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Silva Balcanica, 16(1)/2015
HARVEST RESIDUES ASSESSMENT IN PINE PLANTATIONS HARVESTED BY
WHOLE TREE AND CUT-TO-LENGTH HARVESTING
METHODS (A CASE STUDY IN QUEENSLAND, AUSTRALIA)
Mohammad Reza GhaffariyanForest Industries Research Centre,
Faculty of Arts and Business, University of the
Sunshine Coast, Locked Bag 4, Maroochydore DC, Queensland
Robin ApolitUniversite de Lorraine ENSTIB, Épinal
Abstract
Forest harvesting residues can be an additional fibre source if
recovered during harvest but it can also play an important role in
maintaining soil quality over the next rotations if left on the
cut-over area. The first step in properly managing harvesting
residues is to know how much is left after harvesting operations
and is potentially available for sustainable recovery. There has
been little information available on quantity and quality of
plantation harvest residues in Queensland, thus this project aimed
to measure the average weigh of slash left on two harvesting sites;
a whole tree extraction to road side and a cut-to-length harvest
site. The results indicated a significant difference between
average weights of residues from the two harvesting methods. The
percentage of other elements of harvesting residues were measured
and documented in this study. The results can help land owners and
forestry planners to set a proper strategy for woody waste
management and additional fibre recovery.
Key words: whole tree extraction, cut-to-length harvesting,
t-test, residue composition
INTRODUCTION
Previous studies in Australian plantations (Ghaffariyan, 2013)
have shown that there was a significant amount of harvesting
residues left in different clear-felled pine plantations. These are
a potential renewable, sustainable source of bio-mass currently in
use in Europe and North America for bioenergy largely in biomass
heating systems (Cuchet et al., 2004). The harvesting residues left
on the site after merchantable wood extraction (left-slash) depend
on various parameters, such as the harvesting method and equipment,
stand species, site and stand quality. A previous study conducted
in pine plantations indicated that processing trees at the stump
(cut-
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to-length method (CTL)) can result in higher harvesting
restudies than processing trees at road side (whole tree method
(WT)) (Ghaffariyan, 2013).
Extracting whole trees from the site can reduce tree growth
(Mann et al., 1988) due to high nutrient removal (Hendrickson et
al., 1989; Ouro et al., 2001 cited in Hacker, 2005). Koelling et
al. (2007) indicated that high level of biomass utilization may
reduce soil fertility and uncontrolled biomass recovery may impact
stand growth which results in reduction of long term site
productivity. There is also concern regarding to potential damage
to soil during the harvesting operations (Skin-ner et. al., 1989;
Senyk, Smith, 1991 cited in Proe et al., 1994). Negative ecological
impacts can be reduced by appropriate timing of operations,
minimizing the nutrient removals from the forest sites and
recycling of ash from combustion installation. The removal of
forest residues from poor sites should be avoided in all cases,
because this would further reduce the nutrients availability in
these already nutrient poor sites (Burgers, 2002; Hakkila,
2002).
There have been several studies on quantifying the amount of
harvesting resi-dues in softwood plantations. In Finland, Nurmi
(2002-2004) studied the residue removal in mature Norway spruce
stands (Pinus abies L.) harvested by a combina-tion of harvester
and forwarder. The author used 2×2 m sample plots on a 30×30 m
grid. Under different working methods the uncovered slash
(remaining slash after biomass recovery) ranged from 19.4 to 49.9
GMt/ha. Steele et al. (2008) studied the post-harvest residue
collection by slash bundler for southern pine species. For the
first thinning, second thinning and clear cut the amount of
recovered residues was 14.3, 11.7 and 26.9 GMt/ha respectively.
Stem wood had larger share (74-93% of bundle weight) than needles
and residuals in the bundle fraction test. The remain-ing slash
after residue recovery was not reported in this study. Cuchet et
al. (2004) used 1×1m sample square plot distributed at 10 m
intervals along transects. They reported remaining slash varying
from 12.3 GMt/ha (in Maritime pine stand) to 81.5 GMt/ha (for
spruce stands) after residue recovery by a slash-bundler in France.
The initial amount of residues varied from 22.1 to 178.1 GMt/ha.
Watson et al. (1986) quantified the energy-wood biomass available
on two 22-year-old slash pine planta-tions and a 45-year-old
natural stand of mixed slash and loblolly pine in Alabama. All
stands were clear-cut for pulpwood. Average harvesting residues
weight for both pine plantation sites was 75.4 GMt/ha while for the
natural mature stand the harvest-ing residue weight averaged at
61.5 GMt/ha. Smethurst, Nambiar (1990) reported a weight of 52
GMt/ha for the remaining slash in a clear-felled Pinus radiata D.
Don plantation by cut-to-length harvest method (CTL) (harvester and
forwarder) in Mount Gambier (South Australia). They compared their
results with the range reported for other coniferous forests; 43
GMt/ha on a Pinus elliottii Englem.- Pinus palustris Mill. site in
Florida (Morris, Pritchet 1982), 54.6 GMt/ha on a
high-produc-tivity Douglas-fir site in Washington (Bigger, Cole
1983).
Retaining woody debris is an essential component to site
fertility and planta-tion growth through nutrient recycling,
conserving soil moisture and mitigating ero-
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sion (Schnepf et al., 2009; Graham et al., 1994; Evans, 2011;
Thiffault et al., 2011) thus it is important to know the weight of
the harvesting residues in order to define if sufficient level of
slashes has been left to ensure future site sustainability. There
has been little information available on quantity and quality of
harvesting residues in south-east Queensland. This project aimed to
meet the following objectives:
Measuring the average weight of harvesting residues (in green
metric tonnes per hectare (GMt/ha)) in two harvesting sites
operated by whole tree and cut-to-length methods);
Evaluating the composition of residues (such as branch, bark,
cone …) for both sites;
Assessing the impact of harvesting method on quantity of
left-slashes to iden-tify potential additional recovery of the
residues.
STUDY AREA
The study sites were located in the Toolara Forest in Bungawatta
plantation near Gympie (Queensland). Fig. 1 and 2 show the map for
both harvesting sites in-cluding an indication of where one hectare
study blocks were located within them. The species was mainly slash
pine (Pinus elliottii Engelm.) and Honduras Caribbean Pine (Pinus
caribaea var. hondurensis (Sénécl.) W.H.G.)). The stand was 30
years old. The average DBH in both sites was 31 cm. The small end
diameter for log pro-
Fig. 2. One hectare study block for cut-to-lengthfor whole tree
extraction
Fig. 1. One hectare study block
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cessing was 7 cm. WT harvest site was sampled on July 23th 2014
and CTL site on July 24th 2014.
The WT operation consisted of a feller-buncher to fell the trees
and a skidder to extract whole trees to road side where the trees
were processed into logs at the landing (Fig. 3). The CTL operation
included a harvester to fell and process the trees to short logs at
the stump while leaving residues on the site. Then a forwarder was
used to extract the logs to the landing (Fig. 4). Both sites were
harvested in April/May 2014. Sawlog and pulpwood were produced at
both sites.
Fig. 3. Whole tree harvesting site Fig. 4. Cut-to-length
harvesting site
STUDY METHOD
The method used in a previous pine residue study within a mobile
chipper harvesting site (Ghaffariyan et al., 2014; Ghaffariyan et
al., 2012b) was used to mea-sure the amount of left-slash after
industrial wood recovery to improve our under-standing of its
quality and quantity (Fig. 5). After a brief investigation of each
site, a representative area was selected for sampling. An
assessment of the left slash was conducted using line transects,
within and adjacent to the study site, along which 18 plots
(0.5×0.5m) were established where 5 of them were fractioned to
evaluate the proportion of needles, bark, cones and branches
(including small branches with di-ameter at midpoint 3 cm) (Fig.
6). The green weight of samples was measured with a portable scale
that had an accuracy of 0.01 kg. The moisture content of the
samples was not evaluated in this case study. To compare the
average weight of harvesting residues in two harvesting site a
Student’s t-test was applied to compare the means at α = 0.05. This
test was applied to verify the impact of harvesting method on
average weight of harvesting residues in pine plantations.
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RESULTSThe primary analysis for both sites was based on 18
samples collected from
each site. The average weight of harvesting residues in WT and
CTL sites was 31.7 GMt/ha and 77.6 GMt/ha (Table 1). The weight of
harvesting residues in CTL site ranged from 20.0 GMt/ha to 235.8
GMt/ha while for WT site the minimum weight of slash was 12.8
GMt/ha and maximum weight of slash was 64.0 GMt/ha.
Using the Student’s t-test (independent samples) the average
weight of left-slash on both sites was compared. From Levene’s test
(Table 2), as the significant level of 0.01 is less than α = 0.05
threshold, the variance of both groups was signifi-cantly
different. This indicated to use the significance level of 0.01
(t=-2.85) in the t-test result (Rezai, A. 2008.
http://academic.udayton.edu/gregelvers/psy216/spss/ttests.htm).
Since the computed value is less than α = 0.05 there is a
significant dif-ference between the average weight of residues in
the two harvesting sites (Table 2). This demonstrates the
harvesting method can significantly affect the weight of
Fig. 5. Weighing residues in each plot
Fig. 6. Fractioning method
Table 1 Results of left-slash assessment in both sites
Whole tree method Cut-to-length methodNumber of plots 18 18Min
(GMt/ha) 12.8 20.0Max (GMt/ha) 64.0 235.8
Average (GMt/ha) 31.7 77.6
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harvesting residues. Processing trees at stump resulted in
higher harvesting residues scattered on the site compared to whole
tree harvesting where most of the residues were piled at road sides
while little were scattered on the site. The geometric volume of
one pile close to the one hectare study area in the WT operation at
road side was measured about 110 m3. To convert volume of residue
piles to green metric tonnes further studies on slash density of
road side piles will be required.
Fig. 7 and 8 show the composition of residues for the WT and CTL
harvest-ing methods, respectively. The CTL harvesting method
resulted in a higher weight of residues especially in the amount of
small and large branches compared to WT method. In CTL site 41% of
residues were large branches (about 31.5 GMt/ha) while in WT site
(Fig. 7, 8 and 9, 10) there was no large branch, only small
branches
Table 2Independent samples t-test result
Levene’s Test for Equality of Variances
t-test for Equality of Means
F Sig. t df Sig. (2-tailed)
Mean Differ-ence
Std. Error Differ-ence
95% Confidence Interval of the
DifferenceLower Upper
Equal variances assumed
21.3 0.00 -2.85 34 0.007 -45.87 16.07 -78.53 -13.22
Equal variances not assumed
-2.85 18.89 0.010 -45.87 16.07 -79.52 -12.23
Fig. 7. Residues of whole tree extraction Fig. 8. Residues of
cut-to-length extraction
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(diameter
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was less than for a Spruce case study in France (259.6 GMt/ha)
reported by Cuchet et al. (2004) and another case study of radiata
pine in Tasmania harvested by CTL operations (238.7 GMt/ha)
(Ghaffariyan et al., 2012a). The weight of residues from the WT
site (31.7 GMt/ha) is relatively high compared to a whole tree
operation in Western Australian Eucalypt plantation, where left
slash was found to be 6.1 GMt/ha. The difference can be explained
by the high amount residue needles in this case study compared to
leaves in Eucalypt trees (Fig. 7) and no other study on pine whole
tree operation was available to compare.
Considering the level of slash removal on both sites meets or
exceeds avail-able recommendations for maintenance of site quality
(Table 3) and there is a rela-tively large quantity of residue logs
and branches on the CTL site (31.5 GMt/ha) there may scope for
additional fibre recovery (Walsh, Strandgard, 2014; Ghaffariyan et
al., 2015) using conventional and modified bin-load forwarders
(Ghaffariyan et al., 2012c) and mobile chippers (Desrochers et al.,
1993). Recovered fibre have po-tential use in producing chips for
medium-density fibreboard (MDF) or biorefinery/bioenergy depending
on the quality of fibre and local markets. If bioenergy is a target
application for the recovered residues, the material can be stored
to dry naturally on site to reduce moisture content and increase
calorific value (Acuna et al., 2012; Ghaffariyan et al., 2013)
while all fine materials such as needles, small branches or barks
can be retained on the site to maintain soil fertility for next
rotation (Rothe, 2013).
CONCLUSIONS
Harvesting method significantly impacts the weight of harvesting
residues in pine plantations. In this study cut-to-length
harvesting yielded significantly higher residue levels compared to
whole-tree harvesting due to leaving large branches on the site
when processing trees at the stump. The weight of large branches
left follow-ing CTL harvesting was 31.5 GMt/ha in this case study.
Depending on the economics of harvesting and haulage operations,
this may represent a viable resource of fibre to be extracted for
industrial use.
Table 3Wood recovery and left-slash percentage
Industrial wood recovery (GMt/ha)
Left slash (GMt/ha)
Total biomass (GMt/ha)
Left sash share of total biomass (%)
Whole Tree 262.2 31.7 293.9 10.8
Cut-to-length 334.5 77.6 412.1 18.8
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Acknowledgments: The authors would like to acknowledge Mr Ian
Last, Mr Tim Don and Mr Tim Lee (from HQP) for the assistance with
this project by providing information and access to the
plantations. They also thank Prof. Mark Brown for editing the
English language of this article.
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