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158 IAWAIAWA
Journal 35 (2),
Journal 352014: 158–169
(2), 2014
ANATOMICAL, CHEMICAL AND MECHANICAL PROPERTIES OF
FAST-GROWING Populus × euramericana cv. ‘74 / 76’
Zhao Rongjun1, Yao Chunli 2, Cheng Xianbao1, Lu Jianxiong1, Fei Benhua3
and Wang Yurong 1,*
1 State Key Laboratory of Tree Genetics and Breeding, Research Institute of Wood Industry of
the Chinese Academy of Forestry, 1 Dong Xiao Fu, Xiang Shan Road, Hai Dian,
Beijing 100091, P.R. China
2 Beijing Forestry University, 35 Qinghua East Road, Hai Dian, Beijing 100083, P. R. China
3 International Center for Bamboo and Rattan, 8 Futong Dongdajie, Wang Jing, Chao Yang,
Beijing 100102, P. R. China
*Corresponding author; e-mail: yurwang@caf.ac.cn
ABSTRACT
The anatomical characteristics, chemical composition, and physical and me-
chanical properties of fast-growing Populus × euramericana cv. ‘74/76’ juve-
nile wood were investigated. Four- to five-year-old clonal plantation trees were
harvested from two different experimental sites in the suburbs of Beijing. The
Shunyi site had black alkali soil with a planting density of 4 × 6 m and the
Miyun site had sandy loam soil with a planting density of 3 × 5 m. The test
results showed that the poplar trees from the two sites were both fast growing,
with poplar at Shunyi growing faster than at Miyun. There were no significant
differences in wood properties between trees grown at the two sites. Fiber length
at breast height varied from 872 to 1300 µm between growth rings, average
fiber width varied from 21.0 to 25.5 µm and double wall thickness varied
from 5.0 to 6.6 µm. Average cellulose, lignin and hemicellulose contents in
the samples were 48.9%, 25.4%, and 18.8%, respectively. MFA was higher
in the first two growth rings (20–25°), and then decreased rapidly to 12° close
to the bark. The average air-dry density at breast height was 401 kg/m3 while
the average MOE at breast height was 9.3 GPa. The trees showed large growth
rates in both height and stem diameter during the growing season. However,
wood properties of the juvenile poplar appeared to be similar to those of poplars
with a slower growth rate.
Keywords: Poplar, fiber characteristics, microfibril angle, density, modulus of
elasticity, juvenile wood.
INTRODUCTION
Poplar trees are among the most widely planted in subtropical and temperate regions.
The characteristics of fast growth rate, short rotation cycle, prolific sexual reproduc-
tion, small genome size, ease of cloning, and tight coupling between physiological
traits and biomass productivity makes poplar a well suited forest tree (Joshi et al. 2004;
© International Association of Wood Anatomists, 2014 DOI 10.1163/22941932-00000057
Published by Koninklijke Brill NV, Leiden
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via free accessZhao et al. – Properties in fast-growing poplar 159
Jansson & Douglas 2007). Poplar wood is a traditional raw material for pulp and paper,
wood-based composites (fiberboard and plywood), building, furniture and construc-
tion (Balatinecz & Kretschmann 2001).
The woody biomass currently available is limited compared with its dramatically
increasing demand (Fenning & Gershenzon 2002). To increase the quantity and quality
of wood production, fast-growing plantations are desirable because woody biomass
can be accumulated in short rotation coppicing systems in a relatively short period of
time through intensive management. So, some hybrid poplars, for example, poplar 741,
poplar I-69/55, poplar I-214, and poplar 107 were planted and researched in China.
Poplar 741 is fast-growing and its wood properties have been assessed (Tian et al.
2013). The wood quality of poplar I-69/55, poplar I-214 and Populus tomentosa has
been compared indicating that the density of I-69/55 is similar to that of P. tomen-
tosa (Cown et al.1999; Fang et al. 2008). A fast growth rate can result in changes to
microfibril angle (MFA) and stiffness (Cave & Walker 1994; Lindstrom et al 1998).
Populus × euramericana is also an important fast-growing plantation clone in China.
It can be propagated easily by cuttings, grown at high planting density and with high
yield, and can be harvested after a few years (Lin et al. 2006; He et al. 2008). Therefore,
it is necessary to study various properties of this species to maximize its economic value
and utilization. Prior studies on Populus × euramericana have focused on the radial
variation of its chemical composition (Zhou et al. 2010), the effect of stand density on
growth (Tian et al. 2011), MFA, basic density and longitudinal shrinkage (Liu & Liu
2011). The chemical constituents could provide an indication of wood quality (Longui
et al. 2012 ).These studies were based on samples from Shandong, Henan and Hebei
provinces. However, there are few studies on the properties of the poplar wood grow-
ing in the Beijing region, especially on the properties of juvenile wood of Populus ×
euramericana from different sites. Site, hybrid crosses and clone effects on growth and
wood properties of Populus have shown that site can significantly influence growth
traits and wood density (Monteoliva & Senisterra 2008). General climatic site condi-
tions have indicated a weak influence on vessel variables and strong influence on fiber
and parenchyma tissue in other species (Fichtler & Worbes 2012).
The objectives of this study were to determine the fiber and vessel characteristics,
MFA, chemical composition, density and MOE for bending of Populus × euramericana
cv. ‘74/76’ near Beijing and investigate juvenile wood properties of the fast-growing
poplar and the effects of site on its growth rate and wood properties. The results pro-
vide new data not only for juvenile wood properties but also for fast-growing poplar
planting near Beijing.
MATERIALS AND METHODS
Sample trees of a Populus × euramericana cv. ‘74/76’ clone were harvested from the
Shunyi (SY) and Miyun (MY) experimental plantation sites near Beijing, China (ISO
4471). These sites were established and maintained by the Research Institute of For-
estry, Chinese Academy of Forestry. They were planned as important planting sites for
fast-growing poplar. The soil of SY is black alkali soil and is located in the north-east
of Beijing City suburbs at 116° 60 ' east longitude and 40° 10 ' north latitude. The
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annual average temperature is 11.5 °C, the average annual precipitation is 625 mm
and the frost-free period is about 195 days. The MY site is sandy loam soil at 116° 80 '
east longitude and 40° 30 ' north latitude. The annual average temperature is 10.8 °C,
the average annual rainfall is 661 mm and the frost-free period is more than 180 days.
MY and SY are about 50 km apart.
Four-year-old poplar with a planting density of 4 × 6 m in SY, and five-year-old
poplar with a planting density of 3 × 5 m in MY, were harvested. Five trees were selected
from each site. From each sample tree, two sample logs with a length of 450 mm were
sawn at breast height and 1.5 m sample discs with a thickness of 5 cm without obvious
defects were removed, and then sawn into two strips. In strip 1, 1.5 × 10 mm (radial
[R] × tangential [T]) slices were continuously sawn from pith to bark and 5 locations
in each ring were selected to determine the average MFA by X-ray diffraction (XRD).
Slices and sticks were cut from strip 2 for anatomical and chemical analysis. Above a
height of 1.5 m, stems with a length of 450 mm were sawn from north to south in dif-
ferent rings to produce samples with dimensions of 20 × 20 × 300 mm (R × T × L) for
mechanical testing. Small cubes with dimensions of 20 × 20 × 20 mm were cut from
each end of the mechanical test samples to determine density (Fig. 1).
South North S N
strip 2
B B B B
A A A A strip 2
L/450 mm
C L/20 mm
South North
L/380 mm
D L/300 mm
A - MFA sample
50 mm (L) x 10 mm (T) x 1.5 mm (R)
B - Anatomical and chemical sample
C - Air-dry density sample
20 mm (L) x 20 mm (T) x 20 mm (R)
D - MOE sample
300 mm (L) x 20 mm (T) x 20 mm (R) Figure 1. Preparation of poplar wood samples.
Anatomical analysis
For measurement of morphological properties of fibers and vessels, sticks from each
annual ring were separately macerated in a solution of glacial acetic acid and hydrogen
peroxide (1:1, v/v) at 60 °C for 48 h. A suspension of washed fibers was placed on
microscope slides and the length and width of 50 unbroken fibers from each ring were
measured using a fiber length tester from the images captured with a digital camera
attached to a light microscope.
To measure cell wall thickness and cell lumen diameter, samples were also sectioned
at a thickness of 15 µm using a sliding microtome, and the sections were dehydrated
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through an alcohol series and stained with safranine for photography and observation
using a Leica DMLB light microscope and an image analysis system (Japan, Q570).
MFA determination
An X-ray diffractometer (Netherlands Panalytical X’Pert Pro) was used to measure
the MFA of samples using a sample size of 1.5 ×10 mm (R × T). Each wood sample
was attached to a holder that held the wood sample perpendicular to the incident
X-ray beam, which passed through the tangential face near the center of the sample.
The sample was rotated and the intensity curve was measured as a function of the
rotation angle φ with a step of 0.5° and a measuring time of 180 s per point. A CuKα
radiation source was used with λ = 0.154 nm, voltage = 40 kV, and current = 40 mA.
The aperture of the incident beam was 2 × 4 mm. Mean MFAs were determined ac-
cording to the method developed by Cave (1966) and Meylan (1967), namely, 0.6T,
where T is an XRD parameter taken from the tangents drawn at the points of inflection
(Stuart & Evans1994; Andersson et al. 2000). This method has been widely used to
determine the fibril angle from measurements of the 002 peak made in transmission
mode (Zhang et al. 2007).
Analysis of wood chemical components
To measure the contents of α-cellulose, lignin, alcohol benzene extractives and ash
in poplar wood, air-dried samples were first cut into matchsticks and then milled into
fine powder. Chemical compositions were determined following a standard procedure
(ASTM 1103–1107).
Physical measurement
Wood density was measured based on oven-dry weight and 12% water content,
and was determined at 20 °C and 65% RH. All samples were cut from each end of the
mechanical samples. The nominal size of each sample was approximately 20 × 20 ×
20 mm, although actual dimensions were precisely measured for each sample. Weight
was measured using precision balances (0.001).
Mechanical testing
Samples with dimensions of 20 (R) × 20 (T) × 300 (L) mm were used to test me-
chanical properties according to a standard procedure (ISO 3349). Before testing, all
samples were dried and weighed at 20 °C and 65% RH, which generated a moisture
content of 12%. A total of 60 samples were prepared for mechanical testing.
RESULTS AND DISCUSSION
Tree growth rate
As shown in Table 1, the sample trees from SY had an average diameter of 19.8 cm
at breast height (DBH) and an average height of 17.9 m. The sample trees from MY
had an average DBH of 20.9 cm and an average height of 19.3 m. The annual increase
in volume of a tree at the two sites was 0.18 m3. The poplar trees from both sites near
Beijing therefore had high growth rates. The plant height, stem diameter and growth
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Table 1. Growth characters of sample trees in the SY and MY sites.
Height of first branch (m)
Volume per year (m3)
Planting density (m)
Average DBH (cm)
Age (years)
Soil texture
Height (m)
Number
1 Black alkali soil 4 4×6 17.8 2.7 19.5
2 Black alkali soil 4 4×6 18.0 2.8 21.0
Shunyi site 3 Black alkali soil 4 4×6 17.9 2.5 19,7 0.18
4 Black alkali soil 4 4×6 19.7 2.4 19.6
5 Black alkali soil 4 4×6 16.2 2.2 19.0
1 Sandy soil 5 3×5 18.9 2.1 19.3
2 Sandy soil 5 3×5 18.8 2.2 19.4
Miyun site 3 Sandy soil 5 3×5 19.4 2.5 21.3 0.18
4 Sandy soil 5 3×5 19.3 2.2 23.2
5 Sandy soil 5 3×5 20.3 2.3 21.4
rate of trees had obvious characteristics of fast growth, even with different soil fertility
and planting density. The poplar trees were much taller than slower growing trees of
similar age such as Chinese pine and Chinese poplar (Jiang & Jiang 2008) and grew
faster in Beijing than similar trees in Hebei. The poplar trees in Hebei were 6 years
old, planted in sandy loam soil, and had an average diameter at DBH of 15.8 cm and
an average height of 13.5 m (Tian et al. 2011).
For the poplar trees at Beijing, the average annual increase in DBH of poplar trees
at SY was 4.94 cm, and the annual increase in height was 4.48 m. The average annual
increase in DBH of poplar trees at MY was 4.18 cm, and the annual increase in height
was 3.87 m. It can be seen that the poplar trees at SY grew faster than those at MY.
Anatomical characteristics
Populus × euramericana cv. ‘74/76’ is a typical diffuse-porous wood. Vessels are
almost uniform in size and distribution, and the transition from earlywood to latewood
is gradual throughout each annual ring. The microstructure of poplar wood from the two
sites was not exactly the same. The vessel size of the poplar wood was more uniform at
SY than at MY (Fig. 2). The anatomical parameters of the samples are given in Tables
2 and 3. The mean vessel proportion varied from 22% to 30%, and fiber proportion
varied from 65% in the first annual ring to 62% in the fifth annual ring. Fiber length at
breast height varied from 872 to 1300 µm between growth rings, and the average fiber
width varied from 21.0 to 25.5 µm. The double wall thickness of the samples varied
from 5.0 to 6.6 µm. It can be concluded that mean values of anatomical properties of
4- to 5-year-old fast grown poplar are similar to those of 22-year-old Sanbei poplar
(Populus nigra × P. simonii cv. “Zhonglin Sanbei-1”). Bao et al. (2001) have reported
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Figure 2. Typical cross-sectional images of fourth growth rings of Populus × euramericana cv.
‘74 /76’. – A: SY site; B: MY site.
Table 2. Anatomical parameters of Populus × euramericana cv. ‘74/76’ in the SY site.
Annual rings
Variable _______________________________________________________________
1 2 3 4
Vessel (%) 22.37 ± 8.96 24.31 ± 6.67 27.17 ± 5.24 30.08 ± 6.14
Fiber (%) 65.81 ± 9.11 62.88 ± 5.18 62.13 ± 7.30 61.52 ± 4.74
Fiber wall (%) 46.21 ± 5.76 42.48 ± 5.62 41.87 ± 7.86 41.08 ± 4.78
Fiber length (μm) 872.79 ± 99.71 1018.10 ± 131.32 1098.94 ± 127.73 1252.95 ± 147.66
Fiber width (μm) 21.10 ± 2.81 22.63 ± 3.03 23.53 ± 3.20 23.90 ± 3.24
Cell wall thickness (μm) 5.03 ± 0.76 5.21 ± 0.79 5.20 ± 0.83 5.67 ± 0.88
Microfibril angle (°) 19.16 ± 3.9 22.90 ± 2.0 17.63 ± 2.0 13.74 ± 1.7
Table 3. Anatomical parameters of Populus × euramericana cv. ‘74/76’ in the MY site.
Annual rings
Variable _________________________________________________________________
1 2 3 4 5
Vessel (%) 22.54 ± 9.15 24.53 ± 7.65 28.43 ± 7.45 28.27 ± 7.11 28.75 ± 5.93
Fiber (%) 65.43 ± 8.01 63.57 ± 7.55 62.42 ± 6.11 62.36 ± 7.24 62.76 ± 6.95
Fiber wall (%) 47.16 ± 5.21 43.21 ± 6.02 41.27 ± 5.35 41.80 ± 7.22 42.16 ± 4.95
Fiber length (μm) 898.33 ± 107.25 988.04 ± 112.89 1079.36 ± 133.68 1191.58 ± 138.61 1298.13 ± 167.14
Fiber width (μm) 20.69 ± 3.51 22.93 ± 3.51 24.00 ± 3.71 25.23 ± 3.45 24.76 ± 2.62
Cell wall thickness (μm) 5.39 ± 1.07 5.66 ± 0.77 5.70 ± 0.90 6.31 ± 0.92 6.59 ± 1.25
Microfibril angle (°) 19.24 ± 4.7 21.35 ± 3.3 17.08 ± 2.3 14.19 ± 1.3 12.41 ± 1.2
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1500 35
A B
1400
30
1300
Fiber length (µm)
Fiber width (µm)
1200 25
1100
1000 20
900 SY
15 SY
MY MY
800
700 10
1 2 3 4 5 1 2 3 4 5
Growth rings Growth rings
10 30
C D
8 25
Cell wall thickness (µm)
20
6
MFA (°)
15
4
10
2 SY SY
MY 5
MY
0 0
1 2 3 4 5 1 2 3 4 5
Growth rings Growth rings
Figure 3. Radial variation of fiber morphology and microfibril angle in different rings of Populus
× euramericana cv. ‘74 /76’. – A: The variation of fiber length in different rings from two sites. –
B: The variation of fiber width in different rings from two sites. – C: The variation of fiber cell
wall thickness in different rings from two sites. – D: The variation of microfibril angle in dif-
ferent rings from two sites.
anatomical properties of Sanbei poplar. Their average fiber length, fiber width, fiber wall
thickness, fiber content and vessel content were 1252 µm, 20.50 µm, 6.8 µm, 55.7%
and 36.0% respectively in mature wood. Kojima et al. (2009) also found that xylem
maturation depends on diameter growth and begins after a certain diameter is reached.
They suggested that the rapid lateral growth during the early growth stage will produce
mature wood in Acacia spp. and Paraserianthes spp. after only a few growth seasons.
For samples from both sites, fiber length increased almost linearly with age for young
cambial age (Fig. 3), indicating that cambial age has an important effect on the length
of wood fibers in fast-growing plantation poplars. Fiber width and cell wall thickness
increased slightly from the pith towards the bark, but the rate slowed down in outer rings
(Fig. 3). For tissue proportions, a “step” transition appeared to take place at the second
ring, which then remained stable. The influence of site conditions on the properties of
Populus × euramericana cv. ‘74/76’ was not statistically significant, in agreement with
previous research involving young Pinus brutia where similar mean tracheid length
values were found for good and medium sites (Adamopoulos et al. 2012).
The variance analysis of anatomical properties of the samples is presented in
Table 4. It can be concluded that variations in fiber length are significant in rings 1–5,
and variations in fiber width are greater in rings 1–3 than in rings 4 and 5. The fiber
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Table 4. Variance analysis of tissue proportion and fiber morphology at different rings.
fiber cell lumen (μm)
Microfibrillar angle
fiber cell wall (μm)
Cell wall area (%)
Fiber length (μm)
Fiber width (μm)
Vessel area (%)
Fiber area (%)
Thickness of
Diameter of
Rings
1 A A A A A A A A
2 A AB A AB B B B A
3 A B A AB C BC B B
4 A B A BC D C B C
5 A AB A C E BC B D
F 2.186 5.927 1.466 5.211 29.208 4.54 11.527 28.164
P 0.075 0.00022 0.217 0.005 4.25E-08 4.54E-05 5.12E-05 6.70E-15
length of fast-growing trees tends to be shorter when the diameter of the stem is small,
but it is almost the same as that in wood from 22-year-old Sanbei poplar for larger
diameters when a comparison is made based on the distance from the pith (Bao et al.
2001). In contrast, if the comparison is made based on the same annual rings, there are no
significant differences in fiber length between fast and slower growing poplar samples.
The variation of MFA in Populus × euramericana cv. ‘74/76’ at breast height fol-
lowed the same pattern in all samples. It was high in the first two years, reached its
maximum (around 22°) in the second year, and then decreased rapidly to 12° close to
the bark (Fig. 3). This variation agreed with that of the MFA of Norway spruce (Sarén
et al. 2004). In the present study, MFA was around 10° in the fifth year, which is close
to that in mature Populus wood from trees more than 15 years old (Fang et al. 2004).
This means MFA is correlated with cambial age and suggests that MFA had almost
reached mature-wood values in the 5-year-old poplar samples. The area of juvenile
wood depends on the diameter of the tree and suggests that maturation starts after a
certain diameter is reached. This is consistent with the fact that the MFA in young trees
of small diameter was around 20°, whereas it was 10° for stems of larger diameter, even
though it was still juvenile wood.
MFA plays an important role in controlling certain fiber qualities, including fiber
stiffness and dimensional stability (Lachenbruch et al. 2010). MFA has significant
negative curvilinear correlations with all the mechanical properties (Yin et al. 2011).
MFA can be measured on small samples from within a growth ring so that the mechani-
cal properties of the fast-growing poplar can be predicted by MFA. The mechanical
properties of outer rings in the sample trees must be higher than those of the inner
rings. The influence of site conditions on the properties of Populus × euramericana
cv. ‘74/76’ was not statistically significant, in agreement with similar MFA values at
both sites.
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Table 5. Main chemical components of Populus × euramericana cv. ‘74/76’.
Experimental Cellulose (%) Lignin (%) Hemicellulose (%) Alcohol benzene Ash (%)
sites extractive (%)
SY 46.55 ± 3.68 26.83 ± 2.56 19.05 ± 0.78 4.03 ± 0.90 0.44 ± 0.09
MY 49.49 ±1.39 25.01 ± 1.53 18.50 ± 0.87 3.82 ± 0.57 0.51 ± 0.10
Chemical composition
Cellulose is the most important chemical component of wood cell walls, and is also
essential in pulp and paper. As shown in Table 5, the average cellulose content of Populus
× euramericana cv. ‘74/76’ was 48.9%, and lignin and hemicellulose contents were
25.4% and 18.8% averaged for ten trees, respectively. No clear difference was observ-
ed between poplars grown at the two sites. These average cellulose and lignin con-
tents are comparable to fast growing poplar from Henan province (Zhou et al. 2010)
but are higher than those of slower growing Populus nigra × P. simonii, which has
cellulose and lignin contents of 45.4% and 19.7 %, respectively (Jiang & Jiang 2008).
It has also been reported that juvenile wood and mature wood have less difference
in chemical composition than in anatomical and mechanical properties (Bao et al.
2001).
Table 6. Comparison of physical and mechanical properties of poplar wood in the SY and
MY sites.
Experimental sites MOE (GPa) Air-dry density (kg / m3 )
SY 9.26 ± 0.59 403 ± 21
MY 9.30 ± 0.67 400 ± 20
Physical and mechanical properties
The average density of the wood samples from the SY and MY sites based on vol-
ume at 12% moisture content and oven-dry weight, are 403 kg/m 3 and 400 kg/m3,
respectively (Table 6). In our study, the wood density of the poplar clone is higher
than that reported previously for clones of P. balsamifera (Ivkovich 1996), P. deltoi-
des (Olson et al. 1985), P. maximowiczii and P. balsamifera (Pliura et al. 2007). How-
ever, the present densities are in the same range of those reported for stands of P. tremula
(Kärki 2001). Higher mean wood densities were reported for hybrids of P. deltoides
(Zhang et al. 2003). These differences may be caused by the methods used to determine
wood density, clone variation within hybrids, different environmental conditions, and
variation in anatomical features. Moreover, results show that fast growth has no obvious
effect on wood density of the poplar trees. It is also reported that the basic density of
Scots pine is independent of growth rate (Kärenlampi & Riekkinen 2004).
There is no clear difference in average wood density of Populus × euramericana cv.
‘74/76’ from the two sites. The average wood density trend of sample trees at each site
is for an increase from pith to bark.
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The modulus of elasticity (MOE) of Populus × euramericana cv. ‘74/76’ is presented
in Table 6. The average MOE of ten trees was 9.3 GPa at breast height, which is similar
to other poplars with normal growth, such as 8.1 GPa for P. cathayana and 10.4 GPa
for P. tomentosa (Zhang 1997). MOE was therefore unaffected by fast growth. No
significant differences in MOE were observed between poplars grown at the two sites.
The average MOE of sample trees increases from pith to bark at both sites. Mechanical
properties of the selected young poplar hybrid crosses show no uniform trends relating
growth rate to either higher or lower modulus of elasticity or modulus of rupture (Yu
et al. 2008). Apart from anatomical features, tissue type also influences mechanical
properties. If all other factors are equal, wood tissue stiffness is closely correlated with
density and MFA. In several eucalypt species, both MFA and density have significant
and independent effects on the modulus of elasticity in static bending of small clear
specimens (Yang & Evans 2003).
CONCLUSIONS
The anatomical, chemical-physical and mechanical properties of Populus × eurameri-
cana cv. ‘74/76’ juvenile wood from two planting sites near Beijing were investigated.
The average DBH of the sample trees from both sites is 20.4 cm and average height
18.6 m. The annual increase in stem volume was 0.18 m3. Poplar trees from the two
sites both had a high growth rate, including both plant height and stem diameter in-
dicating fast-growth characteristics. The poplar trees at SY grew faster than those at
MY based on the annual increase in height and DBH. No significant differences in
these properties were observed between poplars grown at the two sites. Juvenile wood
properties of these fast growing trees appeared to be similar to those of poplars with a
slower growth rate, so the high growth rate does not seem to have any adverse effects
on wood properties. The Beijing region is therefore suitable for growing Populus ×
euramericana cv. ‘74/76’.
ACKNOWLEDGEMENTS
The authors thank the National “973” project foundation of China (2012CB114506) and the National
Natural Science Foundation (31370562) for financial support.
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Accepted: 19 November 2013
Associate Editor: Lloyd Donaldson
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