Growth Response of Trees with Different Growth Statuses to Pruning on a Pinus massoniana Lamb. Plantation

Abstract

Pruning is an important technique in culturing good knot-free timber. However, to make more accurate pruning plans, it is necessary to consider the growing status of trees and set reasonable pruning intensities based on this. In a seven-year-old Pinus massoniana Lamb. plantation, we carried out pruning twice with a time interval of two years. The treatments included one unpruned treatment (CK) and five pruned treatments (from P1 to P5, representing the live branch height being kept at 68%, 55%, 60%, 55% and 45% of the tree height). CK, P1 and P2 were conducted in the first pruning in March 2019, and the remaining treatments were conducted in the second pruning in January 2021. The growth investigations were carried out in March 2019, December 2019, December 2020 and December 2021. Another investigation was carried out only for the measurement of live branch heights and crown widths in January 2021 just after the second pruning. The results showed that pruning resulted in a 15.08% to 60.62% increase in diameter growth and an 10.28% to 29.87% increase in volume growth. The stem form was also improved. Significant differences were recorded in live branch heights after green pruning but gradually recovered to the same level in two years by branch senescence. Pruning also resulted in a faster extension of the crown width with an enlarged growing space by the removal of green branches. We highlighted that trees with different growth statuses responded differently to pruning intensities: trees of weak growth statuses grew better under a light pruning intensity, while those with a strong growth status grew better under a severe pruning intensity. Overall, for the seven-year-old young mason pine plantation, keeping four rounds of branches in trees with diameters of less than 8 cm and keeping three rounds of branches in trees with diameters of more than 8 cm were appropriate measures.

1. Introduction

Pruning is a traditional measure in forest management. The main purpose of pruning is to cultivate knot-free quality timber of good form by removing useless, excessively growing and competitive branches [1,2]. Applications of pruning have verified that pruning is an effective way to regulate the assimilate partitioning of trunks, branches and leaves, and to improve trunk form [3]. The branch is the primary member of the crown and is the essential organ that holds leaves to proceed with photosynthesis to produce organics; therefore, the removal of leaves will certainly affect the growth of trees [4,5]. However, the effects differed with the character of species, intensity of pruning and stand quality of the forests [6,7,8]. Coniferous species might have a more complicated response system for pruning. A study of radiata pine (Pinus radiata D. Don) indicated that the negative effects on height and diameter growth lasted for 11 years [9]. Considering the inhibitory effects of severe pruning on growth and the weak promotion that results from light pruning, an appropriate pruning intensity is important for the cultivation of ideal timber [10,11,12]. The optimal pruning intensity should be determined with a consideration of species, the age of forests, growth conditions, crown ratio, etc.

Masson pine (Pinus massoniana Lamb.) is a native tree species in China. With its ecological characteristic of wide adaptability, it spreads to more than 17 provinces and cities across the subtropics and north tropic zone [13,14]. Thus, it has become the most widely distributed pine in China. In suitable habitats, it grows a full and straight stem with a small taper, large form factor, and good wood quality, making it an ideal timber species [15]. However, dead knots easily form during the growth of P. massoniana, which lowers the wood quality. In the early fast-growing stage of P. massoniana, the tree height and diameter show accelerated growth, and the crown gradually closes, causing a competition between individual trees. In this period, stands grow quickly in lateral branches. However, the resin in the dead branches makes them hard to shed [16], and the dead branches might form dead knots, affect the wood quality and also easily burn, which would require proper pruning for weak self-pruning. Therefore, pruning is an important and necessary cultivation measure for the growth of P. massoniana, it leaves more growing space for trees to make the trunks straighter and have fewer knots, keeps the stand in a sensible density structure, improves light conditions above the ground and increases nutrition space under the ground. However, the growth statuses of trees of the same stand and of the same age differ because of the competition between trees, resulting in different diameters, heights and crown widths. For a young P. massoniana plantation which is in a fast-growing stage, it is important to set the optimal pruning intensity, but the optimal pruning intensity may differ between large trees and small trees. In terms of how to determine the optimal pruning intensity by the growth status of trees, there is still a lack of relevant studies. Based on this, we designed a pruning trial on a young Masson pine plantation to study the following questions: How does young Masson pine respond to pruning during growth? Are the responses of different tree statuses to the same pruning intensity the same? In accordance with the research results, a scientific pruning plan was made for P. massoniana stands.

2. Materials and Methods

2.1. Description of the Study Area

The trial was located in Mochong town, a southern suburb of Duyun city, Guizhou Province. The climate of the area corresponds to a humid subtropical monsoon climate with abundant rainfall, and the average rainfall reaches 1431 mm. The average temperature is 5.6 °C in the coldest month (January) and 24.8 °C in the hottest month (July), with no extreme weather in winter and summer. The terrain is mainly mountainous, and the soil in the trial area is fertile yellow soil. The trial started in 2019 in a 7-year-old P. massoniana plantation that was planted in 2012 with an initial planting density of 2 m × 2 m, which was the same as the existing density. The canopy density of the stands was 0.85 when they were pruned in March 2019.

2.2. Experimental Design

Considering the growth of one branch per year of P. massoniana, it is insufficient to express the pruning intensity by the percentage of removed live crown or live branch height (LBH) in a 7-year-old young P. massoniana forest, so the rounds of branches we kept were used to represent the pruning intensity, which was clearer to understand and easier to operate. The pruning was carried out twice with a time interval of two years. The first pruning experiment was carried out in March 2019, with a randomized block design with three treatments and three replicates in three blocks. In each block, there were three plots corresponding to three treatments, involving keeping all branches (control, CK), keeping four rounds (P1, equal to LBH at 68% of tree height) and keeping three rounds (P2, equal to LBH at 55% of tree height). Each plot had 81 trees in an area of 20 m × 20 m (Figure 1). Trees in the same plot were treated with the same pruning intensity regardless of the growth status.

In January 2021, based on the first pruning, under treatment P1, which consisted of four branch rounds, the number of branch rounds had grown to six branch rounds, and under P2, which consisted of three branch rounds, the number of branch rounds had grown to five branch rounds. The second pruning experiment was conducted, including keeping five rounds (P3, equal to LBH at 60% of tree height), keeping four rounds (P4, equal to LBH at 55% of tree height), and keeping three rounds (P5, LBH at 45% of tree height). Specifically, each plot was divided into two equal areas in the horizontal direction, and half of the plots of CK, P1 and P2 were carried out with P3, P4 and P5, respectively (Figure 2).

2.3. Data Collection

In March 2019, all trees in each plot were measured for the first time with the following traits: total height (H), diameter at breast height (DBH), live branch height (LBH) and crown width (CW). H was measured with a height-measuring lever (range: 12 m, precision: 0.01 m), the DBH was measured at 1.3 m above the ground with a diameter tape, the LBH was measured with a height-measuring lever at the first branch growing green needles, and the CW was the average value of the vertical projection width of the crown in two directions (north–south direction and east–west direction) measured with a measuring tape. According to the first survey, all trees were divided into three groups by DBH: diameter class A consisted of trees with DBHs from 10 cm to 12 cm, diameter class B consisted of trees with DBHs from 8 cm to 10 cm, and diameter class C consisted of trees with DBHs from 6 cm to 8 cm. Trees in the diagonal line of each plot were numbered with spray paint, and approximately 30 trees in each plot were marked. The following surveys mainly measured the marked trees with the above traits in each December month from 2019 to 2021, and the final measurement was conducted in December 2021. Another survey was carried out in January 2021, just after the second pruning. This survey only measured two traits, LBH and CW.

Tree volume (V) was calculated with the following equation [17]:

$$V=0.00006789·DB{H}^{\left[1.974-0.005206·\left(DBH+H\right)\right]}·H^{\left[0.7927+0.006951·\left(DBH+H\right)\right]}$$

The breast-height form factor (f_1.3) was calculated with the equation: $f_{1.3}=\frac{\mathrm{v}}{\frac{\pi }{4}DB{H}^2· H}$.

2.4. Data Analysis

Data obtained were evaluated using an analysis of variance (ANOVA), first undergoing tests of normality and homogeneity in the SPSS software (Version 22.0; SPSS Inc., Chicago, IL, USA), the Tukey HSD test was used to compare the differences among different treatments, and p < 0.05 was considered statistically significant.

3. Results

3.1. Effects of Pruning on Timber Growth

The height increase (ΔH) from March 2019 to December 2021 was the largest in CK compared to that in the other pruning treatments, revealing the negative effect of pruning on height growth. The difference in height growth between CK and treatment P2 in diameter class A was significant. After the second pruning, the height increase in P5 plots was less than that in P2 in diameter classes B and C, while the height increase in P4 plots was larger than that in P1 in diameter class B and C, showing the positive effect of suitable further pruning, but this could also been an adverse effect when intensity pruning was too strong (Figure 3A).

Pruning resulted in a larger increase in DBH and showed positive effects on radial growth. In diameter classes A and B, the DBH (ΔDBH) under the P2 treatment increased by 60.62% and 19.60% compared with that without pruning, respectively. However, in diameter class C, ΔDBH was larger under treatment P1 than under treatment P2, increasing by 25.32% and 15.08%, respectively, compared with that under CK. With the P1 treatment in diameter class A and the P2 treatment in diameter class B, considerable increases were shown in DBH. The results indicated that trees with a stronger growth status grew better under a stronger pruning intensity, while trees with a weak growth status grew better under a light pruning intensity. After the second pruning, the ΔDBH of the trees differed with different treatments and diameter classes, and the diameter growth under the P5 treatment was smaller than that under the P2 treatment, indicating the negative effect of a severe pruning intensity on diameter growth. Unlike the other two diameter classes, trees under pruning treatments all grew better than under CK in diameter class A, indicating the stronger adaptability of larger trees to pruning (Figure 3B).

Despite the negative effects on height growth, pruning still promoted individual volume growth of appropriate intensity due to its positive effects on diameter growth. After pruning for two years, the individual volume increases (ΔV) in pruned trees were larger than those in unpruned trees in diameter class A, and the ΔV was increased by 22.73% under P1 and 25.27% under P2. In diameter class B, the P2 treatment caused a stronger acceleration in individual volume growth 3 years after pruning. It caused an improvement of 16.96% in ΔV compared with CK. In diameter class C, trees under the P1 treatment had a larger ΔV than those under P2, which was improved by 29.87% and 10.28%, respectively, compared with CK. After the second pruning, the ΔV under treatment P5 was always less than that under treatment P2, but the ΔV under treatment P4 was larger than that under treatment P1 in diameter classes B and C, which also shows the positive effect of suitable further pruning, but this could have been an adverse effect when the intensity of pruning was too strong. According to the performance of the individual volume in the different treatments and grades, the effect of pruning was similar to that of the diameter growth, revealing a closer correlation with DBH than with height (Figure 3C).

3.2. Effects of Pruning on Stem Form

The change in the height-to-diameter ratio (ΔH/D) in diameter class A showed that the pruned trees had a smaller ΔH/D than the unpruned trees did with significant differences, indicating the positive effect of pruning on reducing H/D. In diameter classes B and C, most of the pruned trees had a smaller ΔH/D than that under CK, and the difference was that the ΔH/D under treatment P2 was the smallest in class B while the ΔH/D under treatment P1 was smaller than that under P2 in diameter class C, indicating that the P1 treatment was more appropriate for small trees. After the second pruning, the ΔH/D of pruned plots was smaller than that of unpruned plots, without any rules regarding the pruning intensity (Figure 4A).

In diameter class C, the breast height form factor changes (Δf_1.3) in the CK plots were the smallest, while those of the pruned plots were larger, indicating the positive effect of pruning on the trunk form. There were significant differences between the CK and two pruning treatments, P3 and P4. In diameter class B, the Δf_1.3 values of pruned plots were still larger than those under CK without significant differences. In diameter grade A, the Δf_1.3 values under P1, P3 and P5 were significantly larger than those under CK, also demonstrating the positive effects of pruning (Figure 4B).

3.3. Effects of Pruning on Crown Characteristics

After the first pruning, the LBH values under P2 showed significant differences from those under CK and P1 in March 2019 in all classes, and the difference between CK and P1 was significant in diameter class C. However, the difference between P1 and P2 in this class was not more significant in December 2019, and there was no significant difference between each treatment in diameter class A. Furthermore, the difference in the LBH under three treatments in any grade was not significant in January 2021, showing the rule that the differences between pruned and unpruned plots were reduced with time due to the natural senescence of branches. After the second pruning, the differences in LBH values between P5 and other treatments were significant in January 2021 in diameter class B. In diameter class A, the LBH values under P5 also showed significant differences from those under CK, the no-pruning plot, and P3, the weakest pruning intensity plot. However, in December 2021, the difference between each treatment was no longer significant (Figure 5A). In 2019, the live branch height change rates under CK were similar to those under P1 with a change rate of approximately 70%, while there were no changes in the LBH under P2. Therefore, the differences between P2 and the other two treatments in LBH were lessened, indicating that the speed of the natural senescence of live branches was less than two rounds per year. In 2020, the change rate under P2 was less than that under CK in the three classes, generally being 14% less than that under CK, while P1 had a change rate that was 11% lower, further reducing the difference between pruned and unpruned trees in LBH and eliminating the significance of the difference. In 2021, there were no changes in LBH under P5, reconfirming that the speed of the natural senescence of live branches was less than two rounds per year (Figure 5B).

Although parts of green needles were pruned, there were no significant differences in crown width (CW) between the pruned and unpruned trees. After the first pruning, with no live branches being pruned, the CW under CK was the largest in the three classes. However, in January 2021, two growing years after pruning, the CW under P2 was the largest in diameter class A, and that under P1 was the largest in the other two classes, showing the acceleration effects on crown growth after pruning, also indicating that a light pruning intensity was the most appropriate measure for recovering crown width in small trees, while a stronger pruning intensity might be more appropriate for larger trees. After the second pruning, the differences in CW between each treatment were still not significant. Although green branches in the P5 plot were pruned the most, their crown widths were not the smallest in diameter classes B and C, showing no rules regarding pruning (Figure 5C).

4. Discussion

When live needles that participate in photosynthesis are removed by green pruning, resulting in a reduction in photosynthetic products, the growth of trees will be affected by pruning [18,19]. However, no significant differences were found in growth between the pruning and no pruning treatments in our study. This was because the photosynthesis capacity gradually decreased from the upper crown to the lower parts, and the net photosynthetic rates of the remaining leaves were enhanced, so the production of photosynthetic organic matter of entire trees under different treatments was similar [20,21]. The effects of pruning on tree growth differed with species, forest age, pruning intensity and so on. For example, the results of a pruning study on 2-year-old Populus deltoides showed a decrease in DBH and individual volume in pruned trees compared with unpruned trees [22]; another negative effect was reported in research on pruning effects in 3-year-old Populus plantations, and the height and diameter increases in pruned trees with different pruning intensities were all smaller than those in unpruned trees [23]. However, our results showed the positive effects of pruning on diameter growth and volume growth, regardless of the diameter classes of trees. It also had a positive effect on height growth; the acceleration was only shown in the first year after pruning, and the height growth was larger in unpruned trees in later years. These results can be explained by carbon allocation and ecological competition. On the one hand, the photosynthetic productivity of the entire tree was not reduced by moderate pruning, but the supply to removal branches, especially those with negative photosynthetic rates [24], was redirected to other organs and increased the growth of trees. On the other hand, as the branches were removed, the canopy density was reduced from 0.85 to 0.70 and 0.55, the competition of growing space between trees was weakened, and the height growth slowed during pruning [25,26].

It was remarkable that small trees were more sensitive to pruning than large trees. The diameter and volume grew more under P1 in the diameter class 6–8 cm, while trees in the diameter classes 8–10 cm and 10–12 cm grew better under treatment P2. This was because the large trees had wider crowns and stronger photosynthetic capacity, and the loss of photosynthate by the removal of lower branches was negligible because no significant changes in crown width were caused by pruning. Similar to other studies, severe pruning caused decreases in the growth of Masson pine [27]. Our study also found that growth was restrained with severe pruning. However, volume growth was restrained the least in these large trees; the diameter growth increased but the height growth decreased more, possibly because large trees were higher and already in a dominant position, and the removal of branches reduced competitiveness between trees and slowed their height growth [28]. For small trees, pruning mitigated the suppression by large trees and gave them the opportunity to occupy the dominant position [29], resulting in a larger difference in height growth between unpruned trees and severely pruned trees. The results showed that appropriate pruning could improve the growth in diameter and volume, and is an advisable way to cultivate larger-diameter timber trees, but considering that the effects of pruning on the growth of trees differed with the status of trees, the pruning strategy should be made in accordance with the actual growth status of trees.

Pruning is an effective way to culture full and straight trunks [30]. Although our study showed no significant changes in the H/D ratio changes, there were significant differences in breast height form factor changes after pruning, and the form of the trunk was improved. The same results were also found in Pinus elliottii [31] and Paulow nia spp. [32], in which it was also reported that the stem was improved by pruning. The reason was that as the transportation of photosynthate was intercepted by cuts, it was accumulated more in the upside of cuts and promoted the radial growth of the upside, increasing the fullness of the trunk [33]. In addition, the positive effect on diameter growth and negative effect on height growth of pruning also reduced the tapering of stems.

As the lower live branches were removed by pruning, live branch height was significantly changed by pruning, but the differences between pruned and unpruned trees gradually diminished. This was caused by the natural senescence of branches in Masson pine [34]. As the new branches grew out, the lowest old branches and the needles on them gradually dying, the natural change rate in the live branch height was approximately 70%, including one or two rounds of natural senescence a year. In the first two years after pruning, as the lower branches were artificially removed by pruning, which should have naturally died in one or two years, one of the change rates in pruned trees was 0 in the first year after pruning, and the other one was close to that in unpruned trees but only had one round of dead branches according to the variation in live branch height. Therefore, the live branch heights of different treatments were close after two years of pruning. However, the crown widths of the pruned trees did not change significantly even though more than two rounds of branches were removed. As the gap was enlarged by pruning, crown width in the pruned plots grew faster than that in the unpruned plots due to the former’s wider growth space, compensating for the photosynthetic capacity loss of the removed branches [35,36]. These results also indicated that the negative effects of pruning on growth were temporary because the live branch height was recovered to a consistent level in two years.

5. Conclusions

We confirmed that pruning caused an increase in diameter and volume growth, and improved the fullness of the trunk. Artificial removal of lower branches brought forward the annual natural replacement of branches, leading to low rates and low height changes in live branch changes in the pruned trees. It also enlarged the stretch space of the crown, resulting in the stimulation of growth in the crown width. However, trees with different growth statuses responded differently to the pruning. It is convenient to formulate pruning plans in accordance with the DBH of trees in plantation management. Based on our results, we can conclude the following: (1) Pruning in P. massoniana improved the diameter, volume growth, and stem form. (2) For a 7-year-old young P. massoniana plantation, keeping four rounds of branches in trees with a weak growth status (a diameter of less than 8 cm) or keeping three rounds of branches in trees with a strong growth status (a diameter of more than 8 cm) was appropriate.