Journal of
Plant Ecology
Pure and mixed plantations of Persian walnut (Juglans regia L.)
for high quality timber production in Chile, South America
Verónica Loewe-Muñoz1,*, , Mónica Balzarini2 and Marta Ortega González3
*Correspondence author. E-mail: vloewe@infor.cl
Handling Editor: Michael O’Brien
Received: 26 February 2019, Revised: 17 July 2019, Accepted: 20 August 2019, Advanced Access publication: 22 August 2019
Citation: Loewe-Muñoz V, Balzarini M, González MO (2020) Pure and mixed plantations of Persian walnut (Juglans regia L.) for high quality timber production in
Chile, South America. J Plant Ecol 13:12–19. https://doi.org/10.1093/jpe/rtz042
Abstract
Aims Persian walnut (Juglans regia L.), an interesting forest species for the veneering industry, requires adequate management to produce
valuable high-quality logs. Since species associations and management level can improve stand productivity, the novelty of this work was to
assess Persian walnut performance in different planting mixtures and in pure plantations conditioned to management intensity.
Methods Growth, straightness and survival measurements were taken annually for 7 years after planting pure and mixed plantations under
two contrasting management scenarios. Diseases were recorded at Age 7 in all plantations. Under each management intensity, besides the
monoculture, three mixtures were tested: a mixture of only main forest species, main forest species plus one arboreal companion species,
Black alder (Alnus glutinosa L.) and main species plus the shrub Russian olive (Elaeagnus angustifolia L.) as nurse species. A test of interaction
between plantation type and management scenario was conducted using repeated growth data.
Important Findings The interaction was significant, indicating the presence of different mechanisms underlying plantation effects under
high and low management level. Compared with pure plantations, Persian walnut associated with the nurse shrub exhibited 78% higher
height and 53% higher diameter growth in plantations under low management. Health benefits (lower presence of walnut blight than in
the monoculture) and better straightness were also found in the association including the shrub when the management intensity was not
high. These beneficial effects in the presence of Russian olive were not present under high management intensity (irrigation, fertilization,
tutoring and frequent pruning). Site-specific designs for Persian walnut plantations would depend on the foreseen management intensity.
Keywords: noble wood, monoculture, forest associations, companion species, Russian olive
摘要: 波斯胡桃(Juglans regia L.)是用于装饰工业的一种森林物种，需要适当的管理才能生产出经济效益好的高质量原木。由于种间关联和管
理水平可以提高林分生产力，因此本研究的创新之处在于评估波斯胡桃在不同混合种植中以及在适度的管理强度的纯人工林中的表现。在两
种截然不同的管理方案下种植纯林和混交林后，连续7年每年对波斯胡桃的生长、直度和存活率进行测定。在第7年时，记录所有人工林中病
害情况。在每种管理强度下，除纯林外，检验三种混交林：仅与主要森林物种混交、主要森林物种和黑桤木(Alnus glutinosa L.，伴生种)混
交，以及主要物种和灌木俄罗斯橄榄(Elaeagnus angustifolia L.，保育种)混交。利用重复生长数据对人工林类型与管理方案之间的交互作用进
行检验。研究发现人工林类型与管理方案的交互作用显著，表明在管理水平高和低的条件下，人工林效应存在不同的机制。与纯林相比，在
低管理水平下，有保育灌木混交的波斯胡桃的树高增加了78%、直径生长增加了53%。同样，在管理强度不高的情况下，与灌木混交有更多
的健康益处(胡桃枯萎病的发生率低于单一栽培)和更好的直度。在高管理强度下(灌溉、施肥、抚育和频繁修剪)，与俄罗斯橄榄混交时这些
有益效应消失。因此，波斯胡桃种植园的具体选址设计取决于可预见的管理强度。
关键词: 高质量木材、单一栽培、森林协会、伴生种、俄罗斯橄榄
© The Author(s) 2019. Published by Oxford University Press on behalf of the Institute of Botany, Chinese Academy of Sciences and the Botanical Society of China.
All rights reserved. For permissions, please email: journals.permissions@oup.com
JOURNAL OF PLANT ECOLOGY | doi:10.1093/jpe/rtz042
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Chilean Forest Institute (INFOR), Metropolitan Office, Sucre 2397, Ñuñoa, Santiago, Chile, 2CONICET Biometry Unit, College of
Agriculture, Universidad Nacional de Córdoba, Ciudad Universitaria, CC509, CP 5000, Córdoba, Argentina, 3Chilean Forest Institute
(INFOR), Biobío Office, Camino a Coronel Km 7.5, San Pedro de la Paz, Concepción, Chile
1
Research Article
INTRODUCTION
JOURNAL OF PLANT ECOLOGY | VOL 13 | February 2020 | 12–19
be controlled (Muncharaz 2001). Walnut blister mite (Eriophyes erinea
Nalepa) occasionally occurs in Persian walnut orchards (Pickel et al.
2014). It feeds on the lower surface of leaflets, causing characteristic
blister-like swellings on the upper surface of leaflets; later in the
season, these areas turn brown. Its damage is primarily aesthetic,
since it does not cause economic damage to walnuts. On the other
hand, crown and root rots caused by Phytophthora sp. are among the
most serious diseases of Persian walnut worldwide (Vitale et al. 2018),
favored by wet or waterlogged soils. It causes reduction in growth
and early senescence and leaf fall, terminal shoot death and foliage
reduction; root crown rot is characterized by decayed bark and a black
exudate with a typical smell (Browne et al. 2011), causing tree decline,
affecting root system development and increasing host vulnerability to
environmental stress (Belisario et al. 2006).
Even though Persian walnut pure plantations can be profitable,
mixed plantations can be a better option in some environments or
management conditions, being preferred by some stakeholders for
their aesthetic values (Grilli et al. 2016). Additionally, landowners
may consider mixed plantations to be a practical option given their
superior resilience to disturbances (Knoke et al. 2008), including
climate change (Mohni et al. 2009), increasing the likelihood of a good
return on investment. However, Persian walnut mixed plantations
can be differently designed, including only main arboreal species, or
incorporating N-fixing companion species, either arboreal or shrubs.
Consequently, the objective of this article was to compare growth, pest
and disease infection, stem form and survival among monocultures and
distinct mixtures of Persian walnut trees under levels of management
intensity. We posit that site-specific designs for Persian walnut depend
on the management intensity proposed for the plantation.
MATERIALS AND METHODS
Experimental material
The study examined growth, sanitary conditions, stem form and
survival of Persian walnut trees in monoculture (pure plantation, T1)
and mixtures involving only main forest species (T2, MS), and main
forest species associated with one arboreal (Black alder, Alnus glutinosa
L.) (T3, MS/AC), or with a shrub (Russian olive, Elaeagnus angustifolia
L.) companion species (T4, MS/SC) (Table 1).
All plantation types were established during winter in two sites
of Chile (Yacal and Saval) and monitored annually for 7 years after
planting. Site characterization is given in Table 2. The main differences
between sites were associated with arboriculture management
intensity, since in Yacal periodic intensive interventions were provided,
including spring and summer irrigation, fertilization, use of tutors and
pruning twice a year, whereas in Saval nonintensive management was
applied (one annual pruning, no fertilization or irrigation).
Experimental design
Plantations were established under a complete randomized block
design with three replications at each site and a plot area of 428.5
m2 with a 3.5 m × 3.5 m tree spacing. The number of Persian walnut
trees ranged between 4 (mixed plantations with several main species
and a companion tree or shrub) and 35 (pure plantations) per plot.
Plantation design considers a tree-by-tree species mixture (Fig. 1), with
two-row buffer between treatments.
Measurements
Height and diameter measurements of Persian walnut trees were
taken yearly for the first 7 years after planting, always in winter
time; in Saval, diameter-at-collar-height (DCH) was measured
throughout the study period because average tree height was lower
than 1.3 m, whereas in Yacal, growth development was faster so
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Mixed plantations established to produce high-value timber can
contribute to a sustainable forestry through a better use of soils and
increased tree species diversity and system stability. These forest
associations include main species that generate valued products at
harvest (i.e. veneer logs), and companion species that may improve
growth and straightness of main species, increasing the quality of final
products.
If mixed plantations are properly established and maintained,
they diversify the production, diminish phytosanitary risks,
facilitate management (such as pruning and weed control), increase
productivity and timber quality, landscape aesthetic values (Gabriel
et al. 2005; Piotto 2008; Zuppinger-Dingley et al. 2014), and provide
food and cover for wildlife (Burde 1989). In fact, timber production
may increase with tree species richness (Vilà et al. 2007). Besides
species diversity, the complexity in forest structure may also foster selfregulation and provide higher adaptability to cope with climate change
effects (Del Río et al. 2015).
Some forest species have shown higher diameter and height
growth when associated with Nitrogen (N)-fixing species than in pure
plantations (Forrester et al. 2006). The positive effect of companion
N-fixing species—either shrubs or trees—on Persian walnut (Juglans
regia L.) growth has been reported (Buresti and De Meo 1998;
Pelleri et al. 2013). In Latin America, some production experiences
and research initiatives on mixed species plantations have shown
interesting results, such as those reported by Piotto et al. (2004) for
Costa Rica and by Loewe and González (2006a, 2006b) and Loewe
et al. (2006, 2008, 2013) for Chile.
Persian walnut is valued not only for producing a highly healthy
nut (Bolling et al. 2011), but also for its high quality timber (Cambria
and Pierangeli 2012). It is a very interesting commercial forest species,
since it can produce priced timber for noble uses, such as cabinetmaking, veneer, and sawn wood, valued for its aesthetic quality and
color; other common uses include furniture, gunstocks, parquet floors,
interior panels, cultural heritage objects (Bachtiar et al. 2018) and other
small wooden objects and novelties. Even small diameter logs (15–
25 cm) from intermediate thinning in young arboriculture plantations
may be used for making chairs or solid timber panels (Domini et al.
2018). The use of Persian walnut in Europe started in the 15th century
(Giannini and Mercurio 1997), providing one of the most valued noble
timbers, characterized by high prices (IDF 1983) that can reach US$
2500 m−3 and even close to US$ 5000 m−3 (Lupi 2016; Torreggiani 2017).
Adequate cultural practices are necessary to produce valuable logs that
must be branch-free to the first 2–4 m of the lower trunk, cylindrical,
straight, with a diameter >40 cm, free of defects (Loewe and González
2001). In particular, stem straightness is important in determining tree
and log value, and the ability to make an effective assessment before
harvesting is useful for forest managers and practitioners to improve
forecasting, planning, marketing and resource use (Price et al. 2017).
However, and given that growth and timber quality are essential
characteristics for an attractive economic return in Persian walnut
plantations, prices vary enormously for same-dimension pieces due to
the presence of defects, such as nodes, mechanical or biotic damages,
curved fiber, grafting and irregular diametric growth (Berti et al. 2003).
In fact, Buresti and Mori (2009) evidenced that Persian walnut timber
prices present the highest sensitivity to quality changes, with a price
relationship among categories that decreases from 1 to 0.08.
Walnut bacterial blight (Xanthomonas campestris pv. Juglandis (Pierce)
Dye), one the main diseases affecting the species, is the largest problem
in Persian walnut production, since it causes losses in fruit yield, vigor
reduction and even tree death if not controlled. The disease affects
leaves, stalks and fruits (Penrose and Fahy 1987). The main control
is preventive, since once the infection has occurred it can no longer
Journal of Plant Ecology
Journal of Plant Ecology
Research Article
Table 1: Pure and mixed Persian walnut plantations established at two sites of Chile
Pure
Mixed
Site
(T1)
MS
(T2)
MS/AC
(T3)
MS/SC
(T4)
Yacal
Walnut (100%)
Walnut (38%)
Cherry (38%)
Pear (14%)
Apple (11%)
Saval
Walnut (100%)
Walnut (51%)
Cherry (49%)
Walnut (11%)
Cherry (6%)
Pear (3%)
Apple (2%)
Black alder (77%)
Walnut (17%)
Cherry (17%)
Black alder (66%)
Walnut (11%)
Cherry (6%)
Pear (3%)
Apple (2%)
Russian olive (77%)
Walnut (17%)
Cherry (17%)
Russian olive (66%)
Table 2: Plantation site characteristics
Location
Site
Latitude
Longitude
Altitude (m asl)
Annual rainfall (mm)
Annual average temperature (°C)
Management intensity†
Yacal
Saval
35°12′S
39°50′S
71°7′W
73°15′W
536
20
701.9
1,871.0
13
11
High
Low
High management included spring and summer irrigation, fertilization, use of tutors and pruning twice a year.
a
diameter-at-breast-height (DBH) at 1.3 m above the ground was
measured starting from Year 2. The stem form was observed at the
end of the study as straightness following the visual methodology
described by Wienstroer et al. (2003) (0: straight tree; 1: curved less
than average; 2: curved more than average; 3: strongly curved tree).
Several authors (Blackburn et al. 2013; MacDonald et al. 2009; Price
et al. 2017), have pointed out for several species that the visual stem
straightness assessment method in standing trees is an accurate and
cost-effective measurement tool.
Phytosanitary variables were measured at the end of the study, and
considered any disease or pest observed in each Persian walnut tree.
The identified disease was bacterial blight, and the pests present were
Phytophtora sp. and blister mite.
Statistical analyses
A linear mixed model was used to fit the repeated growth measurements.
It included fixed effects of site (indicating two management scenarios,
Yacal: high intensity management/Saval: low intensity management),
plantation type (pure and associations 1–3), time (year), along with
all interactions between these main effects. Initial tree size was used
as covariates. Linear mixed models were used to take into account
temporal correlations in the model error terms. Growth models in
forests are usually characterized by heteroscedasticity (variances
increase through tree age); therefore, mixed models were also used
to account for such heterogeneous variances. The model also included
random effects of blocks within sites. To account for the autocorrelation
among growth measurements taken from the same plots, we tested
several structures for the residual (co) variance matrix, including: (i)
unstructured (UN), (ii) compound symmetry (CS), (iii) heteroscedastic
compound symmetry (CSH), (iv) first-order autoregressive (AR1), (v)
heteroscedastic first-order autoregressive (ARH1). Parameters were
estimated using restricted maximum likelihood (REML). The best
model was selected using Akaike information criterion (AIC; smaller
is better). Models were estimated by using SAS PROCMIXED (version
9.3). Pairwise comparisons between treatments were tested with the
adjusted LSMeans.
14
Generalized linear models were used to evaluate plantation type
differences in straightness and proportion of pest and disease damage.
Finally, Kaplan–Meier survival curves and log rank test (α = 0.05) were
used to compare survival among plantation types within sites.
RESULTS
Growth
The interaction was significant, indicating the presence of different
mechanisms underlying plantation effects in both sites. Then Persian
walnut growth under different plantation designs (mixtures and pure
plantations) was compared in each site, and within each site was
modeled using repeated measurement analysis. Initial tree size (height
and DCH) was compared among plantations, and nonsignificant
differences were found (Supplementary Table S1; P > 0.05).
Height
Population average profiles for the tested treatments were built from
the predicted values of fitted models (Fig. 2). Covariance among height
data taken from the same tree was best fitted with an ARH1model.
Differences among treatments were observed in both sites (P < 0.0001).
In Yacal, at Age 7, average height was 3.4 m, with performance of
T1 (pure plantations) and T3 (mixed plantation with black alder) being
greater than that of T2 (mixed plantation of main species) and to T4 (mixed
plantation with a companion shrub) (P = 0.0301). On the contrary, height
growth in Saval was poor, reaching an average height of 1.2 m at the same
age. There, the highest growth rate was recorded in T4 (mixed plantation
with the companion shrub Russian olive), where Persian walnut tree
height was 78.6% greater than in the pure plantation, with differences
from the remaining treatments being also significant (Table 3).
Diameter
In both sites, the effects of the species associations interacted with time
(P < 0.001) (Fig. 3). Covariance among diameter data taken from the
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Persian Walnut (Juglans regia), Cherry (Prunus avium), Pear (Pyrus communis), Apple (Malus communis), Black Alder (Alnus glutinosa), and Russian olive
(Elaeagnus angustifolia). Abbreviations: MS = main species association, MS/AC = main species association and an arboreal companion species (Black alder),
MS/SC = main species association and a shrub companion species (Russian olive).
Research Article
Journal of Plant Ecology
Figure 2: Persian walnut tree height growth curves fitted for pure walnut and several mixed plantations in two sites of central (Yacal, a) and south (Saval,
b) Chile (three plots per treatment). T1: pure walnut (solid bold line); T2: main species association (dashed line); T3: main species association and an arboreal
companion species (dotted line); T4: main species association and a shrub companion species (solid thin line).
same tree was best fitted with an ARH1 model. Differences among
treatments were observed in both sites (P < 0.0001).
In Yacal, DBH at Age 7 did not show statistical differences among
treatments. In Saval, the significance of the interaction treatment×time
was linked to an increase of the positive effects of T4 on monoculture
over time. Statistical differences among treatments T1 and T4
(P = 0.0406) were observed. DCH at Age 7 was higher in T4 than in T1
(53.4%) (Table 3).
Pest and diseases
In both study sites, Walnut blight affected the plantations, with
statistically significant differences in percentage of damage among
treatments being detected only in Yacal (Table 4), where the mixed
plantation involving the Russian olive shrub as companion presented
a 94.2% lower proportion of affected trees than the pure plantation
(P = 0.0004).
Two pests were recorded only in Yacal, Walnut blister mite
and Phytophtora sp., with statistical differences among treatments
(P < 0.0001). Walnut blister mite affected the monoculture and the
associations T2 and T3, while in T4 it was totally absent (0% of affected
JOURNAL OF PLANT ECOLOGY | VOL 13 | February 2020 | 12–19
trees). Phytophtora sp. showed statistically greater infestation in T4 than
in T1 (P = 0.0007) and T2 (P = 0.0006).
Stem form
None of the plantations presented statistical differences in trunk
straightness in Yacal (Table 5) (P > 0.05). However, in Saval, the
association with the shrub had a 56.9% higher percentage of straight
trees than the monoculture.
Survival analysis
The log rank test applied to the estimated survival curves for the tested
treatments showed significant differences among Persian walnut
plantation types only in Yacal (χ 2 = 16.0, P < 0.0011), where the lowest
survival was registered in T4 throughout the study period (Fig. 4).
DISCUSSION
Height differences between 7-year old pure and mixed Persian walnut
plantations were clear in both sites. Interactions between treatments
and time were detected, showing changes in the differences between
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Figure 1: Trials design: Yacal (top) and Saval (bottom). W: Persian walnut; C: Common cherry; Pe: Pear tree; Ap: Apple tree; ba: black alder; ro: Russian olive.
Journal of Plant Ecology
Research Article
Table 3: Persian walnut height and diameter at Age 7 in pure and mixed plantations in two sites of Chile
Treatment
Site
Height (m ± SD)
Yacal
Saval
Diameter (cm ± SD)
Yacal (DBH)
Saval (DCH)
Pure
(T1)
MS
(T2)
MS/AC
(T3)
MS/SC
(T4)
3.66 ± 1.04a
0.98 ± 0.19b
3.26 ± 1.02b
1.25 ± 0.79b
3.72 ± 0.94a
0.85 ± 0.03b
3.15 ± 1.56b
1.75 ± 0.52a
4.58 ± 1.63a
2.32 ± 0.45b
4.03 ± 1.39a
2.54 ± 1.00b
4.45 ± 1.69a
2.80 ± 0.03b
3.73 ± 1.83a
3.56 ± 0.41a
See Table 1 for abbreviations.
a,b
Statistically significant differences (P ≤ 0.05).
the tested plantation types over time. Balandier et al. (2008) found
that companion species facilitate the increase of log length. In
addition, Gabriel et al. (2005) reported that broadleaved species in
mixed plantations could have up to 24% greater heights than in pure
plantations, and Mohni et al. (2009) stated that dense plantations
with companion species often exhibit superior height and diameter
growth than pure plantations. Our results showed that in Saval, a
low-intensity management site, one association—the combination
with the shrub Russian olive (T4)—significantly improved Persian
walnut height growth by 79% with respect to the pure plantation after
7 years. Tani et al. (2006a) also reported a higher height growth (45%)
of Persian walnut associated with the same shrub than in the pure
plantation at Age 9.
Burde (1989) stated that interplanting nurse trees and shrubs with
Persian walnuts can greatly improve their growth and quality. This
finding was clearly observed in Saval (low-intensity management)
for the association with Russian olive, but it was not observed in
Yacal (high-intensity management), suggesting the need for a match
between associations, site and management intensity. N-fixing species,
whose mechanisms are described in detail by Forrester et al. (2006),
can have different effects. In particular, the performance of Persian
walnut trees in association with black alder and Russian olive have
evidenced that a higher growth is determined not only by a superior
Nitrogen supply, but also by light and space availability (Cutini and
Giannini 2009).
In Yacal, an intensively managed site, Persian walnut height
growth showed significant differences between T1 (pure) and T3,
16
superior to T2 and T4, whereas in Saval, a low-intensity management
site, the treatment that included Russian olive had the best Persian
walnut height growth. This is in agreement with Clark et al. (2008),
who reported that Russian olive’s architecture has a significant positive
effect on the height growth of black walnut (Juglans nigra L.). Tani et al.
(2006b) also reported a higher height growth for mixed plantations
than in the monoculture. Similarly, Piotto (2008) conducted a metaanalysis of studies comparing mixed plantations and monocultures
for several species, and reported that stand composition affects tree
diameter and height growth. These evidences suggest that mixing tree
species generally increases plantation growth rate, especially after the
first years (Nunes et al. 2013), probably due to crown stratification and
to a better site use by the species associations.
Regarding diameter, in Saval there was an interesting gain
in Persian walnut diameter in mixed plantations, with the pure
plantation having the lowest growth over time, showing that
interspecific competition was lower than intraspecific competition in
these associations, in agreement with Forrester et al. (2006). However,
differences among associations depended on the year of evaluation,
showed by the significant interaction between treatments and time. In
the association with the shrub Russian olive, Persian walnut diameter
growth was 53% higher than in the monoculture; this finding is in
agreement with results reported by Bianchetto et al. (2013), who
indicated 83% higher DBH growth in Persian walnut associated with
a high proportion of olive trees (Elaeagnus umbellata Thumb.) than in
the monoculture; the finding is also in agreement with Pedlar et al.
(2006), who indicated that E. umbellata stimulates black walnut growth
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Figure 3: Persian walnut tree diameter growth curves fitted for pure walnut and several mixed plantations in two sites of central (Yacal, a) and south (Saval,
b) (three plots per treatment). T1: pure walnut (solid bold line); T2: main species association (dashed line); T3: main species association and an arboreal
companion species (dotted line); T4: main species association and a shrub companion species (solid thin line).
Research Article
Journal of Plant Ecology
Table 4: Incidence of walnut blight, walnut blister, and Phytophtora in pure and mixed Persian walnut plantations expressed as percentage of damaged trees,
by site
Walnut blight
Walnut blistera
Phytophtora sp.a
Plantation type
Yacal
Saval
Yacal
Yacal
Pure (T1)
MS (T2)
MS/AC (T3)
MS/SC (T4)
47.0b
44.0b
52.9b
24.2c
37.1c
22.2c
16.7c
27.8c
11.4b
10.0b
11.8b
0.0c
12.9c
6.0c
14.4b,c
35.6b
See Table 1 for abbreviations.
a
Walnut Blister and Phytophtora were not present in Saval.
b,c
Statistically significant differences (P ≤ 0.05).
Plantation type
Yacal
Saval
Pure (T1)
MS (T2)
MS/AC (T3)
MS/SC (T4)
50.0a
59.0a
69.6a
69.2a
28.3a
34.6a
20.0a
44.4b
See Table 1 for abbreviations.
a,b
Statistically significant differences (P ≤ 0.05).
better than other companion species, even though its invasive nature
has been highlighted (Johnson et al. 2006) in North America where
its invasiveness has severely reduced its use as a nurse crop, which
has not been observed in Chile. In addition, our results on Persian
walnut growth in Saval being improved by this association agree with
Piotto (2008), who stated that the presence of N-fixing species has a
positive effect on diameter growth of nonfixing species. Schlesinger
and Williams (1984) reported that E. umbellata stimulated J. nigra
growth in 80% of the sites included in the study, being especially
effective in those where the species growth was slower in pure plots,
as it was evidenced in Saval. This stimulation on Persian walnut can
be explained by the N-fixing ability of Elaeagnus, being more effective
on low-intensity management sites. In Yacal, management included
irrigation, fertilization and frequent pruning, which can explain the
lower impact of the associations. In this site, diameter growth reached
0.63 cm year−1 in the mixed plantations, with similar values for the pure
plantation. Therefore, the observed diameter growth is comparable
to the species performance in medium-quality soils for the species in
Ferrara province, Italy (Buresti et al. 2007), but lower than the average
diameter growth reported in Ancona province, Italy (1.3 cm year−1)
(De Meo et al. 1996) and in Burgos province, Spain (0.8 cm year−1).
Our findings on growth performance for Saval, a low-intensity
management site, agree with those of Tani et al. (2006a) who indicated
best performance in Persian walnuts planted especially with N-fixing
companion trees. These benefits are probably a consequence of several
co-occurring favorable factors, including the higher Nitrogen availability.
In poor soils, the benefits of companion species are even more
relevant than in fertile sites (Balandier et al. 2008; Chifflot et al. 2005).
Accordingly, in Saval height and DBH were enhanced by the tested
associations, whereas in Yacal, where natural conditions were altered
by anthropogenic management interventions, the mixed plantations
did not enhance growth.
Regarding diseases, Walnut bacterial blight was found in the two
study sites. There were significant differences among treatments only in
Yacal, where the association including main species and Russian olive
presented the lowest number of affected trees, showing a protective
effect of this shrub. These results agree with findings of Miller et al.
JOURNAL OF PLANT ECOLOGY | VOL 13 | February 2020 | 12–19
(1993) in broadleaved plantations associated with red alder, a species
that induces more resistance to pests and diseases. Walnut blister mite
was detected only in Yacal, affecting the monoculture and two of the
three associations. Affected trees were observed in pure and mixed
plantations, except for T4, where there were no affected trees. On
the other hand, the association with the shrub showed a statistically
higher Phytophtora incidence than that observed in the pure plantation
and in T2. Montagnini et al. (1995) also reported cases in which pure
stands are more resistant to certain insects than mixed associations,
evidencing that that depending on the causal agent, growth conditions
in pure or mixed plantations can affect the occurrence of pests and
diseases; that is fungal diseases may spread rapidly in pure plantations,
with high infestation intensity. Mixed plantations can contribute to
damage intensity reduction depending on both the tree species and the
insect type and species, while pure plantations would present a higher
phytosanitary risk, favoring the insects spread.
The most important variables related to stem form, straightness,
which determines the economic result of the plantation at harvest
(Buresti and Mori 2009), showed significant differences between
treatments in Saval, where the nurse shrub companion species
improved the percentage of straight trees by 56.9% compared with the
monoculture. However, no differences were found among pure and
mixed plantations in Yacal, probably due to the intensive management
scheme that included pruning and the use of tutors.
Even though it was not quantified in this study, we did observe that
pruning intensity was considerably lower and easier in mixed plantations
compared to monoculture, since average branch diameter was lower
and branch insertion angle was higher. These results are in agreement
with findings of Pelleri et al. (2013), who stated that Persian walnut trees
associated with poplar showed cone-shaped crowns, light branching and
a good stem quality in comparison with trees grown in monoculture.
Regarding survival, on the Yacal study site it was observed the
lowest survival in the association with a shrub companion species and
the highest in the pure plantations, while in Saval, the site with poorer
growth and the low-intensity management, the opposite trend was
found. These results would indicate that inter-specific competition in
associations and intra-specific competition in pure plantations varies
according to sites and management intensity.
Even though both studied sites are located in different climate
conditions, especially annual rainfall, considering that the highintensity management site was irrigated during spring and summer,
we assign the observed Persian walnut performance differences to the
management intensity levels.
This study was not designed to determine which factors
potentially altered by companion species (such as soil, microclimate,
vegetation, pest behavior) contributed to the observed Persian
walnut response, we conclude that associations can perform
differently in different sites, in agreement with Bauhus (2017),
which in turn can be modified by management intensity, which is
17
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Table 5: Stem form as percentage of straight Persian walnut trees in
different plantation types in two sites
Journal of Plant Ecology
Research Article
the innovation this study brings into the study of mixed plantations
versus monoculture.
CONCLUSIONS
In Persian walnut cultivation, mixed plantations including a shrub
may have economic advantages, especially in poor sites or under
low-intensive management, where this companion species can bring
beneficial effects. Our results showed that Persian walnut trees in
association with cherry and Russian olive as companion shrub species
had greater height (up to 79%) and DBH (up to 53%) than in pure
plantations, as well as less severe phytosanitary damages even under
intensive management schemes.
Some associations can favor Persian walnut development and improve
its health status, being the plantation management intensity important in
the species performance under a specific planting mixture. Thus, we suggest
selecting and matching growth characteristics of companion and main
species for each combination of site quality and management intensity.
Supplementary Material
Supplementary material is available at Journal of Plant Ecology online.
Table S1: P-values from the variance analysis for growth variables
among pure Persian walnut and several mixed plantations in two sites
of central and south Chile.
Funding
This work was supported by the Chilean Ministry of Agriculture and
trial establishment and management were supported by the Foundation
for the Agriculture Innovation (FIA), Ministry of Agriculture, Chile,
project “Mixed plantations: productivity, diversity and sustainability
for the forest development” [C00-1-F-028].
Acknowledgements
The authors thank the private owners and companies that actively
collaborated with this initiative.
Conflict of interest statement. None declared.
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