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Leaf water extracts from invasive Acer negundo do not inhibit seed germination more than leaf extracts from native species

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This study was undertaken to test the hypothesis about the allelopathic activity of the alien (invasive) tree species Acer negundo in Eurasia compare with native tree species. Research of allelopathic effects of invasive plants is important for its management because of their influence on native communities. Two experiments in Petri dishes were conducted. The effect of water extracts from leaves on the seed germination of herbaceous plants was assessed. Leaves were collected in the summer and autumn season in areas invaded by A. negundo in Yekaterinburg, Russian Federation. Four treatments (invasive A. negundo and native tree species Sorbus aucuparia, Prunus padus, and Salix caprea) were tested on seeds of three recipient plant species (Festuca rubra, Sinapis alba, and Trifolium repens). We found that water extracts from A. negundo leaves weakly inhibit seed germination compared to distilled water. However, the inhibitory effect of A. negundo was no greater than effects of extracts from leaves of native P. padus and S. caprea. Seed germination was most strongly inhibited with extracts from a native shrub S. aucuparia, and the delay in comparison with distilled water was 1–2 days. Therefore, in Petri dishes water extracts from leaves of A. negundo do not inhibit seed germination of test plants more than native tree species. Our data do not support a hypothesis that allelopathy can explain the ability of A. negundo to influence native communities.
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Management of Biological Invasions (2022) Volume 13, Issue 4: 705–7
23
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 705
CORRECTED PROOF
Research Article
Leaf water extracts from invasive Acer negundo do not inhibit seed germination
more than leaf extracts from native species
Olesya S. Rafikova* and Denis V. Veselkin
Institute of Plant and Animal Ecology, Ural Branch of Russian Academy of Sciences, 8 Marta St. 202, 620144, Yekaterinburg, Russia
*Corresponding author
E-mail: rafikova_os@mail.ru
Abstract
This study was undertaken to test the hypothesis about the allelopathic activity of
the alien (invasive) tree species Acer negundo in Eurasia compare with native tree
species. Research of allelopathic effects of invasive plants is important for its
management because of their influence on native communities. Two experiments in
Petri dishes were conducted. The effect of water extracts from leaves on the seed
germination of herbaceous plants was assessed. Leaves were collected in the summer
and autumn season in areas invaded by A. negundo in Yekaterinburg, Russian
Federation. Four treatments (invasive A. negundo and native tree species Sorbus
aucuparia, Prunus padus, and Salix caprea) were tested on seeds of three recipient
plant species (Festuca rubra, Sinapis alba, and Trifolium repens). We found that
water extracts from A. negundo leaves weakly inhibit seed germination compared
to distilled water. However, the inhibitory effect of A. negundo was no greater than
effects of extracts from leaves of native P. padus and S. caprea. Seed germination
was most strongly inhibited with extracts from a native shrub S. aucuparia, and the
delay in comparison with distilled water was 12 days. Therefore, in Petri dishes
water extracts from leaves of A. negundo do not inhibit seed germination of test
plants more than native tree species. Our data do not support a hypothesis that
allelopathy can explain the ability of A. negundo to influence native communities.
Key words: allelopathy, non-native plants, plant invasion, inhibitory effect, invasion
success
Introduction
Plant invasions result from multiple mechanisms. Major ecological invasion
hypotheses are “enemy release”, “EICA” (the evolution of increased
competitive ability), “novel weapons”, “naive prey”, “new associations”,
“missed mutualisms” and “Darwin’s naturalization hypothesis” (Catford et
al. 2009; Saul et al. 2013). For example, according to the Novel Weapons
Hypothesis (NWH) some invasive species release unique chemical compounds
which are new to the invaded community. Releasing of compounds which
native species are not adapted to can give a competitive advantage to non-
native plants (Callaway and Ridenour 2004; Weidenhamer and Callaway
2010). In terms of the Novel Weapons Hypothesis allelopathy is studied as
one of the mechanisms of invasive success for plants in their non-native
22) Leaf water extracts from invasive
do not inhibit seed
. Management of Biological
13(4): 705723, https://doi.org/10.
2 February 2022
15 September 2022
24 October 2022
Ileana Herrera
Catherine Jarnevich
© Rafikova and Veselkin
Attribution 4.0 International - CC BY 4.0).
OPEN ACCESS.
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 706
range (Klironomos 2002; Callaway and Ridenour 2004; Anacker et al. 2014;
Brouwer et. al. 2015).
Allelopathy is studied using different methods such as laboratory bioassays
(including Petri dishes), greenhouse and field experiments. A laboratory
experiment is primarily aimed at studying the allelopathic effect of extracts
from roots and leaves (Sharma et al. 2000; Tseng et al. 2003; Nasir et al.
2005). The methodological difficulties of these bioassays have been discussed
frequently (Inderjit and Dakshini 1995; Inderjit and Weston 2000; John et
al. 2006). It is possible that some laboratory bioassays do not allow assessment
of interactions in nature. Laboratory conditions do not correspond to
natural ones. And frequently there are not standardized methods or proper
control treatment in a number of bioassays (John et al. 2006). It is believed
that laboratory bioassays cannot demonstrate that allelopathy works in
vivo (Inderjit and Weston 2000).
The results of allelopathy tests vary not only depending on the methods,
but also depending on the species of invasive plants. Evidence for direct
allelopathic effects of invasive plants on native plants and communities is
abundant (Kumar and Bais 2010; Cipollini et al. 2012). Evidence for
allelopathic effects has been obtained for Lonicera maackii (Rupr.) Maxim.
(Dorning and Cipollini 2006); Ailanthus altissima (Mill.) Swingle (Gómez-
Aparicio and Canham 2008); Solidago canadensis L. (Lu et al. 2020). However,
an inhibitory effect of extracts from tissues of alien plants on the
development of recipient plants is not always observed. For example, no
confirmation of allelopathic effects has been obtained for Euphorbia esula L.
and Centaurea stoebe L. (Olson and Wallander 2002), Impatiens glandulifera
Royle (Gruntman et al. 2017). Thus, despite the prevalence of results
confirming the assumption of the allelopathic activity for invasive plants,
this is not a universal explanation of their invasive success.
There is a lack of clarity regarding allelopathy as a mechanism for the
invasive success of a particular plant species. This may be partially related
to the imperfection of experimental methods. For example, this statement
is true for the invasive tree Acer negundo L., we are studying. Its common
name is box elder or American maple. This is a transformer species that
can transform native ecosystems (Vinogradova et al. 2010). In Russia, box
elder invades local ecosystems, mainly in disturbed and semi-natural areas
(Vinogradova et al. 2010). The diversity of native plants decreases in
communities dominated by A. negundo (Emelyanov and Frolova 2011). In
urbanized communities where it is dominant, the α- and γ-diversity of
herbs also decreases (Veselkin and Dubrovin 2019; Dubrovin 2018). Therefore,
elucidating the mechanisms of the invasive success of A. negundo is an
urgent task.
A wide range of results about the allelopathic activity of A. negundo have
been obtained. There is evidence of inhibitory allelopathic activity (Csiszár
2009; Yerіоmenko 2012; Csiszár et al. 2013; Aleksandrov and Kalashnikov
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 707
2019), and there are descriptions of unclear effects or their absence
(Panasenko et al. 2018; Veselkin et al. 2019). Some research demonstrated
that allelochemicals from leaf litter of A. negundo inhibit the germination
of seeds and the growth of seedlings. However, the effect depends on
concentration of allelochemicals. For the concentrations of extracts taken
to be considered close to natural (1:100), no negative effect on the test
objects was revealed (Nikolaeva et al. 2021). There are also cases of
stimulation of seed germination for recipient plants with A. negundo
compounds (Tsandekova 2019, 2020). Simultaneously, a number of results
were obtained using only water as a control treatment (Yerіоmenko 2012;
Aleksandrov and Kalashnikov 2019; Nikolaeva et al. 2021) without
comparison with native plant species (Panasenko et al. 2018). Thus, several
studies of the allelopathic activity of A. negundo are known. However, their
results are contradictory and these studies may have methodological issues
mentioned above. For a reliable assessment of the allelopathic effects of
A. negundo, it is necessary to accumulate methodically rigorous results of
experiments of various designs.
Presumably, the effect of box elder allelopathy varies during a vegetation
season. Seasonal variation of chemical compounds in leaves is known
phenomenon (Königer et al. 2000; Altyar et al. 2020; Passarinho et al. 2006;
Morais et al. 2021; Sachse et al. 2009). For one of our donor plants Salix
caprea, there was a study about seasonal changes in chemical compounds
of twigs. It has showed the deposition periods for starch and protein differ
clearly. Starch accumulates from May until October while protein is deposited
primarily during leaf senescence in fall. Sugar content of the living cells of
the wood increases drastically in late October and in November (Sauter
and Wellenkamp 1998). Differences in other plant organs can be expected
as well. Sorbus aucuparia also was tested for seasonal variation in compounds
of leaf extracts (Olszewska 2011). From May to October, the best antioxidant
capacities and the highest phenolic contents were found for the leaf
samples harvested during the three summer months. For the study species
A. negundo leaf volatiles depend on time of the day and month (Jian-
Guang et al. 2003). The analysis revealed that the diurnal rhythm of release
of volatile compounds from maple differs in July and in August. In July,
the releasing of most volatile compounds reached the peak at 14 o’clock,
while in August it was at 10 o’clock. This can be explained by maturation
of leaves. Therefore, we expected some differences between summer and
autumn leaf samples.
Research of invasive species influence including allelopathy on native
plant communities is the first step towards its management. Most studies
paid particular attention to invasive species removal and lacked an evaluation
of native revegetation following removal. Minimal focus on revegetation of
natives has been identified as limitation to successful invasive species
management (Kettenring and Adams 2011). Therefore, this study of
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 708
A. negundo allelopathy may help us to understand what the specific
mechanisms of invasion and long-term consequences for native species are.
For example, there is the potential to use A. negundo biomass in the
production of fuel pellets (Mudryk 2011; Nobre et al. 2015). A growing
demand for solid biofuels, in particular from agrobiomass, increases day by
day due to the need of renewable energy sources. Energy plants are a
possible way to produce fuel from biomass. Agriculture faces the challenge
of mass growing the most energy-efficient crops on energy plantations.
Production of pellets from box elder could manage the problem of biomass
utilization from areas invaded by it. The possibility of using A. negundo for
pellets and growing it on energy plantations raises the question of the
environmental consequences for local ecosystems.
Some of our recipient plants (Festuca rubra and Trifolium repens) are
used in grass seed mixtures. These species of lawn grasses can grow in the
conditions of the modern urban and suburban landscape and potentially
resist the spread of invasive plants (Alexandrov and Kalashnikov 2019;
Klimenko and Djachenko 2012). Therefore it is important to understand if
they are affected by allelopathy of invasive A. negundo because this could
help us to assess their potential use in management and restoration of
invaded areas.
This study aims to evaluate the effect of water extracts from leaves of
A. negundo (collected in the summer and autumn seasons) on seed
germination of different herbaceous plants compare with native tree extracts.
Materials and methods
For studying allelopathic effects in experiments with water extracts, it is
recommended to avoid: 1) the use of distilled water as the only control
treatment, 2) lack of comparison with native plants, 3) grinding plant
material (Inderjit and Dakshini 1995; Inderjit and Weston 2000; John et al.
2006). We considered these requirements when planning the experiments.
Yekaterinburg (56°50N; 60°35E) is a city within the Russian Federation
with a population of 1.4 million residents. It is the administrative center of
Sverdlovsk oblast. Yekaterinburg is located in the southern taiga subzone
of the boreal forest zone. The plant communities are dominated by pine
forests (Pinus sylvestris L.) on sod-podzolic soils and burozems (Kulikov et
al. 2013). The climate is temperate continental, winter is long and cold with
a stable snow cover, and summer is short. The average annual temperature
is +3.0 °C, while the average temperature in January is −12.6 °C, and +19.0 °C
in July. The average annual precipitation is 550650 mm. The maximum
precipitation occurs during the warm season (MayAugust), during which
approximately 6070% of the annual amount falls. The leaves of donor
plants were collected in a large public park named after V.I. Mayakovsky
(56°48N; 60°38E). The primary vegetation is pine forests, with an
occasional admixture of Betula spp., Populus tremula L., and Tilia sp. There
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 709
Table 1. Donor and recipient plants description (Gubanov et al. 2002, 2003; Ovesnov and Efimik 2009; Kattge at al. 2020; Kew
2022; Pladias 2022)
Plant traits Donor plants Recipient plants
Acer negundo Sorbus aucuparia Prunus padus Salix caprea Festuca rubra Sinapis alba Trifolium repens
Family Sapindaceae Rosaceae Rosaceae Salicaceae Poaceae Brassicaceae Fabaceae
Height, m 10-20 1-15 0.5-12 3-12 0.2-1 0.3-1.2 0.1-0.2
Growth form tree tree (shrub) tree (shrub) tree (shrub) clonal herb annual herb clonal herb
Life form macrophanerophyte
macrophanerophyte,
nanophanerophyte
macrophanerophyte
(nanophanerophyte)
nanophanerophyte
(macrophanerophyte)
hemicryptophyte therophyte
hemicryptophyte
(chamaephyte)
Life strategy C – competitor C – competitor C competitor C – competitor
C – competitor,
CS competitor/
stress-tolerator
CR
competitor/
ruderal
CSR
competitor/stress-
tolerator/ruderal
Root type shallow roots intermediate taproot shallow roots fibrous taproot fibrous
Mycorrhiza arbuscular arbuscular arbuscular ectomycorrhiza arbuscular
non-
mycorrhizal
arbuscular
Average 1000 seed
weight (g)
36.0 6.0 78.0 0.129 1.1 5.2 0.7
Humidity
requirements
mesophyte mesophyte mesophyte mesophyte mesophyte mesophyte mesophyte
Nutrient
requirements
mesotrophic mesotrophic mesotrophic mesotrophic mesotrophic eutrophic eutrophic
Tolerance to shade photophilous shade-tolerant shade-to lerant shade-tolerant shade-tolerant photophilous shade-tolerant
Native range
North America
from Europe to the
Urals and the
Caucasus
Europe and Asia
Europe, the
Caucasus, China
and Japan
Europe, Asia, the
Caucasus
Europe,
North Africa
and Asia
Europe and Asia
Habitats
in continental
Europe, urban areas,
along rivers, and
invades protected
ecosystems
undergrowth in
coniferous and
mixed forests, in
glades and forest
edges, and is often
cultivated
undergrowth in
coniferous and
mixed forests, along
ravines, riverbanks,
and valleys, where it
often grows in
continuous thickets
damp forests,
glades, forest edges,
along roads, and
often near dwellings
meadows, glades,
on sands, pebbles,
and vario us
secondary
habitats
in weedy
places, along
roads, in
fields, and in
gardens
on dry and
floodplain
meadows, on
pastures, along
river-banks and
streams, along
roadsides, near
dwellings, on
wastelands, and
as a weed in
crops
are many ornamental plantings in the park, including introduced the
following species: Ulmus laevis Pall., Malus baccata (L.) Borkh., Acer
negundo L., Syringa vulgaris L., Cotoneaster lucidus Schltdl., and Caragana
arborescens Lam.
Donor plants
We studied the allelopathic activity of one invasive species, box elder Acer
negundo, and three local species, mountain-ash Sorbus aucuparia L., bird
cherry Prunus padus L., and goat willow Salix caprea L. All four species are
common within the undergrowth of urbanized forests in Yekaterinburg
(Veselkin et al. 2018b).
Recipient plants
Typical local plants were chosen as recipient species red fescue Festuca
rubra L., creeping clover Trifolium repens L., and white mustard Sinapis
alba L., which is often used as a model plant in experiments to test
germination (Table 1). These species are also used in our field and
greenhouse experiments. According to the literature, these herbaceous
plants are also used as model species in allelopathy experiments. This
allows us to compare our results with other research. In addition, these
species germinate quickly in the laboratory conditions.
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 710
Collection of leaves and water extract preparation
Fresh mature leaves of each donor species were harvested at a height of
1.53 m from at least five mature donor trees. The distance between
individuals of donor plants were at least 50 m. The samples were taken in
two sites of local urbanized forests of Yekaterinburg. Summer leaves were
collected on date 08.04.2020. Autumn leaves were collected on date
09.07.2020. The leaves were not ground in order to avoid tissue disruption
as much as possible. Extracts were prepared by soaking leaves with distilled
water in a ratio of 1:10 by weight (100 g of leaves and 1000 ml of water)
(John et al. 2006). The extracts were incubated for 24 h in a dark place at
room temperature, and then filtered through a filter paper. Flasks with
made extracts were stored in a refrigerator at +4 °C. Fresh extracts were
prepared every seven days.
Seed germination
Petri dishes (9 cm dia) and filter paper were sterilized for 2 h in a drying
cabinet at +110 °C. The seeds of recipient species were purchased in 2020
year. They met the National Standard of the Russian Federation (№ 52325
2005) and were preliminarily tested for germination. Average seed germination
on 4 day was 0.55 ± 0.11 for T. repens, 0.88 ± 0.04 for S. alba, and 0.22 ±
0.04 for F. rubra (it increased up to 0.80 ± 0.02 on 11 day).
Seeds were surface sterilized with 0.1% NaOCl (sodium hypochlorite)
solution for two minutes, then rinsed with distilled water. In each dish, 50
seeds of F. rubra, S. alba, or T. repens were placed on two layers of filter
paper moistened with 2 ml of the extract or water. The dishes were exposed
at a constant temperature of +23 °C and 12 h light / 12 h dark photoperiod
in a grow room. Seeds were regularly moistened to keep the paper from
drying out. The seeds were considered germinated when root length was at
least 1 mm. The germination rate was recorded for 14 days.
Experimental design
In total, there were five treatments: one invasive tree species (A. negundo),
three native tree species (S. aucuparia, P. padus, and S. caprea), and
distilled water as a negative control. There were three species of recipient
plants: F. rubra, S. alba, T. repens. Each “extract” × “plant-recipient”
combination was replicated three times. Thus, there were a total of 45 Petri
dishes simultaneously exposed on one table. The dishes were randomly
placed and mixed daily. Two experiments were performed sequentially
according to this design. The first experiment was conducted between
08.06.202008.21.2020 with summer (green) tree leaves, and the second
experiment was between 09.11.202009.25.2020 with autumn (colored)
tree leaves.
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 711
Figure 1. Germination rate of Festuca rubra in a random Petri dish in distilled water in the green
leaves experiment; xy10, xy50, and xy90 are lower, middle, and upper critical points; хуmax is the
point of the maximum number of germinated seeds.
Statistical analysis
Seed germination rate was estimated with a logistic curve for dose effect
relationship (Equation 1) (Vorobeichik et al. 1994):
=
 (αβ)+ (1)
where y is the estimation of the proportion of germinated seeds, x is the
estimation of the time after the start of germination, α and β are
coefficients that were found by the iterative method, a0 is the minimum
germination rate (considered as zero for all Petri dishes), and A is the
maximum germination (i.e. the maximum percentage of germinated seeds
recorded for 14 days of the experiment). Based on the initial data of
germinated seeds and days of maximum germination, an equation for each
Petri dish was compiled using the logistic curve equation formula (1).
Using the User Specification Regression (least squares) in STATISTICA we
input the Equation 1 for every Petri dish to find out α and β coefficients.
After finding the coefficients α and β, the lower (x10), the middle (x50),
and the upper (x90), critical points were analytically found. These
determined the germination time of 10, 50, and 90% of seeds respectively.
Critical point’s coordinates were (Vorobeichik et al. 1994):
10 = 0.1 (A )+ (2)
50 = 0.5 (A )+ (3)
90 = 0.9 (A )+ (4)
10, 50, 90 =󰇡
10, 50, 90󰇢
(5)
A logistic approximation was built for each of 45 Petri dishes in every
experiment, as shown in Figure 1 (example of random Festuca rubra dish).
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 712
All coefficients and coordinates for each dish are averaged for each extract
to build a final curve.
Comparison of the abscissas of the critical points x10, x50, and x90 enabled
the direct characterization and comparison of the germination rates of
seeds of different recipient plants and the germination rates of one
recipient plant under the influence of leaf extracts from different woody
plants. In this case, one can compare the rate of the beginning of seed
germination (x10), the rate of germination of half seeds (x50), and the rate of
the end of active germination (x90). The abscissas of the maximum
germination points (xmax) allow comparing the time of the maximum
germination for different recipient plants. Comparison of the ordinates of
the maximum germination points (ymax) enables the comparison between
the absolute germination of different species and the absolute germination
for one recipient plant under the influence of leaf extracts from different
woody plants. The coordinates of these points (x10, x50, x90, xmax, ymax) were
further analyzed using a two and three-way ANOVA. The homogeneity of
the variances was controlled using the Leuven test, and the pairwise
differences between treatments were assessed using the Tukey test. A
statistical analysis was performed using the STATISTICA 10.0 software
(StatSoft, USA).
Results
Extracts from donor plants
The maximum germination did not differ depending on leaf extracts or
water: water: 87 ± 2%, S. caprea: 85 ± 3%, P. padus: 87 ± 2%, S. aucuparia:
83 ± 2%, and A. negundo: 86 ± 2%. However, the germination rate depended
on the donor plant species.
The effect of the extract on seed germination has not been identified for
the beginning of germination (parameter x10). The average values of x10
were water: 2.3 ± 0.3 days, S. caprea: 2.7 ± 0.3 days, P. padus: 2.5 ± 0.3 days,
S. aucuparia: 2.7 ± 0.3 days, and A. negundo: 2.4 ± 0.3 days. There were no
significant differences according to Tukeys test.
For the parameters x50, x90, and xmax, a delay in seed germination in the
extracts of various donor plants was noticeable. The average values of the
parameter x50 were water: 3.6 ± 0.4 days, S. caprea: 4.0 ± 0.5 days, P. padus:
3.8 ± 0.4 days, S. aucuparia: 4.8 ± 0.5 days, and A. negundo: 4.0 ± 0.4 days.
According to Tukeys test, significant differences were identified between
the “S. aucuparia” and all other treatments (P = 0.0001), as well as between
“water” and “A. negundo” (P = 0.0049), and between “water” and “S. caprea
(P = 0.0004). The average values of the parameter x90 were water: 4.9 ± 0.6
days, S. caprea: 5.4 ± 0.6 days, P. padus: 5.2 ± 0.6 days, S. aucuparia: 6.9 ±
0.7 days, and A. negundo: 5.5 ± 0.7 days. According to Tukeys test,
significant differences were discovered between the “S. aucuparia” and all
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 713
Figure 2. Seed germination of recipient plants in extracts from summer (green) leaves. Black
points are lower, middle, and upper critical points; red points are maximum points of the function;
intervals SE.
other treatments (P < 0.0001), between “water” and “A. negundo” (P = 0.0041),
and between “water” and “S. caprea” treatments (P = 0.0183). The values of
the parameter xmax were water: 9.3 ± 0.8 days, S. caprea: 9.8 ± 0.8 days,
P. padus: 9.8 ± 1.0 days, S. aucuparia: 11.1 ± 0.4 days, and A. negundo: 10.1 ±
0.8 days. The significant differences were found with Tukeys test only
between the treatments “water” and “S. aucuparia” (P = 0.0178). Thus,
according to the parameter xmax, seeds germinated fastest in water and
slowest in extracts from S. aucuparia.
The use of native plants as controls suggested that our method allows
recording responses that can be interpreted as an indication of allelopathic
activity. In both summer and autumn, a pronounced inhibitory effect of
leaf extracts from a local shrub S. aucuparia was observed (Figures 2, 3).
This inhibition of seed germination was seen in a standard comparison of
S. aucuparia extracts with distilled water. However, it is important that for
some parametersin particular for the time of the fastest germination (x50)
and time of 90% seeds germinated (x90)—it was possible to see inhibition of
seed germination by S. aucuparia extracts in comparison with other tested
woody plants, both local and invasive. Thus, water leaf extracts of S. aucuparia
have allelopathic activityalthough not highleading to a delay in the
germination, but not to a decrease in the total number of germinated seeds.
Extracts from summer (green) and autumn (colored) leaves
The seasonal pattern of germination was not expressed to absolute
germination. The average maximum germination for all treatments of all
recipient plants was 84 ± 1% in summer leaf extracts and 87 ± 2% in autumn
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 714
Figure 3. Seed germination of recipient plants in extracts from autumn (colored) leaves. Black
points are lower, middle, and upper critical points; red points are maximum points of the function;
intervals SE.
ones. However, the seasonal characteristics of germination are well expressed
in terms of germination rate. In summer leaf extracts, seeds germinated
somewhat faster (the average value for all treatments was x50 = 3.2 ± 0.3
days and xmax = 8.5 ± 0.6 days) than in autumn leaf extracts (x50 = 4.9 ± 0.2
days and xmax = 11.5 ± 0.4 days). Faster seed germination in summer leaf
extracts was observed both for extracts from leaves (the average value for
all extracts from summer leaves is x50 = 3.3 ± 0.3 days, xmax = 8.9 ± 0.6 days,
and from autumn leaves x50 = 5.0 ± 0.2 days, and xmax = 11.6 ± 0.5 days)
and in water (summer leaf extracts x50 = 2.8 ± 0.6 days, xmax = 7.2 ± 1.1
days, autumn leaf extracts x50 = 4.4 ± 0.5 days, and xmax = 11.3 ± 0.7 days).
Despite the significant interaction among the factors “season” × “recipient
plants” (Table 2), the differences between the recipient plantsin terms of
germination rateswere the same for both seasons. S. alba germinated
quickly, T. repens did slower, and F. rubra did even slower (Figures 2, 3).
Thus, the seed germination of recipient plants was slower in autumn leaf
extracts, but overalldespite a 1.54 day delayit was no less successful
than in summer leaf extracts.
Seed germination of recipient plants
The specific features of germination and its rate were well expressed, as
indicated by the great significance of the influence of the factor “recipient
plants” on all parameters of seed germination (Table 2). The average
maximum germination rate for all treatments (ymax) was 95 ± 1% for S. alba,
87 ± 2% for F. rubra, and 74 ± 1% for T. repens (Table 3). Moreover, the
seeds of the recipient plants also germinated at different rates. Seeds of
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 715
Table 2. Significance of the influence of factors “recipient plants” (Sinapis alba, Festuca rubra, Trifolium repens), “season”
(summer or autumn leaves) and “extract” (Acer negundo, Prunus padus, Salix caprea, Sorbus aucuparia, water) in a three-way
ANOVA on seed germination of recipient plants.
Parameter
Factors
Interaction among the factors
extract
(dF=4)
season
(dF=1)
recipient
plants
(dF=2)
extract ×
season
(dF = 4)
extract ×
recipient
plants
(dF = 8)
season ×
recipient
plants
(dF = 2)
extract × season
× recipient
plants (dF = 8)
Maximum germination rate (ymax)
0.062
0.0409
< 0.0001
0.844
0.8826
0.0682
0.3043
Days:
maximum germination (xma x)
0.0329
< 0.0001
< 0.0001
0.2418
0.4312
< 0.0001
0.8169
10% seeds (x10)
0.0326
< 0.0001
< 0.0001
0.2758
0.0368
0.0265
0.2887
50% seeds (x50)
< 0.0001
< 0.0001
< 0.0001
0.1869
0.0333
< 0.0001
0.0006
90% seeds (x90)
< 0.0001
< 0.0001
< 0.0001
0.8345
0.6034
< 0.0001
0.0211
Note: maximum germination rate the maximum percentage of germinated seeds recorded for 14 days of the experiment. Days
the day of maximum seed germination (xmax), beginning of seed germination (x10), half of seed germination (x50), and the end of
active germination (x90).
Table 3. The absolute mean values of the seed germination and days of maximum germination, ± SE.
Recipient plants
Season
Extract
Max germination rate, %
Day of maximum germination
1
Festuca rubra
autumn
Acer negundo
85 ± 3
12.7 ± 0.9
2
Festuca rubra
autumn
Prunus padus
89 ± 4
13.7 ± 0.3
3
Festuca rubra
autumn
Salix caprea
89 ± 4
13.7 ± 0.3
4
Festuca rubra
autumn
Sorbus aucuparia
85 ± 4
14.0 ± 0.1
5
Festuca rubra
autumn
water
89 ± 5
12.3 ± 0.9
6
Festuca rubra
summer
Acer negundo
88 ± 4
13.7 ± 0.3
7
Festuca rubra
summer
Prunus padus
87 ± 3
14.0 ± 0.1
8
Festuca rubra
summer
Salix caprea
86 ± 1
12.3 ± 0.3
9
Festuca rubra
summer
Sorbus aucuparia
81 ± 3
13.3 ± 0.3
10
Festuca rubra
summer
water
91 ± 2
11.3 ± 0.3
11
Sinapis alba
autumn
Acer negundo
98 ± 1
9.3 ± 1.2
12
Sinapis alba
autumn
Prunus padus
99 ± 1
7.0 ± 1.5
13
Sinapis alba
autumn
Salix caprea
99 ± 1
9.3 ± 1.2
14
Sinapis alba
autumn
Sorbus aucuparia
95 ± 2
9.3 ± 1.7
15
Sinapis alba
autumn
water
99 ± 1
9.3 ± 1.2
16
Sinapis alba
summer
Acer negundo
94 ± 1
6.0 ± 0.6
17
Sinapis alba
summer
Prunus padus
93 ± 2
4.7 ± 0.7
18
Sinapis alba
summer
Salix caprea
91 ± 3
4.0 ± 0.1
19
Sinapis alba
summer
Sorbus aucuparia
91 ± 1
6.7 ± 0.3
20
Sinapis alba
summer
water
92 ± 2
4.7 ± 0.7
21
Trifolium repens
autumn
Acer negundo
78 ± 1
12.7 ± 0.3
22
Trifolium repens
autumn
Prunus padus
77 ± 4
11.7 ± 2.3
23
Trifolium repens
autumn
Salix caprea
67 ± 3
12.0 ± 1.2
24
Trifolium repens
autumn
Sorbus aucuparia
75 ± 1
13.3 ± 0.7
25
Trifolium repens
autumn
water
76 ± 5
12.3 ± 1.2
26
Trifolium repens
summer
Acer negundo
73 ± 5
6.3 ± 0.9
27
Trifolium repens
summer
Prunus padus
77 ± 4
8.0 ± 2.0
28
Trifolium repens
summer
Salix caprea
76 ± 1
7.3 ± 0.7
29
Trifolium repens
summer
Sorbus aucuparia
70 ± 3
10.0 ± 1.0
30
Trifolium repens
summer
water
76 ± 2
5.7 ± 1.2
S. alba germinated the fastest (x50 = 2.2 ± 0.2 days and xmax = 7.0 ± 0.5),
T. repens was slower (x50 = 4.1 ± 0.2 days and xmax = 9.9 ± 0.6 days), and
F. rubra was even slower (x50 = 5.9 ± 0.1 days and xmax = 13.1 ± 0.6). Thus,
the most active and fastest germination rate was for S. alba, and F. rubra
were identified as having the slowest germination rate (Figures 2, 3).
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 716
Discussion
We found only a weak effect of invasive A. negundo on the seed germination
of recipient plants. Extracts from A. negundo slowed the germination by
roughly half a day in comparison with distilled water. If we compared
A. negundo extracts with water only, then we would get a statistical
confirmation of the inhibitory effect. For example, the differences between
the mean values of the parameter x50 for A. negundo extracts (4.0 ± 0.4 days)
and for water (3.6 ± 0.4 days) would be significant (according to Tukeys
test P = 0.0006, obtained in a three-factor ANOVA with exclusion from the
analysis extracts of S. caprea, P. padus, and S. aucuparia). A similar deceleration
of seed germination for the x90 parameter in A. negundo extracts (5.5 ± 0.7
days) and in water (4.9 ± 0.6 days) would also be significant (P = 0.0040).
In our study the inhibitory effect of A. negundo was no stronger than the
similar effect of local S. caprea and P. padus. The most pronounced the
inhibitory effect was found in the local shrub S. aucuparia, which was
established both in summer and autumn leaf extracts reliably (i.e.
persistently manifested itself during the vegetation season). Therefore,
allelopathy cannot be a convincing explanation for the ability of A. negundo
to influence local communities.
The conclusion about the absence of an allelopathic effect of A. negundo
extracts is supported by our earlier experiments (Veselkin and Rafikova
2022). We performed an experiment with a similar design but using roll
culture method which allows estimating root and shoot length of recipient
plants. The effect of A. negundo was similarly strong as those of extracts
from the leaves of native P. padus and S. caprea, while the leaf extract of the
native shrub S. aucuparia proved to have a distinct inhibitory effect. We
found that the seed germination of native plants is not suppressed in soils
transformed by A. negundo (Veselkin et al. 2019).
Our results showed that the invasive tree A. negundo weakly inhibited
the seed germination of local herbaceous species. Is that conclusion
consistent with the other allelopathy studies? Results of 384 studies that
measured allelopathic effects were summarized in a meta-analysis (Zhang
et al. 2021). The authors concluded allelopathy reduced plant performance
by 25%, but the variation in allelopathy was high. The synthesis reveals that
allelopathy could contribute to the success of alien plants. Moreover, native
plants suffered more from leachates of naturalized alien plants than from
leachates of other native plants. However, laboratory and greenhouse
experiments often do not yield identical results (Nuismer and Gandon 2008;
Heinze et al. 2016; Florianová and Munzbergová 2018; Aldorfová and
Munzbergová 2019). Not all invasive plants demonstrate allelopathic
activity (Olson and Wallander 2002; Gruntman et al. 2017). There are studies
that do not show any evidence for the Novel Weapons Hypothesis. Instead,
they suggest that invasive species release allelopathic compounds to a
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 717
similar degree as the native plant community (Del Fabbro et al. 2014;
Chobot et al. 2009). Moreover, some of non-native plants can positively affect
seed germination of native species (Tsandekova 2020). In our case, the
conclusion about the absence of the inhibitory effect of A. negundo compare
to native trees is consistent with various similar results (Rafikova and
Ekshibarov 2017; Veselkin et al. 2019; Tsandekova 2019). However, there is
evidence of A. negundo allelopathy compare to distilled water (Yeriomenko
2012; Aleksandrov and Kalashnikov 2019; Nikolaeva et al. 2021). As was
mentioned above, if we compared A. negundo extracts with water only,
then we would get a statistical confirmation of the inhibitory effect as well.
Allelopathic effect on recipient plants can be explained by allelochemicals
compounds in plant tissues. The secondary metabolites in the leaves of
A. negundo and the local tree species are diverse. For A. negundo, different
publications (Ping et al. 2001a, b; Li et al. 2003; Bi et al. 2016; Barrales-
Cureño et al. 2020) indicate 2060 components, including alkaloids, aldehydes,
aromatic and heterocyclic compounds, carboxylic and fatty acids, ketones,
esters, alcohols, tannins, terpenoids, phytosterols, and flavonoids. There is
a smaller variety of secondary metabolites in the leaves of local woody
species, most likely due to the lack of studies performed. In the leaves of
S. aucuparia, 15 metabolites were confirmed, including phenol-carboxylic
acids and flavonoids (Budantsev 2009). Approximately 20 metabolites were
confirmed in the leaves of P. padus, including aldehydes, lignans, flavonoids,
cyanogen compounds, and cerebrosides (Budantsev 2009). Roughly 20
metabolites were registered in the S. caprea leaves, including catechins, organic
acids, proanthocyanidins, phenols, and flavonoids (Budantsev 2009).
Therefore, all our donor plants synthesize compounds that can potentially
have allelopathic activity. Based on the knowledge about secondary
metabolites, the ability to inhibit seed germination for A. negundo was
most commonly found, since the list of its known secondary metabolites is
the widest. However, the hypothesis was unconfirmed, and the most
pronounced inhibitory effect was found for extracts from the local shrub
S. aucuparia.
The weak effect of A. negundo leaf extracts on F. rubra and T. repens
allows us to recommend them in grass seed mixtures in urban areas invaded
by A. negundo. Sowing lawn grasses can be a part of invasive tree management
in urban ecosystems. Such species additions not only help restore native
species lost from the ecosystem but can also increase the number of
competitors, and these may act to reduce invasive species population size,
recruitment and spread (Hulme 2006). Also S. alba and T. repens are used
as green manure to provide a nutrient source for crops. An additional
mechanism to encourage increased suppression of invasive alien species by
native species is through restoring soil nutrient balance. Soil conditions are
usually nutrient poor or even toxic and suffer a high risk of erosion, with
the result that establishing native vegetation cover is often a struggle
Leaf water extracts from invasive Acer negundo
Rafikova and Veselkin (2022), Management of Biological Invasions 13(4): 705723, https://doi.org/10.3391/mbi.2022.13.4.08 718
(Hulme 2006). And low exposure of our recipient species to water leaf
extracts can be an advantage in use on invaded areas.
Therefore, we have not identified convincing evidence of the phytotoxic
effect from water leaf extracts under the experimental conditions used,
although the hypothesis of its allelopathy has previously received a number
of experimental confirmations. The ability to manage and restore plant
communities transformed by A. negundo probably does not significantly
differ from the ability to restore communities disturbed for other reasons.
This point is also confirmed by the slight influence of A. negundo on the
soil seed bank (Veselkin et al. 2018a). Therefore, the existence of long-term
effects caused by leaf leachates is unlikely for A. negundo.
Future research should be aimed to search for the mechanisms of box
elder invasion by analyzing other biological and ecological properties that
determine its environment-transforming or competitive activity or ability
to spread. Consequently, it is necessary to perform further studies on a wider
geographical range and/or using other experimental designs, for example,
to germinate seeds in natural communities transformed by A. negundo.
Acknowledgements
Authors thank reviewers whose comments and suggestions helped to improve and clarify this
manuscript.
Funding declaration
The authors disclosed receipt of the following financial support for the research: the study was
carried out as part of the project of the Russian Foundation for Basic Research, No. 20-34-90084
(Rafikova O.S.) and was performed under the State Assignment of the Institute of Plant and Animal
Ecology, Ural Branch of the Russian Academy of Sciences AAAA-A19-119031890084-6
(Veselkin D.V.). The funders had no role in study design, data collection and analysis, decision
to publish, or preparation of the manuscript.
Authors’ contribution
Rafikova O.S. investigation and data collection, sample design and methodology, data analysis
and interpretation, original draft; writing review and editing. Veselkin D.V. research
conceptualization, sample design and methodology, data analysis and interpretation, ethics
approval, funding provision, original draft; writing review and editing.
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The α-diversity of the grass layer was compared for communities exposed to Acer negundo and unexposed communities. The communities were aligned by characteristics by their degrees of urbanization, fragmentation, and anthropogenic disturbance. The research was carried out in the city of Yekaterinburg (the southern taiga subzone, Russia) at 13 sites. Each site included two sampling plots: one in communities dominated by A. negundo and the other in communities dominated by other tree species (total of 26 communities). It is established that the key factors of variation in the grass layer characteristics are the dominant tree species (A. negundo or other trees) and the stand area. The number of grass species per 400 m² was lower in A. negundo thickets than under the crowns of other trees: 17 ± 3 and 28 ± 3, respectively. However, communities with and without A. negundo did not differ in the values of the Shannon index, the degree of dominance, or the ratios of annual/perennial and graminoid/forb species. An increase in the degree of habitat fragmentation was accompanied by an increase in the proportion of synanthropic species, both under the canopy of A. negundo and in communities dominated by other trees. Methodically, the results showed that assessment of the consequences of plant invasion should always take into account the spatial patterns of determination of the structure of communities.
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В статье анализируются результаты биохимического состава растительного опада различных фитогенных зон Acer negundo L. в условиях нарушенных пойменных растительных сообществ. Отбор образцов проводили на пробных площадках в различных условиях сомкнутости крон с учетом зон влияния деревьев. В качестве контроля выбрана внешняя зона одиночных деревьев. Определение зольности проводили путем сухого озоления; содержание азота и фосфора – из одной навески после мокрого озоления: азот – по методу Къельдаля, фосфор – по методу Мерфи и Райли; накопление лигнина – с использованием 72% раствора серной кислоты. Статистическая обработка полученных данных и построение графиков выполнены с помощью стандартного пакета программ StatSoft STATISTICA 8.0. for Windows и Microsoft Office Excel 2007. Выявлены некоторые особенности химического состава растительного опада A. negundo в условиях нарушенных пойменных сообществ. У одиночных деревьев в несомкнутых древостоях в подкроновой и прикроновой зонах выявлено наибольшее количество золы, а у деревьев с сомкнутостью крон 50-60% – более высокие показатели по азоту, фосфору и лигнину в сравнении с другими группами деревьев и с контролем. Наиболее сильно различавшимся показателем химического состава растительного опада на пробных площадках было содержание золы и лигнина, в меньшей степени варьировало содержание азота и фосфора. Экспериментальные данные можно использовать для оценки состояния напочвенного покрова и образования органического вещества почвы в лесных сообществах.
Article
We investigated the assumption of transformation of soil seed banks under the influence of invasive plants. For this purpose, we analyzed the taxonomic diversity and abundance of seedlings from the soil seed bank in the thickets of the invasive ash-leaved maple Acer negundo L. We performed our experiments using the seedlings emerged from soil seed banks collected in two types of habitats in Yekaterinburg (dense thickets of A. negundo and habitats with similar geomorphological and edaphic features but without A. negundo). In addition, we analyzed the seedlings emerged from sod-podzolic soils collected from suburban meadow areas. We observed a small negative effect of A. negundo on the abundance of seedlings from the soil seed bank. The amount of seedlings on the soil from the thickets of A. negundo was lower by 1.5–2.5 times than in urban habitats without this species. The taxonomic diversity of seedlings differed between suburban and urban habitats but did not depend on the presence of habitat transformation by A. negundo.