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Highlights:
Four pruning intensities were evaluated in a seven-year-old Pinus patula plantation.
Increases in height, diameter and volume were measured one year after pruning.
Pruning the lower third of the crown generated the greatest increase in growth.
The number of epicormic shoots was greater when the lower two thirds of the crown were pruned.
Introduction
Pruning as a silvicultural tool is implemented with the main objective of producing knot-free wood, which translates into good quality timber when other intermediate cuts such as thinning are also applied (Baders, 2017; Fernández et al., 2016; Nyland et al., 2016). These are usually applied when stands are going to be managed intensively, since it is a silvicultural practice that requires a high investment and will be recovered until the end of the rotation (Ashton & Kelty, 2018; Nyland et al., 2016).
Classic silvicultural literature indicates that trees should be pruned when they have reached at least the height of the first commercial log and in stands on high quality sites that have been thinned (Ashton & Kelty, 2018; Daniel et al., 1982; Nyland et al., 2016; Smith et al., 1997).
The removal of inefficient branches for photosynthesis in the lower parts of trees can benefit tree growth (Nyland et al., 2016; Tonguc & Guner, 2017). On the other hand, excessive pruning promote a photosynthetic deficiency condition in the tree stand that is compensated by vigorous emission of photosynthetic structures, which can compromise growth and survival (Amateis & Burkhart, 2011; Masatoshi & Velez-Mesa, 1992).
Because of its physiological activity, the tree crown is usually divided vertically into three sections, of which the upper two thirds remain photosynthetically active, while the last third produces a lower amount of photosynthates, thus depending on the supply of metabolites produced by the upper two thirds for its subsistence. Accordingly, the theory suggests pruning the last third of the crown, since it is not only freed from significant photosynthate production, but also receives energy supply that could be used for other tree development processes, such as diameter and height growth (Ashton & Kelty, 2018).
In the ejido Llano Grande, municipality of Chignahuapan, Puebla, the clearcut regeneration method is applied in Pinus patula Schiede ex Schltdl. & Cham. stands, with immediate assisted regeneration, and when the saplings growth between two or three years these are pruned. Pruning is based on removing the portion of the crown located below half of its total height; however, when the prune is applied, usually the removal is over the lower two thirds of the crown, which can cause the reduction of sapling growth.
Therefore, the objective of this study was to evaluate the effect of applying four pruning intensities on the growth of a seven-year-old P. patula plantation in the Llano Grande ejido, to determine the pruning intensity that promotes greater growth in diameter at breast height, total height and volume, as well as the one that produces the greatest number of epicormic shoots.
Materials and Methods
Study area
The study was carried out in the ejido Llano Grande, municipality of Chignahuapan, in the northwestern region of the state of Puebla (19° 43’ 08.39’’ N, 104° 09’ 29.40’’ W) (Figure 1). The ejido is located in the Sierra Norte de Puebla in the foothills of the Sierra Madre Oriental, Physiographic Province V, specifically, in subprovince 57 called Lago y Volcanes de Anáhuac (Instituto Nacional de Estadística y Geografía [INEGI], 2001). Elevation ranges from 2 200 a 3 400 m.
The formation of the mineral matrix of the area dates from the Cenozoic era, specifically the Tertiary period and the Miocene epoch. The origin is volcanic, with a predominance of basaltic and andesitic rocks, and medium to high permeability. Regarding soil types, the area has a ditic Regosol and humic Andosol (INEGI, 2014).
According to the Köppen climate system, modified by Garcia (1964), the ejido has climates C(w2) and Cb’(w2) (Comisión Nacional para el Conocimiento y Uso de la Biodiversidad [CONABIO], 1998). The average annual temperature is 13.1 °C with an average annual precipitation of 1 463 mm, most of which is distributed between June and September (INEGI, 2018).
The dominant species in the ejido is P. patula, but it is also possible to find species of the genus Quercus and Abies religiosa (Kunth) Schltdl. & Cham. (Comisión Nacional Forestal [CONAFOR], 2016).
Figure 1 Location of the Llano Grande ejido, municipality of Chignahuapan, in the northwestern region of the state of Puebla. Source: Soto-Gil et al. (2022).
The experimental area has an area of 7 317 m2 with a homogeneous slope of 10 %. The plantation was established in mid-2011 with a density of 1 100 plants∙ha-1, so this property should have a number of individuals ranging between 800 and 1 100 trees. However, at the time of the establishment of the study (seven years), a series of gaps were observed.
Previously, the plantation was pruned in two periods (2014 and 2016) under the intensity scheme of approximately two thirds of the live crown. The sampling intensity was 10 %, resulting in 12 trees per treatment (four treatments with three replicates) and a total of 48 trees sampled (Table 1; Figure 2). Pruning was carried out in December 2018, under the premise that tree metabolism is reduced in winter and, therefore, these are prevented from negatively influencing wood development.
Table 1 Pruning treatments applied to a Pinus patula plantation in the ejido Llano Grande, municipality of Chignahuapan, Puebla
| Treatment | Description | Plots |
|---|---|---|
| 1 | No pruning (control) | 7, 8 and 11 |
| 2 | Pruning of the lower third of the tree crown | 3, 4 and 12 |
| 3 | Pruning of the lower half of the tree crown | 2, 6 and 9 |
| 4 | Pruning of the lower two thirds of the tree crown | 1, 5 and 10 |
Evaluated variables
The variables evaluated were: diameter at breast height of 1.30 m from ground level, measured with a diameter tape; height of the individuals from ground level to the terminal shoot, measured with a Bitterlich relascope (Spiegel, Model MS) and taking as reference its scale of 15 m distance to the tree; tree volume, determined from the standardized model to estimate the total volume of a tree of the species P. patula in the UMAFOR 2108 ( V= 0.000101dcc 1.893733 ∗ h 0.841048 ); and number of epicormic shoots by direct count. Each variable was recorded immediately after the pruning treatment and one year later to evaluate the increase in the number of pruning treatments.
Statistical analysis
The study was carried out under a completely randomized experimental design, with four treatments and three replications, with a total of 12 plots. The experimental unit was 400 m2 (20 m x 20 m square), consisting of no more than 40 trees.
The statistical model of the experimental design is shown as follows:
where,
Y ij = random variable representing the j-th observation of the i-th treatment (response variable)
µ = constant effect, common to all the levels of the factor, called global mean
τ i = part of Y ij caused by the action of the i-th level, which will be common to all the elements subjected to that level of the factor, called the effect of the i-th treatment.
µ ij = random variables comprising a set of factors, each influencing the response in only a small magnitude, but which should be considered.
The random variables µ ij are called perturbations or experimental error and can be interpreted as variations caused by all the non-analyzed factors that within the same treatment will vary from one element to another.
An ANOVA (P < 0.05) was performed to detect differences in the effect of the treatments using Excel 2016. Subsequently, and using the same program, a Tukey mean comparison was performed to identify groups with different or similar means.
Results and Discussion
Pruning intensity and its effect on height and diameter
According to Table 2, the ANOVA showed significant differences between treatments in all variables. Table 3 shows the average increments of the variables analyzed, in which the treatments with the highest pruning intensity (T3 and T4) had the lowest increments.
Table 2 Analysis of variance summary of the variables evaluated for Pinus patula after one year of pruning treatment.
| Variable | F | P- value | Critical value for F |
|---|---|---|---|
| Height | 5.91687 | 0.00175 | 2.81646 |
| Diameter | 14.7559 | 8.68E-07 | 2.81646 |
| Volume | 3.39993 | 0.02585 | 2.81646 |
| Epicormic shoots | 11.5277 | 1.05E-05 | 2.81646 |
Table 3 Average increase in growth variables and number of shoots of Pinus patula by pruning intensity treatment.
| Treatment | Description | Height (m) | Diameter (cm) | Volume (m3) | Epocormic shoots |
|---|---|---|---|---|---|
| 1 | No pruning | 0.92 ± 0.09 ab | 1.13 ± 0.12 b | 0.0097 ± 0.0021 b | 0.33 ± 0.65 dc |
| 2 | Pruning the lower third of the tree crown | 0.93 ± 0.17 a | 1.29 ± 0.21 a | 0.1080 ± 0.0031 a | 0.42 ± 0.67 cb |
| 3 | Pruning of the lower half of the tree crown | 0.84 ± 0.11 c | 1.08 ± 0.14 cb | 0.0093 ± 0.0022 cb | 1.42 ± 1.38 bc |
| 4 | Pruning of the lower two thirds of the tree crown | 0.74 ± 0.12 d | 0.88 ± 0.14 cd | 0.0077 ± 0.0022 d | 3.33 ± 2.31 a |
Means (± standard error, n = 12) with different letters indicate significant differences between pruning treatments according to Tukey's test (P = 0.05).
Tukey's test shows that the greatest height increment was recorded in T2 (0.93 m) (pruning the lower third of the tree crown); however, it was not statistically different from the control (0.92 m). According to Figure 3, T4 was the treatment with the lowest height increment (0.74 m) and was statistically different from the rest of the treatments (Table 3)
Figure 3 Height increment variation of Pinus patula by pruning intensity treatment (T1 = no pruning [control], T2 = lower third of the tree crown, T3 = lower half of the tree crown and T4 = lower two thirds of the tree crown).
The lower height increment trend at higher pruning intensities is consistent with the idea that pruning more than 50 % has a negative effect on growth. This agrees with the results of Fassola et al. (2002) in Pinus taeda L., who report that pruning intensities of 50 % of the live crown in early stages of development result in lower height increment, while when 30 % of the live crown is removed the height increment is not affected. In addition, the results of this study coincide with those of Davel (2013), who reported that light (25 %) and moderate (50 %) intensities had no significant effect on height increment in plantations of Pseudotsuga menziesii (Mirb.) Franco; even when there was no significant difference with the most intense treatment (65 % pruning), the trend was lower height increment in pruning’s higher than 50 %. In the case of Pinus pinaster Ait. and Pinus radiata D. Don., light pruning (12 to 15 %) had no significant effect on height growth (Hevia et al., 2016).
The same trend was observed for diameter increment as for height growth (Table 3). Figure 4 shows that the treatment causing the greatest increment was T2 (pruning of the lower third of the crown, 1.29 cm). On the other hand, the treatment without pruning (T1, 1.13 cm) showed no significant differences compared to T3 (1.08 cm), but the treatment with the highest intensity (T4) had the lowest diameter growth.
Figure 4 Variation in diameter at breast height (DBH) of Pinus patula by pruning intensity treatment (T1 = no pruning [control], T2 = lower third of the crown, T3 = lower half of the crown and T4 = lower two thirds of the crown).
Height increment showed significant differences (P = 0.05) between treatments T2 (removal of the lower third of the tree crown) and T1 (control) compared to T3 (pruning of the lower half of the tree crown) and T4 (removal of the lower two thirds of the tree crown), while diameter at breast height increment showed significant differences between T2 and the rest of the treatments. This is consistent with the idea that diameter growth is significantly more sensitive to intensive pruning than height increment (Amateis & Burkhart, 2011; Erkan et al., 2016).
Pruning the lower third of the tree crown (T2) was higher in diameter growth, indicating that this pruning intensity has no negative effect on radial increment, but rather benefits its increase. This agrees with that found by Tonguc and Guner (2017) in Pinus nigra J. F. Arnold, who reported that the effect of pruning 25 % of the crown showed a tendency of a higher increment than the rest of the treatments.
Table 3 shows a trend in the reduction of radial increment as pruning intensity increases. This coincides with the findings of Schneider et al. (1999) in Pinus elliottii Engelm, who consider the existence of an inverse relationship between pruning intensity and diameter increment, recommending pruning less than 40 % of the live crown. In the case of Eucalyptus grandis x Eucalyptus urophylla in Brazil, pruning of branches up to 70 % of the live crown has no effect on diameter growth, while more intense pruning (85 %) has a negative effect (Ferraz et al., 2016).
Although there is a negative relationship between pruning intensity and diameter increment, except for the light pruning treatment (T2), it can be assumed that the removal of ineffective branches may benefit tree growth (Nyland et al., 2016; Tonguc & Guner, 2017). The results coincide with the studies of Cown (1972) and Ferrere et al. (2015) in Pinus radiata D. Don, who report that pruning the lower third of the tree crown benefits radial growth. This could also be achieved by planting at a higher density that promotes natural pruning (Wang et al., 2019).
Pruning intensity and its effect on volume
Regarding volume, Figure 5 shows that the treatment causing the greatest increment was T2 (pruning of the lower third of the crown, 0.0108 m3). According to Table 3, this treatment had significant differences with the rest of the treatments. Volume, as determined by the increment in height and diameter at breast height, had similar trends with these variables; that is, less increment was observed as the intensity of pruning increased. Irschick et al. (2005) report that light pruning in Pinus taeda L. results in greater increment in volume per tree, while in Pinus radiata, pruning more than 50 % results in reduced growth (Ferrere et al., 2015; Neilsen & Pinkard, 2003). In this study, the difference between the average volume resulting from the upper (T2) and lower (T4) treatment was 0.10033 m3, which can be taken as the loss in volume that the trees had due to intense pruning.
Figure 5 Variation in volume increment of Pinus patula by pruning intensity treatment (T1 = no pruning [control], T2 = lower third of the crown, T3 = lower half of the crown and T4 = lower two thirds of the crown).
Based on the average increment in volume per tree resulting from the superior treatment (T2), extrapolated to the estimated number of trees in the experimental property (800 trees), it is calculated that each year there would be an increase of 8.64 m3. On the other hand, based on the average increase in volume per tree, obtained by applying the lower treatment (T4), there would be an annual increase of 6.16 m3. This indicates that the application of the lower pruning treatment results in a loss of 2.48 m3 of timber volume. Similar results have been obtained by York (2019) in Sequoia giganteum (Lindl. J. Buchholz).
The results of this study support the fact that maintaining the upper two thirds of the tree crown shows a constant photosynthetic activity, so it can be inferred that the increment in height and diameter and, therefore, in volume, is negatively affected when part of the crown undergoing effective photosynthesis is removed (Nyland et al., 2016). The results of Ferrere et al. (2015) in P. radiata plantations, with density equal to that of this study (around 1 100 plants∙ha-1), reveal that reducing the plantation density to 50 % plus a 40 % pruning of the live crown results in a loss of productivity. On the other hand, it has been reported that trees with different growth capacity respond differently to pruning intensity; for example, in Pinus massoniana Lamb., increment and height respond better with light pruning in weak trees, while vigorous trees respond better with severe pruning (Zhao et al., 2023).
Pruning intensity and its effect on epicormic shoot emission
The emission of epicormic shoots also showed different trends depending on pruning intensity. Figure 6 shows that the treatment with the lowest number of epicormic shoots was the control (T1, 0.33 shoots∙tree-1) which was only significantly different with the most intense treatment (T4, 3.3 shoots∙tree-1). Overall, the rest of the treatments were similar to each other and differed compared to T4 (Table 3).
Figure 6 Emission of epicormic shoots of Pinus patula by pruning intensity treatment (T1 = no pruning [control], T2 = lower third of the crown, T3 = lower half of the crown and T4 = lower two thirds of the crown).
The results indicate that high pruning intensity favors the emission of photosynthetic structures. The results are similar to studies reported by Masatoshi and Velez-Mesa (1982), Amateis and Burkhart (2011) and Desrochers et al. (2015), which indicate that, in addition to increased shoot emission, intensive branch pruning also results in vigorous growth of photosynthetic structures. Therefore, pruning should be applied at one third of the crown length to avoid the formation of epicormic branches that eventually decrease growth, specifically stem growth (Desrochers et al., 2015).
Conclusions
Pruning treatments had a significant effect on Pinus patula; in general, pruning of greater intensity (lower two thirds of the tree crown) had a negative effect on growth variables, so pruning in early development stages is not recommended. Since the plantation studied had already been pruned previously, it is recommended to study pruning in all growth stages where it is applied to define the most convenient pruning system.
