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Can bioplastic or woodchip groundcover replace
herbicides or plastic mulching for valuable broadleaf
plantations in Mediterranean areas?
Jaime Coello
1
•Lluı
´s Coll
1,2
•Mı
´riam Pique
´
1
Received: 6 July 2016 / Accepted: 12 January 2017
ÓSpringer Science+Business Media Dordrecht 2017
Abstract Weed control is fundamental in plantations of valuable broadleaved species. The
most common weeding techniques are repeatedly applied herbicides and removable plastic
mulching, both raising environmental concerns. We studied the performance of these
techniques on a hybrid walnut plantation, compared with three biodegradable mulch
alternatives: a prototype bioplastic film, a layer of composted woodchips and a layer of
ramial chips. The durability and effect of the treatments on tree performance (survival,
growth, physiological traits) and soil features (moisture and temperature) were evaluated
over 4 years. Herbicide yielded the best results, while all the mulching treatments provided
better results than controls for nearly all the variables. The performance of plastic and
bioplastic films was similar, suggesting that the latter could replace plastic mulching. The
performance of the two chip mulches was similar and slightly below that of the films,
probably because of the excessive thickness of the former (13–14 cm). In summary,
biodegradable mulches showed high effectiveness in controlling weeds and so could offer
an alternative to herbicide application and plastic mulching when these are contra-indi-
cated technically (accessibility, repeatability), economically (labour cost), legally or
environmentally.
Keywords Afforestation Biodegradable mulch Herbicide Juglans Restoration
Weed competition
&Jaime Coello
jaime.coello@ctfc.es
1
Sustainable Forest Management Unit, Forest Sciences Centre of Catalonia, Solsona, Lleida 25280,
Spain
2
Department of Agriculture and Forest Engineering, University of Lleida, Lleida 25198, Spain
123
New Forests
DOI 10.1007/s11056-017-9567-7
Introduction
In Mediterranean conditions, the primary limiting factor for tree growth is water shortage
during summer drought (Villar-Salvador et al. 2012) which is expected to worsen in the
future (Vallejo et al. 2012; Dumroese et al. 2015), and whose negative effect can be
exacerbated by a poor soil preparation (Lo
¨f et al. 2012) and by weeds. The competition
effect of unwanted vegetation is especially intense on highly fertile sites such as former
agricultural fields (Kogan and Alister 2010; Olivera et al. 2014), and is particularly harmful
to young seedlings with underdeveloped root systems and insufficient leaf area to shade
and control weed development unaided (Coll et al. 2003; Devine et al. 2007). The use of
herbicides, especially glyphosate, is the most common weed control technique applied in
forest plantations in Europe (Willoughby et al. 2009) and most temperate areas worldwide,
because of its efficacy against a wide range of weed types (Kogan and Alister 2010).
However, growing social and environmental concern about herbicides is curbing their use
(Ammer et al. 2011). They are banned in public forests in Quebec (Thiffault and Roy 2011)
and in a broad range of forest areas in various European countries, for example Germany,
Denmark, Czech Republic and Slovakia (Willoughby et al. 2009). Also, the need for
repeated application, at least once a year, and during one particular stage of weed
development, limits herbicide use in plantations under low intensity management schemes,
when minimizing invested resources is fundamental. Alternatives or supplements to her-
bicide application include the use of planting strategies to reduce water or nutrient stress
from competing vegetation such as the use of nursery nutrient loading or direct application
of controlled-release fertilizer to the root zone (Jacobs et al. 2005; Uscola et al. 2015;
Schott et al. 2016). Another alternative to repeated, time-consuming chemical or
mechanical weeding in tree plantations is one-time application of groundcover or
‘‘mulching’’ (Chalker-Scott 2007). This technique consists of covering the ground around
the tree to prevent the germination and growth of weeds (Maggard et al. 2012), being a
physical barrier that stops light reaching the soil (Bond and Grundy 2001). The most
significant benefit of mulching is increased soil water content, especially during the driest
periods (Maggard et al. 2012; McConkey et al. 2012), through preventing water transpi-
ration by weeds and reducing soil water evaporation (Percival et al. 2009; Kumar and Dey
2010; Barajas-Guzma
´n and Barradas 2011; Zegada-Lizarazu and Berliner 2011). Other
reported benefits are extreme temperature buffering (Cregg et al. 2009; Barajas-Guzma
´n
and Barradas 2011; Arentoft et al. 2013) and improvement of soil physical properties
(Chalker-Scott 2007), which helps trees regulate root respiration and favours water and
nutrient uptake (Dodd et al. 2000). Organic mulches may increase soil nutrient and organic
matter content during their decomposition (Merwin et al. 2001; Van Sambeek and Garrett
2004). The resulting positive effect of mulching on tree survival and growth is cumulative
and perceptible over subsequent decades (George and Brennan 2002; Pedlar et al. 2006).
To date, most studies on the use of this technique and its effects on vegetation and soil have
been conducted in temperate conditions. In areas limited by water, such as prevail in
Mediterranean sites, the efficacy of this technique is still unproven.
The most widespread mulching material is black polyethylene film (Barajas-Guzma
´n
et al. 2006; Arentoft et al. 2013), a low-cost homogeneous material with proven positive
effects on forest plantations (Green et al. 2003). Its use is, however, limited by concerns
about using long-lifespan plastics in the open air, by its unsightliness, and especially by its
high cost of removal and disposal (Shogren and Rousseau 2005). As a result, a wide range
of biodegradable alternatives are being developed, including bio-based plastics (A
´lvarez-
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123
Cha
´vez et al. 2012), notably polylactic acid (PLA) (Finkenstadt and Tisserat 2010) and
polyhydroxyalkanoate (PHA). These materials show homogeneous behaviour but have not
yet been widely evaluated in outdoor conditions for forest restoration purposes. Other
biodegradable mulches used in tree plantations include those based on locally abundant
(often waste) organic materials, such as straw, woodchips and paper.
The aim of this work was to evaluate, on a 5-year basis, whether groundcover based on
renewable, biodegradable materials could offer a suitable alternative to herbicide appli-
cation or plastic-based mulches on valuable broadleaf plantations in Mediterranean con-
ditions. Such plantations are being increasingly considered as an alternative use, with both
economic and environmental advantages, for agricultural land facing abandonment (Aleta
`
et al. 2003; Coello et al. 2009) in Mediterranean areas.
Our hypothesis was that these novel mulches would yield technical outcomes similar to
those of plastic mulching for tree performance and micro-site features, while being more
environmentally friendly and not needing removal. They could also usefully replace her-
bicide application, being cost-saving (less labour-intensive) and ecologically and socially
more acceptable.
Methods
Study area and experimental design
The study was conducted in Solsona, NE Spain (41°590N; 1°310E), in a flat, homogeneous,
former agricultural field, located at an elevation of 670 m a.s.l., and farmed until the year
of planting for winter cereal production (wheat, barley and oats). The study area had a
Mediterranean continental sub-humid climate (Martı
´n-Vide 1992) or Mediterranean con-
tinental Csb (Temperate, dry mild summer) in the Ko
¨ppen classification. Mean annual
temperature is 12.0 °C and mean annual and summer precipitation are 670 mm and
171 mm, respectively (Ninyerola et al. 2005). The analysis of soil samples taken at three
different points at depth 5–30 cm revealed a loamy texture (21% clay, 45% silt, 34% sand)
and pH 8.2. Initial soil organic matter content was 2.3% and active limestone was 5.2%.
The main weed species were first Avena fatua and second Lactuca serriola.
One experimental plantation was set up in March 2011 with 1-year-old hybrid walnut
(Juglans 9intermedia) MJ-209xRa, 40–60 cm high, bare-rooted, planted on a 4 94m
frame. Soil preparation consisted in deep (50 cm), crossed sub-soiling with a 150 HP
tractor with chisel, and pits were opened manually just before tree establishment. Six
different vegetation control conditions were then applied: (1) chemical weeding (gly-
phosate, 22.5 cm
3
/tree at 1.25%) applied yearly in May with a backpack sprayer
(HERBICIDE), (2) commercial black polyethylene film, treated against UV radiation,
80 lthick (PLASTIC), (3) prototype black PHA (polyhydroxyalkanoate) film, 100%
biodegradable, 80 lthick (BIOFILM), (4) a layer of woodchips made with woody debris
from pine forest harvesting operations, composted for 8 months, size 15/35 mm, layer
thickness 13/14 cm (WOODCHIPS), (5) a layer of fresh ramial woodchips (branches and
twigs from urban pruning), size 15/35 mm, layer thickness 13/14 cm (RAMIALCHIPS),
and (6) no treatment (CONTROL). Each weeding treatment was applied on
100 9100 cm of soil (centred on the tree). No artificial watering or fertilization was
applied to the experimental area.
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123
These treatments were deployed in a randomized complete block design with sub-
samples. Each of the five blocks consisted on six randomly distributed plots (one per
treatment) with 12 trees each (192 m
2
), for a total of 72 trees per block (1152 m
2
) and 360
experimental trees in all (5760 m
2
).
Weather and soil variables
Daily temperature and precipitation data were obtained during the study period from a
nearby weather station belonging to the Catalan Meteorological Service, located less than
1 km away from the study site at a similar altitude. The first two vegetative periods (lasting
normally from April–May to October), namely 2011 and 2012, were notably drier and
warmer than the historical average, especially during summer, with precipitation 60 and
70% lower than the historical average. Precipitation was not only low but also unevenly
distributed, with few episodes of significant rainfall. However, 2013, 2014 and 2015 were
much wetter, with summer precipitation close to the historical average (2013 and 2015)
and 36% higher (2014) and with a fairly regular distribution of rainfall episodes (Table 1).
Soil volumetric water content (l water per l soil) at depth 5–20 cm was estimated
gravimetrically on two different dates: June 2012 and August 2014. Each sampling
campaign was conducted after at least 2 weeks without rain, and consisted of the extraction
of four soil samples per treatment in 3 blocks (144 in all). Sampling points were located
30 cm away from the tree stem at aspect 225°(SW); approximately 150 g of soil was
collected with a soil auger. Samples were kept in tared, zip-closed plastic bags, and
weighed within 3 h at the laboratory on a precision scale (0.1 g) to obtain fresh weight,
after subtracting the bag weight. Dry weight was obtained after leaving the bags fully open
at 65 °C for 96 h, using the same procedure.
Soil temperature at depth 7.5 cm was recorded continuously from the time of tree
planting using thermometers with built-in dataloggers. There were three thermometers per
treatment (one per block, in 3 blocks), located 30 cm away from the tree in aspect 225°
(SW), each tied to a short rope for easy retrieval. The results for soil temperature are shown
for three key periods: a representative flushing period (1–20 May 2012), the warmest spell
(11–26 August 2011) and the coldest spell (2–23 February 2012) after the time of planting.
Table 1 Weather summary 2011–2015, summarizing the variables most relevant to tree growth. Source:
Servei Meteorolo
`gic de Catalunya
Year Mean daily
temperature in
summer (°C)
Mean maximum
daily temperature
in summer (°C)
Annual
precipitation
(mm)
Summer
precipitation
(mm)
Number of
summer days
with precipitation
[10 mm [25 mm
Reference 20.3 27.2 670 171 n.a. n.a.
2011 20.9 28.8 568 68 2 0
2012 21.8 30.0 464 52 1 1
2013 20.5 29.3 637 173 6 3
2014 20.2 27.9 713 233 7 5
2015 21.8 30.2 520 195 5 2
Summer extends from 21 June to 20 September (92 days)
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123
Tree measurements and mulch durability
Seedling mortality was monitored at the end of each vegetative period (2011–2015),
together with any signs of vegetative problems (death of the apical shoot or presence of
basal sprouting). Plant basal diameter and total height were recorded at the time of
planting, and then at the end of each growing season using a digital calliper and a mea-
suring tape, respectively. The diameter and height growth of a living tree during a vege-
tative period was calculated as the difference between two consecutive measurements. Tree
height growth was considered to be zero for a given year when the apical shoot was dead,
but the height of the highest living bud was measured to have the initial size for the
subsequent year.
Physiological variables were measured within the growing seasons. During July
2012, predawn (07:00 solar time) and midday (12:00 solar time) leaf water potential
were measured with a pressure chamber (Solfranc technologies, Vila-Seca, Spain) using
ten leaves per treatment collected from different trees from 3 blocks. In July and
August 2014 midday leaf water potential was measured likewise. In all cases the
measuring day was chosen after at least two weeks without rain. Finally, leaf chloro-
phyll content was estimated using a Minolta SPAD-502 instrument (Minolta Camera
Co. Osaka, Japan) in both July 2012 and July 2014 on fully elongated leaves exposed
to direct sunlight. For each treatment, 10 trees from 3 blocks were sampled, and the
SPAD (Soil Plant Analysis Development) value, a relative indicator of leaf greenness
(Djumaeva et al. 2012) was obtained for each tree as the average of three fully
elongated, sun-exposed leaves.
Forty months after the start of the experiment (June 2014), the durability of the four
mulching treatments was assessed by visual estimation of the percentage (rounded to tens)
of mulched area free of weeds (chip mulches) or free of weeds and also physically intact
(film mulches). Mulch status was rated effective (showing at least 80% intact surface),
partially damaged (40–70% intact surface) or ineffective (30% or less intact surface).
Statistical analysis
The data related to soil moisture, tree growth, tree water status and leaf chlorophyll were
analysed independently for each measuring date. We used an analysis of variance
(ANOVA), considering both treatment and block as fixed factors, and following the model:
Yi¼lþaiþbjþeij þdijk
where Y is the dependent variable; lis the population mean for all treatments; a
i
is the
treatment effect; b
j
is the block effect; e
ij
is plot error and d
ijk
is subsample error.
Normality of residuals was confirmed with the Kolmogorov–Smirnov test. To meet this
condition we used a square root transformation of the diameter growth values and the zero
values of height growth were transformed to 0.0000001 (Kilmartin and Peterson 1972).
Differences between treatments were examined by a post hoc Tukey test with the signif-
icance threshold set at p\0.05. All these analyses were performed with SPSS v.19.0
software (IBM SPSS Statistics for Windows, Armonk, NY, USA 2010). The rest of
variables (soil temperature, tree survival and mulch durability) were analysed with
descriptive statistics.
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123
Results
Plant survival and growth
In general, mortality rates were low, less than 6% for all the treatments together along the
five growing seasons. Most mortality occurred in unweeded plants (CONTROL), which
showed an overall mortality of 33% at the end of 2012. This year was the driest and
warmest, showing 94% of all casualties; there was no mortality thereafter.
Vegetative problems (loss of apical shoot or basal sprouting) were especially frequent in
the CONTROL (25% on average in the five vegetative periods) and chip groundcover
(RAMIALCHIPS—26% and WOODCHIPS—23%) plots, with the other treatments below
15%. Most of the vegetative problems (90%) arose during 2011 to 2013, affecting 26% of
the trees considering all treatments, this figure falling to 6% in 2014 and 4% in 2015
(Table 2).
Throughout the study period there was a consistent and remarkable treatment effect
(p\0.001 in all cases) on annual tree diameter and height growth. The HERBICIDE plots
gave predominantly the best results of all the treatments in terms of seedling growth, while
CONTROL gave the poorest results (Fig. 1). Mulching treatments provided intermediate
outcomes, with films (PLASTIC and BIOFILM) resulting in higher growth rates than chip
layers (WOODCHIPS, RAMIALCHIPS) during the driest years (2011 and 2012).
Through 2013–2015 the average annual diameter growth was 15.7 mm for HERBI-
CIDE, 10.9 mm for the mulches taken together and 6.5 for CONTROL. Annual height
growth values in the last three growing seasons showed similar trends, with HERBICIDE
at 83 cm, followed by 65 cm for mulches and 39 cm for CONTROL.
Plant and soil water status and chlorophyll content
During the driest year of the study period (2012), HERBICIDE provided the highest levels
of soil moisture and the least negative soil and plant water potential values, while CON-
TROL gave the poorest results for all these variables (Table 3). The only significant
differences in the effects of mulching treatments on plant and soil water potential were
obtained in 2012, when in June BIOFILM plots showed more favourable pre-dawn leaf
water potential values (a proxy of soil water status) than the other mulches, and in July, a
more favourable midday leaf water potential than RAMIALCHIPS. In 2014, a particularly
Table 2 Number of hybrid walnut trees suffering from vegetative problems and dying during the first five
growing seasons, for each weeding technique
Alive, with vegetative problems Dead
2011 2012 2013 2014 2015 2011 2012 2013 2014 2015
HERBICIDE 16 4 4 0 0 0 0 0 0 0
PLASTIC 17 6 8 2 3 0 0 0 0 0
BIOFILM 18 5 7 4 3 2 2 2 2 2
WOODCHIPS 21 17 24 4 2 0 3 3 3 3
RAMIALCHIPS 14 24 25 10 2 0 5 5 5 5
CONTROL 26 27 18 2 2 0 33 33 33 33
Mortality is cumulative, while the occurrence of vegetative problems in a tree could vary from year to year
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123
wet year, both HERBICIDE and RAMIALCHIPS resulted in better plant water status than
CONTROL, while the other treatments showed no significant difference. Finally, soil
moisture produced minor differences between mulching treatments, consisting in preva-
lently higher values in the case of chip mulches compared with film mulches (Table 3).
The measurements of leaf chlorophyll content made in July 2012 and July 2014 showed no
significant difference between treatments. In 2012, HERBICIDE showed the highest value
(35.4 ±0.9) followed by CONTROL (34.5 ±1.05), while the average value of mulch
treatments ranged between 33.1 and 33.6. Trees showed slightly higher SPAD values in
2014, led by BIOFILM (41.3 ±1.9), HERBICIDE (40.7 ±2.2) and CONTROL
Fig. 1 Diameter and height growth time course of hybrid walnut during the first five growing seasons.
Growth was calculated as the difference between the basal diameter (mm) or total height (cm) at the end and
at the beginning of each growing season. Significant differences (p\0.05) between treatments found each
year are indicated by different letters, and were grouped according to Tukey test
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123
(40.2 ±1.7), while the rest of mulch treatments had averages ranging between 38.9 and
39.2.
Soil temperature
Mulches had a remarkable buffer effect on soil temperature, especially chips (Fig. 2). For
example, during a representative flushing period (May 2012) the minimum soil tempera-
tures were 2 °C higher in chip mulch plots than in the other treatments. During the warmest
period of the study (August 2011) the buffering effect of chip layer mulches was even
stronger, with mean and average maximum temperatures respectively 4 °C and 6 °C lower
than the average for the other treatments. Finally, during the coldest period of the series
(February 2012) the effects of chip mulches were less noteworthy, with minimum soil
temperatures 1 °C higher than in the case of film mulches, and 2 °C higher than for
HERBICIDE and CONTROL, which were the only treatments in which soil temperatures
fell below 0 °C. The most noticeable effect of film mulches was the increase, by 2–4 °C, in
the maximum soil temperatures during the flushing period (May 2012) compared with
CONTROL and chip layer mulches. Finally, HERBICIDE and CONTROL in general
showed similar soil temperature trends, with slightly higher maximum temperatures during
the flushing period for CONTROL, and during summer and winter for HERBICIDE.
Durability of mulches
The durability assessment of mulching performed after 40 months (Fig. 3) gave PLASTIC
the highest proportion of effective units: two-thirds of polyethylene mulches kept at least
c
Fig. 2 Distribution of soil temperatures at depth 7.5 cm during the warmest spell (16 days in August 2011),
the coldest spell (23 days in February 2012), and a representative flushing period (21 days in May 2012).
The box length represents the interquartile range (Q1–Q3) while the horizontal line inside the box indicates
the median
Table 3 Plant and soil water status, expressed as leaf water potential and volumetric soil water content,
respectively
Leaf water potential (bar) Soil moisture (%
volume)
Predawn w
June GS2
Midday w
June GS2
Midday w
July GS4
Midday w
August GS4
June
GS2
August
GS4
F-value 81.8** 11.9** 4.7* 2.35* 25.9** 2.87*
HERBICIDE –2.56a –13.4a –12.7a –15.0a 16.3a 11.7ab
PLASTIC –5.61c –16.1b –13.6ab –16.4ab 10.7bc 10.0b
BIOFILM –3.81b –15.9b –12.9a –16.0ab 10.4c 11.3ab
WOODCHIPS –6.03c –16.3b –13.6ab –16.0ab 12.1b 11.4ab
RAMIALCHIPS –5.13c –15.7b –14.7bc –15.5a 12.1b 11.9a
CONTROL –12.3d –18.1c –16.3c –18.0b 8.7d 10.1b
Each measurement of leaf water potential refers to 60 trees (10 per treatment). Each soil moisture moni-
toring refers to 144 trees (24 per treatment). F-values and the level of significance of each analysis are
provided
* Significant at 0.05 level; ** significant at 0.01 level
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123
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123
80% of their surface being effective, whereas only 6% were rated ineffective (30% or less
of their surface being effective). By contrast, approximately one-third of BIOFILM mul-
ches were found effective at the evaluation time and a similar proportion were ineffective.
The results for chip mulches fell between the two film mulches, with RAMIALCHIPS
showing slightly higher durability than WOODCHIPS, with 47 and 37% of effective units
and 6 and 14% of ineffective units.
Discussion
Hybrid walnut is characterised by late flushing, second half of May in our study area,
which is four to five weeks after most weed emergence. This property allowed herbicide
application to be postponed until early-to-mid-May. Chemical weeding was therefore
applied on grown weeds (around 50–70 cm high on average), which left not a bare soil but
a dry grass cover that was (1) free from transpiring vegetation, (2) dense enough to impede
new weed proliferation and stop sunlight reaching the soil, thus mitigating soil water
evaporation, and (3) highly permeable to water infiltration. These three features are
especially beneficial during summers with infrequent precipitation distributed in low-
volume episodes (typical in Mediterranean climates, and most common in 2011 and 2012
in our case) and might in part explain the outstanding results of the HERBICIDE treatment
found in this study.
Mulching provided better results than CONTROL for most of the variables and mea-
suring dates, as reported in most previous studies (Johansson et al. 2006; Abouziena et al.
2008; Maggard et al. 2012). Among the different mulching treatments, the relatively poor
results of chip mulches during the dry years (2011 and 2012) might be linked to the
thickness of the layer used (13–14 cm). This thickness was chosen based on previous
studies (Granatstein and Mullinix 2008; Percival et al. 2009) carried out in wetter areas.
Mulches made of chips and other organic materials need to be moistened adequately before
they become permeable to water and let it through to the soil. In particularly dry summers
Fig. 3 Durability of mulching treatments after 40 months (March 2011 to June 2014), based on visual
evaluation (percentage of intact area, rounded to tens)
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123
with precipitation in small volume episodes as those which occurred in our study in
2011–2012, excessively thick organic mulches can prevent water reaching the soil (Gilman
and Grabosky 2004). Film mulches (PLASTIC and BIOFILM) may also have kept water
from reaching the soil during the low-volume rainfalls of 2011–2012. The two mulches
yielded relatively similar results between them for both plant and soil variables. The
similar mechanical properties of biofilms and plastic were also reported by Garlotta (2001).
In general terms, black biofilm groundcover can be considered as a useful substitute for
black polyethylene film: it is made from renewable raw materials and is biodegradable, so
there is no disposal cost at the end of its service life. PHA belongs to the group of preferred
bioplastics as regards environmental, health and safety impacts (A
´lvarez-Cha
´vez et al.
2012) and highest production (Iles and Martin 2012). In addition, the growing demand for
bioplastics in multiple applications is expected to bring down prices in the near future (Iles
and Martin 2012). The choice of one formulation (plastic or bioplastic) over another could
depend on differences in cost of purchase (lower for polyethylene) and removal (nil for
biodegradable mulches).
Van Sambeek (2010) analysed 50 papers for the effect in terms of growth and fruit
production of different weeding techniques on walnut, expressed as a relative response.
The herbicide/control response ratio was 178, i.e. trees treated with herbicide yielded 78%
more than those that were unmanaged. The figure was 267 for synthetic mulches (mostly
polyethylene) and 265 for organic ones. In our study the average growth response (di-
ameter and height) of HERBICIDE relative to CONTROL (unmanaged) was 272, much
higher than the above reported response. However, mulches resulted in poorer relative
responses than in that study, with PLASTIC giving a value of 196 and chip mulches
averaging 180. These divergences could arise from (1) the delayed application of the
herbicide treatment adapted to a late-flushing tree species, and (2) the occurrence in our
study of particularly dry summers with very infrequent rainfall events in the two first years
after planting, when (as stated above) mulches may have hindered water infiltration into
the soil.
The trial presented mortality rates below 6% considering all treatments, indicating the
suitability of the species for the site, despite the unusually dry 2011 and 2012. While
mortality in 2011 was negligible (one seedling in all), it rose to 15 further seedlings at the
end of 2012, probably as a consequence of the poor root development in 2011 (Watson
2005) and the depletion of reserves after two consecutive harsh years. The CONTROL
treatment accounted for 75% of the dead seedlings of the study, highlighting the effect of
weeding on survival (Green et al. 2003; Van Sambeek and Garrett 2004). Similarly, Green
et al. (2003) and Chaar et al. (2008) found that the positive effect of weeding with respect
to unweeded trees was especially noticeable in the second year after plantation. Vegetative
problems, especially loss of apical shoots and symptoms such as basal sprout emergence,
were especially frequent (one-third of the trees) during the first vegetative period, probably
owing to the particular harsh conditions in 2011 and post-transplant shock effects (Oliet
et al. 2013). However, during the following years the number of trees with vegetative
problems diminished, indicating sound acclimatisation of the trees to the site conditions.
We found a clear difference in the time course of growth in the plots under weed man-
agement (HERBICIDE and mulching) and the CONTROL plots between 2011 and 2012.
Whereas the weeded trees kept a relatively similar aboveground growth rate during both
years, CONTROL tree growth slowed dramatically in 2012, as did survival rate, in line
with the results of Coll et al. (2007) in hybrid poplar plantations established in forest sites.
The ranking of treatment performance (HERBICIDE [mulching [CONTROL) was
consistent throughout the period (Fig. 1).
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123
With regard to plant and soil water status, HERBICIDE yielded better results than
mulching during the driest year of the study period (2012), probably owing to the sparse
rainfall episodes in that summer, which may have limited the amount of water reaching the
soil in mulched trees. A similar situation was observed by Ceacero et al. (2012) in a dry
year in southern Spain. However, during wet years (2014) only HERBICIDE only provided
better plant water status than CONTROL, while not being consistently better than any of
the mulch models. Similarly, in areas with moderate-to-high water availability, such as
Central USA (Maggard et al. 2012) or in irrigated orchards in NW India (Thakur et al.
2012) and SW Russia (Solomakhin et al. 2012), mulching did not result in lower soil water
content than after herbicide application. On the other hand, mulching increased soil
moisture compared with CONTROL during warm dry years (2012) as a result of buffered
maximum temperatures, lower evaporation and decreased transpiration due to weed sup-
pression (Barajas-Guzma
´n et al. 2006; Zhang et al. 2009; Maggard et al. 2012; McConkey
et al. 2012). The lack of effect of any treatment on the SPAD measurements indicates that
weed competition did not affect the chlorophyll content of walnut leaves, a variable closely
related to nutritional status, especially with regard to nitrogen. This was also found by
Cregg et al. (2009) after 2 years with both polyethylene and chip mulches. In addition, the
process of degradation of chip mulches did not lead to either an increase or a decrease in
nitrogen content at plant level. Nitrogen release may be expected in subsequent years,
especially with RAMIALCHIPS, composed mostly of thin branches of both broadleaf and
conifer species, while nitrogen shortage is more likely with WOODCHIPS, rich in pine
bark. Finally, the buffering effect of the mulches on extreme temperatures was especially
noticeable in the case of chip mulches (WOODCHIPS, RAMIALCHIPS) in all seasons, in
line with Cregg et al. (2009); Barajas-Guzma
´n and Barradas (2011) and Arentoft et al.
(2013). By contrast, film mulches (PLASTIC, BIOFILM) did not buffer, but instead
increased maximal summer soil temperatures (Dı
´az-Pe
´rez and Batal 2002;Dı
´az-Pe
´rez et al.
2005).
The effectiveness of mulching techniques was evaluated after 40 months, a period long
enough to let trees develop a root system and/or sufficient leaf area to shade and mitigate
weed development unaided (Coll et al. 2003; Devine et al. 2007), and thus adequate to
estimate whether or not the mulches had reached their expected service life (Coello and
Pique
´2016). PLASTIC mulches were found to be especially durable, with 66% still
effective (80% or more intact surface), as in Haywood (2000), who observed 70% of
plastic mulches free of weeds after 5 years. RAMIALCHIPS and WOODCHIPS were
close to the end of their service life in view of the incipient weed development on their
areas, with roughly 50 and 40% of effective units, respectively. Finally, BIOFILM was
approaching the end of acceptable service life, given that approximately one-third of the
mulches fell into each of the three damage categories (effective, partially damaged or
ineffective). This particular model of a pre-commercial prototype probably needs slight
modification in composition or thickness to offer the desirable durability for afforestation
in areas subject to higher sun and heat radiation.
These results correspond to a single trial installed in a homogeneous, flat field, repre-
sentative of Mediterranean continental sub-humid conditions. However, they are to be
complemented by further research in additional trials and pedoclimatic conditions in order
to have more consistent results on the efficacy of the treatments tested.
New Forests
123
Conclusion
Our study shows that on highly productive Mediterranean continental sites, weed control is
critical for the success of valuable broadleaf plantations since it has a decisive effect on
survival, growth and vigour of young seedlings. The optimized application of herbicides to
late-flushing hybrid walnut gave the best results of all the techniques with regard to tree
performance (all 5 years of study) and soil moisture (during dry years). However,
mulching proved an effective alternative, especially considering that repeated weeding
interventions are obviated, which could be a major advantage in minimal management
schemes. The case of biodegradable mulches (biofilm, chips) is particularly beneficial in
this regard, as they do not need to be removed at the end of their service life. This
advantage, together with their composition based on waste or renewable raw materials,
makes them a socially and environmentally valuable alternative to plastic mulching.
However, further studies are needed to investigate the optimal properties of biodegradable
mulches, both film and particle-based, in various sites, especially in terms of water balance
(notably permeability) and durability. We also need to study, from an economic and
operational point of view, the relation between the productive outcomes of each treatment
and the inputs linked to their repeated application (e.g. herbicide) or need for removal (e.g.
plastic mulching), compared with one-time mulch application (e.g. biodegradable models).
Acknowledgements The experimental design was prepared in collaboration with Philippe Van Lerberghe
(IDF-Midi-Pyre
´ne
´es, France) and Eric Le Boulenge
´(UCL, Belgium). The authors thank Guillem Martı
´,
Eduard Mauri, Sı
´lvia Busquet, Carla Fuentes, Miquel Sala, Fernando Valencia, Rosalı
´a Domı
´nguez, So
´nia
Navarro, A
`ngel Cunill, Alejandro Borque, Sergio Martı
´nez, Toni Go
´mez and Aleix Guille
´n for indis-
pensable support during field trial design and/or data collection, Aitor Ame
´ztegui for support on data
analysis and Terrezu SL and Groencreatie BVBA for providing the prototype biofilm for the trial.
Funding This work was supported by project Poctefa 93/08 PIRINOBLE: Valuable broadleaves for
restoring and enhancing economic development of rural areas: innovation and technology transfer on
sustainable plantation techniques.
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