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Cultivation Potential and Uses of Paulownia Wood: A Review

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Abstract

This review aimed to determine the current state of research on the growth conditions and use pertaining to paulownia wood, mainly in European countries where paulownia has been introduced only relatively recently. Several studies carried out on Paulownia hybrids have shown significant differences in the growth dynamics of individual clones in their response to local environmental and climatic conditions. For example, dry biomass production yields in the second year of cultivation range from 1.5 t ha−1 to as much as 14 t ha−1. This diversity has manifested itself not only in growth characteristics but also in the properties of the wood and the possibilities for its use. Despite having clear similarities to the genus Paulownia, the cultivation of species and hybrids under different conditions has produced varying results. The best growing conditions for this wood (that make economic sense) are in the Middle East and Southern Europe. These regions have accumulated the most experience because of the earlier establishment of the crop. Today, paulownia cultivation is dominated by hybrids with selected traits that are propagated mainly in vitro. The most commonly planted hybrids include the clones in vitro 112, Cotevisa 2 and Shan Tong. The growth results and production capacity in central European countries are lower compared to Southern Europe. Experiments on paulownia cultivation are still relatively young, mainly consisting of replicating the cultivation of hybrids developed in Asia or Southern Europe. However, agronomic procedures are being developed and reactions to local climatic conditions are being studied. It is likely that, in the next few years, the profitability of growing paulownia in these regions will become apparent.
Citation: Jakubowski, M. Cultivation
Potential and Uses of Paulownia
Wood: A Review. Forests 2022,13, 668.
https://doi.org/10.3390/f13050668
Academic Editor: Miha Humar
Received: 4 April 2022
Accepted: 24 April 2022
Published: 26 April 2022
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Review
Cultivation Potential and Uses of Paulownia Wood: A Review
Marcin Jakubowski
Department of Forest Utilization, Faculty of Forestry and Wood Technology, Pozna´n University of Life Sciences,
Wojska Polskiego 71 A, 60-625 Poznan, Poland; marcin.jakubowski@up.poznan.pl
Abstract:
This review aimed to determine the current state of research on the growth conditions
and use pertaining to paulownia wood, mainly in European countries where paulownia has been
introduced only relatively recently. Several studies carried out on Paulownia hybrids have shown
significant differences in the growth dynamics of individual clones in their response to local environ-
mental and climatic conditions. For example, dry biomass production yields in the second year of
cultivation range from 1.5 t ha
1
to as much as 14 t ha
1
. This diversity has manifested itself not
only in growth characteristics but also in the properties of the wood and the possibilities for its use.
Despite having clear similarities to the genus Paulownia, the cultivation of species and hybrids under
different conditions has produced varying results. The best growing conditions for this wood (that
make economic sense) are in the Middle East and Southern Europe. These regions have accumulated
the most experience because of the earlier establishment of the crop. Today, paulownia cultivation is
dominated by hybrids with selected traits that are propagated mainly in vitro. The most commonly
planted hybrids include the clones
in vitro
112, Cotevisa 2 and Shan Tong. The growth results and
production capacity in central European countries are lower compared to Southern Europe. Ex-
periments on paulownia cultivation are still relatively young, mainly consisting of replicating the
cultivation of hybrids developed in Asia or Southern Europe. However, agronomic procedures are
being developed and reactions to local climatic conditions are being studied. It is likely that, in the
next few years, the profitability of growing paulownia in these regions will become apparent.
Keywords: hybrid; biomass; wood properties; fast-growing; plantation; cultivation
1. Introduction
Climate change has led to a rapid increase in research into reducing carbon dioxide
(CO
2
) emissions. One form of reducing CO
2
emissions is the use of alternative energy
sources, including biomass. Various forecasts have predicted a steady increase in de-
mand for the use of wood and wood-based materials until at least 2050 [
1
5
]. There is a
rapidly growing interest, worldwide, in fast-growing wood species that can be used for
biomass [
6
8
]. As one of the fastest-growing tree species in the world, the genus Paulownia
has attracted enormous interest from academia and industry in recent years. Several re-
search programs have been launched and experiments performed in order to verify the
possibility of growing and using paulownia wood as a raw material [
9
14
]. These research
programs are being carried out simultaneously across a variety of fields, and their number
is growing by the month. On the one hand, this is a positive development, leading to a
broad exploration of the possibilities of using paulownia, but on the other hand, numerous
limitations are being discovered. Doubts have also been expressed about the previous
optimism regarding universal fast-growing species. The genus Paulownia is native to China
but has quickly gained popularity throughout Asia, the USA, Australia, and Europe [
15
].
Research on its cultivation has also been conducted in central Africa [
16
]. The aim of this
review was to determine the current state of the research on its growing conditions and
areas in which paulownia wood can be used, mainly in European countries and the Middle
East, as these areas have seen increased interest in paulownia wood over the last 20 years.
Forests 2022,13, 668. https://doi.org/10.3390/f13050668 https://www.mdpi.com/journal/forests
Forests 2022,13, 668 2 of 15
Botanical Description
Paulownia tomentosa (Thunb.) Steud and P. fortunei (Seem.) Hemsl. are genera/species
of plants that belong to the family Paulowniaceae but that were historically included in
the genus Catalpa Scop. and the family Scrophulariaceae. Modern genetic research has
updated the systematic affiliation of Paulownia. Further distinctions were made in the
families Scrophulariaceae and Bignoniaceae, leading to the new family Paulowniaceae
being separated out, the Paulowniaceae being genetically closer to the Lamiaceae than
the Scrophulariaceae [
17
,
18
]. There is currently no consensus on the number of species of
Paulownia, with between six and a dozen species being listed [15,19].
The name ‘paulownia’ was given to the tree in honor of Anna Pavlovna Romanova,
Grand Duchess of Russia, and later, Queen of the Netherlands, who sponsored Philipp
von Siebold’s second expedition to Japan in 1861 [
20
,
21
]. The tree has an umbrella-shaped,
low-set crown, and the green parts of the plant are covered with fine hairs. The bark is
grey-brown or black, smooth, and covered with numerous lenticels, with vertical cracks
appearing with age. The young plant develops large leaves that are 15–30 cm long and
10–20 cm wide (P. tomentosa); in older plants, these are smaller [
22
,
23
]. Paulownias develop
many branches in the wild but will develop a single trunk in close quarters or with specific
pruning techniques. Eight years of research into different techniques, including budding
branches and pruning development, have yielded varying results. Optimal results have
been obtained for moderate-intensity pruning as the best solution for rapid growth in
budding branches and the development of an extended trunk [
24
]. Paulownias have a deep
and well-developed root system that forms several branches and usually reaches depths of
2 m. Recorded examples of root systems that are nearly three times as wide as the crown
have been reported [
23
]. Their strong and rapidly growing root systems can also penetrate
to greater depths under favorable conditions and can be used, for example, to stabilize
landslides [25].
2. Cultivation
2.1. Reproduction
Paulownias reproduce generatively and vegetatively; however, under industrial con-
ditions, reproduction is almost entirely vegetative. Historically, the oldest reproduction
method has been via root-splitting, which is also used for natural species [
23
]. Root-splitting
at an early developmental stage—known as the mini-cuttings technique [
26
]—or activating
the rooting process for green cuttings [
27
] have also been used. However, the primary
means of propagation for many clones is via
in vitro
propagation [
28
33
]. One of the most
important steps in the reproduction phase is the production of a healthy, well-developed
root system, so some of the research has focused primarily on this issue [3436].
2.2. Hybrids
The most commonly cultivated species are P. tomentosa,P. elongata,P. fortunei,
P. taiwaniana,P. fargesii,P. galbrata and P. catalpifolia [
15
,
37
]. In the early introduction
of paulownias worldwide, pure botanical species were used. One of the first countries to
introduce it on a large scale was the US, with paulownias (P. tomentosa) being imported
around 1840. Because of its quick growth, it was named “the tree of the future”. Over the
past 150 years, it has spread throughout the various states, causing a great deal of trouble
that has resulted in a heated debate regarding all species of Paulownia.Paulownia tomentosa
has officially been declared an invasive species, so much so that it has been eradicated
from many states. More lenient treatment has been given to P. elongata, which is not as
invasive but is also accepted reluctantly. Paulownia in the US has as many opponents as
supporters, and discussions regarding the genus are heated because the existing crops
make their owners substantial profits [38].
Recent studies have shown that P. tomentosa can spread in areas where stands have
been damaged by various disasters and disturbances, especially in the canopy [
39
]. In
some countries, certain species of Paulownia have been declared dangerous, such as
Forests 2022,13, 668 3 of 15
P. tomentosa, which has been recognized as an invasive species in Austria [
40
]. The Czech
Republic, too, has taken note of it, giving it the status of an alien species requiring constant
monitoring [
41
]. State authorities in Poland have also been cautious about the introduc-
tion of paulownias on a mass scale [
42
]. Natural Paulownia species are still being grown
throughout Asia, as far as Turkey, but are increasingly being replaced by hybrids. In some
countries, such as Bulgaria, hybrids have only gained importance as a potential product af-
ter previous attempts to cultivate pure species failed [
14
,
29
]. For the production of hybrids,
individuals are selected from several popular species that exhibit high productivity and
high environmental adaptability, including P. elongata
×
P. fortunei [
43
,
44
] and P. fortunei
×
P. tomentosa [
45
]. Some of the best-known hybrids are the clones
in vitro
112 [
10
,
19
,
46
],
Cotevisa 2, Sundsu 11 [
47
] and Shan Tong [
48
,
49
]. Examples of less well-known hybrids
include Arctic [
37
] and the selected genotypes PWCOT-2, PW-105, PWL-1, PWST-33 and
PWST-11 [
50
]. There are also naturally occurring hybrids, such as P. taiwaniana from a
cross between P. kawakamii and P. fortunei [
51
,
52
]. Sometimes, more unusual hybrids are
found, such as the 9501 ((P. fortunei
×
P. elongate)
×
(P. fortunei
×
P. tomentosa)) [
49
]. The
paulownia plantations are established at a specific density depending on the end-use. They
are usually planted at a spacing of 2
×
1.5 m
2
to 4
×
4 m
2
. For biomass production, about
2000–3300 plants/ha are planted, while for timber production, far fewer are planted, about
550–750 trees/ha [
10
,
19
]. Paulownia hybrids are grown in short, 6–10-year cycles for their
roundwood, but these cycles can be even shorter for biomass [10].
2.3. Growth Conditions
The most well-known feature of paulownia is its ability to reach gigantic sizes in a very
short time. In China, it has been said of the paulownia that it “looks like a pole in one year,
an umbrella in three years and can be cut into boards in five years” [
23
]. Record-breaking
specimens have been described in China, such as the 80-year-old P. fortunei growing in
Kweichow Province, which reached a height of 49.5 m, a diameter at breast height (DBH)
of 202 cm and a wood volume of 34 m
3
; another, at 90 years of age, had a 224 cm DBH and
44 m
3
wood volume. Among the younger trees, an 11-year-old P. fortunei, grown in the
Guangxi Zhuang Autonomous Region of southern China, measured 22 m tall and had a
DBH of 75.1 cm with a wood volume of 3.69 m
3
. Similar sizes have also been achieved by
P. elongata [
23
]. Under native conditions in China, paulownias typically attain a 30–40 cm
DBH in 10 years and produce around 0.3–0.5 m
3
of wood, although in optimal conditions,
useful wood can be produced in 5–6 years [
23
]. Using natural species as crops has been a
popular undertaking in China and Southeast Asia for centuries. Compared to other species,
paulownias perform very well in terms of growth dynamics, even compared to the fastest-
growing poplars [
53
]. Other studies have confirmed that, compared to other fast-growing
trees, such as willow, poplar, eucalyptus and red oak, paulownias achieve by far the
greatest growth under optimum conditions [
54
]. These conditions—-especially light levels—
-impact photosynthesis, which paulownia growth benefits from under optimal conditions
and is limited by unfavorable conditions. This tree requires light intensities of between
20,000 and 30,000 lux for optimal growth [
19
]. Paulownias perform photosynthesis based on
C4-cycle enzymes [
15
], as opposed to the classical C3-cycle characteristic of most plants [
55
].
Their greater photosynthetic efficiency in the right conditions enables paulownias to gain
weight quickly in a short period. Although C4 mechanisms are present in paulownias,
hybrid lines tend to exhibit C3 activity [
56
]. The activity of enzymes involved in the
C4 cycle is highly variable and often limited. Paulownias are greatly affected by their
growth and development conditions, especially the stress caused by drought or salinity,
for example [
55
57
]. Through these mechanisms, paulownias can show high adaptation
to environmental stress conditions. However, their intensive growth requires a great deal
of water—from 1000 to 2000 L per seedling in the first growing season [
43
]. Studies have
emphasized the particular importance of watering in the first months of growth [
58
,
59
].
The water supply can also contribute to the production of more leaves and, consequently,
increased shoot growth [
60
]. Paulownias require permeable soil with a pH value above
Forests 2022,13, 668 4 of 15
5 (5–8.9) and an optimum temperature of 15–16
C. Researchers have pointed out, however,
that mass production greatly depends on soil quality, as expressed by the soil quality
index [
61
]. Because of the rapidity of paulownia biomass production, there may be notable
changes in the soil. After one year of soil monitoring, it was found that some microbiological
parameters had decreased around the trees, which was related to a decrease in the nutrient
content [
62
]. Other researchers have reported similar changes related to soil microbial
activity and the soil’s microbial community [
63
]. One proposed solution has been the use of
residues produced by the forest biomass industry. This mainly involves the use of pine bark
and ash biomass. Soil enriched with these components can maintain a balance between
microbial activity and the community [63].
Paulownias have shown sensitivity to soil salinity, which is often a characteristic of
semi-arid regions. Sometimes, this condition can be intensified by the evapotranspiration
of the paulownia. This susceptibility is not the same for all Paulownia species; it has been
recommended that lines of resistant hybrids should be developed [
64
]. Stress caused by soil
salinity can enhance the effect of high light stress, consequently reducing photosynthetic
efficiency [
65
]. The optimum air temperature for rapid paulownia growth is 27
C but the
spectrum of extreme temperatures that they can tolerate is broad, ranging from
25 to
47
C [
19
]. However, these values vary by species, the natural tolerance of minimum tem-
peratures being
20
C for P. tomentosa,
15
C to
18
C for P. elongata and P. catalpifolia,
and
5
C to
10
C for P. fortunei,P. kawakamii and P. fargesii. Introduction experiments
conducted in China have resulted in only partial survival at lower temperatures [
23
]. As
hybrids are currently being grown mainly in southern Europe, where severe frosts do not
occur, there is no confirmed data on the frost resistance of introduced hybrids. However,
some observations have been made on the survival of young seedlings in regions where
frosts and freezing occur. Seedling survival in Turkish experiments on seedlings that
originated in China was determined to be 60–90%, with one experiment resulting in only
a 50% survival rate [
66
]. In Northern Ireland, where the conditions are quite different,
survival ranged from 70% to 95% for hybrids with Spanish origins, except for one study
where the survival rate was only 20%, while the survival of hybrids with Moroccan origins
ranged from 30% to 33.3% [
50
]. Some authors have also reported that all their seedlings
died in winter. Ulu et al. [
66
], in one of their experiments, reported that all the seedlings
growing in Gölköy, Turkey (at 1400 m) died during the winter of 1999. Other authors have
also reported significant numbers of young seedlings that did not survive the winter [
67
].
Some studies have pointed to frost as being particularly damaging to the growth of young
paulownia seedlings when the shoots are green—-that is, at the beginning of winter or
during spring frosts. According to the study by Ayan [
45
], all the top shoots froze in winter.
Research carried out in the Czech Republic has shown unsatisfactory growth due to frost
in the clone
in vitro
112 in the first two years of cultivation [
46
,
68
]. Other researchers have
recorded heavy snow damage in the winter of 2001 at the Ulubey site in Turkey [66].
Another factor limiting the growth of paulownias is the length of the growing season,
which is variable in Central European climate conditions. In comparing the length of the
growing season of the Paulownia Oxytree (clone
in vitro
112) in north-eastern Poland, a
28-day shorter growing season was recorded in 2019 compared to 2018 [
69
]. Paulownias
have very large leaves in their juvenile state, and the stem is not sufficiently woody at
that stage, making it susceptible to mechanical damage. Therefore, another damaging
factor can be strong winds, which, in open conditions, can destroy an entire plantation [
70
].
Experiments in New Zealand have shown that young trees and branches are susceptible to
damage from wind speeds as low as 40 km hr
1
, and that wind can restrict tree growth
even after 3–4 years of age [71].
2.4. Diseases
Like any plant, paulownias can get sick and are susceptible to various pathogens. The
most well-known disease in paulownia is witches’ broom, a condition that has been observed
in Chinafor many years [
23
,
72
]. Modern studies have determined phytoplasma (a parasite) to
Forests 2022,13, 668 5 of 15
be the causal agent of witches’ broom, which is potentially transmissible between paulownia
trees [
73
,
74
]. The mechanism of activity is probably related to gene-expression changes in
response to the phytoplasma [
75
,
76
]. Some studies have suggested that there may be genetic
resistance to the disease in the cultivar P. tomentosa
×
P. fortunei [
77
]. In addition, other
diseases have been found, such as Phytophthora root and collar rot [
78
] and rot caused by
Trametes hirsuta in Serbia [
79
]. Recent reports have indicated the presence of nematodes
(Meloidogyne hapla) in the roots of P. tomentosa in Poland [80,81].
3. Paulownia Wood Properties and Uses
3.1. Wood Structure
Paulownias have light yellow to light red heartwood. The boundary between the
sapwood and heartwood is not clearly defined. The sapwood is very narrow and usually
contains one or two annual rings. Annual rings are clearly visible in all cross-sections.
The wood has a ring-porous or semi-ring-porous structure. The vessels are either barely
visible or are not visible at all, and the tree rays are visible only under magnification [
23
].
The vessels are oval in shape and can be divided into early-wood vessels and late-wood
vessels, the latter being 3 to 10 times smaller. Surrounded by a broad band of parenchyma
of varying shapes, the rays are narrow, usually occupying a single row up to 0.5 mm
high, although multi-seriate rays do also occur [
23
,
82
,
83
]. The heartwood is sensitive to
discoloration. Several factors can cause such color changes, which can be divided into three
groups—-chemical discoloration, microbial discoloration and photo-discoloration [
84
]. The
aging of the wood also causes distinct color changes, with natural changes being less
obvious than those induced artificially by ultraviolet light or high temperature [
85
]. It is
well known that thermal modification causes similar color changes in other wood species.
Suri et al. [86] described these changes in paulownia wood.
3.2. Wood Properties
In terms of their physical and mechanical properties, paulownias are most similar
to willows and poplars. However, the findings of many of the works on this subject
relate to different crop origins and locations. The experiments that have been carried out
often employed different methodological assumptions; therefore, the results are sometimes
difficult to compare. Paulownia wood density, with a 12% moisture content, ranges from
220 to 350 kg m
3
, but most often oscillates around 270 kg m
3
[
87
91
]. Variability in
paulownia wood density is caused mainly by different growth conditions, although this
differs between the species P. tomentosa,P. elongata and P. fortunei, with a slightly higher
density commonly being attributed to P. tomentosa [
87
,
89
]. Occasionally, as indicated by
certain authors [
92
], a density of above 400 kg m
3
has been measured (Shan Tong, Bulgaria).
Paulownia wood static bending strength ranges from 23.98 to 43.56 MPa, depending on
the species, while the modulus of elasticity ranges from 2651 to 4917 MPa [
44
,
89
,
93
,
94
], or
even up to 5900 MPa for P. tomentosa [
87
]. In both standing-tree and log tests performed
using non-destructive methods, a higher modulus of elasticity has been reported for trees
with larger diameters [95].
The physical and mechanical properties of paulownias have been found to be subject to
a number of variations during the drying of the sawn timber. Depending on the technological
process used, these variations may be either highly significant or non-significant [
96
]. Paulow-
nias have a high strength-quality factor, which equates to a high strength-to-density ratio.
For the Cote-2 hybrid, this has been measured as up to 9.2 km [
91
]. For specific applications,
this is a most useful parameter, especially where very lightweight but robust structures are
required, such as in composite construction panels [
45
,
97
]. All these data on paulownia wood
indicate quite wide variations in the different species’ properties; therefore, when talking
about paulownia wood, it is important to have a specific species in mind. This issue has been
highlighted by Feng et al. [
98
], who tested 23 clones and demonstrated a phenotypic variation
in the wood properties of 11.75% and a genetic variation exceeding 19.04%.
Forests 2022,13, 668 6 of 15
3.3. Traditional Uses of Paulownia Wood
The first descriptions of traditions related to the use of paulownia wood date back
to several centuries BCE [
21
]. The tree was used for religious and medical purposes and
was held in high esteem, being associated with birth and death. Many legends have
been associated with the tree, mainly in China and Japan. The widespread cultivation
of paulownias has been known since the third century CE [
23
]. Today, the wood is often
used because of its popularity and its ability to grow under a wide range of conditions.
Paulownia wood is used in plywood, engineered wood (other than construction wood),
paper, veneers, hand carvings, clogs, furniture and kitchen items, such as rice pots, water
buckets, bowls, spoons and sticks [
22
,
38
,
99
]. A frequently mentioned use for paulownia
wood is in the manufacture of musical instruments [
22
,
38
,
100
]. Specific acoustic parameters
that are not found in spruce wood may cause it to be replaced by paulownia in some
instruments, resulting in new sounds. However, as a material that might be suitable for
musical instruments, paulownia wood does not work well as a sound absorber. Paulownias
have large vessels, but low through-pore porosity because of the large number of tyloses in
the vessels. Therefore, gas permeability is low and sound absorption is poor. This reduces
its quality as a soundproofing material compared to balsa or binuang [
101
]. It also limits
the use of paulownia in applications where wood saturation is required [102,103].
One solution to this problem is the thermal modification of the wood. As determined by
Kang [
104
], the gas permeability and sound absorption coefficient of heat-treated P. tomentosa
increased as a result of heat treatment, depending on the temperature. Kolya and Kang [
105
]
reached a similar conclusion, recommending hydrothermally treated paulownia for sound-
absorption boards in housing applications. Yet another method, based on a supercritical CO
2
treatment, has been proposed by Xu et al. [
106
]. Researchers have shown that this method
significantly improves the gas permeability of P. fortunei as a result of the reduction in the
proportion of cells with tyloses [
106
]. Tests for suitability in the manufacture of pencils and
crayons have also been positive, with tests comparing paulownia (P. elongata) wood to poplar
(Populus tremula) and juniper (Juniperus excelsa), which are commonly used in these products,
having shown great promise in terms of wood properties [107].
3.4. Pulp Industry
The utility of P. tomentosa in the pulp industry was pointed out decades ago in the US;
however, researchers have stressed that paulownia fibers are short and are only suitable
for certain grades of paper [
108
]. Contemporary research has verified this knowledge
in a broader context. As determined by San et al. [
44
], based on a study conducted on
a 3-year paulownia plantation, the fiber sizes take on values typical of the deciduous
species useful in this industry. However, it is always important to bear in mind the specific
species in question because, while the fiber lengths of the various species of Paulownia
are similar, ranging from around 0.82 to 1.002 mm, the thickness of the cell wall varies
considerably. For example, the average thickness of the fiber cell wall can vary from 3.8 to
8.6
µ
m between the different species of Paulownia [
44
]. Its high cellulose content (47.85%)
makes it useful for the pulp industry, as indicated by Popovic and Radosevic [
109
], who
also, however, noted that the chemical composition differed between the various species.
Similar relationships and the suitability of Paulownia for cellulose production have also
been pointed out in other studies [
110
112
], whereas in later studies, the biorefinery of
paulownia wood has been improved in order to obtain lignocellulosic biomass for fuels,
solvents and chemicals, etc. [
113
]. Due to paulownia wood’s short production cycles, both
the stem and branch wood can be used, although the latter is of lower value and is often
accompanied by reaction wood; however, this can be used in paper, nanocellulose, charcoal
and other applications [82,114,115].
Forests 2022,13, 668 7 of 15
3.5. Energy Goals
Nowadays, paulownia plantations oriented toward biomass production are gaining
in popularity [
12
]. This tree can produce as much biomass in a year as other species can
in a few years [
47
]. Suitable hybrids work best in this field, but the differences between
the hybrids and the regions where the experiments were carried out are quite significant.
Studies have shown that, for example, the clone Cotevisia 2, grown in Spain, has 1.8 times
the productivity of the clone Suntzu 11. However, in two locations out of the six stud-
ied, it was Suntzu that yielded higher productivity. Both clones in the 2-year experiment
showed record productivity in the Villanueva del Río y Minas region (Sevilla province,
Spain), where between 7.2 and 14 t of dry matter were harvested per hectare (i.e., 3.2 and
7.4 t of C, respectively). The same clones in another region of Spain (Cordoba) showed
productivity ranging from 1.7 to 2.3 t of dry matter [
47
]. By comparison, a 16-year ex-
periment conducted on P. tomentosa in Asia yielded 38.8 t C ha
1
, while a 21-year experi-
ment yielded over 105 t C ha
1
[
90
]. The experience of other researchers has also indicated
that biomass production greatly depends on the hybrid used for the crop. As an exam-
ple, Berdon et al. [
10
] determined very poor productivity from clone X1 compared to three
others (112, COT2 and L1) based on a 3-year experiment conducted in southern Spain.
Baier et al. [
116
], in a study conducted near Lake Issyk-Kul (Kyrgyzstan), demonstrated
biomass production of between 1.52 and 3.41 kg per tree per season, with water consumption
of between 433 and 613 l. Gyuleva et al. [
117
], reporting on a 5-year experiment that compared
the productivity of two paulownias introduced into Bulgaria, showed that the best results
came from the southwestern part of the country. Paulownia tomentosa had higher productivity
of 3.479 t ha
1
(oven-dried biomass) after 2 years of growth and 36.995 t ha
1
after 4 years,
while the productivity of P. elongata
×
P. fortunei was 2.730 t ha
1
after 2 years and 19.964 t ha
1
after 4 years. On degraded soils, paulownias show a very low growth rate. In a 3-year study
in Spain, a biomass yield of P. fortunei of only 3.34 t ha
1
was achieved, which was very low
compared to the parallel-grown Eucalyptus globulus (40.4 t ha1) [118].
As a tree with a high growth rate but low wood density, paulownias do not perform
well at producing efficient fuel. A comparison of pellets, in terms of European standards,
produced from young P. elongata
×
P. fortunei plantations showed their poor quality
compared to Pinus radiata and Eucalyptus nitens pellets [
119
]. In another study, however,
evaluating the production of briquettes and pellets from sawdust, satisfactory energy
effects were obtained for P. tomentosa and P. elongata [
120
]. It may be appropriate to
first apply biomass torrefaction, as proposed by ´
Swiechowski et al. [
121
,
122
], in order
to get a better starting quality for fuel production. The calorific value of paulownia
alone is close to that of the energy species already grown in Europe. The tested hybrids
(9501 and Shan Tong) produced only slightly lower gross calorific values of 19.5 MJ
kg
1
(hybrid 9501) and 19.6 MJ kg
1
(Shan Tong) than willow (19.9 MJ kg
1
) and
poplar (19.8 MJ kg
1
). By contrast, the OXI hybrid (19.2 MJ kg
1
) had a lower calorific
value [
123
]. Similar values have been provided by Zachar et al. [
124
] for P. tomentosa
(19.71 MJ kg1).
However, today’s economies require much more rapid results than are obtainable
from years of observation during various experiments. To some extent, this process can
be, and has been, accelerated by building mathematical models and forecasting crop
development under different local conditions. As experience has shown, such models
have a great future and can highlight favorable vs. unfavorable phenomena at an early
stage. Stankova et al. [
125
,
126
] showed using a model that variability in dendromass
production depended on the species and crop type, with variability ranging from 0.3 to
4.5 t ha
1
of dry matter. They also proved that most biomass is accumulated in the trunk,
and only 35% of juvenile trees have branches. However, modeling faces several problems
due to the complexity of the criteria that can affect forecasting, although the models are
constantly developing. This is especially true for biomass planning and the forecasting
of wood properties [
90
,
127
130
]. In a recent analysis, Iran, which has some experience
in introducing paulownia cultivation, forecast the capacity for paulownia cultivation in
Forests 2022,13, 668 8 of 15
an area of 160,000 km
2
[
128
,
129
]. Introducing paulownias as an important element in
biomass production needs particular consideration in countries with favorable conditions
for the growth of hybrids that may be competitive with native species. Research in this
area is ongoing in Spain [
131
,
132
], Romania [
133
], Portugal [
9
], Italy [
134
], Iran [
129
],
Kyrgyzstan [
135
], Serbia [
54
], Ukraine [
136
,
137
] and Northern Ireland [
15
,
50
], among others.
3.6. Other Modern Uses
There is currently much research being conducted that is aimed at furthering the use
of paulownia wood in the production of wood plastics and composites [
97
,
138
,
139
] and in
the production of biopolymers [
140
]. Paulownia wood also performs well in the production
of blockboards, which act as a core layer between veneers [
141
], and as an ingredient for
the production of lightweight particleboards [
142
,
143
]. It can also be subjected to thermal
modification [144146] or can be improved by other methods, such as high-pressure treat-
ment, which works well for low-density woods such as paulownia [
147
,
148
]. Paulownia
wood can undergo pyrolysis and conversion to gases as an energy source [
149
] and can be
used as a feedstock for bioethanol production [
13
,
150
]. Work is also underway to examine
the use of paulownia waste as a substrate for producing biohydrogen [
151
]. In addition to
the wood, the remaining parts of the plant, such as the leaves and flowers, can be used for
medicinal purposes [152157] and as a food source for animals [153,158160].
Attempts to use Paulownia species for the phytoremediation of heavy metals in con-
taminated soils have indicated a significant accumulation of metals, such as copper, zinc
and cadmium, although this was due to their high biomass productivity rather than their
metal accumulation potential [
161
,
162
]. Another study found a significant difference in
lead and zinc accumulations between different hybrids [163].
4. Summary
Based on the literature review presented here, it can be concluded that the main
purpose of growing paulownia in short cycles is to produce woody biomass for the energy
and pulp industries, as well as for other industries related to wood processing, the creation
of wood composites and biopolymers, and wood gasification, etc. To a minor extent,
research is ongoing into the use of other parts of the plant, such as the leaves and flowers, in
the pharmaceutical industry or for animal feed. The traditional uses of wood for furniture-
making and wider mechanical processing is rather limited by the natural occurrence
of Paulownia species and the locations where paulownia trees reach large dimensions.
Experience has shown that pure species, such as P. tomentosa and even P. fortunei, are
invasive so only some hybrids have been accepted into mass cultivation. Considering the
regions involved, the best conditions for the growth of Paulownia hybrids are in southern
Europe, particularly Spain, Portugal, Italy and the Balkans, and in Middle Eastern countries
such as Turkey and Iran, where conditions are also much better than in countries further
north. The number of positive experiences indicates great potential, which, however, varies
strongly locally.
The reported technical parameters, chemical composition, requirements for growing
conditions and biomass production differ significantly between different species and hy-
brids. The values obtained from the published studies sometimes differ by up to several
dozen percent. Each time, a trial should determine the suitability of the selected species
or hybrid for each specific purpose. Experiments in Central and Eastern Europe are still
in the early stages due to the late migration of paulownias to these areas. However, there
are already indications of lower production efficiency than in southern Europe. Several
factors contribute to this finding, the most important being the shorter growing season,
which significantly reduces seedling growth and biomass production. The second is low
temperatures and frosts in spring and autumn. This does not rule out the possibility of
introducing paulownias into this region of Europe, but further research is required, mainly
to improve cultivation methods in order to best adapt the tree to the specific climatic
conditions. The use of appropriate frost protection and various agrotechnical treatments
Forests 2022,13, 668 9 of 15
can aid in this. Paulownia and its hybrids offer a serious alternative to many native tree
species in Europe, but it is not a completely universal species and requires further research,
variety selection and improved cultivation methods for introduction into production in
specific regions.
Funding: This research received no external funding.
Conflicts of Interest: The author declares no conflict of interest.
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... With its light, silky texture, high dimensional stability, and good acoustic resonance, Paulownia wood is used in the manufacture of furniture, decorative materials, musical instruments, handicrafts, and other finished products [2,3]. Paulownia species have been introduced from China to many countries, including the United States, Japan, Poland, Turkey, Israel, India, Mexico, Canada, and Australia [4][5][6]. The original Paulownia species that were introduced worldwide are increasingly being replaced by superior clones such as Cotevisa 2 (Paulownia elongata S. Y. ...
... Steud. × Paulownia fortunei), and 9501 (natural hybrid of Paulownia fortunei), among others [5]. ...
... trunk diameter at 2.6 m above ground (D2.6), the height of the extended trunk (H1), the diameter of the extended trunk at 0.5 m above H0 (De0.5), and the diameter of the extended trunk at 1.5 m above H0 (De1. 5). An analysis of covariance was used to adjust heights and diameters to compensate for the influence of initial seedling height and DBH on experimental results. ...
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Clonal forestry has developed rapidly in recent years and already plays a significant role in commercial tree plantations worldwide. Clonal breeding requires accurate assessments of genetic parameters, together with measurements of clonal productivity, stability, and adaptably. However, relevant studies for clones of Paulownia spp. genotypes are rare. We therefore conducted clonal tests on twenty Paulownia clones established at three sites in the temperate and subtropical regions of China. Trees were planted in a randomized block design, with four replications in each site, twenty plots in each block, and six to eight individuals of the same clone in each plot. We measured the trunk diameter at breast height (DBH), total trunk height (Ht), and individual stand volume of 7-year-old trees to estimate genetic parameters and analyze genotype–environment interactions. A combined analysis of variance indicated that clonal, site, and clone–site interactions significantly affected the three growth traits. Clonal heritability and individual heritability were 0.35–0.84 and 0.07–0.30, respectively. The phenotypic and genetic correlation coefficients among the growth traits were 0.46–0.93 and 0.85–0.99, respectively. There were extremely significant positive linear relationships between the best linear unbiased predictors for DBH and the original DBH values (R2 > 0.98). Clones 10, 2, 18, and 13 were selected for deployment based on a selection intensity of 1.4, GGE biplots, and the relative performance of harmonic means on genotypic values analysis. For these clones, the genetic gains in DBH, Ht, and volume were 18.05%, 21.46%, and 46.03%, respectively. These results provide useful information for the selection of Paulownia clones at the target sites and will provide a sound basis for improving Paulownia clonal breeding programs in the future.
... Paulownia was also introduced in North America, Australia and Japan [3], and is cultivated worldwide in more than 40 countries [4]. Other objectives of Paulownia plantations are to reduce soil hazards by tree-crop intercropping in farmlands [5], to protect systems against erosion, flooding or wind damage [6], to reduce air pollution and to secure the increasing energy demand [7]. Paulownia trees have exceptional root systems and can adapt easily to various soil conditions [8]. ...
... At 12% moisture content, Paulownia wood density varies from 220 to 350 kg/m 3 , with an average of 270 kg/m 3 [5]. This variability in density is determined by growth conditions. ...
... This behaviour of Paulownia wood can be attributed to narrower core rays. The rays are narrow, occupying a single row up to 0.5 mm, but also multi-seriate rays can occur [5]. Firstly, the core rays control the wood in a radial direction and ensure values of swelling up to 4% [35], such as for most species (at this density). ...
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The aim of this research is the characterization of physical and mechanical properties of Paulownia sawn wood from three plantation sites in Europe, namely Spain, Bulgaria and Serbia. As a fast-growing wood species, Paulownia has a significant positive forecast for the European markets and a wide range of possible applications that still need to be explored. For this purpose, Paulownia tomentosa(Tunb.) x elongata(S.Y. Hu) wood species was investigated. Sorption behaviour, Brinell hardness, 3-point bending strength, flexural modulus of elasticity, tensile strength, compressive strength and screw withdrawal resistance were examined in detail. The samples from Spain have the higher average bulk density (266 kg/m3), 3-point flexural strength (~40 N/mm2), 3-point flexural modulus of elasticity (~4900 N/mm2), compressive strength (~23 N/mm2), tensile strength (~44 N/mm2) and screw withdrawal resistance (~56 N/mm). The plantation wood from Bulgaria has the highest average of annual ring width (46 mm). Paulownia wood has potential in lightweight applications and can replace successfully expensive tropical species as Balsa.
... Due to the high productivity of this plant, it can be used for biofuel goals (Rodríguez-Seoane et al., 2020;Jakubowski, 2022). The wood chemical composition of P. tomentosa (up to 3 years) showed 40% of cellulose, 36% of hemicellulose, and 24% of lignin content (Esteves et al., 2021). ...
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The review of previous studies on Paulownia spp. showed that these plants are valuable raw materials with polyfunctional use among which are medicinal, forage, energetic, etc. This study demonstrated the accumulation of selected biochemical components in the different parts of Paulownia tomentosa (Thunb.) Steud. genotypes plants by the end of vegetation collected from experimental collections of M. M. Gryshko National Botanical Garden of the National Academy of Sciences of Ukraine. The accumulation of selected nutrients in the leaves was the following: a dry matter of 24.09–29.44%, lipid content of 5.01–8.58%, total sugar content of 5.51–9.82%, mono sugar content of 1.73–6.17%, ash content of 1.09–8.96%, phosphorus content of 0.47–1.60%, calcium content of 1.41–3.43%, and heating value of raw material of 4,083.09–4,353.11 Kcal.kg-1. In the branches of investigated genotypes on average accumulated 37.5–46.0% of dry matter, 3.69–6.52% of lipids, 6.66–19.96% of total sugar content, 1.95–5.75% of mono sugar content, 1.43–2.93% of ash content, 0.38–0.89% of phosphorus content, 0.515–1.61% of calcium content, and 3,911.45–4,290.78 Kcal.kg-1of heating value. The trunks had 40.09–51.5% of dry matter, 2.0–6.14% of lipids, 6.44–20.48% of sugars, 1.6–3.67% of mono sugars, 1.18–2.53% of ash, 0.22–0.40% of phosphorus, 0.37–0.63% of calcium, and 4,073.45–4,525.28 Kcal.kg-1 of heating value. A very strong correlation was found between sugars and mono sugars content in the leaves (r = 0.859), lipids and phosphorus (r = 0.864) in the branches, heating value, and calcium (r = 0.820) in the trunks. Due to the increasing interest in the growth and use of P. tomentosa during the last time, this study can be useful for further breeding work with this species as biofuel, forage, and medicinal plants.
... The average density of the studied variety of Paulownia at 12% moisture is slightly lower than usual for this species. The density for Oxytree should be between 220 and 350 kg/m 3 with an average value of 270 kg/m 3 [26][27][28][29]. The average density for all densified samples of hornbeam wood increased by an average of about 320 kg/m 3 . ...
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The aim of the study was to densify samples of Paulownia Clone wood in vitro 112 and hornbeam (Carpinus betulus L.) by compression in the radial direction. Before the specimens were densified, they were subjected to plastic treatment in an ammonia solution. After densification, the compressive strength in the radial direction and the determination of the Brinell hardness in all three anatomical directions of the wood were determined. The wood swelling in humid air (98% RH) and liquid water was also determined. Paulownia wood density increased by about 280% and hornbeam wood density by 40%. The Brinell hardness parallel to the fibres increased by 49 and 390%, perpendicular by 80 and 388% for hornbeam and Paulownia, respectively. A significant increase in the compressive strength of wood in the radial direction was also observed. Densified hornbeam wood exposed to water showed a high swelling value of 153, while Paulownia wood exhibited 107%.
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This article describes the characteristics of th Oxytree (Paulownia) plant, both in terms of its impact on GHG emissions and its potential use to produce biofuel, i.e., biogas. The described research involved the physico-chemical and elemental analysis of the Oxytree leaf composition and its biogas efficiency depending on the harvesting method. Three different scenarios were considered: the freshest possible leaves—processed immediately after stripping from the living tree; after the first day of collection from pruned or harvested wood; after the first week of collection from pruned or harvested wood. The best results were achieved for the harvest of the freshest leaves—on average 430 m3/Mg (biogas) and 223 m3/Mg (methane) per dry organic mass. The highest yield of biogas in terms of fresh mass (FM) was obtained for leaves fallen and collected after 1 day—123 m3/Mg FM, and 59 m3/Mg FM (methane). Processing Oxytree leaves through anaerobic digestion will contribute to reducing the carbon footprint of wood biomass production and is an additional source of renewable energy and fertilizer product.
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The shift away from fossil fuels needed to reduce CO2 emissions requires the use of renewable carbon and energy sources, including biomass in the bioeconomy. Already today, the bioeconomy has a significant share in the EU economy with traditionally bio-based sectors. For the future, the energy, mobility and chemical sectors have additional high expectations of the bioeconomy, especially for agriculture and forestry to produce biomass as an industrial feedstock. Numerous studies have been published on the availability of feedstocks, but these often only look at individual applications. Looking at the total demand and considering the sustainability limits of biomass production leads to the conclusion that the expected demand for all industries that could process biomass exceeds the sustainably available capacity. To mitigate this conflict between feedstock demand and availability, it is proposed that the organic chemical sector be fully integrated into the bioeconomy and the energy sector be only partially integrated. In addition, recycling of wastes and residues including CO2 should lead to a circular bioeconomy. The purpose of this manuscript is to help fill the research gap of quantitatively assessing the demand and supply of biomass, to derive economic trends for the current transition phase, and to further develop the theoretical concept of the bioeconomy towards circularity.
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Bioenergy crops play an ecologically and economically fundamental role as an alternative to agri-food productions and as renewable energy sources. Thus far, less attention has been given to assessing microbiological indicators of soil quality in bioenergy crops on abandoned land. The current study assessed microbial and biochemical properties of two soils with different textures in agroforestry plantations of Paulownia elongata x Paulownia fortunei, with regard to the analysis of potential for the reclamation and redevelopment of abandoned lands. The soil samples were characterised by measuring microbial biomass C and N, key enzyme activities, and determining the community-level physiological profiles (CLPP) using Biolog EcoPlates. Soil texture, sampling time (June and October), and distance of sampling (0.1 m and 1 m from a tree) had significant effects on microbiological properties. Moreover, dehydrogenases and acid phosphatase activities as well as microbial biomass C and N decreased with distance from the trees, and were significantly higher in the October than in the June. The community-level physiological profiles (CLPP) and diversity indices showed a similar trend to other parameters of biological activity. The results showed that there were significant differences in the AWCD (average well-colour development) of all carbon sources among the Paulownia microbial communities (p < 0.05). In summary, already after one year of tree planting, a statistically significant increase in microbial activity was found, regardless of soil texture, when evaluated by various methods. This proves the value of the Paulownia as fast-growing plant for recultivation and improvement of soil quality on abandoned land.
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This paper aims to investigate the effect of financial development and renewable energy consumption on consumption-based CO2 emissions in Chile while controlling for economic growth and electricity consumption. Based on the aim of the paper, autoregressive distributed lag (ARDL) bounds with Kripfganz and Schneider's (2018) approximations, fully modified ordinary least square (FMOLS), dynamic ordinary least square (DOLS), and gradual shift causality tests are applied in this study. The outcomes clearly reveal that while financial development and renewable energy consumption reduce the consumption-based CO2 emissions in Chile, economic growth and electricity consumption increase consumption-based carbon emissions. The gradual shift causality test provides consistent results with ARDL, FMOLS, and DOLS estimators. Therefore, policymakers in Chile should dynamically encourage the research and development of low-carbon technologies and renewable energy investments while imported nonrenewable energy sources level should be targeted, and especially those sectors which are more energy-intensive and causing to increase in consumption-based CO2 emissions.
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This article describes the effect of a growth stimulant on the rooting of Paulownia and tulip tree saplings in the conditions of the Tashkent region. In addition, the article discusses the method of propagation of sapling from lignified and annual green cuttings, the influence of the preparation of the substrate 3: 3: 1 from sand, klinec (crushed stone), vermicompost on the development of saplings. In this case, the usual water (control), root SP, basfoliar Kel-SL and heteroauxin stimulants were used.
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Paulownia species are fast growing trees native to China, which are being grown in managed plantings in several European countries for the production of wood and biomasses. In 2018, wilting, stunting, leaf yellowing, and collapse, as a consequence of root and crown rot, were observed in around 40% of trees of a 2-year-old planting of Paulownia elongata × P. fortunei in Calabria (Southern Italy). Two species of Phytophthora were consistently recovered from roots, basal stem bark, and rhizosphere soil of symptomatic trees and were identified as Ph. nicotianae and Ph. palmivora on the basis of both morphological characteristics and phylogenetic analysis of rDNA ITS sequences. Koch’s postulates were fulfilled by reproducing the symptoms on potted paulownia saplings transplanted into infested soil or stem-inoculated by wounding. Both Phytophthora species were pathogenic and caused root rot and stem cankers. Even though P. palmivora was the only species recovered from roots of naturally infected plants, in pathogenicity tests through infested soil P. nicotianae was more virulent. This is the first report of Phytophthora root and crown rot of a Paulownia species in Europe. Strategies to prevent this emerging disease include the use of healthy nursery plants, choice of well-drained soils for new plantations, and proper irrigation management.
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The increase of renewable energy production worldwide, occurred in the last years, is also attributable to some agroforestry species cultivated according to the short rotation coppice technique. Although these species are able of enhancing abandoned or marginal land leading to numerous environmental benefits, an increasing number of farmers are introducing them in place of agricultural crops. Therefore, since for a farmer economic sustainability is one of the main factors to introduce a biomass crop, the study aimed at evaluating the profitability of Paulownia, a species that has been spreading in recent years. In particular, an economic analysis has been carried out in a Southern Italian farm in which Paulownia has recently replaced a vineyard, by adopting a discounted cash flow approach. The results show that the Paulownia for both timber and woodchip production, with an annual gross margin equal to 357.91 € ha⁻¹, can represent a valid alternative compared to wine grape (237.41 € ha⁻¹), while Paulownia for exclusively biomass production has almost zero profitability (4.22 € ha⁻¹). However, the profitability depends not only on the product typology but also on the future market price fluctuations, on the subsidies or incentives, as well as on adequate choices by entrepreneurs for the creation of sustainable supply chains also from an environmental and social point of view.
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Paulownia biomass is rich in carbohydrates, making which a potential feedstock for biohydrogen production. In the study, different parts and varieties of Paulownia were chose as substrates to evaluate hydrogen production potential of paulownia lignocellulose via biohydrogen production by photo fermentation (BHPPF) and energy conversion efficiency (ECE). Results showed the highest hydrogen yield (CHY) of 67.11 mL/g total solids (TS) and ECE of 4.74% were obtained from leaves of Paulownia, which were 121.06% and 115.45% higher than those of the branches. Moreover, Paulownia jianshiensis leaves were found to be the best variety for BHPPF, with the maximum CHY of 98.83 mL/g TS and ECE of 7.18%. Using Paulownia waste as the substrate to produce hydrogen helps broaden the range of raw materials for BHPPF and improve the economic utilization of forestry waste.