthiness Index

To investigate the effects of girdling and thinning on tree size and growth, 44 trees of five clones in Block A were marked and monitored. We randomly selected those trees and assigned them to each treatment class, to investigate the girdling and thinning effects by avoiding the intrinsic effect of initial tree status, i.e., tree sizes did not differ between girdling class (F = 2.167, p > 0.05) nor between thinning class (F = 0.007, p > 0.05). Their height, diameter at breast height (DBH) and crown width were first measured in 2017 and then again in 2021. For the analysis of tree growth, we calculated the relative growth rate as the change in tree size from 2017 to 2021 divided by the initial size. Trees exhibiting normal growth and outliers were identified, and the outliers were excluded from the analysis.
To investigate the effects of girdling, thinning and pruning on tree health, we scored the status of each tree based on crown vigor and healthiness, as follows: trees that died (referred as to index 1), trees that almost died, or trees that produced current-year shoots and leaves that were sparsely distributed on a few living branches only (index 2), trees that produced fresh shoots and leaves that were relatively sparsely distributed within the crown (index 3), trees that produced fresh and well-elongated shoots with abundant leaves on most branches (index 4), and trees that produced numerous fresh shoots and leaves on almost all branches and showed perfect vigor (index 5).
Girdling wound depths in bark were measured for each girdling position on trees in Block A and B in July 2021. All measurements were performed in quadruplicate, and mean values were calculated for each tree. We also measured girdling wound depths immediately after girdling. The annual recovery rate (mm/year) was calculated as: (initial wound depth—remaining depth)/year.

2.7. Data Analysis

We used linear mixed effect models (LMM) [41] to analyze data in R 3.2.5 [42]. First, the seed orchard status in terms of light environments and tree sizes are summarized (Table 1), and we compared the population means of rPPFD values and tree sizes (height, diameter and crown width) between years by using LMM with ‘year’ and ‘clone’ set as fixed- and random-effect explanatory variables, respectively.
To evaluate whether changes in the light environment from 2017 to 2021 were positively affected by thinning, the rPPFD value around each tree canopy was analyzed by using LMM with ‘year’ and ‘thinning,’ and their interaction terms treated as fixed-effect explanatory variables, and ‘clone‘ included as a random-effect factor.
The effects of girdling and thinning on the relative growth rates for tree height, diameter and crown width were analyzed by LMM, with ‘girdling’ and ‘thinning’ treated as fixed effects, and ‘clone’ included as a random effect. The effects of girdling, thinning and pruning on the tree healthiness index and post-girdling recovery rate were analyzed by LMM, with ‘girdling’, ‘thinning’ and ‘pruning’ treated as fixed-effects, and ‘clone’ and ‘block’ included as random effects. We also considered inputting the fixed-effect interaction terms into these models, but since none of the interaction terms was significant, we omitted the interactions from the final models. After testing the main fixed effects, post hoc tests were conducted based on the Tukey-HSD method.
To estimate the time series trends in the remaining girdling depth and the half decay period, we used a nonlinear regression model, assuming a logistic response of post-girdling recovery over time. The logistic function simulated the remaining wound depths after girdling, as follows:
Dt = D0 − a/(1 + b exp( (c + g × Girdling + p1 × Pruningw + p2 × Prunings) × t),
where, Dt is the remaining wound depth at time (year) t, D0 is the initial wound depth, Girdling is the explanatory vector for girdling, Pruningw is the vector for winter pruning, Prunings is the vector for summer pruning, a, b and c are the basic parameters of the logistic functions, and g, p1 and p2 are coefficients. To obtain well-converged estimates, we set Girdling as a numeric vector rather than a dummy variable. Using the estimated parameters, we plotted the response curves and obtained estimates of half decay periods.

3. Results

3.1. Light Intensity

The orchard status in terms of light environments is summarized in Table 1; there was a significant difference in the light intensity over the course of the study period, with the mean rPPFD being 49.2% in 2017 and 35.9% in 2021. When comparing the temporal changes between the thinning manipulation classes (Figure 1), there was a marked reduction in the intact class, but there was no significant change in the thinning class.

3.2. Tree Size Growth Characteristics

The summary of orchard status in terms of tree sizes are shown in Table 1. Despite a significant difference in mean tree height and crown radius between 2017 and 2021, no significant difference was observed in the DBH between both years. Figure 2 shows the results of girdling and thinning treatment on the relative growth rates of tree sizes. There was a significant and positive effect of girdling on relative height growth rate (F = 4.938, p = 0.034; Figure 2B), but the effects of girdling on the rates of DBH (F = 1.004, p = 0.324; Figure 2C) and crown width (F = 0.000, p = 0.987; Figure 2A) were not significant. Thinning did not have a significant effect on any of the traits examined (Figure 2D–F; crown width: F = 0.08, p = 0.78; height: F = 0.27, p = 0.61; diameter: F = 0.01, p = 0.91). The random effect of clone was not significant (crown width: Varclone = 0.0016, χ2 = 0.00, p > 0.99; height: Varclone = 0.000, χ2 = 0.00, p > 0.99; diameter: Varclone = 0.0002, χ2 = 0.59, p = 0.44).

3.3. Healthiness Index and Recovery from Girdling Damage

The healthiness index varied in response to the severity of girdling (i.e., girdling level) across different pruning seasons (Figure 3A, Table 2). Significant negative effects were associated with girdling and pruning on tree healthiness, but the effects of thinning and interaction terms were not significant (Table 2). Compared to girdling levels 0–1 (i.e., intact or less-injured trees), the healthiness index for girdling level 3 trees (i.e., severely injured trees) decreased markedly. Even in the no-pruning classes (pruning “−” in Figure 3A), two out of 12 trees subjected to girdling level 3 died, but none of the trees subjected to girdling levels 0–2 died. Even at girdling level 0 (i.e., not girdled), the healthiness of trees in the summer pruning class (pruning “S” in Figure 3A) was lower than that in the other pruning classes (pruning “−”/“W” in Figure 3A). There were significant differences between girdling levels and pruning seasons for the recovery rate from girdling damage (Figure 3B, Table 2), but the effects of thinning and interaction terms were not significant. Post hoc comparisons of the main effects revealed that the recovery rate for trees subjected to girdling level 3 (severely injured) was lower than that for trees subjected to girdling levels 1 and 2. Moreover, the recovery rate in the summer pruning class (pruning “S” in Figure 3B) was significantly lower than that in the winter pruning class (pruning “W”). The random effects of clone on healthiness index (Varclone = 0.003, χ2 = 0.02, p = 0.88) and recovery rate (Varclone = 0.002, χ2 = 0.00, p > 0.99) were not significant.

3.4. Time Series Analysis of Girdling Depth Recovery

Figure 4 shows the results of a nonlinear regression model for time series trends in girdling depth remaining in the tree after manipulation, assuming a logistic response in their recovery from injury over time (the parameter estimates were shown in Figure 4 left). Each line indicates the response curve estimated for each girdling level. In the treatment class with winter pruning under thinning, the median times for remaining girdling depths of the girdled trunks were 2.3, 2.5 and 2.8 years for girdling levels 1, 2 and 3, respectively (Figure 4 left). The median times for girdling depth in the summer pruning class were 2.9, 3.3 and 3.8 years for girdling levels 1, 2 and 3, respectively (Figure 4 right).

4. Discussion

4.1. Effect of Thinning

The findings of this study showed that thinning counteracts the gradual decrease in light intensity associated with canopy growth; there was a marked reduction in light intensity in the un-thinned class over time, while the light intensity in the thinned class did not show significant decreases and remained at ca. 50% rPPFD. In a previous study, we showed that more than half of the trees in a stand—even ungirdled trees—tend to produce female cones when the light intensity reaches rPPFD ≥ 50%. Another study reported that a light intensity of ≥50% rPPFD is optimal for L. gmelinii var. japonica orchards [10]. The practice of thinning to target a light intensity level of 50% rPPFD appeared to be effective for maintaining light conditions in old seed orchards.
Previous studies reported that thinning has the effect of increasing tree growth [26,27,43]. However, despite increasing the light intensity, we did not find that thinning had a significant effect on tree growth. Previous studies have also reported that the growth response to increasing light intensity by thinning occurs only in a time window spanning several years after thinning, e.g., the growth rates for trees in a plot were increased at four years after thinning, but not at two years [25]. Some studies have found that thinning had a relatively short-term effect, and that although growth responses were observed after thinning, these responses lasted for less than a decade [26,44]. Similarly, the duration of growth responses was found to be related to thinning intensity [43,45]. Since the crowns of the remaining trees often expand, gaps are often filled soon after thinning [27]. In our study, even in a four-year study period, since tree sizes increased both upwardly (tree height) and laterally (crown enlargement), the light intensities in the un-thinned class decreased. In the old seed orchards, the upright branches often re-sprouted dense shoots and tree height was recovered. In previous studies on Larix, ensuring that orchard trees maintain a wider crown and relatively low height was found to improve cone production [46,47]. The present study examined the growth response of trees within the first two- to four-year periods after thinning, and the longer-term effects are still unclear. Further monitoring will be useful for exploring long-term responses in tree growth and canopy closure due to thinning.

4.2. Effect of Pruning

Pruning is a management technique that involves the removal of dense branches or tree tops by cutting, allowing trees to stay small and receive more light. However, removing photosynthetically active leaves from the crown also reduces tree productivity [32]. As the apex has a higher priority for carbon allocation than the stem, the response to pruning tends to recover photosynthetic capacity by rebuilding the crown, rather than by increasing stem growth [48]. The effect of pruning varies depending on the pruning intensity, pruning season and species [33]. If crown removal is not severe, then pruning has no, or little, effect on tree height and crown enlargement [48]. Even after low-intensity pruning, growth reduction has been observed [49]. In other cases, crown removal of 50–60% did not affect long-term growth [31,48]. A ten-year study on Larix occidentalis Nutt. found that pruning leaving 60% of the crown has only a minor effect on tree growth, suggesting that employing moderately severe pruning is an optimal pruning regime for crown management in this species [50].
In the present study, there was no significant difference in crown vigor between unpruned and winter-pruned trees, both of which showed almost perfect vigor. However, the summer-pruned trees showed significantly lower vigor, and most of the trees produced fresh leaves sparsely within their crown or on a few branches only. Thus, the effects of pruning can vary depending on the timing [33]. It has been recommended that pruning be conducted in late autumn to early spring when conifer trees grow slowly and have low physiological activity [33].

4.3. Effect of Girdling

Although Fajstavr et al. [51,52] modeled post-girdling radial growth of the trunk above and below the girdled section, our study is the first to model the annual trends in the remaining wound depth of recently girdled trunk sections. The half decay period was 2–3 years for the wound to half heal, but this was longer in cases of severe girdling. The typical response of girdled trees is to produce a callus from the cambium at the margins surrounding girdling wounds. This physiological response has been shown to be affected by girdling depth and width, and callus production is greater in vigorous trees [53]. As was observed in this study on L. kaempferi, if girdling is not too severe, trees continue to grow by producing a callus, which can differentiate into wounded wood [53,54]. Cambial activity has been shown to be affected directly by girdling when healing was prioritized [55]. After 2 cm wide girdling on P. strobus L., secondary cell growth was inhibited and finally stopped; the cells that had formed before girdling was performed differentiated during the remaining growth season, but no new cells were produced [56].
There is thus a tradeoff between healing through the resumption of cambial activity and tree vigor, and competition for assimilates [57]. After girdling, the belowground parts no longer act as a sink for assimilates, triggering an increase in tree growth above the girdle [58]. The present study found that height growth of girdled L. kaempferi trees was greater than that observed in intact trees. Overgrowth of above-girdled parts has similarly been reported in girdling studies [35,36,58]. Our study showed that, in a little as five years after girdling, L. kaempferi trees subjected to girdling levels 1 and 2 retained high crown vigor, similar to intact trees, while the vigor of trees subjected to girdling level 3 was significantly lower. Since the consumption of stored carbohydrates in roots is a response to damage to aboveground parts [59], severe girdling has a negative impact on xylem transport from the root system, resulting in reduced aboveground vigor [58]. Depending on the girdling intensity, the duration over which tree health can be maintained after girdling differs among species [40], and several conifers have been reported to remain alive for 1–5 years after girdling. Larix and Pinus species appear to be more resistant to girdling than other trees; for example, L. laricina (Du Roi) K. Koch and P. resinosa L. remained alive for two years after girdling [60], and P. strobus L. remained alive even after 60% girdling. Other species showed decreased survival if the girdling level was more than 25% [61], and pine trees died immediately after girdling [62]. Studies have also shown that most trees survived after 25% girdling, few trees survived after more than 75% girdling, and almost half survived within this range, even in the absence of crown damage [63].
Girdling is regarded as a cost-efficient and useful technique for achieving an increase in cone production, but the technique needs to be balanced against the detrimental effects associated with the damage induced by excessive girdling. We found that the magnitude of girdling has a marked effect on tree vigor. Even in the absence of pruning, two out of 12 trees subjected to girdling level 3 died, while none of the trees subjected to girdling levels 1 and 2 died. Our model of time series trends in the remaining girdling depth showed that the half decay period was ca. 2–3 years, and that it was longer for severe girdling. Although cone production increased as the girdling level increased [47], the positive effects of girdling on stimulating reproduction are relatively limited in duration, and severe girdling adversely affects tree vigor and decreases total long-term reproductive output. These findings suggest girdling level 3 may be too severe, while girdling level 2 might be better suited for balancing tree vigor and reproduction.

5. Conclusions

This study evaluated the effect of thinning, girdling intensity and pruning season on tree growth and vigor in an old L. kaempferi seed orchard. Even over a four-year study period, tree sizes (crown width and height) increased, and light intensity of the un-thinned class decreased. Thinning counteracted the gradual decrease in light intensity and kept rPPFD levels at ca. 50%. While the effect of thinning on tree growth was not significant, girdling had a positive effect on increases in tree height. Tree vigor was almost perfect in both unpruned and winter-pruned treatments, but vigor was significantly lower in summer-pruned trees. Five years after girdling, trees subjected to girdling levels 1 and 2 retained high vigor, similar to intact trees, while the vigor of trees subjected to girdling level 3 was significantly lower. Our results suggest that management procedures that combine moderate-level girdling, winter pruning and thinning to keep rPPFD at 50% are suitable for balancing tree vigor and reproduction in old seed orchards. In addition, since seed costs are lower when orchard lifetimes are longer, intensive management of thinning, girdling intensity and pruning can improve the cost-effectiveness of old orchards.

Author Contributions

M.M., H.N. and A.T. conceived and designed research. M.M., H.N. and A.T. performed field survey. H.N. managed study site. M.M. conducted data analyses and wrote original draft. A.T. and H.N. conducted supervision and project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by grants from the Project of the NARO Bio-oriented Technology Research Advancement Institution (the special scheme project on regional developing strategy; Forestry C105) and JSPS KAKENHI Grant Number 17K15291.

Acknowledgments

We thank the staff of the Fujisan Orchard for their assistance with field investigations.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Forests 13 00449 g0a1 550
Figure A1. Example photos for girdling levels 1–3.
Figure A1. Example photos for girdling levels 1–3.
Forests 13 00449 g0a1

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Forests 13 00449 g001 550
Figure 1. Changes in mean relative photosynthetic photon flux density (rPPFD %) values (±SE) in the intact and thinning classes within the seed orchard from 2017 to 2021. Different letters show statistical significances.
Figure 1. Changes in mean relative photosynthetic photon flux density (rPPFD %) values (±SE) in the intact and thinning classes within the seed orchard from 2017 to 2021. Different letters show statistical significances.
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Forests 13 00449 g002 550
Figure 2. Effects of girdling (A–C) and thinning (D–F) on the relative growth rates of crown width, height and diameter. In each panel, changes in size are shown for each class from 2017 to 2021. The asterisk within the panel indicates a statistical significance.
Figure 2. Effects of girdling (A–C) and thinning (D–F) on the relative growth rates of crown width, height and diameter. In each panel, changes in size are shown for each class from 2017 to 2021. The asterisk within the panel indicates a statistical significance.
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Forests 13 00449 g003 550
Figure 3. Treatment effects of girdling, thinning and pruning on healthiness index (A) and recovery from girdling damage (B). Means (±SE) are shown in each panel. Different lowercase letters above each panel indicate significant differences between pruning classes based on a post hoc comparison. The sample sizes (n) for girdling levels were 66, 27, 35 and 26 trees to levels 0, 1, 2 and 3, respectively; n for thinning manipulation: 26 and 128 trees in intact and thinned classes, respectively; n for pruning manipulation: 41, 75 and 38 trees in intact, winter- and summer-pruned classes, respectively. Different symbols show different treatment classes. Different letters show its statistical significances.
Figure 3. Treatment effects of girdling, thinning and pruning on healthiness index (A) and recovery from girdling damage (B). Means (±SE) are shown in each panel. Different lowercase letters above each panel indicate significant differences between pruning classes based on a post hoc comparison. The sample sizes (n) for girdling levels were 66, 27, 35 and 26 trees to levels 0, 1, 2 and 3, respectively; n for thinning manipulation: 26 and 128 trees in intact and thinned classes, respectively; n for pruning manipulation: 41, 75 and 38 trees in intact, winter- and summer-pruned classes, respectively. Different symbols show different treatment classes. Different letters show its statistical significances.
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Forests 13 00449 g004 550
Figure 4. Dependence of remaining girdling depths on time after girdling manipulation across different pruning seasons. In each graph, each line indicates one of the three girdling levels.
Figure 4. Dependence of remaining girdling depths on time after girdling manipulation across different pruning seasons. In each graph, each line indicates one of the three girdling levels.
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Table
Table 1. Summary of the orchard status in terms of relative photosynthetic photon flux density (rPPFD), diameter at breast height (DBH), tree height and crown radius in 2017 and 2021.
Table 1. Summary of the orchard status in terms of relative photosynthetic photon flux density (rPPFD), diameter at breast height (DBH), tree height and crown radius in 2017 and 2021.
rPPFD (%)DBH (cm)Height (m)Crown Width (m)
2017Mean49.238.810.14.4
(n = 39)S.E.2.470.950.400.13
2021Mean35.939.011.45.3
(n = 39)S.E.2.570.880.480.17
F-value13.9080.0155.58817.788
p-value<0.0010.9030.021<0.001
Table
Table 2. Summary of linear mixed model analysis of healthiness index and recovery rates.
Table 2. Summary of linear mixed model analysis of healthiness index and recovery rates.
Healthiness Index Recovery Rate
F-Valuep-ValueF-Valuep-Value
Thinning0.2040.5620.1370.712
Girdling4.2380.00713.030<0.001
Pruning61.512<0.00120.593<0.001
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Matsushita, M.; Nishikawa, H.; Tamura, A. Effects of Girdling Intensity, Pruning Season and Thinning on Tree Growth, Crown Vigor and Wound Recovery in Japanese Larch. Forests 2022, 13, 449. https://doi.org/10.3390/f13030449

AMA Style

Matsushita M, Nishikawa H, Tamura A. Effects of Girdling Intensity, Pruning Season and Thinning on Tree Growth, Crown Vigor and Wound Recovery in Japanese Larch. Forests. 2022; 13(3):449. https://doi.org/10.3390/f13030449

Chicago/Turabian Style

Matsushita, Michinari, Hiroki Nishikawa, and Akira Tamura. 2022. "Effects of Girdling Intensity, Pruning Season and Thinning on Tree Growth, Crown Vigor and Wound Recovery in Japanese Larch" Forests 13, no. 3: 449. https://doi.org/10.3390/f13030449

APA Style

Matsushita, M., Nishikawa, H., & Tamura, A. (2022). Effects of Girdling Intensity, Pruning Season and Thinning on Tree Growth, Crown Vigor and Wound Recovery in Japanese Larch. Forests, 13(3), 449. https://doi.org/10.3390/f13030449

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