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Simultaneous applications of thinning and pruning are common silvicultural practices in radiata pine (

All silvicultural practices produce effects on wood properties and thereafter on market value. For instance practices such as initial planting, growing space, thinning and/ or timing and intensity of pruning may have a great influence on the extent of branching and therefore on the final product (

In radiata pine (^{-1}, with four pruning events when dominant trees reach a height of 7.6, 10, 13 and 17 m, with pruning height at 2, 3, 5.3 and 5.5 m, respectively. Typically, there are around 550 to 650 pruned trees per hectare during the two first prunings and 200 to 350 trees in the remaining two. At the time of the first pruning, a first thinning of the non-pruned trees is completed, leaving 550 to 650 trees per hectare. Later on, when trees reach a dominant height of 13 m, the second thinning is applied. Variations of this silvicultural regime are done depending on site conditions. In low productive sites neither pruning nor thinning is applied if final production is focused on pulp production, or two thinnings with no pruning if sawn timber is intended (

Crown development of many conifers, including radiata pine, follows a rhythmic growth (

The growth trend during the lifespan of radiata pine has been well described by

The study of the tree crown is very important in terms of some wood properties. Branches determine the frequency and size of knots that will be present in the final processed wood, influencing the quality and price of sawn timber. Size and frequency of knots are the most important determinants of wood quality for sawn timber production (

In Chile, size and distribution of knots are considered when classifying sawn timber for both visual and mechanical qualities in radiata pine (

In productive tree plantations in Chile, as well as in many other countries, it is common to manage stands simultaneously with thinning and pruning. Although the effects of each of these practices have been separately studied (for example, see

An 18-year-old radiata pine stand planted in 1982 was chosen for the study. The stand of 38 ha was divided in two plots later on, to carry out the study. The stand was located in Chile, in the “Comuna de Cobquecura” (36° 10′ S, 73° 70′ W), Ñuble province, VIII Region “Del Bío-Bío,” in a temperate Mediterranean sub humid climate. At the beginning both plots were a single stand, growing under the same edafoclimatic conditions, coming from the same lot of seeds, without genetic breeding. The site has a site index of 26 (height attained at an age of 20 years) corresponding to a site of medium productivity. Annual mean precipitation in the location is 1134 mm, with five months of drought; mean annual temperature is 13.8 °C, mean maximum temperature in summer is 23.9 °C, and mean minimum temperature in winter is 5.9 °C (

The stand had an initial density of 2500 trees ha^{-1} and after several years the original stand was split into two smaller plots. One of the plots was left unmanaged (the Unmanaged Stand, UMS - 26 ha), while the other plot was subject to several management treatments (Managed Stand, MS - 12 ha). At year 6, a thinning leaving 1300 trees ha^{-1} and a light pruning up to 1.5 meters high were performed to all the trees in the MS plot; at year 11 another thinning leaving 600 trees ha^{-1} and pruning up to 6 meters high were carried out. There were no unpruned trees left in the pruned stand. At year 17, just one year before the harvesting and measurement of the trees, the last thinning without pruning was done in the managed stand. At the moment of felling and measuring the trees, the MS had a remaining stand density of 450 trees ha^{-1}, all of them pruned up to 6 meters high while the UMS had a stand density of 2250 trees ha^{-1} because of natural mortality. After the measurement of the diameter and height of all trees of the plots, 30 trees from each plot were felled following their diameter distributions. Height and stem diameter at breast height (1.3 meters) of the sample trees were measured.

As radiata pine is a polycyclic species, more than one growth unit can be produced in one year. The whole growth of a circadian year is called an “annual shoot”. The demarcations between growth units and annual shoots in the main stem were identified. The length of every growth unit was recorded from the insertion point of the upper structure (branch or cone) in the previous growth unit to the insertion point of the upper structure in the actually measured growth unit. The base diameter of every growth unit was also recorded 10 cm above the base limit of the growth unit to avoid the stem deformation produced by the presence of branches (

The data analysis consisted in (a) a morphological analyses of the 30 trees from each plot, and (b) a characterization of the logs.

The crown morphological analysis was done for the annual shoots from years 11 through 17, corresponding to the annual shoots formed after the second silvicultural intervention and located in the upper part of the crown. Year 11 was the first one analyzed since it was the first year where annual shoots could be clearly identified because of the branches (last pruning was done at year 11). The current annual shoot developing at the time of measurement (annual shoot from year 18) was not considered because it was not completed at the time of harvesting. The two plots were compared for the following variables: total length of the annual shoot, length and number of growth units per annual shoot, number of branches and cones per growth unit, average diameter of branches in the last growth unit of each annual shoot, and stem taper index per year, calculated as the difference in centimeters between the thickest and the thinnest part of an annual shoot, divided by the length in meters.

An initial classification or regression tree using Tree package of R (

To express the potential yield of wood pieces free of knots along the stem, an assessment proposed by

For the characterization of the logs, we derived five virtual or composite logs of 4 meters each from the average values of each plot. Thereafter, and based on different wood property parameters, logs with average characteristics from the MS and UMS were statistically compared (F-test and t-test). This analysis included the lower stem part of the tree (growth prior to 11^{th} annual shoot), which was not considered in the morphological study. We compared the composite logs for, taper index (

where

where ^{3},

We also compared the logs’ internode index (

where

For the characterization of the whole stem, the average characteristics of the stems were calculated. The limits for the annual growth were not identified, but the branches/knots of the growth units were clear and easy to measure. Therefore, it was possible to calculate the average branch number in each log.

At the beginning of year 11 (before the second management actions), the average height of the UMS trees was slightly smaller than the height of the MS trees (^{3} in the UMS and 0.73 ± 0.30 m^{3} in the MS. Based on the branch diameter measurements and assuming the allometric relationship between branch diameter and branch length described by

During the studied period (years 11 through 17), trees in the MS had more growth units per annual shoots than those in the UMS. The robust ANCOVA (non-parametric) showed that these differences were significant (p<0.005) in all the evaluated years (^{th}.

^{th} to 13^{th}, and a second period between year 14^{th} to 17^{th}. The regression tree and the repeated measures ANCOVA also showed this difference. Until year 13^{th} both the year and the treatment were statistically significant (p=0.0356 and p=0.0001 respectively), showing the UMS longer annual shoots. On the contrary, from year 14^{th }to 17^{th} the MS had longer annual shoots, being the treatment not statistically significant, but only the year 16 (p=2.6e-5). The shoots were shorter when they were grown later on time.

During the whole considered period (year 11^{th}-17^{th}) the mean growth unit length of the UMS were longer than the growth units of the MS (p<0.0002 - ^{th} as a break point. After year 12^{th} the difference between the treatments was lower but still significant (p=0.0157). Summarizing, the MS had shorter growth units but more of them during the period between the year 11^{th}-17^{th}.

To further study the effects of silvicultural management on branches, mean branch diameter in the last cluster of each annual shoot was compared. In radiata pine, the last cluster of the year is generally the most vigorous one, showing bigger branches (the acrotony effect of radiata pine - ^{th}, being the branches of the MS stand larger than those of the UMS (p=0.0187 - ^{th} and 17^{th} for the UMS.

Regarding the number of growth units longer than or equal to 75 cm both UMS and MS showed a decreasing number of long growth units through the years (

For the characterization of the logs, the stems were divided in 5 logs of 4 meters each. The first log was accounted starting from the bottom of the stem. In most of the logs (2^{nd} to 5^{th}) the Internode Index (^{st} to 4^{th}, p<0.001), showing logs with more taper. Considering the size of the logs, volume was higher in all the MS logs.

Generally, the effect of forest management on radiata pine has been evaluated in terms of dry matter production (

Given that the MS was simultaneously thinned and pruned at years 6^{th} and 11^{th}, the results showed that the combined management actions increased the number of growth units per annual shoot and decreased their internode length. Annual shoots were larger in the UMS during years 11^{th} to 13^{th}; afterwards both stands presented similar annual shoots length, slightly larger in the MS (but without significant differences). Thus, during years 14^{th} to 17^{th}, annual shoots were similar but the number of growth units was larger in the MS (and therefore the growth units were shorter - ^{-1}, which is a high value for the species and site conditions. First pruning was done together with a thinning reducing the tree number from 2500 to 1300 trees ha^{-1}.

Within the first few years after the second management practices there were statistical differences for the studied variables, but those differences disappeared in later years in the annual shoot length and the mean branch diameter, corresponding with the estimated canopy closure of the MS (year 14^{th}). It might be possible that several years after the management practices, their effect on the trees were diminished as the crown repaired the foliage loss. Also, as the two plots approached a similar level of canopy closure in the later years of the study period, light and temperature levels in the two stands became more similar. This may also reduce growth differences between the plots, as growth units’ production depends on these environmental factors (

The frequency of internodes longer than 75 cm was clearly higher in the UMS than in the MS among annual shoots of similar size, which indicates that pruning together with thinning leads to trees with more and shorter growth units, but annual shoots of similar total size. This contradicts the findings of

Looking at the results of the branch analysis (

According to our results, the effect of stand management on branch diameter shows that when thinning is performed, the growing space and resources for the remaining trees increase, generally allowing the enlargement of branches, as could be seen during years 11^{th} to 13^{th}, with significant differences. It has been also reported by other authors like

Previous studies regarding the effects of thinning on tree taper produced similar results to ours (

Concerning the effects of pruning on taper form,

In our assessment of stem characteristics, using the five virtual logs per stand (^{nd} to 5^{th}) and lower taper index (logs 2^{nd} to 4^{th}). Thus, thinning and pruning together produced logs with more branch clusters, shorter internodes, and more taper (

The study focused on the effect of the classical management practice of thinning together with pruning in radiata pine. Even if this practice is generally used in Chile, the real practical effects were not yet studied for this region. The combined management generated larger volumes of wood and clear wood in the pruned logs, and changed the wood quality properties of the unpruned stem section for a number of years after the silvicultural interventions. The managed trees after the treatments showed more growth units per annual shoot with shorter internodes, thus generating more knotty wood. Although differences between trees in MS and UMS decreased or disappeared as trees of the managed plot restored the foliar biomass lost due to pruning, and as the canopy closure in the two plots approached an equal level, the deleterious effects of management on sawn wood quality are clear in the commercial logs between 4 and 16 m height. The study is based in an un-replicated experiment and mixing both treatments (pruning and thinning). Therefore, we suggest further research with replicated samples and separating the effect of both management practices in Chilean growing conditions.

This research were partially financed by Chilean FONDECYT Grant 11085008, by the Marie Curie Action ForEAdapt project funded by the European Union Seventh Framework Programme (FP7-PEOPLE-2010-IRSES) under Grant Agreement No. PIRSES-GA-2010-269257 and the SuFoRun project funded by the European Union’s H2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant Agreement No. 691149. Ane Zubizarreta-Gerendiain also acknowledges the financial support of the FCT (SFRH/BPD/63979/ 2009) and the UEF Foundation (Project 930341). Ricardo Olea also acknowledges the financial support of the FONDECYT Grant 11121128. We thanks Forestal Bagaro S.A. for the possibility to work with their radiata pine plantations.

(a) Stem portion showing an annual shoot with three growth units. (b) Detail of growth unit limits from the upper part of the previous branch cluster to the upper part of the current cluster, the internode corresponds to the portion inside a growth unit that is free of branches (free of knots).

(a) Number of growth units per annual shoot; (b) average number of branches and cones per growth unit. Error bars represent the standard deviation. MS corresponds to dark columns, UMS to light gray columns.

Annual shoot length (cm) in MS (a) and UMS (b) showing with continuous line the corresponding fitted model and with dashed line the model for the opposite treatment; mean growth unit length per annual shoot (cm) in MS (c) and UMS (d); mean branch diameter in the last growth unit per annual shoot in MS (e) and UMS (f); annual shoot taper for each year in MS (g) and UMS (h).

Frequency distribution of growth units’ length and growth units longer than 75 cm (inserted histogram) for (a) MS and (b) UMS. Average growth unit lengths along the stems for (c) MS and (d) UMS. The x-axis shows the growth unit position as age “dot” position in the annual shoot (

(a) Standing trees characteristics and (b) average log characterization with minimum and maximum log diameter and average branches per log for the managed (MS) and unmanaged stands (UMS).

Mean tree characteristics (total tree height - Height, m; diameter at breast height - DBH, cm; individual tree volume, m^{3}; number of growth units per annual shoot - Annual GU), number of branches per growth unit (Branches per GU), taper of the stem (Taper) and their standard deviation in unmanaged stands (UMS) and managed stands (MS) at year 11 (before the second management) and year 18 (before harvesting). Note: merchantable volume was obtained using 10 cm as top diameter.

Year | Treatment | Height(m) | DBH(cm) | Volume(m^{3}) |
AnnualGU | Branchesper GU | Taper |
---|---|---|---|---|---|---|---|

11 | UMS | 11.8 ± 2.9 | - | - | 3.10 ± 1.15 | 6.5 ± 1.4 | 0.79 ± 0.42 |

MS | 13.9 ± 2.9 | - | - | 3.77 ± 0.97 | 6.0 ± 1.2 | 1.06 ± 0.34 | |

18 | UMS | 24.7 ± 3.4 | 24.8 ± 6.5 | 0.53 ± 0.35 | 2.73 ± 1.26 | 5.2 ± 1.8 | 1.21 ± 0.64 |

MS | 25.0 ± 2.6 | 28.7 ± 5.4 | 0.73 ± 0.30 | 3.21 ± 1.14 | 4.5 ± 1.7 | 1.55 ± 0.87 |

Mean and standard deviation of internode index (^{-1}), minimum diameter of the log (cm), and log volume (m^{3}) for the first 5 logs of 4 m each. Statistically significant differences (p<0.05) between managed (MS) and unmanaged stand (UMS) within a log marked with an asterisk (*).

Log | Variable | InternodeIndex | TaperIndex | Min. diameter log | Logvolume |
---|---|---|---|---|---|

1st log (0 to 4 m) | UMS | 0.30 ± 0.18 | 2.05 ± 0.71 | 20.5 ± 5.9 | 0.21 ± 0.11 |

MS | 0.36 ± 0.23 | 2.20 ± 0.66 | 25.2 ± 5.2* | 0.29 ± 0.11* | |

2nd log (4 to 8 m) | UMS | 0.48 ± 0.27 | 0.61 ± 0.30 | 18.3 ± 5.8 | 0.13 ± 0.08 |

MS | 0.38 ± 0.24 | 0.84 ± 0.35* | 21.7 ± 4.3* | 0.18 ± 0.07* | |

3rd log (8 to 12 m) | UMS | 0.44 ± 0.27 | 0.67 ± 0.21 | 15.6 ± 5.2 | 0.10 ± 0.06 |

MS | 0.27 ± 0.28* | 0.82 ± 0.28* | 18.3 ± 4.0* | 0.13 ± 0.05* | |

4th log (12 to 16 m) | UMS | 0.31 ± 0.29 | 0.82 ± 0.22 | 12.3 ± 4.8 | 0.07 ± 0.05 |

MS | 0.14 ± 0.19* | 0.96 ± 0.26* | 14.3 ± 3.9 | 0.09 ± 0.04 | |

5th log (16 to 20 m) | UMS | 0.25 ± 0.26 | 1.13 ± 0.27 | 8.4 ± 4.3 | 0.04 ± 0.03 |

MS | 0.11 ± 0.16* | 0.97 ± 1.52 | 10.3 ± 5.2 | 0.06 ± 0.03 |