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Relative allelopathic potential of invasive plant species in a young disturbed woodland 1

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Invasive plant species are often more successful within introduced areas when compared to their natural ranges. Allelopathy has been suggested as a potential mechanism for this success because invasive plants frequently establish monocultures and may produce allelochemicals evolutionarily novel to the recipient community. However, species are typically tested in isolation making the relative strength of allelopathy difficult to assess. We conducted laboratory bioassays for 10 co-occurring non-native species to determine the relative strength of their allelopathic potential. These species represented a suite of successful invaders within a young forest and were from a variety of plant life forms: trees, lianas, shrubs, and herbs. We determined the germination responses of a target species to a gradient of leaf extract concentrations to assess relative allelopathic potential. The relative strength of germination inhibition was quantified by the slope (b) of the germination response to plant extract concentration. Ailanthus altissima extract had the greatest inhibitory effect on target species germination out of all 10 species (b 5 20.55) while the other tree species, Acer platanoides extract, had small effects on germination (b 5 20.14). For lianas, Celastrus orbiculatus extract inhibited the target species (b 5 20.28) more than Lonicera japonica extract (b 5 20.06). All invasive shrub extracts had very small effects on seed germination (b value 20.03 to 20.19). Extracts of the two herbaceous species, Alliaria petiolata and Microstegium vimineum, had very large inhibitory effects (b520.37 and 20.38, respectively). In this system, we screened a suite of invasive species for allelopathic potential and determined the relative strength of germination inhibition. Most species, particularly invasive shrubs, did not exhibit sufficient allelopathic potential to suggest allelopathy would occur in the field. Four species, Ailanthus altissima, Alliaria petiolata, C. orbiculatus, and M. vimineum all exhibited strong germination inhibition and warrant additional study in the field.
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Oldest Botanical Journal in the Western Hemisphere
Relative allelopathic potential of invasive plant species in a
young disturbed woodland
1
Nikki L. Pisula
2
,
3
and Scott J. Meiners
Department of Biological Sciences, Eastern Illinois University, Charleston, IL 61920
Relative allelopathic potential of invasive plant species in a
young disturbed woodland
1
Nikki L. Pisula
2
,
3
and Scott J. Meiners
Department of Biological Sciences, Eastern Illinois University, Charleston, IL 61920
P
ISULA
,N.L.
AND
S. J. M
EINERS
. (Department of Biological Sciences, Eastern Illinois University,
Charleston, IL 61920). Relative allelopathic potential of invasive plant species in a young disturbed
woodland. J. Torrey Bot. Soc. 137: 81–87. 2010—Invasive plant species are often more successful within
introduced areas when compared to their natural ranges. Allelopathy has been suggested as a potential
mechanism for this success because invasive plants frequently establish monocultures and may produce
allelochemicals evolutionarily novel to the recipient community. However, species are typically tested in
isolation making the relative strength of allelopathy difficult to assess. We conducted laboratory bioassays
for 10 co-occurring non-native species to determine the relative strength of their allelopathic potential. These
species represented a suite of successful invaders within a young forest and were from a variety of plant life
forms: trees, lianas, shrubs, and herbs. We determined the germination responses of a target species to a
gradient of leaf extract concentrations to assess relative allelopathic potential. The relative strength of
germination inhibition was quantified by the slope (b) of the germination response to plant extract
concentration. Ailanthus altissima extract had the greatest inhibitory effect on target species germination out
of all 10 species (b 520.55) while the other tree species, Acer platanoides extract, had small effects on
germination (b 520.14). For lianas, Celastrus orbiculatus extract inhibited the target species (b 520.28)
more than Lonicera japonica extract (b 520.06). All invasive shrub extracts had very small effects on seed
germination (b value 20.03 to 20.19). Extracts of the two herbaceous species, Alliaria petiolata and
Microstegium vimineum, had very large inhibitory effects (b 520.37 and 20.38, respectively). In this system,
we screened a suite of invasive species for allelopathic potential and determined the relative strength of
germination inhibition. Most species, particularly invasive shrubs, did not exhibit sufficient allelopathic
potential to suggest allelopathy would occur in the field. Four species, Ailanthus altissima, Alliaria petiolata,
C. orbiculatus,andM. vimineum all exhibited strong germination inhibition and warrant additional studyin
the field.
Key words: allelopathy, bioassay, comparative ecology, germination inhibition, invasive species.
Invasive plant species are responsible for
disrupting ecosystems and are of concern to
conservationists worldwide. Once an invader
enters a recipient community, it has the
potential to occupy large areas, posing numer-
ous problems for native flora and fauna.
Because of these problems, the determinants
of species invasiveness and the invasibility of
communities are vital for understanding the
ecology of non-native species (Crawley 1987,
Rejma´nek 1999, Mack et al. 2000, Ehrenfeld et
al. 2001, Byers et al. 2002, Huston 2004).
Several mechanisms have been proposed to
explain invasive plant species success within
introduced areas compared to their natural
ranges. Among these mechanisms, life-history
characteristics, physiological properties, rapid
changes in genetics, and escape from natural
enemies, may all contribute to the invasibility
of non-natives (Rejma´nek and Richardson
1995, Williamson 1996, Reichard and Hamil-
ton 1997, Ehrenfeld et al. 2001). More
recently, allelopathy has been suggested as a
potentially important mechanism of plant
invasion success, particularly when the invad-
ers produce evolutionarily novel chemicals
(Hierro and Callaway 2003, Inderjit et al.
2008).
Allelopathy has been broadly defined as the
production of chemical compounds by one
plant species that influence another plant
species, mostly in a deleterious way (Newman
1983, Rice 1984, Lawrence et al. 1991).
Allelopathic plants interfere with nearby
plants by dispersing chemicals into the soil
that may inhibit neighboring plant growth,
nutrient uptake, or germination (Inderjit and
Dakshini 1994, 1995, Inderjit 1996, Abhilasha
et al. 2008). Allelochemicals are released into
the environment via leachates and volatiles
from live or dead plant roots and leaves
(Jackson and Willemsen 1976, Rice 1984,
Inderjit and Dakshini 1994, 1995, Inderjit
1
This work was supported by NSF DEB
0424605.
2
We thank L. M. Ladwig for comments on
previous drafts of this manuscript. We also thank J.
Lyerla-Kirkton for help with laboratory bioassays.
3
Author for correspondence. E-mail: nlpisula@
eiu.edu
Received for publication July 14, 2009, and in
revised form December 18, 2009.
Journal of the Torrey Botanical Society 137(1), 2010, pp. 81–87
81
1996). In most cases, the release of these
chemicals results in more resources available
to the allelopathic plant for uptake, which
produces a net benefit. However, allelopathic
plants can also be affected by their own
chemicals (autoallelopathy), resulting in re-
duced growth (Kumari and Kohli 1987).
Because of observed depression of species
richness and failed recruitment in heavily
invaded areas, allelopathy is often invoked as
a potential mechanism of invasive plant
impact and success (Hierro and Callaway
2003, Prati and Bossdorf 2004, Abhilasha et
al. 2008, Inderjit et al. 2008). However, these
studies often focus on individual invaders and
employ a variety of methods. This variation
makes comparison among studies difficult and
assessment of the toxicity of a particular
invader relative to other species nearly im-
possible. We screened for the allelopathic
potential of a suite of 10 non-native woodland
invaders to assess the relative importance of
allelopathy as a potential determinant of
invasion success. By using standard methods
across a suite of species, we can compare the
relative strength of allelopathy and better
predict which invasive species may exhibit
allelopathy in the field.
Materials and Methods. The non-native
invasive species all co-occurred within the
young deciduous forests of the Hutcheson
Memorial Forest Center (HMFC) in the
Piedmont region of New Jersey, USA (40u309
N, 74u349 W). The research area was com-
posed of 10 agricultural fields that had been
abandoned 43–51 years prior. Successional
processes in these fields have been described in
detail from a series of permanent plots—the
Buell-Small Succession Study (Pickett 1982,
Myster and Pickett 1990, Meiners et al. 2002).
The species of focus were from a variety of
plant life forms (trees, lianas, shrubs, and
herbs) and represent all of the major forest
invaders at the site. The species were: trees
(Acer platanoides L. and Ailanthus altissima
Miller (Swingle)), lianas (Celastrus orbiculatus
Thunb. and Lonicera japonica Thunb.), shrubs
(Elaeagnus angustifolia L., Ligustrum vulgare
L., Rosa multiflora Thunb., and Rubus phoe-
nocolasius Maxim), and herbs (Alliaria petio-
lata (Bieb.) Cavara & Grande and Microste-
gium vimineum (Trin.) A. Camus). These
species are all natives of Eurasia and found
in disturbed woods, roadsides, and old fields
throughout the northeastern United States
(Gleason and Cronquist 1991, Rhoads and
Block 2000). The species are also a concern to
conservationists as they are able to occupy
large areas within many plant communities.
All of these species are considered invasive and
many have been shown to have direct negative
impacts on native communities (Fike and
Niering 1999, Martin 1999, Hunter and
Mattice 2002, Yurkonis et al. 2005, Morrison
et al. 2007, Go´ mez-Aparicio and Canham
2008).
L
ABORATORY
B
IOASSAYS
. The effect of po-
tentially allelopathic plants is often tested
through bioassays, typically by testing the
effects of plant tissue extracts on the germina-
tion of a target species. However, there are
difficulties in relating laboratory bioassays to
allelopathic interactions in the field related to
slow release, low concentrations, or degrada-
tion of the allelopathic agent (Gibson 2002). In
spite of these limitations, laboratory bioassays
are useful for screening allelopathic potential,
from which species can be selected for more
detailed field studies to verify allelopathy. We
employed a simple germination bioassay to
survey the invasive plant species of HMFC for
allelopathic potential.
Species were tested with laboratory bioas-
says following the methods of Butcko and
Jensen (2002). For each species, leaves from at
least 20 plants were collected from HMFC and
dried at 60 uC for two days. Leaf tissue was
used as it can disperse a greater distance than
material from roots and often contains allelo-
chemicals (Butcko and Jensen 2002). Extracts
were made from 12.5 g of dried leaf tissue in
500 ml of deionized water (Butcko and Jensen
2002). This ratio of biomass to water generates
plant extracts that affect germination of target
species and allows for differentiation among
allelopathic species (Butcko and Jensen 2002,
Pisula and Meiners 2010). Leaves were not
ground as doing so results in the release of
enzymes, salts, amino acids, and other com-
pounds that may not be released under natural
circumstances (Chou and Muller 1972, Inderjit
and Dakshini 1995). The mixture was placed
on a magnetic stirrer for 24 hours at room
temperature and was strained through cheese-
cloth to remove particulate plant material.
Dilutions of each extract, ranging from 0% to
100% in 10% increments, were made. Filter
paper was placed in 90 mm Petri plates with 20
82
JOURNAL OF THE TORREY BOTANICAL SOCIETY [V
OL
.137
seeds of the target species. We used radish
(Raphanus sativus L. ‘Early Scarlet Globe’;
Bay Farm Services, Inc., Bay City, MI) as the
target species in all trials. We used this species
to indicate allelopathic potential because it
germinates quickly, is commonly used in
allelopathic studies, is sensitive to allelopathic
inhibition, and detects differences among
species extracts (Butcko and Jensen 2002,
Pisula and Meiners 2010).
Five trials were run at each dilution for each
species tested. Four ml of extract was added to
each plate and incubated at 25 uC for a 12/12 h
light/dark cycle. Petri plates were placed in
bags to retain moisture. The plates were
removed after four days and germinated seeds
were counted. An analysis of covariance
(ANCOVA) was used to measure the overall
effect of plant extract concentration and
species identity on germination (SAS 9.1;
SAS Institute Inc., Cary, NC). Individual
linear regressions of percentage germination
as a function of extract concentration were
conducted for each species. Coefficients (b
values) from these regressions were used to
compare the relative strength of plant extracts
on seed germination. We used t-tests to
identify the threshold at which effects ap-
peared relative to the control (SAS 9.1 SAS
Institute Inc., Cary, NC). Due to limited
replication at each concentration for threshold
determination we chose to focus on regression
b values as a means of comparing species.
Results. Inhibition of germination in target
seeds was influenced by the concentration of
extract and the interaction of concentration of
extract and species (Table 1, Fig. 1). However,
species identity by itself was not significant in
the ANCOVA. Regression analyses of indivi-
dual species showed marked variation (Ta-
ble 2). Plant extracts reduced target seed
germination (P , 0.05) for all but one species
(Elaeagnus angustifolia). Ailanthus altissima
extracts had the greatest inhibitory effect
(based on beta values) out of all 10 species
while extracts from Acer platanoides, the other
tree species, had small effects on germination.
For the two lianas, Celastrus orbiculatus
inhibited target seeds (b 520.28), more than
Lonicera japonica (b 520.06; Table 2). Shrub
extracts from Ligustrum vulgare, Rosa multi-
flora and Rubus phoenocolasius had uniformly
small inhibitory effects on seed germination,
even at higher concentrations. Extracts of the
fourth shrub species, E. angustifolia,hadno
significant effect on target seed germination.
Extracts of the herbaceous invaders Alliaria
petiolata and Microstegium vimineum had
large inhibitory effects on target species
germination, only exceeded by the tree A.
altissima. Threshold concentrations varied
from 40% to 80% of full extract concentration
and did not correspond to the rankings
determined through regression analyses.
Table 1. Effects of species identity (categorical
variable) and extract concentration (continuous
variable) on target species germination. Overall
ANCOVA F
19,530
5 43.16; P , 0.0001; R
2
5 0.61.
Bolding indicates a significant P-value.
Factor df MS FP
Species 9 78.52 1.01 0.4281
Concentration 1 30333.68 391.39 , 0.0001
Species 3
concentration 9 1422.51 18.35 , 0.0001
Error 530 77.50
Table 2. Regression coefficients (b 6 standard error) of the relationship between plant extract
concentration and percent germination to quantify the strength of allelopathy for each invasive species.
Threshold concentration refers to the lowest concentration at which germination was significantly decreased
compared to the control using a t-test. Ranking is based on beta values: a ranking of 10 is the most toxic and
1 is the least toxic species. Bolding indicates a significant P-value.
Species Rank
Threshold
concentration (%) b
Standard
error F
1,53
R
2
P
Ailanthus altissima 10 60 20.55 0.05 121.18 0.70 , 0.0001
Microstegium vimineum 97020.38 0.04 72.66 0.58 , 0.0001
Alliaria petiolata 84020.37 0.05 62.44 0.54 , 0.0001
Celastrus orbiculatus 77020.28 0.04 50.42 0.49 , 0.0001
Ligustrum vulgare 66020.19 0.04 22.05 0.30 , 0.0001
Rosa multiflora 55020.18 0.03 37.72 0.46 , 0.0001
Rubus phoenocolasius 47020.16 0.03 21.98 0.29 , 0.0001
Acer platanoides 38020.14 0.03 25.38 0.32 , 0.0001
Lonicera japonica 27020.06 0.03 4.82 0.08 0.0326
Elaeagnus angustifolia 1—20.03 0.02 1.53 0.03 0.2216
2010]
PISULA AND MEINERS: ALLELOPATHIC POTENTIAL OF INVASIVE PLANTS
83
Within plant life forms, shrubs and herbs
had consistent levels of germination inhibition.
Shrubs were all weakly allelopathic or had no
effects while both herbaceous invaders had
strong allelopathic potential. In contrast, liana
and tree species had a mix of strong and weak
effects on germination.
Discussion. In general, our results have
confirmed the allelopathic potential previously
found through varying methods for some of
the species tested. In individual studies,
bioassays of Ailanthus altissima (Lawrence et
al. 1991, Go´ mez-Aparicio and Canham 2008)
and Alliaria petiolata (Hanson and McCarthy
1998, Prati and Bossdorf 2004, Stinson et al.
2006, Cipollini and McClain 2008) revealed
that they exhibited allelopathy in the lab or
field. While these studies documented the
potential of allelopathy for individual species,
our study allows for direct comparison of
species allelopathic potential as the same
methods were employed for all species in a
controlled setting. Ranking a large number of
species based on allelopathic potential is a
need for invasion centered studies, as predict-
ing which species will be allelopathic in the
field has been difficult to judge in the past. As
more species are tested, the range of allelo-
pathic potential will be refined and it will
become easier to assess whether a plant is
likely to exhibit strong allelopathic properties.
Our laboratory bioassays confirmed the
presence of some allelopathic capability for
all but one of the invasive specie’s extract
tested (Elaeagnus angustifolia). However, the
response to plant extract concentration was
not consistent within all life forms. The tree
species, Ailanthus altissima, was the most
inhibitory overall while the other invasive tree,
Acer platanoides, reduced germination much
less. Similarly, the two lianas differed in their
allelopathic potential with Celastrus orbicula-
tus having a much greater effect on seed
germination than Lonicera japonica.Incon-
trast to these life forms, extracts of herbaceous
and shrub species were largely consistent in
their effects on target species germination.
Herbaceous species consistently and strongly
inhibited germination, whereas shrubs had a
uniformly small effect (or no effect) on the
germination of target seeds. Though based on
a few species, this variation within life forms
suggests limited utility of plant life form in
predicting the presence of allelopathy.
Allelopathy is considered to be a competi-
tive plant strategy as it allows for greater
acquisition of resources (Grime 2001). For this
reason, allelopathy may be more important in
certain species rather than vary consistently
across life forms. The invasive herbs Alliaria
petiolata and Microstegium vimineum spread
rapidly in forest understories in a variety of
light conditions (Morrison et al. 2007) and
appear to be strong competitors relative to
many native understory herbs (Carlson and
Gorchov 2004, Stinson et al. 2006, Morrison
et al. 2007, Oswalt et al. 2007, but see Meekins
and McCarthy 1999). Likewise, allelopathy
may enhance the competitive success of the
weedy and shade intolerant tree, Ailanthus
altissima, which often regenerates in large
canopy openings where competition may be
intense. In contrast, the shade tolerance of
Acer platanoides (Martin 1999), a stress
tolerant strategy in Grime’s C-S-R scheme
(Grime 2001), would minimize the importance
of allelopathy for the regeneration of this
species. However, the allelopathic potential of
shrubs does not appear to follow the expecta-
tions of Grime’s plant strategies as the
allelopathic potential of shrubs was uniformly
low. The aggressive nature of these shrubs in
abandoned agricultural land, particularly
Rosa multiflora and Elaeagnus angustifolia,
would suggest that they employ a competitive
plant strategy and might have been expected
to be strongly allelopathic.
If allelopathy is a broad mechanism con-
tributing to species invasibility, we would
expect native species on average to be less
toxic than non-native species within the same
community. However, the range of allelo-
pathic potential for the invasive species tested
here is similar to that documented for some
native species from the same site. For example,
native goldenrod (Solidago) species are often
linked with allelopathy (Abhilasha et al. 2008,
Pisula and Meiners 2010). Using the same
method as this study, three out of six gold-
enrod species had beta values that were within
the range found in this survey of non-natives
(Pisula and Meiners 2010). The remaining
goldenrod species were more toxic than any of
the invasive species tested (b values ranging
from 20.59 to 20.74, Pisula and Meiners
2010). These results are important as gold-
enrods are considered invasive in Europe
(Weber 2001, Abhilasha et al. 2008) and
allelopathy has been invoked to explain their
84
JOURNAL OF THE TORREY BOTANICAL SOCIETY [V
OL
.137
success in displacing native European flora.
While allelopathy can clearly be important in
some non-native plant invasions, it also
appears to be a common component of native
plant strategies at this site.
Moreover, the evolutionary experience of
resident plants to the chemicals produced by
invasive species may also be important in
determining responses to allelopathy (Abhila-
sha et al. 2008, Thorpe et al. 2009). The novel
weapons hypothesis suggests that native spe-
cies that lack evolutionary exposure to a non-
native plant’s allelochemicals should be more
affected than plants from the native range of
the invader (Callaway et al. 2008, Thorpe et al.
2009). While this effect may be important in
invaded systems, we did not use both native
and non-native target species and therefore
cannot address the novel weapons hypothesis.
The role of evolutionary exposure in determin-
ing allelopathic responses needs to be assessed
in a broader range of species.
The bioassay data presented here represent
a preliminary screening of invasive species for
their allelopathic potential only. From these
data, species with sufficiently large inhibitory
effects on germination may be selected for
more detailed analyses in the field or in the
laboratory. Allelochemicals in the field are
likely released at different rates and may be
lost from the soil through degradation, leach-
ing, or binding with other chemical com-
pounds (Inderjit and Dakshini 1995, Gibson
2002, Pisula and Meiners 2010). The balance
between release and loss will determine the
concentration, and therefore the efficacy, of
allelochemicals within plant communities.
Determining the persistence and concentration
F
IG
. 1. Response of radish to plant extracts from 10 co-occurring invasive species. Data plotted are
mean and standard error of the proportion germinated at extract concentrations ranging from 0–100%. N 5
5 for each species at each concentration.
2010]
PISULA AND MEINERS: ALLELOPATHIC POTENTIAL OF INVASIVE PLANTS
85
of allelochemicals is an important, but rarely
done step as the identity of active chemicals is
not always known. Of the species tested here,
only Alliaria petiolata has been sufficiently
characterized (Barto and Cipollini 2009).
Alternatives to detailed chemical analyses of
putative allelopathic invaders include germi-
nation bioassays of co-occurring native species
and the use of activated carbon or plant
leachate additions in greenhouse and field
experiments (Cipollini and McClain 2008,
Go´mez-Aparicio and Canham 2008). All of
these approaches can help to evaluate impor-
tance of allelopathy in species invasion.
Conclusions. While allelopathy has been
implicated in many invasion case studies, we
still lack a clear understanding of its preva-
lence and importance in plant communities.
The majority of species we tested showed some
allelopathic potential. However, there was no
clear pattern among life forms that would
allow a prediction of which invasive species
should be expected to exhibit allelopathy. We
encourage the use of this or similar methods to
compare species’ relative allelopathic potential
to screen and prioritize invasive species for
further study. Four species, Ailanthus altis-
sima, Alliaria petiolata, Celastrus orbiculatus,
and Microstegium vimineum all exhibited
strong germination inhibition and warrant
additional study in the field. A broad suite of
native species should also be tested to deter-
mine the importance of allelopathy in struc-
turing native communities and to develop
predictions of which species have a strong
potential for allelopathy. Comparative studies
provide a critical tool that will allow us to
assess the importance of allelopathy and its
effects on population and community dy-
namics.
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2010]
PISULA AND MEINERS: ALLELOPATHIC POTENTIAL OF INVASIVE PLANTS
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... Weed seedlings die in contact with "herbicidal film" in the soil, before or soon after emergence (Ostojić and Šilješ, 1982). Very strong inhibitory effect on radicle and shoot length of testspecies was consistent with the results of De Feo et al. (2003) and Pisula and Meiners (2010). These authors also pointed to the tree of heaven as a plant with very strong allelopathic potential, among 10 studied plant species. ...
... The inhibitory effect on radicle and shoot length for all 3 test-species was proportional to concentration increase in all treatments. The association of ailanthone concentration and allelopathic effect on susceptible test-species are consistent with many authors who state that different ailanthone content is directly related to the inhibitory effect on investigated properties of test-species (Heisey, 1996;Pisula and Meiners, 2010;Pedersini et al., 2011;Novak, 2017;Novak et al., 2018). with results from other authors is difficult for the reasons already mentioned, but they are consistent with the theory presented by Heisey (2003) andDe Feo et al. (2003). ...
Article
The objective of this paper was to analyse allelopathic effect of 3 different aqueous solutions from tree of heaven’s (Ailanthus altissima (Mill.) Swingle) root extract and their dilutions on common wheat (Triticum aestivum L.), pigweed (Amaranthus retroflexus L.) and red bristlegrass (Setaria pumila L.). Investigated aqueous solutions obtained from tree of heaven’s root extract were root aqueous solution, aqueous solution with isolated ailanthone and root aqueous solution without ailanthone. Each of these solutions was diluted with water in ratio 1:4 and 1:16 before application on seeds of 3 test-plant species. Concentrated root and ailanthone aqueous solution and dilutions were equivalent to concentration of 0.48 mg/mL ailanthone. High allelopathic effect on radicle and shoot length of all test-species was proven for all investigated aqueous solution and their dilutions. Inhibitory effect on initial growth of all test-species was proportional to the increase in concentration in all treatments. The effect on initial growth was stronger in relation to the effect on germination of test-species. Lower concentrations caused stimulation of red bristlegrass germination. Pigweed was the most sensitive and common wheat the least sensitive test-species. The least significant effect was measured in the aqueous solution of isolated ailanthone.
... One definite consequence of increasing rates and volumes of such biotic exchange is the co-occurrence of multiple invasive species across different habitats (Kuebbing et al., 2013). Yet, research has mainly focused on the effects of single, individual species (Hulme et al., 2013;Kuebbing et al., 2013;Stricker et al., 2015;D'Antonio et al., 2017;Tekiela and Barney, 2017), although multispecies invasion is potentially more detrimental to ecosystems compared to single species invasion (Simberloff and Von Holle, 1999;Inderjit et al., 2005;Pisula and Meiners, 2010). Such a knowledge gap impairs our ability to understand the mechanisms underlying the potentially amplifying effects of multiple invasive species. ...
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Despite increasing evidence indicating that invasive species are harming biodiversity, ecological systems and processes, impacts of multiple species invasion and their links with changes in plant and soil communities are inadequately documented and remain poorly understood. Addressing multiple invaders would help to ward against community-wide, synergistic effects, aiding in designing more effective control strategies. In this work, correlative relationships are examined for potential impacts of three co-occurring invasive plant species, Amorpha fruticosa, Fraxinus pennsylvanica, and Acer negundo, on soil conditions and native plant diversity. The research was conducted in riparian ecosystems and included the following treatments: (1) co-occurrence of the three invasive plant species, (2) occurrence of a single invasive species, and (3) control, i.e., absence of invasive species. Co-occurrence of three invasive plant species caused higher direct impact on soil properties, soil functioning, and native plant diversity. Soil in mixed plots (those populated with all three invaders) contained higher levels of nitrifying bacteria, organic matter, nitrogen, and carbon as well as lower carbon to nitrogen ratio as compared to single species invaded plots and control plots. Furthermore, native plant diversity decreased with invasive plants co-occurrence. Differences in soil conditions and lower native plant diversity revealed the interactive potential of multiple invasive species in depleting biodiversity and eroding soil functionality, ultimately affecting ecological and biogeochemical processes both below and above ground. Our results highlight the need to prevent the impact of multispecies invasion, suggesting that riparian ecosystems affected by co-occurring invaders should be prioritized for invasion monitoring and ecological restoration.
... Garlic mustard has a unique phytochemical profile compared with native North American mustard plants ( Barto et al. 2010, Frisch et al. 2014, which suggests that the novel weapons hypothesis may play a role in garlic mustard success (Callaway and Ridenour 2007). The secondary compounds in garlic mustard have been identified and widely studied, and are known to have variable efficacy as allelopathic chemicals (Pisula andMeiners 2010, Cipollini andBohrer 2016). In particular, the degradation of sinigrin glucosinolate results in the allelopathic allyl isothiocyanate, which accounts for nearly half of the volatile contents of fresh garlic mustard leaves (Blažvić and Mastelić 2008). ...
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The invasive plant Alliaria petiolata (garlic mustard) has spread throughout forest understory and edge communities in much of North America, but its persistence, density, and impacts have varied across sites and time. Surveying the literature since 2008, we evaluated both previously proposed and new mechanisms for garlic mustard's invasion success and note how they interact and vary across ecological contexts. We analyzed how and where garlic mustard has been studied and found a lack of multisite and longitudinal studies, as well as regions that may be under- or overstudied, leading to poor representation for understanding and predicting future invasion dynamics. Inconsistencies in how sampling units are scaled and defined can also hamper our understanding of invasive species. We present new conceptual models for garlic mustard invasion from a macrosystems perspective, emphasizing the importance of synergies and feedbacks among mechanisms across spatial and temporal scales to produce variable ecological contexts.
... As invasive plant species become dominant in an ecosystem, they can dramatically reduce the abundance and diversity of native species (Aguilera et al. 2010;Aronson and Handel 2011;Powell et al. 2013), disrupting ecosystem functions and structure (Pritekel et al. 2006;Hejda 2009). Many invasive plant species actively restrict the germination and growth of their native competitors by secreting allelopathic substances into the soil (Hierro and Callaway 2003;Pisula and Meiners 2010;Bauer et al. 2012;Cipollini et al. 2012;Del Fabbro and Prati 2015). Furthermore, the competitive adaptations of invasive species frequently render them difficult to remove from an ecosystem once they have become established (Simberloff 2003). ...
... They are complex mixtures of monoterpenes, sesquiterpenes and aromatic phenols, oxides, ethers, alcohols, esters, aldehydes and ketones. The main terpenoids of volatile essential oils are monoterpenes and sesquiterpenes (79). Some constituents of essential oils (i.e. ...
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Herbicides are a crucial tool for weed control in crops. However, their continuous and indiscriminate uses have caused environmental pollution and development of weed resistance. Hence, there is an urgent need to minimise the dependence on synthetic herbicides. Allelopathy may be used to develop new bioherbicides to inhibit germination and growth of weeds. There is a great potential to develop eco-friendly herbicides from plants, but little research has been done so far in this context. Identification and quantification of natural weed control compounds (allelochemicals) of plants may help to develop natural herbicides (based on indigenous/invasive weed species), to reduce the dependence on synthetic herbicides and improve the integrated weed management programme in crops. This review describes thepotential allelochemicals present in plants, which may be used as a tool to develop new natural herbicides.
... Multispecies invasion has been identified in the vast majority of habitats (Kuebbing et al., 2013). Although multispecies invasion is potentially more detrimental to ecosystems compared to single species invasion (Simberloff and Von Holle, 1999;Inderjit et al., 2005;Pisula and Meiners, 2010), extant research on the ecological impact of invaders has primarily focused on the effects of individual invaders (Hulme et al., 2013;Kuebbing et al., 2013;D'Antonio et al., 2017;Tekiela and Barney, 2017), with studies on woody invaders being relatively scarce (Stricker et al., 2015). Co-occurrence of invasive plants can be explained by the very same introduction pathway, or by interspecific facilitation, whereby one invader facilitates the establishment and spread of another (Simberloff and Von Holle, 1999;Richardson et al., 2000;Flory and Bauer, 2014;Kuebbing and Nuñez, 2016;Zhang et al., 2020). ...
Preprint
Full-text available
Despite increasing evidence indicating that invasive species are harming ecological systems and processes, impacts of multiple invasions, and the linkages between these events and changes in vegetation and soil are inadequately documented and remain poorly understood. Addressing multiple invasions would help to highlight high priority invaders and would aid in designing more effective control strategies, contributing to environmental restoration and sustainability. In this work, we tested the impact of three concurring invasive plant species, Amorpha fruticosa, Fraxinus pennsylvanica and Acer negundo , on soil conditions and native plant diversity. The research was conducted in riparian ecosystem and included the following treatments: (1) co-occurrence of the three invasive plant species, (2) occurrence of a single invasive species, and (3) control, i.e., absence of invasive species. Our findings revealed that the impact of invasive plants on soil properties and native plant diversity is magnified by their co-occurrence. Soil in mixed plots (those populated with all three invaders) contained much higher levels of nitrifying bacteria (NB), organic matter (Om), nitrogen (N), and carbon (C) as well as lower carbon to nitrogen ratio (C:N) levels, compared to single species invaded plots and control plots. Mixed plots were also characterized by reduced native plant diversity compared to single species invaded and control plots. Differences in soil conditions and native plant diversity revealed the interactive potential of invasive plants in depleting biodiversity, and thus in affecting ecological and biogeochemical processes. Our results highlight the need to study the impact of multispecies invasion and suggest that sites in riparian areas affected by co-occurring invaders, should be prioritized for ecosystem restoration.
... Allelopathic compounds do not always interfere in a significative way with all species and variables analyzed [61,62], which explains why each germination variable (GP, GSI, and AGT) shows a meaningful difference in one species only. Ferreira and Borguetti (2004) [60] state that germination is less sensitive to allelochemicals, while length is the variable which best expresses the effects of allelochemical compounds on seedlings, for they are sensitive to toxicity and stay in direct contact with extracts. ...
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Full-text available
Allelopathic potential, which is characterized by harmful or beneficial effects on a species, is a hypothesis that could explain the success of a vegetable species in process of invasion. For that reason, this study aims to investigate the allelopathic effect of the fresh leaf extract from Azadirachta indica on the germination and early growth of native species (Mimosa caesalpiniifolia, Pityrocarpa moniliformis, Astronium urundeuva, Cenostigma pyramidale, and Libidibia ferrea) in the seasonally dry tropical forest (caatinga). The raw extract was prepared with 200g of fresh leaves in 1L of distilled water, diluted in six treatments. The experiment followed a completely randomized design (CRD), with four repetitions of 25 seeds, and paper towel moistened with extract as substrate. The number of germinated seeds was counted daily. At the end of the experiment, radicle length (RL), germination percentage (GP), germination speed index (GSI), and average germination time (AGT) were evaluated. P. stipulacea’s GP and GSI averages and A. urundeuva’s AGT decreased as extract concentration increased. As far as RL is concerned, in intermediate concentrations the extract hindered P. moniliformis and L. ferrea’s growth but favored C. pyramidale and A. urundeuva’s. Results allow us to infer that A. indica has an allelopathic effect on native species and might interfere with their rooting in the field, which in turn would affect the succession and balance of the forest over time, since the community structure and the ecosystemic processes would be impacted.
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Abstract Plant litter can alter ecosystems and promote plant invasions by altering resource availability, depositing phytotoxins, and transmitting microorganisms to living plants. Transmission of microorganisms from invasive plant litter to live plants may gain importance as invasive plants, which often escape pathogens upon introduction to a new range, acquire new pathogens over time. It is unclear, however, if invasive plant litter affects native plant communities by promoting disease. Microstegium vimineum is an invasive grass that suppresses native populations, in part through litter production, and has acquired new fungal leaf spot diseases since its introduction to the United States. In a greenhouse experiment, we evaluated how M. vimineum litter and its pathogens mediated competition with the native grass Elymus virginicus. M. vimineum litter promoted disease on E. virginicus and suppressed establishment and biomass of both species. Litter had stronger negative effects on E. virginicus than M. vimineum, increasing the relative biomass of M. vimineum. Live plant competition reduced biomass of both species and live M. vimineum increased disease incidence on E. virginicus. Altogether, invasive grass litter suppressed both species, ultimately favoring the invasive species in competition, and increased disease incidence on the native species.
Article
Many invasive species show allelopathic effects that contribute to competitive impacts on native vegetation for which few control measures exist. We investigated the potential for pyrolized organic material used as a soil amendment (“biochar”) to sorb allelochemicals and mitigate allelopathic effects on seed germination and early seedling development in three common invasive plants in Canada, garlic mustard (Alliaria petiolata), tree of heaven (Ailanthus altissima) and yellow sweetgrass (Melilotus officinalis). We hypothesized that biochars would mitigate effects on germination and early seedling development (radicle extension and cotyledon development) through sorption of allelochemicals. Laboratory assays of seed germination and early seedling development of two agricultural crops (Lactuca sativa and Raphinus raphanistrum) and two native grass species (Andropogon gerardi and Poa palustris) were conducted using water extracts from leaves. Seeds were treated with plant extracts exposed to four different biochars (red oak (Quercus rubra), jack pine (Pinus banksiana), shipping pallet and construction waste, and, high-carbon wood ash) using a range of extract and biochar dosages. Treatment of allelopathic plant extracts by biochars significantly (p <0.05) mitigated effects on seed germination for Alliaria and Melilotus. Effects on seed germination and early seedling development depended on extract concentration, biochar dosage, and the target species assessed. Controls treated with biochar leachate alone showed significantly (p <0.05) increased germination for two of the biochars tested (jack pine biochar and high-carbon wood ash). Results indicated that biochars can mitigate allelopathic effects of invasive species through sorption of allelochemicals, however this will be effective in only some cases.
Article
Fragmented suburban forests of the northeastern US are challenged by abundant white-tailed deer and nonindigenous plant invasions. Deer browsing/grazing pressure varies among sites, potentially affecting herbivory on nonindigenous plants and their invasion success. We aimed to identify a useful deer pressure indicator for suburban forests and then use it to relate deer pressure to grazing on and abundance of two herbaceous invaders, Microstegum vimineum and Alliaria petiolata. We compared three indicators: fecal pellet accumulation rate, deer browse on indigenous woody plants, and indigenous shrub layer cover. The pellet method produced estimates generally far below the region’s known deer density. Browse rates and shrub layer cover were negatively correlated, and correlations of the three indicators with evidence of deer pressure from a subsequent 6.5-year exclosure experiment supported shrub layer cover as the better choice. Using that measure in 10 forests, we detected a weak pattern of more grazed stands under greater deer pressure, but few plants per stand were grazed; any negative influence of deer on these species was limited to individuals, without population effects. Alliaria petiolata abundance was unrelated to deer pressure, but M. vimineum abundance was greater in forests with more deer pressure, suggesting facilitation of its invasion.
Article
Compounds inhibitory to the growth of neighboring plant species were found in significant concentrations in the leaves and stems of young Ailanthus altissima ramets. The surrounding soil also contained appreciable concentrations of similarly acting toxins. Individuals of neighboring plant species have either incorporated active portions of inhibitory compounds or responded to Ailanthus by producing growth-inhibiting substances. Under greenhouse conditions, individuals of neighboring plant species previously unexposed to Ailanthus in the field were found to be more susceptible to the Ailanthus toxins than individuals previously exposed. Moreover, seeds produced by unexposed populations were also more susceptible to Ailanthus toxins than seeds produced by previously exposed populations. These differences demonstrated that the allelochemicals of Ailanthus altissima exhibited a measurable impact upon neighboring plant species. Since the progeny of these populations displayed a differential response to Ailanthus toxin, this phenotypic difference between the two populations may have a heritable basis.
Article
Ambrosia artemisiifolia (ragweed) is a dominant species in the first year of old field succession but rarely persists for more than two years. Ragweed and Raphanus raphanistrum (wild radish), also an early invader, failed to become re-established in plots cleared of second stage perennial vegetation (dominated by Aster pilosus), despite the large number of seeds of these primary invaders present in the soil. Edaphic experiments revealed that this pattern of succession was not due to mineral or physical properties of the soil. Field soil from the second stage of succession inhibited the growth and germination of ragweed and wild radish while soil from the first stage had no effect. Inhibitory volatile materials from ragweed were not detected. However, root exudate of ragweed, and shoot extracts of ragweed and aster inhibited the germination and growth of early invaders of abandoned fields. These results indicate that the vegetational change from the first to the second stage of succession may be mediated at least partially by an allelopathic response. Chromatography and bioassay techniques revealed the inhibitory compounds to be phenolic acids, including caffeic and chlorogenic acids.
Article
Alliaria petiolata (M. Bieb.) Cavara and Grande (garlic mustard) is a non-indigenous member of the Brassicaceae that is invading woodlands throughout eastern North America. Previous work has demonstrated that this species is having a negative effect on the diversity of understory communities and is actively displacing native species. The purpose of this study was to evaluate the extent to which allelopathy might be acting as the mechanism of interference. Extracts of garlic mustard root and shoot tissues were applied to seeds and seedlings of four target species used as bioassays: radish, winter rye, vetch, and lettuce. While seed germination rates varied by species and extract concentration, total germination after 5-7 d was largely unaffected by any extract concentration. Only radish seeds treated with the most concentrated root solution exhibited a depressed germination relative to the water control. Likewise, seedling biomass was generally unaffected by any extract treatment. Only shoot biomass for rye was significantly depressed with the highest concentration of leaf extract. Our data provide little evidence that allelopathy in involved in the invasive success or community interference of this non-indigenous species, even though the Brassicaceae are well known to possess potentially biological active compounds with alleleopathic effects.
Article
Arctostaphylos glandulosa var. zacaensis is a strong dominant of chaparral vegetation on Zaca Ridge in the San Rafael Mountains, California. It occurs in pure stands, sometimes openly spaced with the canopies covering 50 to 75% of the soil surface, or it may form completely closed cover. A barren soil surface, without seedlings of any species in spite of sufficient light and soil moisture, characterizes the stands. Fire or artificial removal of shrubs results in growth of herbs and shrub seedlings until shrub dominance is again established. Detrimental biochemical interference (allelopathy) between Arctostaphylos and associated plants is involved in this phenomenon. Aqueous leachate from Arctostaphylos foliar branches and from leaf litter is toxic to the growth of annual grasses. The allelopathic constituents leachable from Arctostaphylos plant organs have been identified as arbutin, hydroquinone, and gallic, protocatechuic, tannic, vanillic, chlorogenic and p-hydroxybenzoic acids. Toxins such as p-hydroxybenzoic, p-coumaric, ferulic, syringic and o-coumaric acids are found in the soils beneath Arctostaphylos shrubs. These toxins can inhibit the germination and suppress the radicle growth of herbaceous plants at concentrations below 400 ppm; e.g., hydroquinone significantly inhibits radicle growth of Avena fatua and Bromus rigidus at a concentration of 50 ppm. Chemical constituents present in leaf litter appear to be more leachable after subjection to temperatures up to 160 C-resulting in increases of toxicity to herb growth-but are denatured at temperatures above 180 C, with complete loss of toxicity at temperatures above 200 C. Allelopathy apparently plays a particularly important role in the fire cycle of chaparral vegetation.
Article
This study documents changes in the distribution of non-native woody species from 1938 to 1999 within 30 forests of a $54\>km^2$ landscape in Monroe County, New York. Within these forests, the mean number of exotic species increased nearly three-fold from 1938 to 1999, and two species not naturalized within the landscape in 1938, Lonicera morrowii and Rosa multiflora, had become widespread by 1999. In 1999, the most abundant exotic was Lonicera morrowii, which had greater cover within forests on wetter portions of the landscape where much of the surrounding agricultural land had been abandoned. Though exotics accounted for < 10 % of relative cover within the shrub layer and < 1 % of the tree layer, their cover was higher in portions of some forests, especially near edges. In 1999, six species had covers > 25 % within patches $> 100\>m^2$: Acer platanoides, Crataegus monogyna, Ligustrum vulgare, Lonicera morrowii, Robinia pseudoacacia and Rosa multiflora. These species in particular may represent on-going invasions that could alter this landscape's forested habitats.
Article
We examined the PCA ordination scores of 10 old fields through 22 yr of succession. Clonal species Dactylis glomerata and Hieracium pratense had the largest correlation with the first principal component in the 1st 18 yr of succession. After 18 yr, Rosa multiflora and Toxicodendron radicans had the largest correlation with the first principal component. Successional pathways of the 10 old fields show great variation in early succession but variation of pathway decreases later. Comparison with a null model reveals that the trend of increasing similarity of old fields through time is real and convergence is occurring. Because mechanisms causing convergence might include the effect of initial abandonment conditions, we divided the 10 old fields into groups defined by the last crop, season of abandonment, year of abandonment, and mode of abandonment (plowed or not). Analysis of the minimum spanning tree needed to connect the old fields of the same age in ordination space showed that the greatest difference between groups was in the 1st 8 yr between old fields that had Dactylis glomerata (orchard grass) as a last crop and those that did not. Of common abandonment conditions, last crop greatly influences successional pathways and convergence in these old fields.