Effect of f i rst thinning and pruning on the individual growth of Pinus patula tree species

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ORIGINAL PAPER

Effect of f i rst thinning and pruning on the individual growth of Pinus patula tree species

Edward Missanjo1•Gift Kamanga-Thole1

©Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2015

The effect of f i rst thinning and pruning on height, diameter at breast height(DBH),and volume growth was studied in individual trees of Pinus patula Schiede and Deppe in Chongoni Plantation,using four plots for thinning trials. Each of the plots was 0.5 ha and subjected to one of the following silvicultural treatments:f i rst thinning and pruning,f i rst thinning and no pruning,pruning and no thinning,and control (no pruning and no thinning).The silviculturaltreatments were randomized in fourreplicates.Fourteen yearsafterplanting,the following parameters were measured including total height, DBH,and volume.The highest DBH and volume growth was observed in thinning and pruning,while the highest growth in height was observed where there was pruning and no thinning. Clearly,both thinning and pruning are an important managementoption in pine speciesplantationsin Malawito maximize the increase in volume productivity.

Pinus patula⋅Thinning and pruning⋅Diameter growth⋅Height growth⋅Volume

Introduction

Thinning is the most common silvicultural treatment,so the effects of thinning have always been studied as part of forest research science(Peltola et al.2007).However, despite the large number of thinning trials analysed,discussion still exists regarding some aspects of thinning on growth and yield(Gilmore et al.2005;Guller 2007; Sharma et al.2006).For example,one subject under discussion is the effect of thinning from below on the diameter growth of individual trees according to their relative size within the stand.Some studies report lack of response to thinning,or only a slight response in the diameter growth of larger trees(Varmola et al.2004), whereas others indicate a positive response in all size classes(Ma¨kinen and Isoma¨ki 2004a).However,the results clearly depend on the studied species and the thinning intensity.

Pinus tree species are of ecological and economic importance all over the world(Rı´o et al.2008)and because of this,a large number of thinning studies have been conducted for this species and mostly in temperate regions. However,the results of these studies differ regarding the response of the species to thinning,both in terms of volume increment(Ma¨kinen and Isoma¨ki 2004b)and in diameter increment of dominant trees(Chroust 2001;Ma¨kinen and Isoma¨ki 2004c).It is probable that these conf l icting conclusions are in part due to differences in trial and statistical methods(Zeide 2001),which makes it diff i cult to generalise the conclusions.However,a large ecological variability of the species could also help to explain the discrepancies.Therefore,it is necessary to determine whether there is a common growth response to thinning for a given species growing under different ecological conditions.

It has also been observed that pruning is usually done to improve the shape of trees during the establishment period and/or to create knot-free wood and increase the value of boles(Neilsen and Pinkard 2003;Pinkard et al.2012; Maurin and DesRochers 2013).Pruning may either reducetree growth(stressful action of trees)or stimulate a tree response by developing secondary shoots and increasing photosynthetic capacity(Springmann et al.2013).Pruning has been demonstrated to enhance the photosynthetic capacity of the remaining foliage in a range of tree species (Medhurst et al.2006).However,the extent and period of response of such increases vary widely.In addition,other studies have reported either no change or a decrease in response to pruning(Turnbull et al.2007).Contradictory reports of the photosynthetic response to pruning suggest that growth environment plays a leading role in determining response(Alcorn et al.2008).

The main objective of this study was to evaluate the effect of f i rst thinning and pruning on height,diameter at breast height(DBH),and volume growth of individualPinus patulaSchiede and Deppe in Malawi,which is found in the tropical region.Our hypothesis was that there is no effect of thinning and pruning on height,DBH,and volume growth of individualPinus patulain Malawi.

Materials and methods

Study site

The study was conducted in Malawi,which is located in Southern Africa in the tropical savannah region at Chongoni Plantation(14°59′S,34°16′E,and 1612 m asl.). Chongoni Plantation receives 1200–1800 mm rainfall per annum,with annual temperature ranging from 7 to 25°C. It is situated about 85 km south east of Lilongwe the capital.

Experimental design and data collection

A total of 10,560Pinus patulaseedlings,which were 6 months old after pricking out,were planted in 1998 in four trial plots of 0.5 ha each.They were subjected to four silvicultural treatments that were completely randomized in four replicates.The silvicultural treatments were:

T1 TP,f i rst thinning and pruning;

T2 TN,f i rst thinning and no pruning;

T3 NP,pruning and no thinning;

T4 NN,no pruning and no thinning(Control).

The general characteristics of the seedlings planted are presented in Table 1.All the sites were situated in an area where the soil is high in ferralsols,acrisols,and nitosols. The slopes of the sites were 5%for site 1,8%for sites 2 and 3,while site 4 was 7%.Hole planting for all treatments was employed using a spacing of 2.75×2.75 m and each plot comprised 660 trees.Low thinning(thinning from below)was carried out by removing 35%of the trees in plots TP and TN at the age of 9 years.Low thinning involved removal of diseased,smaller,and poorer trees.At the ages of 4 and 9 pruning was carried out in plots TP and NP.Trees were pruned for half of the stem height.

At the age of 14 years,a total of 3200 trees(200 trees from each plot)were systematically selected and measured for the following traits:total height,DBH and true volume. Total height was measured using a Suunto clinometer with standard,while DBH was measured at 1.3 m above the ground for each standing tree using a calliper.Tree volume was calculated from the tree diameter and height using a tree-volume function(Ingram and Chipompha 1987).In each trial plot,a total of 5 circular sample plots of 0.05 ha each were constructed and total number of trees were counted.This was done in order to determine the stocking per hectare for each treatment so that the natural mortality affecting different treatments is known.The stocking per hectare was calculated using the formula as expressed in Ingram and Chipompha(1987).

Data analysis

Data obtained was subjected to Kolmogorov–Smirnov D and normal probability plot tests,using Statistical Analysis of Systems software version 9.1.3(SAS 2004).This was done in order to check the normality of the data.After that the data was subjected to analysis of variance(ANOVA) using the same SAS software version 9.1.3(SAS 2004).A criticalPvalue of 0.05 was used.The data was analysed using the following model:

where,Yijis the response variable(height,DBH,volume) inith treatments,μ the overall mean,Tithe fi xed effect of silvicultural treatment(i=1,2,3,4),andeijis the random residual effects,

Results

Our f i ndings showed that there were signif i cant (P＜0.001)differences in the natural mortality rate per year among the silvicultural treatments.The natural mortality rate per year was higher in treatments where no thinning was done than in treatments were thinning was carried out(Table 2).Similarly,there were signif i cant (P＜0.001)differences in height,DBH,as well as volume among the silvicultural treatments.Mean height was highest in pruning and no thinning treatment,followed by no thinning and no pruning treatment.Thinning and pruning treatment had the highest mean DBH and volume, followed by thinning and no pruning(Table 3).

Table 1 General characteristics of the Pinus patula seedlings planted

Table 2 Expected and estimated stocking of Pinus patula per ha and the natural mortality rate per year for each treatment at 14 years

Discussion

The results show that mean height was higher in silvicultural treatments where no thinning was carried out than in treatments where thinning were carried out.The research fi ndings are in line with those reported by Gonzalez et al. (2002),Yatich(2009)and Ngaga(2011).Ngaga(2011) reported that thinning has little impact on height growth in the remaining trees in a stand.This is because in thinned stands there is low stocking ratio compared to unthinned stands.As a result,there was high intra-specif i c competition for sunlight between the trees in the unthinned stands (Evans and Turnbull 2004).However,the results are different from those reported by Zhang et al.(2006).They reported that intensive thinning(2.13 m,or 2212 stem ha-1)in Jack pines stand increased tree height by 13.1%compared with an unthinned stand.The reason for these differences is that the growth height increment of a tree depends on the genetic improvement conducted on the tree(Dieguez-Aranda et al.2006;Ngaga 2011).

One of the objectives of thinning is to produce larger trees with higher DBH(Dykstra and Monserud 2007). According to the results,DBH was higher in silvicultural treatments where thinning was carried out than in the unthinned treatments.Most of the thinning experiments reported similar results.Forrester and Baker(2012) reported higher DBH growth in thinned stands than in unthinned stands.There was no signif i cant difference in mean DBH growth between thinning and pruning(TP)and thinning no pruning(TN).However,TP had the highest mean DBH growth than TN.What this shows is that to maximize DBH growth,it is necessary to carry out boththinning and pruning in the stand.The results concur with Strong and Erdmann(2000),who reported that both thinning and pruning inf l uences DBH growth of trees.

Table 3 Mean height,diameter at breast height(DBH)and volume for Pinus patula with standard errors in parenthesis at the age of 14 years under different silvicultural treatments

The results show that there was a similar trend for DBH growth and volume growth.The reason behind this is that DBH is highly positively correlated to volume(Zeide and Sharer 2002;Louppe et al.2008).

No signif i cant difference existed between DBH growth for NP and NN(Table 3),indicating that DBH responded similarly to both treatments.A similar trend was also observed for TP and TN.This result is consistent with the fi ndings of previous studies(Medhurst et al.2006; Springmann et al.2013).The absence of a signif i cant DBH growth difference is probably due to an increase in photosynthesis in the remaining needles of pruned trees.This phenomenon is known as compensatory photosynthesis (Medhurst et al.2006;Springmann et al.2013).An increase in net photosynthesis is commonly observed after pruning(Medhurst et al.2006).

Conclusion

Thinning and pruning affected the growth of thePinus patula.Highest DBH and volume growth was observed in the treatment where thinning and pruning was carried out, while the highest growth in height was observed in the treatment where pruning and no thinning was carried out. This means that volume is mainly affected by DBH. Therefore,thinning and pruning are recommended as future management options in pine species plantations in Malawi in order to maximise increase in volume productivity.

AcknowledgmentsThe authors thank Mr.Anderson Ndema and his colleagues of Malawi College of Forestry and Wildlife(MCFW)for allowing them to take measurements from the trial plots in Chongoni Plantation.

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31 January 2014/Accepted:25 April 2014/Published online:18 July 2015

The online version is available at http://www.springerlink.com

Corresponding editor:Yu Lei

✉Edward Missanjo

edward.em2@gmail.com

1Malawi College of Forestry and Wildlife, Private Bag 6,Dedza,Malawi