Comparing growth rate in a mixed plantation (walnut, poplar and nurse trees) with different planting designs: Results from an experimental plantation in Northern Italy

Abstract

Results of a mixed plantation with poplar, walnut and nurse trees established in winter 2003 in Northern Italy, are reported. Main tree species (poplar and walnut) were planted according to a rectangular design (10 x 11m), with different spacings and alternate lines. The experimental trial was carried out to verify the following working hypotheses: (i) possibility to combine main trees with different growth levels (common walnut, hybrid walnut, and different poplar clones) and test two different poplar and walnut spacings (5.0 and 7.4 m) in the same plantation; (ii) opportunity to reduce cultivation’s workload, in comparison with poplar monoculture, using mixtures with different poplar clones and N-fixing nurse trees; (iii) verifying the growth pattern of two new poplar clones in comparison with the traditional clones cultivated for different purposes in Italy.The use of different valuable crop trees’ mixtures intercropped with nurse trees and shrubs (including N-fixing trees) allows to decrease the cultivation’s workload. In fact, a heavy reduction of cultural practices - fertilizers, weed control, irrigation and pesticides applications (-61%) are the main concurrent, supplementary benefits. The best growth performances (DBH and tree height), associated with the higher competition towards walnuts, were recorded with the new clones Lena and Neva in comparison with the I214 and Villafranca. The closer spacing (5 m between poplar and walnut trees) was found to be unsuited to get merchantable poplars sized 30 cm without developing a heavy competition towards walnut trees. The wider spacing (7.4 m) resulted vice versa suitable to get poplar trees sized as requested by veneer factories and to maintain an acceptable competitive level with walnut. Within this plantation design, a shorter rotation (8 yrs) is needed for Lena and Neva clones in comparison with I214 and Villafranca (10 yrs). Walnut intercropped with poplar showed cone-shaped crowns, light branching and a good stem quality in comparison with walnut grown in pure plantations. This model of mixed plantation can become an interesting optional choice to walnut's and poplar's monoculture with notable advantages both for farm economics, landscape quality and environment preservation.

Full text

http://dx.doi.org/10.12899/ASR-750
37 (1), 2013: 13-21
1
Consiglio per la Ricerca e la sperimentazione in Agricoltura, Forestry Research Centre, Arezzo (CRA-SEL), Italy
2
Associazione Arboricoltura da Legno Sostenibile per l'Economia e l'Ambiente (A.A.L.S.E.A.)
*
corresponding author: francesco.pelleri@entecra.it
Research paper
Comparing growth rate in a mixed plantation (walnut, poplar and
nurse trees) with different planting designs: results from an experi-
mental plantation in northern Italy
Francesco Pelleri
1*
, Serena Ravagni
2
, Elisa Bianchetto
1
, Claudio Bidini
1
Received 15/11/2013 - Accepted 21/12/2013
Abstract - Results of a mixed plantation with poplar, walnut and nurse trees established in winter 2003 in Northern Italy, are re-
ported. Main tree species (poplar and walnut) were planted according to a rectangular design (10 x 11m), with different spacings
and alternate lines. The experimental trial was carried out to verify the following working hypotheses: (i) possibility to combine main
trees with different growth levels (common walnut, hybrid walnut, and different poplar clones) and test two different poplar and
walnut spacings (5.0 and 7.4 m) in the same plantation; (ii) opportunity to reduce cultivation’s workload, in comparison with poplar
monoculture, using mixtures with different poplar clones and N-fi xing nurse trees; (iii) verifying the growth pattern of two new poplar
clones in comparison with the traditional clones cultivated for different purposes in Italy. The use of different valuable crop trees’
mixtures intercropped with nurse trees and shrubs (including N-fi xing trees) allows to decrease the cultivation’s workload. In fact,
a heavy reduction of cultural practices - fertilizers, weed control, irrigation and pesticides applications (-61%) are the main concur-
rent, supplementary benefi ts. The best growth performances (DBH and tree height), associated with the higher competition towards
walnuts, were recorded with the new clones Lena and Neva in comparison with the I214 and Villafranca. The closer spacing (5 m
between poplar and walnut trees) was found to be unsuited to get merchantable poplars sized 30 cm without developing a heavy
competition towards walnut trees. The wider spacing (7.4 m) resulted vice versa suitable to get poplar trees sized as requested by
veneer factories and to maintain an acceptable competitive level with walnut. Within this plantation design, a shorter rotation (8 yrs)
is needed for Lena and Neva clones in comparison with I214 and Villafranca (10 yrs). Walnut intercropped with poplar showed cone-
shaped crowns, light branching and a good stem quality in comparison with walnut grown in pure plantations. This model of mixed
plantation can become an interesting optional choice to walnut's and poplar's monoculture with notable advantages both for farm
economics, landscape quality and environment preservation.
Keywords - common walnut, hybrid walnut, mixed plantation, poplar clones, sustainable tree farming
http://ojs-cra.cilea.it/index.php/asr
ANNALS OF SILVICULTURAL RESEARCH
Introduction
Poplar cultivation is the main internal source for
timber industry, producing about 50% of the whole
roundwood volume (Facciotto et al. 2003) in Italy.
Since the early nineties, poplar cultivation is suffer-
ing a heavy reduction from 90,000 to 61,381 hectares
(Gasparini e Tabacchi 2011, Coaloa et al. 2012) due
to several factors: (i) the weak farmers’ contractual
position when selling a poplar plantation, (ii) the
lack of any industry planning useful for program-
ming planting investments, (iii) the wider use by
poplar industry of semi-fi nished timber products
imported from other European and extra-European
countries, (iv) the increasing production costs, i.e.
petrol, pesticides, fertilizers, etc. (Nervo et al. 2007).
This condition enhanced research efforts for the
innovation of poplar cultivation systems towards
less energy-consuming plantations (Coaloa and
Vietto 2005) or plantations able to produce, within
the same rotation, larger-sized trees, along with re-
ducing production’s costs (Buresti and Mori 2006).
Another tree species, traditionally cultivated in
Italy for valuable timber production, is common
walnut (Juglans regia L.). In the past, this species
was generally cultivated as a single tree or in linear
plantations, intercropped to agricultural crops, both
for nuts and timber production (Minotta 1990, 1992).
More recently, this twofold production has been
replaced by specialized pure plantations for timber
production, using a square spacing of 5 to 6 m, or
walnut seed orchards (Giannini and Mercurio 1997).
Over the last decades in Europe, under the
fi nancial support of EU rules 2080/92 and of Rural
Development Plan, many mixed plantations were
established with walnut and other valuable broad-
leaved and nurse trees (Becquey 1997, Becquey
and Vidal 2006, Buresti and Mori 2006, Kelty 2006,
Tani et al. 2006, Clark et al. 2008, Mohni et al. 2009).
Recently in Italy, France, and North America, mixed

14
F. Pelleri, S. ravagni, e. Bianchetto, c. Bidini
Comparing growth rate in a mixed plantation (walnut, poplar and nurse trees) with different planting designs:
results from an experimental plantation in northern Italy
Annals of Silvicultural Research - 37 (1), 2013: 13-21
plantations with walnut and poplar clones have
been tested both in plantation forestry (Zsuffa et
al. 1977, Buresti et al. 2008a, Vidal e Becquey 2008a,
Paquette et al. 2008) and in agro-forestry systems
(Balandier and Dupraz 1999, Rivest et al. 2010).
These experiences pointed out the opportunity of
testing the simultaneous cultivation of both species,
characterized by different rotations (short poplar,
medium-long walnut).
In this type of plantation, named in Italy poly-
cyclical plantation (after Buresti et al. 2008b and
Buresti and Mori 2012), the main crop trees with the
same rotation are spaced at a definitive distance, to
reach the merchantable size requested by industry
within a shorter cycle. The reciprocal distance
between main walnut and poplar trees must allow
each tree to complete its rotation without the es-
tablishment of any heavy interspecific competition.
Furthermore, when interplanting distance is cor-
rect, a positive competition can arise and the fast
growing tree species can influence in a beneficial
way the shape of lower growth and shade tolerant
trees because of their low-covering crown (Schütz
2001, Pommerening and Murphy 2004, Kelty 2006,
Buresti et al. 2008a, Buresti et al. 2008b, Vidal and
Becquey 2008a).
A further benefit arising from this type of planta-
tion is economical, by allowing a better distribution
of income, poplar being harvested within 8-10 years
and walnut within 20-30 years (Vidal and Becquey
2008b).
Results obtained in a poplar and walnut polycy-
clical plantation, aged 9 years, are here reported.
The plantation has been carried out to verify the
following hypotheses:
1) opportunity to get poplars sized 30 cm DBH
testing two different distances from walnut (5
and 7.4 m) without heavy interspecific compe-
tition;
2) possibility of poplar cultivation at lower costs
as compared with the traditional technique,
reducing tending operations, using more than
one poplar clone and mixtures with N-fixing
trees;
3) test poplar clones different from the I214 tra-
ditionally planted in Italy, to verify their own
productivity and ability to be competitive on
the wood market;
4) opportunity of harvesting additional valuable
trees established as nurse trees.
Materials and Methods
Site description and plantation management
Planting operations started in February 2003 in a
flood plain of the Oglio river in the Mantua Province
(San Matteo alle Chiaviche). The area is character-
ized by a good site-index, deep, silty-sandy and
moderately alkaline soils, subjected to periodical
flooding. A sub-continental climate with cold winters
and hot wet summers characterizes the area (mean
annual temperature 13.6° C., mean annual precipita-
tion 790 mm with maximum in autumn and minimum
in winter; mean rainfall reaches 145 mm in summer
with a dry period of one month only).
This mixed plantation, extended over 14 hec-
tares, is characterized by the following main crop
trees: four poplar clones (Lena, Neva, I214 e Vil-
lafranca), common walnut (Juglans regia L.) and
hybrid walnut (Juglans x intermedia MJ209) (Tab.
1). The common walnut’s trees have been planted
initially in pairs, each tree being located at 1 m, in
order to select shortly the best tree of each couple.
In this way, the chance to get a good quality tree in
the expected position increases (Buresti et al. 2001,
2003). Walnut and poplar main trees are planted
according to a rectangular design (10 x 11 m) using
alternate lines. In all, 90 poplars, 45 hybrid walnuts,
and 45 common walnut couples were planted per
hectare. The plantation is divided into two areas,
depending on the planting design adopted (A and
B). Each area is divided in monoclonal plots of
about 1.5 hectare for each clone. In design A, wal-
nut trees (both hybrid and common walnuts) are
set in a row with poplar at 5 m. In design B, walnut
trees are staggered to poplars by 7.4 m. Nurse trees
were black alder (Alnus glutinosa (L.) Gaertn.)
and hazel (Corylus avellana L.); along with these
other four valuable nurse trees were introduced,
i.e. narrow-leaved ash (Fraxinus angustifolia L.),
wild service tree (Sorbus torminalis (L.) Cranz.),
English oak (Quercus robur L.), and pear (Pyrus
Table 1 - Main characteristics of poplar clones. Modified from Facciotto et al. (2003).
Clone origin main characteristics
Lena Populus deltoides Bartr. higher productivity in comparison with I214; resistance to leaf diseases (rusts and aphids);
sensitive to wind; high shrinkage and nervousness in comparison with I214.
Neva Hybrid Populus sensitive to spring defoliation and rusts; resistance to bronzing; bark necrosis and brown
X canadensis Monch spots; higher productivity in comparison with I214; sensitive to wind
I 214 Hybrid Populus resistance to spring defoliation; virus and brown spots; green wood density lower to Lena
X canadensis Monch and Neva; resistance to wind; easy to prune
Villafranca Populus alba L. clone resistance to main diseases (rusts. spring defoliation. bronzing. viruses and aphids); hard to
prune; slower growth in comparison with I214

15
F. Pelleri, S. ravagni, e. Bianchetto, c. Bidini
Comparing growth rate in a mixed plantation (walnut, poplar and nurse trees) with different planting designs:
results from an experimental plantation in northern Italy
Annals of Silvicultural Research - 37 (1), 2013: 13-21
pyraster Burgsd.) (Fig.1). Valuable nurse trees, in
addition to their own tending role, act as sort of
insurance for the final outcome of the plantation,
because they are able to replace any main crop tree
in case of death, damage or if they do not achieve
the expected development (Buresti and Mori 2004).
Consequently, the valuable nurse trees must be sub-
jected, at least for 4 to 6 years (pruning phase), to
the same tending operations as the main crop trees
(especially pruning).
In this plantation, the operations have been con-
siderably reduced when compared with traditional
monoclonal poplar plantations by:
1. Reduction of both fertilizers using N-fixing
nurse trees, and pesticides using multi-clonal
mixed plantations. These plantations allowed
the reduction of spreading disease risk by pop-
lar leaf rust (Melampsora spp.) or poplar leaf
and shoot blight (Venturia populina (Vuill.)
Fabric.), as already verified in other areas in-
tensively cultivated with poplars (Coaloa and
Vietto 2005).
2. Stimulation of poplar radial growth, with con-
sequent reduction of the rotation, by planting
more spaced trees that develop longer and
well-lighted crowns over the whole cycle.
Over the first two years, only one treatment
against wood insects’ poplar-and-willow borer
(Cryptorrhynchus lapathi) was performed; in the
following two years a localized treatment to control
longhorn beetle (Saperda carcharis L.) was carried
out in the outside strips (about 20 m) to prevent the
diffusion of this insect from neighbouring traditional
poplar plantations. Weed control was carried out by
polyethylene mulching and using both chemical and
mechanical weeding during the first four years. On
average two mechanical weeding and one chemical
Figure 1 - The planting designs (A and B).
weeding per year were carried out.
No use of fertilizers or irrigations was made,
whereas neighbouring traditional I214 plantations
are managed intensively with one initial fertilization,
3-5 pesticide treatments per year and periodical
emergency irrigations.
Selection of best trees between walnut’s couples
was accomplished at age 5 when trees differences in
vigour and shape were already evident. Pruning of
main trees and of valuable nurse trees was carried
out up to the age of 6, i.e. when the farmer decided
to promote walnut trees because of the good stem
quality and clear boles at least up to 3 meters. Pop-
lars were pruned up to 5.5 meters.
Field survey and data analysis
DBH surveys were performed yearly between the
age of 4 and 9 (2006 to 2011). In the pruning phase
(2006-2007) a sample of walnut and poplar clones
only (at least 30 trees per species, clone and spac-
ing) were considered. Since 2008, when the pruning
ended, a complete survey of walnut and poplar DBH
was made. Total tree and clear bole height of walnut
and poplar clones were measured at ages 6 and 9
(2008 and 2011); in 2011 only 50% of poplar heights
were measured. In 2011 an individual stem quality
evaluation was carried out for walnut trees, rank-
ing stems per quality classes, from A (veneer) to D
(firewood) using the method set up for tree farming
plantations proposed by Nosenzo et al. (2008) that
assigns a quality class, valuing length, trunk axiality,
knots presence and other defects. Only DBHs were
measured on valuable nurse trees in 2008 and 2009.
First, the normality of distributions was evalu-
ated by mean of Kolmogorof-Smirnov’s test, then
data were processed with the analysis of variance
(ANOVA) comparing separately poplar clones, com-
mon walnut and hybrid walnut, but considering
both planting designs (A and B). The comparison of
means was performed by the Tukey’s Test (HSD),
with a significance of 0.05 (Statistica 2005). The
walnut’s distribution per stem quality classes was
compared by χ² Pearson’s test. The performances of
valuable nurse trees and walnut trees were evalu-
ated by comparing mean DBH at 2009 (age 7).
A comparison between traditional poplar plan-
tation (Colaoa and Vietto 2005) and this new type
of poplar and walnut plantation was carried out
comparing the presence (1)/ absence (0) of cultural
practices undertaken during the poplar rotation (10
years).
Results
Poplar
Stem diameter - Poplar clones showed differ-
poplar clone
hybrid walnut
common walnut
black alder
valuable nurse tree
hazel
KEY
10 m
11 m
10 m
11 m
panting design A
panting design B

16
F. Pelleri, S. ravagni, e. Bianchetto, c. Bidini
Comparing growth rate in a mixed plantation (walnut, poplar and nurse trees) with different planting designs:
results from an experimental plantation in northern Italy
Annals of Silvicultural Research - 37 (1), 2013: 13-21
ent growth rates at the age of 7-8 (years 2009-10)
Lena and Neva clones reached the commercial size
(DBH=30 cm), whilst I214 and Villafranca clones
reached the same size 1 or 2 years later (Fig. 2). In
2011, the ANOVA of DBH showed significant differ-
ences among the clones and blocks (designs) rang-
ing from 38.0, 36.1, 31.5 and 39.1 cm respectively for
Lena, Neva, I214 and Villafranca (Tab. 2).
DBH increments keep high, i.e. 3-4 cm yr
-1.
In
2009, Lena pointed out significant higher incre-
ments as compared with all clones in both designs,
differences that decreased in the following years.
Less productive clones showed more steady incre-
ments while a heavy DBH increment reduction was
highlighted in Lena and, to a lesser extent, in Neva.
In the last two years the I214 clone showed higher
DBH increments (Fig. 3).
Tree height - The ANOVA of height is significant
for both years (2008 and 2011) pointing out, in 2011,
a superior height growth of Lena and Neva clones
that reached on average 24.4 m and 24.2 m respec-
tively, in comparison with 20.8 m and 18.2 m of the
I214 and Villafranca clones (Tab. 3). At the age of 9
all poplar clones had built up green and efficient, up
to 12-18 m long, crowns in comparison with poplars
cultivated as usual, where crowns are narrow and
shorter because of their progressive elevation due
to the heavy competition (poplar stem density =
277 tree ha
-1
).
Table 2 - ANOVA of poplar clones DBH 2011 (9 yrs).
DBH ±SD HSD
Poplar clone (cm) (cm)
Lena 38.0 2.2 a
Neva 36.1 2.7 b
I214 31.5 3.0 c
Villafranca 30.1 2.1 d
ANOVA df MS F P value
Clone 3 2559.1 516.4 0.0000
design (block) 1 896.2 180.8 0.0000
clone x design 3 65.9 13.3 0.0000
error 716 5.0
Figure 2 - Poplar clones DBH growth trend ± SE.
25
30
35
40
cm
Poplar clones mean DBH ± SE
10
15
20
25
2008
2009
2010
2011
cm
year
Lena Neva I214 Villafranca
3
,
0
3,5
4,0
4,5
5,0
cm
Poplar clones DBH current increment ±SE
1,0
1,5
2,0
2,5
3
,
0
2009 2010 2011
cm
year
Lena Neva I214 Villafranca
Figure 3 - Poplar clones DBH current increment ± SE.
Walnut
Stem diameter - Data analysis pointed out sig-
nificant differences for walnut DBH grown with
different spacings, it being lower in design A com-
pared to B. Significant variations in DBH were also
found for both walnuts originated by the different
intercropped poplar clones (Fig. 4 and 5). Hybrid
walnut reached on average lower DBH (13.2 cm and
13.7 cm) when intercropped with Lena and Neva in
comparison with I214 and Villafranca intercropping
(14.9 cm and 15.4 cm) in 2011 (age of 9). Common
walnut reached 12.6 and 12.9 cm when intercropped
with Lena and Neva and 13.1 and 13.6 cm when in-
tercropped with I214 and Villafranca (Tab. 4).
A reduction of DBH increment was observed for
both walnuts and designs within all the intercropped
poplar clones in the last years. Since 2010, significant
differences among intercropping and designs were
found for both walnuts. A mean reduction of DBH
increment from 1.6 to 0.6 cm in Lena and from 1.5
to 0.7 cm in Neva intercropping was observed for
hybrid walnut over 2009-2011, whilst a lower DBH
variation of annual increment was detected with the
less productive clones: from 1.9 to 1.2 cm in I214 and
from 2.0 to 1.1 cm in Villafranca intercropping (Fig.
6). In common walnut DBH increment variations
were less significant and only walnut intercropped
with I214 in design B was significantly different.
A mean reduction of common walnut DBH incre-
ment from 1.7 to 0.9 cm in Lena and from 1.8 to 1.1
cm in Neva intercropping was observed along the
full observation period; whilst a DBH increment
variation from 2.0 to 1.3 cm and from 1.9 to 1.2 cm
was recorded, respectively in I214 and Villafranca
intercropping (Fig. 7). The trend of DBH increment
highlights the different specific growth pattern of
both walnuts. Hybrid walnut has an early more
sustained growth followed by a progressive reduc-
tion due to poplar competition, up to a radial (DBH)
growth lower than common walnut (Fig. 8).
Tree height - The ANOVA showed significant
differences in height with common walnut for both
intercropping types in 2008 and 2011, and for plant-
ing design in 2011 only. In hybrid walnut significant
differences in height were noticed for the different

17
F. Pelleri, S. ravagni, e. Bianchetto, c. Bidini
Comparing growth rate in a mixed plantation (walnut, poplar and nurse trees) with different planting designs:
results from an experimental plantation in northern Italy
Annals of Silvicultural Research - 37 (1), 2013: 13-21
Table 3 - ANOVA of poplar clones tree height 2008 - 2011 (6-9 yrs).
h2008 ±SD HSD h2011 ±SD HSD
Poplar clone (m) (m) (m) (m)
Lena 17.43 1.43 a 24.38 2.06 a
Neva 16.80 1.21 b 23.16 1.35 a
I214 14.09 1.49 c 20.75 1.30 b
Villafranca
12.13 0.99 d 18.23 2.00 c
ANOVA df MS F P value df MS F P value
clone 3 1111.5 780.50 0.0000 3 798.6 285.58 0.0000
design (block) 1 114.5 80.40 0.0000 1 4.1 1.48 0.2242
clone x design 3 26.3 18.50 0.0000 3 25.2 9.02 0.0000
726 1.4 357 2.8
13
14
15
16
17
Hybrid walnut DBH ± SE
Lena A
Neva A
I
214
A
Villafranca A
7
8
9
10
11
12
2008 2009 2010 2011
cm
year
Villafranca A
Lena B
Neva B
I214 B
Villafranca B
13
14
15
16
17
Common walnut DBH ± SE
Lena A
Neva A
I
214
A
7
8
9
10
11
12
2008 2009 2010 2011
cm
year
I
214
A
Villafranca A
Lena B
Neva B
I214 B
Villafranca B
Figure 4 - Hybrid walnut DBH growth trend ± SE per intercropping
and design.
Figure 5 - Common walnut DBH growth trend ± SE per intercrop-
ping and design.
intercroppings in 2008 and 2011, and for planting
design in 2011 only.
The best performances of common walnut at
the end of 2011 were measured in the design B with
Lena and Neva intercropping (13.7 and 14.4 m) while
the worst results were found in design A with I214
(11.2 m) and Villafranca (11.6 m) (Tab 5). In 2011, the
best performances of hybrid walnut were recorded
in design B with Lena and Neva intercropping (14.9
and 14.5 m) and the worst results were achieved in
design A with I214 (13.2 m) and Villafranca (12.8
m) (Tab. 6).
Stem quality - The quality of hybrid walnut
showed to be significant superior (χ² = 31.2 p<0.001)
as compared with common walnut. The 72% of
hybrid trees belong to classes A and B, suitable for
veneer and first quality saw-timber production, while
only 53% of common walnut trees reached the same
category (Tab.7). Stem pruning was carried out up
to the height of 3 m. Over this height, crowns were
left to grow free. The percentage of trees suitable
to produce valuable assortments could increase
for both walnut species at higher plantation ages,
because defects due to knots and small stem curves
could be overestimated in trees aged 9 years.
Valuable nurse trees
Close to the end of the pruning phase (2009),
the owner decided to concentrate operations only
on walnut trees showing faster growth and/or fit-
ting the expected stem quality, neglecting valuable
nurse trees. Among these, the best DBH growth
performance was recorded by narrow-leaved ash
followed by pear, oak, and wild service trees. Mean
DBH values measured in 2009 in walnuts and valu-
able nurse trees are reported in Fig. 9.
Cultivation practices
The comparison between cultivation practices
undertaken in the two plantation types (pure and
polycyclical) during the poplar rotation are showed
in Tab. 8 and 9. In the mixed walnut-poplar planta-
tion, the absence of any irrigation and fertilization,
a heavy reduction of the use of pesticides (-90%) and
of mechanical weed control (-60%) was recorded.
On the other hand, the chemical weed control along
walnut lines, the selection between walnut couples,
and the increase of pruning (+25%), were necessary
in this new type of plantation.
Discussion
At the age of 9 (2011), Lena and Neva clones
reached notable diameters (34.7 to 38.4 cm) but
showed already evidence of an incremental reduc-
tion, whilst the I214 and Villafranca maintained a
more regular growth course with lower DBH (29.1
to 33.3 cm). These growth patterns highlight on one
side the early establishment of intraspecific compe-
tition within the fastest growing clones, especially
Lena, and on the other side, the delayed occurrence
of the same condition within the less productive
clones (I214 and Villafranca).
The year 2009 showed to be the right time for

18
F. Pelleri, S. ravagni, e. Bianchetto, c. Bidini
Comparing growth rate in a mixed plantation (walnut, poplar and nurse trees) with different planting designs:
results from an experimental plantation in northern Italy
Annals of Silvicultural Research - 37 (1), 2013: 13-21
Table 4 - DBH 2011 (9 yrs) common and hybrid walnut ANOVA per planting design and intercropped poplar clones.
Hybrid walnut Common walnut
DBH ±SD HSD DBH ±SD HSD
intercropping design (cm) (cm) (cm) (cm)
Lena A 12.8 1.9 c 12.39 1.35 de
Neva A 13.1 2.4 c 11.90 1.03 e
I214 A 14.9 2 ab 12.92 0.86 bcd
Villafranca A 14.8 2.3 ab 13.11 1.68 bcd
Lena B 13.7 2.2 bc 12.82 1.25 cd
Neva B 14.3 2.1 bc 13.80 1.43 ab
I214 B 14.9 2.3 ab 13.34 1.59 abc
Villafranca B 15.9 2.2 a 14.09 1.65 a
ANOVA df MS F P value df MS F P value
design 1 50.93 10.58 0.0013 1 70.06 36.31 0.0000
intercropping 3 79.69 16.55 0.0000 3 14.91 7.73 0.0001
intercropping x design 3 5.70 1.18 0.3160 3 9.42 4.88 0.0025
error 314 4.81 314 1.93
1,5
2,0
2,5
Common walnut DBH current increment ± SE
Lena A
Neva A
I
214
A
0,0
0,5
1,0
2009 2010 2011
cm
year
I
214
A
Villafranca A
Lena B
Neva B
I214 B
Villafranca B
1,5
2,0
2,5
Hybrid walnut DBH current increment ± SE
Lena A
Neva A
I
214
A
0,0
0,5
1,0
2009 2010 2011
cm
year
I
214
A
Villafranca A
Lena B
Neva B
I214 B
Villafranca B
1,5
2,0
2,5
cm
DBH current increment ± SE
0,0
0,5
1,0
2006 2007 2008 2009 2010 2011
cm
yr
hybrid walnut
common walnut
Figure 7 - Common walnut DBH current increment per intercrop-
ping and design.
Figure 6 - Hybrid walnut DBH current increment per intercropping
and design.
Figure 8 - Comparison between common and hybrid walnut DBH
increment.
poplars harvesting, especially for the more produc-
tive clones - Lena and Neva - (Facciotto et al. 2003)
already sized enough for marketing (DBH ≥30 cm),
but adverse concomitant local market conditions
didn’t make profitable their felling to the farmer.
Their maintenance for a longer time affected nega-
tively walnut DBH increment, that is -63% compared
with 2009 (hybrid walnut intercropped with Lena),
-46% (common walnut) and -36% with the I214
intercropping in both walnuts spp. (Tab. 10). At
the end of 2011, walnuts have still long, green func-
tional crowns (12-18 m) and looked able to react to
poplars’ harvesting as compared with pure walnut
plantations undergoing a late thinning, these gener-
ally showing a lower reaction (De Meo et al. 1999,
Marchino and Ravagni 2007). Despite that, no data
exist at now about the incremental DBH reaction of
walnut following a period of heavy competition in
this type of plantation. This points out the need of
using larger poplar-walnut interplanting distances,
especially if we are going to use both slower grow-
ing clones needing rotations of about 10 years to
produce poplars sized more than 40 cm (Buresti
and Mori 2013). The basic importance of defining
suited planting designs and correct walnut-poplar
distances is in this way underlined.
Both walnut trees (common and hybrid) showed
a fast DBH growth. Hybrid walnut presented a su-
perior stem quality in accordance with Paris et al.
(2003), 72% of stems being suitable for good quality
veneer and saw timber production. On the contrary,
common walnut had only 53% of stems in the best
quality classes (Tab. 7). Despite the worse perfor-
mance in terms of growth and stem quality, the use
of common walnut is more frequent since its timber
is more appreciated by the Italian industry and be-
cause this species has been traditionally cultivated
in Italy since the Roman period. The achievement
of a good standard in the trunk quality has been
determined by poplar’s presence (i.e. fast growing
species with low-covering crown) able to nurse,
under his light canopy, walnuts characterized by

19
F. Pelleri, S. ravagni, e. Bianchetto, c. Bidini
Comparing growth rate in a mixed plantation (walnut, poplar and nurse trees) with different planting designs:
results from an experimental plantation in northern Italy
Annals of Silvicultural Research - 37 (1), 2013: 13-21
Table 5 - Tree height 2008 - 2011 (6-9 yrs) common walnut ANOVA per planting design and intercropped poplar clones.
Common walnut
h2008 ±SD HSD h2011 ±SD HSD
intercropping design (cm) (cm) (cm) (cm)
Lena A 8.26 1.06 a 13.13 1.60 a
Neva A 7.89 0.98 a 12.53 1.22 a
I214 A 7.08 0.64 b 11.20 1.08 b
Villafranca A 7.80 1.15 a 11.63 1.50 b
Lena B 8.38 1.33 ab 13.71 1.63 a
Neva B 8.55 1.55 a 14.42 1.99 a
I214 B 7.39 1.19 c 12.38 1.91 b
Villafranca B 7.74 0.89 bc 12.05 1.30 b
ANOVA gdl MS F P value gdl MS F P value
design 1 4.86 3.86 0.0503 1 80.51 33.62 0.000
intercropping 3 16.97 13.48 0.0000 3 68.94 28.79 0.000
intercropping x design 3 1.70 1.35 0.2571 3 8.97 3.75 0.011
error 281 1.26 306 2.39
Table 6 - Tree height 2008 - 2011 (6-9 yrs) hybrid walnut ANOVA per planting design and intercropped poplar clones.
Hybrid walnut
h2008 ±SD HSD h2011 ±SD HSD
intercropping design (cm) (cm) (cm) (cm)
Lena A 8.36 0.97 a 13.57 1.65 a
Neva A 8.14 1.40 a 13.69 2.03 a
I214 A 8.11 0.75 a 13.22 1.44 a
Villafranca A 7.88 1.05 a 12.81 1.54 a
Lena B 8.57 1.40 ab 14.92 2.18 a
Neva B 9.08 1.38 a 14.51 2.03 ab
I214 B 7.94 1.06 b 14.09 1.67 ab
Villafranca B 8.06 0.98 b 13.41 1.28 b
ANOVA df MS F P value df MS F P value
design 1 4.87 2.97 0.0862 1 65.07 21.1 0,0000
intercropping 3 6.01 3.66 0.0131 3 20.45 6.63 0,0002
intercropping .x design 3 3.15 1.92 0.1274 3 2.06 0.67 0.5722
error 238 1.64 307 3.08
Table 7 - Distribution of common and hybrid walnut per stem
quality classes and relative χ² test.
Stem quality common walnut hybrid walnut
classes n % n %
A 35 10.8 63 19.4
B 137 42.4 170 52.5
C 97 30.0 71 21.9
D 54 16.7 20 6.2
tot 323 100.0 324 100.0
χ² = 31.19 p=0.000
Figure 9 - Comparison among walnuts and valuable nurse trees
DBH ±SD at the age of 7 (2009).
6
8
10
12
14
16
cm
Walnuts and valuable nurse trees DBH±SD in 2009
0
2
4
6
Trees species
slender, cone-shaped crowns and small-sized, easily
self-pruned branches (Buresti et al. 2008b). Valu-
able nurse trees were no longer managed (except
the best phenotypes) because of the good walnut
performance both in terms of growth and stem qual-
ity. That is why their cultivation (pruning) has been
abandoned in this trial. In any case, according to
Kelty (2006) and Buresti and Mori (2004), they can
support an improvement of biodiversity and stabil-
ity of mixed plantations and their intercropping is
a sort of farmer’s insurance for the final outcome
of the plantation.
Results highlight that the concurrent cultivation
of walnut and poplar species is possible and advis-
able because of the manifold positive outcomes:
reduced environmental impact of the plantation
type as compared with traditional poplar monocul-
ture, since the trial pointed out a clear reduction of
external inputs in terms of (i) fertilization (-100%),
(ii) irrigation (-100%), (iii) use of pesticides (-90%)
as compared with the traditional cultivation.
This was possible by using multi-clonal mixed
plantations with N-fixing nurse trees that consider-
ably reduce both the spreading of pests and diseases
and also the consequent amount of chemical treat-
ments. The use of fertilizers was cut down, too.
All these achievements are quite positive as

20
F. Pelleri, S. ravagni, e. Bianchetto, c. Bidini
Comparing growth rate in a mixed plantation (walnut, poplar and nurse trees) with different planting designs:
results from an experimental plantation in northern Italy
Annals of Silvicultural Research - 37 (1), 2013: 13-21
compared with the number of cultivation practices
needed in a traditional poplar’s plantations along
the whole poplar rotation (Colaoa and Vietto 2005).
Their occurrence was limited to less than one half
in the studied plantation. The abatement of these
cultivation costs has a notable economic advantage
both for farmers and towards the environment.
A limitation to the use of the Lena and Neva
clones is nowadays the shortage of market for these
clones in Italy, in spite of their significant superior
growth, the lower cultivation needs, in comparison
with I214, and the suitable mechanical timber char-
acteristics. Unfortunately, the higher shrinkage of
timber in these clones decreases the value for veneer
production, this determining a difficult marketing.
At present, veneer industry fully prefers timber from
I214 plantations (Facciotto et al. 2003). According
to Vidal and Becquey (2008b), the mixture between
poplar and walnut appears to be interesting from the
economic point of view too. Further research trials
are necessary to value the overall productivity of the
two main tree species at the end of walnut rotation
and the analytical evaluation of costs.
Table 8 - Presence (1) or absence (0) of cultural practices in traditional poplar plantations - modified from Colaoa and Vietto (2005).
Traditional poplar plantation years of poplar rotation
cultural operations 1° 2° 3° 4° 5° 6° 7° 8° 9° 10° total
mechanical weed control 1 1 1 1 1 1 1 1 1 1 10
irrigation 1 1 1 1 1 1 1 1 1 1 10
pesticide 1 1 1 1 1 1 1 1 1 1 10
chemical weed control 0 0 0 0 0 0 0 0 0 0 0
pruning 1 1 1 0 1 0 0 0 0 0 4
selection double walnut 0 0 0 0 0 0 0 0 0 0 0
fertilization 1 1 1 1 0 0 0 0 0 0 4
total interventions 5 5 5 4 4 3 3 3 3 3 38
Table 9 - Presence (1) or absence (0) of cultural practices in the studied plantation.
Polyciclical plantation years of poplar rotation
cultural operations 1° 2° 3° 4° 5° 6° 7° 8° 9° 10° total
mechanical weed control 1 1 1 1 0 0 0 0 0 0 4
irrigation 0 0 0 0 0 0 0 0 0 0 0
pesticides 0 1 0 0 0 0 0 0 0 0 1
chemical weed control 1 1 1 1 0 0 0 0 0 0 4
pruning 0 1 1 1 1 0 0 0 0 1 5
selection double walnut 0 0 0 0 1 0 0 0 0 0 1
fertilization 0 0 0 0 0 0 0 0 0 0 0
total interventions 2 4 3 3 2 0 0 0 0 1 15
Conclusive remarks
The recent experimental trials on mixed poly-
cyclical plantations (walnut, poplar and nurse
trees) highlighted the notable potential of this type
of plantation in flood plain areas favourable to the
cultivation of both main species, providing interest-
ing productive outcomes as well as ecological-envi-
ronmental considerations. These mixed plantations,
more resistant to external disturbances and less
demanding in terms of energetic inputs (fertilization,
pesticides, irrigation, etc.) proved to be innovative
and more sustainable for poplar and walnut culti-
vation. It can become an advantage for the farmer,
especially within the current unfavourable period
for poplar timber marketing and for the quality of
flood plain environment.
The experiences with this plantation design
are at an early stage and more comparative tests,
widespread trials and screening of results are nec-
essary. The goals in progress are: development of
suitable cultivation models with wide inter-distances
walnut-poplar (≥7.4 m), enough to allow: (i) the
early achievement of poplar’s cultivation goals
before heavy competition with walnut becomes
established; (ii) the use of a fast growing species, i.e.
poplar to nurse walnut trees provided with slender,
conical-shaped crowns, and light branching.
Acknowledgements
Special thanks to Enrico Buresti, who conceived
this type of plantation and to Francesco Mattioli
who managed with competence and care the full
plantation cycle. The authors want to thank also the
anonymous referees for their helpful suggestions.
Table 10 - Reduction of current DBH increments between 2009 and
2011
hybrid walnut common walnut
DBH increment DBH increment
Intercropped reduction reduction
clones % %
Lena 63 46
Neva 53 37
I214 36 36
Villafranca 43 38

21
F. Pelleri, S. ravagni, e. Bianchetto, c. Bidini
Comparing growth rate in a mixed plantation (walnut, poplar and nurse trees) with different planting designs:
results from an experimental plantation in northern Italy
Annals of Silvicultural Research - 37 (1), 2013: 13-21
References
Balandier P. and Dupraz C. 1999 - Growth of widely spaced
trees. A case study from young agroforestry plantations
in France. Agroforestry Systems 43: 151-167.
Becquey J. 1997 - Les noyers à bois. Institut pour le Dévelop-
pement Forestier (IDF), Paris, 144 p.
Becquey J. and Vidal C. 2006 - Quels accompagnements ligneux
choisir pour les plantations de noyer? Forêt Entreprise
170: 35-38.
Buresti E., Mori P., Ravagni S. 2001 - Arboricoltura da legno con
il ciliegio: ridurre i rischi adottando la doppia pianta.
Sherwood - Foreste ed Alberi Oggi 73: 11-16.
Buresti E., Mori P., Ravagni S. 2003 - Quando diradare la doppia
pianta. Un'esperienza con la farnia (Quercus robur L.).
Sherwood - Foreste ed Alberi Oggi 85: 21-24.
Buresti Lattes E. and Mori P. 2004 - Ruolo delle piante, specie e
tipologie d'impianto in arboricoltura. Sherwood - Foreste
ed Alberi Oggi 98: 15-18.
Buresti Lattes E. and Mori P. 2006 - Legname di pregio e bio-
massa nella stessa piantagione. Sherwood - Foreste ed
alberi oggi 127: 5-10.
Buresti Lattes E., Mori P., Pelleri F., Ravagni S. 2008a – Des
peupliers et des noyers en mélange, avec des plants accom-
pagnateurs. Forêt – Enterprise 178: 26-30.
Buresti Lattes E., Cavalli R., Ravagni S., Zuccoli Bergomi L.
2008b - Impianti policiclici di Arboricoltura da legno:
due esempi di progettazione e utilizzazione. Sherwood -
Foreste ed Alberi Oggi 139: 37-39.
Buresti Lattes E. and Mori P. 2012 - Piantagioni policicliche.
Elementi di progettazione e collaudo. Sherwood - Foreste
ed Alberi Oggi 189: 12-16.
Clark J., Hemery G.E., Savill P. 2008 - Early growth and form of
common walnut (Juglans regia L.) in mixture with tree
and shrub nurse species in southern England. Forestry
81: 631-644.
Coaloa D., Tonetti R., Napolitano L. 2012 - Novità della misura
221B del PSR della Lombardia: ciclo medio lungo anche
per il pioppo. Sherwood – Foreste ed Alberi Oggi 186: 23-27.
Coaloa D. and Vietto L. 2005 - Pioppicoltura ecologicamente
disciplinata. Sherwood – Foreste e Alberi Oggi 113: 23-28.
De Meo I., Mori P., Pelleri F., Buresti E. 1999 - Prime indica-
zioni sugli interventi di diradamento nelle piantagioni
di Arboricoltura da Legno. Sherwood – Foreste ed Alberi
Oggi 43: 15-20.
Facciotto G., Minotta G., Zambruno G.P. 2003 - Analisi della
produttività dei cloni di pioppo Dvina, Lena e Neva. In:
Atti IV congresso Nazionale SISEF, Meridiani Forestali,
Università degli Studi della Basilicata, Rifreddo (PZ), 07-10
Ottobre 2003: 151-158.
Gasparini P. and Tabacchi G. (a cura di) 2011 - L’Inventario
Nazionale delle Foreste e dei serbatoi forestali di Carbonio
INFC 2005. Secondo inventario forestale nazionale italia-
no. Metodi e risultati. Ministero delle Politiche Agricole,
Alimentari e Forestali; Corpo Forestale dello Stato, Consi-
glio per la Ricerca e la sperimentazione in Agricoltura, Unità
di ricerca per il Monitoraggio e la Pianificazione Forestale.
Edagricole - Il Sole 24 ore, Bologna, 653 p.
Giannini R. and Mercurio R. 1997 - Il Noce comune per la pro-
duzione legnosa. Avenue media, Bologna, 302 p.
Kelty M.J. 2006 - The role of species mixture in plantation
forestry. Forest Ecology and Management 233: 195-204.
Marchino L. and Ravagni S. 2007 - Effetti del diradamento in
impianti puri di noce. Sherwood - Foreste ed alberi oggi
139: 40-41.
Minotta G. 1990 - La cultura del noce da frutto e a duplice
attitudine produttiva in collina e montagna. Monti e
boschi 41 (1): 27-33.
Minotta G. 1992 - La nocicoltura a duplice attitudine. In: Atti
del 12° convegno pomologico “La coltura del noce”, Caserta
4 luglio: 140-150.
Mohni C., Pelleri F., Hemery G.E. 2009 - The modern silviculture
of Juglans regia L.: a literature review. Die Bodenkultur
60 (3): 19-32.
Nervo G., Magni M., Gazza F., Petrucci B., Carovigno R., Berti S.,
Ducci P. 2007 - Libro bianco pioppicoltura. Commissione
nazionale per il pioppo, 143 p.
Nosenzo A., Berretti R., Boetto G. 2008 - Piantagioni da legno.
Valutazione degli assortimenti ritraibili. Sherwood - Fo-
reste e Alberi Oggi 145: 15-19.
Paquette A., Messier C., Périnet P., Cogliastro A. 2008 - Simulat-
ing light availability under different hybrid poplar clones
in a mixed intensive plantation system. Forest Science
54 (5): 481-489.
Paris P., Ducci F. Brugnoli E., De Rogatis A., Fady B., Malvolti
M.E., Olimpieri G., Pisanelli A., Proietti R., ScartazzaA.
Cannata F. 2003 - Primi risultati di prove comparative
di accessioni di noce da legno. In: Atti del III congresso
Nazionale SISEF, Alberi e foreste per il nuovo millennio,
Viterbo: 181-188.
Pommerening A. and Murphy ST. 2004 - A review of the history,
definition and methods of continuous cover forestry with
special attention to afforestation and restocking. Forestry
77 (1): 27-44.
Ravagni S. and Buresti E. 2003 - Piantagioni con pioppo e noce
comune: accrescimento e sviluppo dopo i primi anni.
Sherwood - Foreste ed alberi oggi 94: 19-24.
Rivest D., Oliver A., Gordon A.M. 2010 - Hardwood intercrop-
ping system: combining wood and agricultural produc-
tion while delivering environmental services. [Online]
URL:http://www.agrireseau.qc.ca/Agroforesterie/docu-
ments/Hardwood_Intercropping_Systems_(1Mo).pdf [2013]
Schütz J.P. 2001 - Opportunities and strategies of transform-
ing regular forest to irregular forest. Forest Ecology and
Management 151: 87-94.
Tani A., Maltoni A., Mariotti B., Buresti Lattes E. 2006 - Gli
impianti da legno di Juglans regia realizzati nell’area
mineraria di S. Barbara (AR). Valutazione dell’effetto di
piante azotofissatrici accessorie. Forest@ 3(4): 588-597.
Statistica 2005 - Statistica 7.1 version , StatSoft Edition, Tulsa,
OK, USA
Vidal C. and Becquey J. 2008a - Enseignements de deux plan-
tations mélangée de peuplier I214 et de noyer hybride.
Institut pour le Développement Forestier – Forêt Entreprise
178: 31-36.
Vidal C. and Becquey J. 2008b - Le mélange peuplier-noyer est
il économiquement intéressant? Institut pour le Dévelop-
pement Forestier – Forêt Entreprise 178: 37-39.
Zsuffa L., Anderson H. W., Jaciw P. 1977 - Trends and Prospects
in Ontario's Poplar Plantation Management. The Forestry
Chronicle 53(4): 195-200.