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Acta Bot. Croat. 71 (2), 249–260, 2012 CODEN: ABCRA 25
ISSN 0365-0588
eISSN 1847-8476
DOI: 10.2478/v10184-011-0065-2
Soil seed bank of the invasive Robinia pseudoacacia
in planted Pinus nigra stands
IMRE CSERESNYÉS*, PÉTER CSONTOS
Institute for Soil Sciences and Agricultural Chemistry, Centre for Agricultural Research,
Hungarian Academy of Sciences, Herman O. út 15, Budapest, H-1022, Hungary
Abstract –Pinus nigra and Robinia pseudoacacia are exotic trees used for afforestation
in Hungary. Pinus nigra was non-invasive, however R. pseudoacacia escaped from culti-
vation and invaded several vegetation types including pine plantations. It has recently
been planned to cut P. nigra plantations and replace them by native tree stands, especially
in nature reserves. The scattered presence of R. pseudoacacia specimens in pine stands
might place constraints on planned tree replacement because of their vegetative resprout-
ing and recolonization from an established seed bank. The aim of this study was to investi-
gate the soil seed bank under the canopy of solitary R. pseudoacacia specimens found in P.
nigra plantations. Altogether 250 soil samples were collected from the 0–6 and 6–12 cm
soil layers under solitary Robinia trees of varying ages (with basal areas between 62.4 and
1089.3 cm2). Seeds were separated by sieving then scarified and germinated. Seed bank
density ranged between 640 and 2285 seeds m–2 with an average distribution of 82.7% and
17.3% in the upper and lower soil layer, respectively. Total density of the seed bank and
also the seed bank ratio of the lower soil layer increased with tree age. The accumulated
seed bank of R. pseudoacacia should be considered in the careful planning of tree replace-
ment operations in Pinus nigra stands.
Keywords: Afforestation, dormancy, Pinus nigra, plantation, Robinia pseudoacacia,
seed germination, soil seed bank
Abbreviations: BA – basal area of tree, DSB – density of seed bank, SBR – seed bank ratio
of the lower soil layer, USB – seed bank density in the upper soil layer
Introduction
Black locust (Robinia pseudoacacia L.) is native to the eastern part of North America.
Its introduction to Hungary dates back to 1710 and it has been intensively used in afforesta-
tion practices since then, due to its diversified utilization (WALKOVSZKI 1998). At present
black locust stands cover about 400,000 hectares, or 23% of the total forested lands of Hun-
gary, a higher coverage than in other European countries (RÉDEI et al. 2008). The black lo-
ACTA BOT. CROAT. 71 (2), 2012 249
* Corresponding author, e-mail: cseresnyes.imre@rissac.hu
Copyright®2012 by Acta Botanica Croatica, the Faculty of Science, University of Zagreb. All rights reserved.
cust has a remarkable spreading capacity due to its rapid vegetative propagation, high
adaptability and nitrogen-fixing character (SWAMY et al. 2002, RICE et al. 2004). The inva-
sion success of Robinia and of other woody Fabaceae species is further enhanced by their
persistent soil seed bank and the physical dormancy of seeds (RICHARDSON and KLUGE
2008). The life expectancy of Robinia seeds in soil is considerably prolonged by antimicro-
bial proteins accumulated in the seed tissues, making them resistant to most pathogens
(TALAS-OGRAS et al. 2005). This species has accordingly become one of the most important
woody plant invaders (CRONK and FULLER 1995) and naturalized in Asia and Australia as
well as in the Western and Central part of North America (HOLLE et al. 2006).
In Hungary, black locust spreads spontaneously mainly in semi-arid sandy areas with
average annual rainfall around 550–600 mm (RÉDEI et al. 2001). Its ever increasing use in
afforestation gives a further impulse to its spread by creating new sources of invasion. As a
consequence, the black locust is now recorded as one of the most dangerous invasive neo-
phyte species in Hungary (BALOGH et al. 2004).
The Austrian pine (Pinus nigra Arn.) is an indigenous tree in the Balkan-Mediterranean
region. In Hungary, it was first introduced in the second half of the 19th century (TAMÁS
2003), and nowadays its stands cover 63,000 hectares (source: Hungarian Forest Manage-
ment Inventory). Alien tree plantations are usually characterized by a poorly developed
herb layer due to the absence of a well organized community of accompanying grasses,
forbs and other species. This phenomenon is especially characteristic of Hungarian P. nigra
plantations, where the strong canopy shading and litter accumulation eliminate the spe-
cies-rich vegetation existing prior to afforestation with the pine (CSONTOS et al. 1996,
2007). The lack of competitive herb- and shrub-layers makes Austrian pine plantations
more susceptible to invasion by aggressively spreading aliens than diverse native forest
communities (ALPERT et al. 2000, MANDRYK and WEIN 2006). In Hungary, the widespread
invasion of R. pseudoacacia in Austrian pine stands is a typical example of this phenome-
non, but further species like Ailanthus altissima (Mill.) Swingle, Asclepias syriaca L. and
Phytolacca americana L. could also be mentioned in this respect.
The low quality timber of the Austrian pine has a limited applicability and it has lost any
economic importance during recent decades. Therefore in Hungary – as in other European
countries (AUGUSTO et al. 2001) – the replacement of alien pine plantations by native for-
ests or grass vegetations has been begun and intensively executed, especially in nature re-
serves and national parks. Obviously, the soil seed banks of alien invasive plants form a
real threat to successful conversion of Austrian pine plantations to native vegetation types.
Among the invasive species listed above, R. pseudoacacia deserves especial attention be-
cause of its pronounced ability to form a long-term persistent seed bank in the soil (THOMP-
SON 1993), due to the physical dormancy of seeds caused by hardseededness (CZIMBER
1980). The soil seed bank of the non-native black locust may cause potential nature conser-
vation and forest management problems on the clear-felled pine stands, even if stubs are
pulled out to prevent re-sprouting from roots and trunks. Therefore, the first aim of our
studies was to quantify the seed bank of the black locust in the soil Austrian pine planta-
tions that it has invaded.
Black locust seeds are known to remain viable for some decades (up to 40 years) in the
soil (TOOLE and BROWN 1946), and thus their accumulation under mature specimens is ex-
250 ACTA BOT. CROAT. 71 (2), 2012
CSERESNYÉS I., CSONTOS P.
pected. The second aim of our studies was to highlight the relationship between the age of
individual trees (represented by basal area) and the soil seed bank density beneath their
canopy.
Seeds buried in deeper soil layers generally remain viable for longer period of time than
those positioned close to the soil surface (FENNER and THOMPSON 2005). If this applies for
black locust, then the ratio of viable seeds in the lower soil layer compared to the total
amount of soil seed bank should be increased under older tree individuals. Investigation of
this presumption formed the third aim of the present studies.
Materials and methods
For soil seed bank studies, five Austrian pine stands invaded by black locust in the
North Hungarian region covered by sandy soil were selected: on the boundary of the settle-
ments of Ács, Isaszeg, Csévharaszt, Tárkány and Komárom (Fig. 1). The information
about localities and main characteristics of the pine plantations were obtained from local
forest inventories (Tab. 1).
ACTA BOT. CROAT. 71 (2), 2012 251
ROBINIA PSEUDOACACIA SEED BANK IN AUSTRIAN PINE STANDS
Fig. 1. Geographical position of Pinus nigra stands involved in soil seed bank sampling. 1 – Ács;
2 – Isaszeg;3–Csévharaszt;4–Tárkány;5–Komárom
Tab. 1. Characteristics of the studied Pinus nigra stands invaded by Robinia pseudoacacia in Hun-
gary.
No. of
sites
Location GPS positions Stand age
(years)
Stand area
(hectares)
1 Ács N 47°44'51.6"; E 18°01'00.2"; 120 m asl. 28 5.12
2 Isaszeg N 47°31'27.1"; E 19°21'25.4"; 205 m asl. 48 4.23
3 Csévharaszt N 47°16'53.3"; E 19°24'56.2"; 129 m asl. 51 2.47
4 Tárkány N 47°35'06.6"; E 17°56'05.1"; 143 m asl. 57 7.76
5 Komárom N 47°44'52.4"; E 18°02'14.8"; 127 m asl. 68 12.35
Soil sampling was carried out between 17 July and 12 August 2009. In each sampling
site five black locust trees were chosen in the interior of the plantation, i.e. at least 20 m dis-
tance from the edge of the pine plantation (to avoid the edge effect). Also, attention was
paid to selecting solitary black locust individuals, thus ensuring that the soil seed bank be-
neath the targeted tree was not influenced by neighbouring black locust specimens. After-
wards the basal area at breast height (BA) was determined by trunk perimeter measure-
ment. Five sampling points were marked out around each tree at a distance of 1.5–2.0 m
from the trunk base. In the sampling points leaf litter and freshly fallen legumes were re-
moved from the soil surface, then soil cores of 80 cm2surface area and 480 cm3volume
were cut from the upper (0–6 cm) and the lower (6–12 cm) soil layers. The five-five
subsamples originating from the same vertical layers were bulked, thus forming a 2400 cm3
total soil volume per layer (4800 cm3per tree). Penetration of Robinia pseudoacacia seeds
into soil layers deeper than 12 cm is negligible (MARJAI 1995), thus the applied sampling
depth was considered to be sufficient.
Samples were transported to the laboratory and washed through a metal sieve with mesh
size of 1.5 mm. After room-temperature (22 °C) drying, black locust seeds were hand-sorted
from the debris and counted. The number of viable seeds (which actually forms the seed
bank) was calculated after performing germination tests. Prior to the germination procedure,
seeds were surface sterilized by soaking them for two minutes in a 20% ethanol solution to
deal with mould (CHUANREN et al. 2004). Hardseededness was reduced by mechanical
scarification carried out with emery paper (BASKIN and BASKIN 1998). Seeds were placed on
filter paper moistened with tap water in Petri dishes and kept at 24 °C temperature for 21
days. Evaporated water was supplied as necessary, and the germinated seeds were removed
daily. On the fifth day the non-swollen seeds were re-scarified and re-germinated. Numbers
of germinated and non-germinated seeds were determined on the 21st day.
The mean seed bank density of each studied Austrian pine stand was calculated (as
seeds m–2) by averaging the results of the five black locust trees sampled. Consequently,
these values can be regarded as the outgrowths of a 12,000 cm3sample volume, which
amount exceeds 2–3-fold the required minimal volume, which is generally 4000–6000 cm3
in climax forest vegetation (CSONTOS 2007). MORIMOTO et al. (2010) collected a 12,500
cm3sample volume from the upper 5 cm soil layer for investigation of the R. pseudoacacia
seed bank.
The relationship between basal area of tree (BA; cm2) and density of seed bank (DSB;
seeds m–2) was evaluated by standard regression methods: linear, power and logarithmic
regression. DSB was calculated by adding the numbers of viable seeds found in the upper
and lower soil layer. Further regression analyses were also carried out in order to relate the
BA (i) to density of seed bank found in the upper layer only (USB), and (ii) to seed bank ra-
tio of lower soil layer (SBR; %). In the latter case, SBR was calculated as seed density of
the lower soil layer divided by the DSB. Statistical significance was assessed at P = 0.05,
and among the regression types the one serving the best fit (greatest R2) was accepted.
Results
Altogether 250 soil core subsamples were collected (125 from each soil layer), thus 50
samples (25 from each soil layer) were analysed after bulking 5–5 subsamples of each tree
sampled. The black locust seed bank was present in both soil layers of each sampled Aus-
252 ACTA BOT. CROAT. 71 (2), 2012
CSERESNYÉS I., CSONTOS P.
trian pine stands. On average 48.6 (ranging from 20 to 96) and 10.1 (ranging from 0 to 38)
seeds per tree were found in the upper (0–6 cm) and the lower (6–12 cm) soil layer, respec-
tively. Among the 25 black locust trees sampled there was only one in the pine stand of Ács,
and below it no seeds were found in the lower soil layer. This black locust tree has the
smallest BA among the studied specimens.
Altogether 1466 seeds were washed out of soil samples and subjected to germination
testing. The seeds expressed a high germination rate: 1371 seeds, or 93.5% of the total
amount, proved to be viable. Seeds showed a greater germination rate (95.6%) in the lower
than in the upper soil layer (93.1%). On the basis of germination results, 1398 seeds m–2,
the mean soil seed bank density was calculated: 1156 and 242 seeds m–2 in the upper and
the lower soil layer, respectively (Fig. 2). Seed bank densities varied among different Aus-
trian pine stands. The greatest seed bank, 2285 seeds m–2 was developed in the pine stand of
Komárom (in 0–6 cm soil layer: 1660 seeds m–2; in 6–12 cm soil layer: 625 seeds m–2),
while the smallest, 640 seeds m–2, was found in the pine stand of Ács (only 600 and 40
seeds m–2 average densities were calculated in the upper and lower soil layer, respectively).
Seed distribution between the two soil depths also had a great variability. The mean per-
centage ratio of seeds found in the upper soil layer was 82.7% of the total amount, conse-
quently 17.3% of seeds appeared from the lower soil layer (Fig. 3). The highest (27.4%)
and smallest (6.3%) seed bank ratio in 6–12 cm soil depths were detected in the pinewoods
of Komárom and Ács, respectively (the same plantations that were responsible for the
highest and the smallest total seed bank densities).
The basal area of the sampled black locust trees ranged from 62 to 1089 cm2(average
value was 488 cm2). The simple regression analysis showed a positive correlation (P <
0.001) between the the basal area of tree (BA, cm2) and the total density of seed bank (DSB;
seeds m–2, Fig. 4). The obtained regression equation [1]indicates a curvilinear relationship:
ACTA BOT. CROAT. 71 (2), 2012 253
ROBINIA PSEUDOACACIA SEED BANK IN AUSTRIAN PINE STANDS
Fig. 2. Average densities of soil seed bank in the upper (0–6 cm) and lower (6–12 cm) soil layers
under Robinia pseudoacacia trees growing in Pinus nigra stands, in Hungary. Sampling
sites are ordered according to age of the plantations (solid bars show the average values of
the five sites).
DSB = 61.87 BA0.504 [1]
adjusted R2= 0.7003. Relationship between BA and seed bank density in the upper soil
layer (USB) proved to be similar (USB = 78.33 BA0.433)butatalowerfit(R
2= 0.5596).
254 ACTA BOT. CROAT. 71 (2), 2012
CSERESNYÉS I., CSONTOS P.
Fig. 3. Percentage share of soil seed bank distribution between the upper (0–6 cm) and lower (6–12
cm) soil layers under Robinia pseudoacacia trees growing in Pinus nigra stands. Sampling
sites are ordered according to age of the plantations (right end bar shows the average of the
five sites).
Fig. 4. Correlation between the total density of seed bank (DSB; seeds m–2) and the basal area (BA;
cm2)ofRobinia pseudoacacia trees growing in Pinus nigra stands, in Hungary. The equa-
tion of curve: DSB = 61.87 BA0.504 (R2= 0.7003)
Regression analysis resulted in a linear and positive relationship (p < 0.001) between
the BA and the seed bank ratio of lower soil layer (SBR) (Fig. 5). Increase in BAcontrib-
utes to the increase of SBR (in %), according to equation [2]:
SBR = 0.0204 BA + 3.0576 [2]
adjusted R2= 0.5579.
Two SBR data had to be excluded from statistical evaluation, because of the performed
outlier analysis: one black locust tree in the sampled pinewood of Komárom and Tárkány
as well.
Discussion
Our investigation verified the ability of the black locust to form a persistent soil seed
bank in Austrian pine stands, and also showed a correlation between seed bank density and
age of tree. The density of Robinia pseudoacacia seed banks ranged from 640 to 2285
seeds m–2 in the pine plantations studied; its mean value was about 1400 seeds m–2. Under
black locust trees in several of the city parks of Budapest, an 871 seeds m–2 soil seed bank
density and – after mechanical scarification – a 94% germination rate were reported by
SIMKÓ and CSONTOS (2009). Our mean germination result (93.5%) also agrees with the
92–98% value stated by MASAKA and YAMADA (2009) on the basis of their ecophysio-
logical research executed in Japan. An extensive field study showed a 96% germination
rate and a 2000–12 000 seeds m–2 seed bank density in monodominant black locust stands
ACTA BOT. CROAT. 71 (2), 2012 255
ROBINIA PSEUDOACACIA SEED BANK IN AUSTRIAN PINE STANDS
Fig. 5. Correlation between the seed bank ratio of lower (6–12 cm) soil layer (SBR; %) and the basal
area (BA; cm2)ofRobinia pseudoacacia trees growing in Pinus nigra stands, in Hungary.
(Empty squares show outliers excluded from the regression analysis.) The equation of line:
SBR = 0.0204 BA + 3.0576 (R2= 0.5579).
in Hungary (MARJAI 1995). In this case, the monodominant character of the studied
Robinia plantations can explain the relatively high seed bank density. As opposed to
MARJAI’s (1995) research, our investigation was carried out under the canopies of solitary
black locust trees in Austrian pine stands, where the amount of seeds incidentally scattered
to greater distances, was not counterbalanced by seed rain of neighbouring trees.
Black locust seed bank was detected in both soil layers. Since the penetration of seeds
to deeper soil layers is time-consuming, a high seed density ratio in the 6–12 cm soil depth
was mainly observable under old black locust trees, i.e. having large basal area (Fig. 5).
The extremely high seed density ratios (shown by the two outlier data) was probably due to
local disturbances caused by forest animals or human activities, that by turning up lower
soil layers resulted in a local »seed bank profile inversion« (CSONTOS 2007).
Higher germination rate was detected in the lower (95.6%) than in the upper one soil
layer (93.1%). Most probably this resulted (i) partly from the favourable environmental
conditions for seed survival associated with deep burial (WITKOWSKI and GARNER 2000,
FENNER and THOMPSON 2005), and (ii) partly from the higher decomposition rate of non-vi-
able seeds in the deeper soil layer. Both the growing seed production of elderly trees and the
long-term accumulation of dormant seeds may interpret the age-dependent increase in soil
seed bank density (MARJAI 1995).
Though the fecundity of R. pseudoacacia generally declines after about 40–50 years,
corresponding to the decrease in tree vigour, the soil seed bank density may continue to in-
crease owing to the considerable accumulation of dormant seeds (MASAKA et al. 2010).
Robinia seed longevity does not exceed 40 years in general (TOOLE and BROWN 1946),
therefore the net seed accumulation rate becomes regressive in the course of time, leading
to a curvilinear relationship between seed bank density and basal area of tree (Fig. 4).
Different natural and human disturbances also facilitate the spreading of the black lo-
cust. The lifetime of Austrian pine is up to hundreds of years in its natural area, but in the
Hungarian stands the decline of trees begins much earlier, mainly as a consequence of the
suboptimal environmental conditions. The resistance of 40–50 year old pines declines
quickly in shallow-soiled habitats and thus rapid destruction can be initiated by a perma-
nent drought or infested pathogen fungi (KOLTAY 1990, 1997). These factors promote the
penetration and spread of invaders in Austrian pine stands. The black locust was natural-
ized in East Asia in the second half of the 19th century; nowadays the invader regularly ap-
pears in indigenous Pinus thunbergii stands and broad-leaved forests, chiefly in the vicinity
of severely disturbed urban regions (MAEKAWAand NAKAGOSHI 1997, LEE et al. 2004, SONG
et al. 2005, TANIGUCHI et al. 2007, MORIMOTO et al. 2010). Frequent fire events prove that
pine plantations in Hungary are highly susceptible to forest fires in view of their accumu-
lated resinous needle litter (CSERESNYÉS and CSONTOS 2004; CSERESNYÉS et al. 2006, 2011).
The heat effect is capable of breaking the physical dormancy of black locust seeds (MARJAI
1995, MASAKAand YAMADA2009), thus initiating a quick colonization by the invader in the
burnt area (AULD and DENHAM 2006, JUNG et al. 2009).
Within the scope of sustainable forest management, the replacement of Austrian pine
stands by natural vegetation types began in nature reserves of Hungary in the past few de-
cades. After coniferous stands are clear cut, the recolonisation of indigenous species from
nearby native forests and grasslands is generally slow, even if the appropriate propagulum
sources are available in its surroundings (MATLACK 1994, RYDGREN et al. 1998, TAMÁS
256 ACTA BOT. CROAT. 71 (2), 2012
CSERESNYÉS I., CSONTOS P.
2001). Under these circumstances regeneration of the native vegetation primarily happens
from the locally available soil seed bank. In soil of Pinus nigra stands the seed bank of spe-
cies of the native flora (that existed prior to afforestation by pine) became impoverished; its
density and richness steadily declined because of their short-term persistent seeds (CSON-
TOS et al. 1996, AUGUSTO et al. 2001). Consequently the diverse natural seed bank could be
replaced gradually by the high density persistent seed bank of non-indigenous species in-
cluding the black locust. The germination of R. pseudoacacia seeds can be continued for
decades, strongly inhibiting or preventing completely the restoration of the native flora af-
ter the removal of an Austrian pine stand. The effective root sprouting also promotes the
rapid spread of locust trees, especially after local disturbances, and thus finally it can hap-
pen that an area just cleared from the non-native pine is occupied by another non-native
tree, the locust, which would be a rather unacceptable result. Since both the density of the
seed bank and the ratio of seeds detected in the lower soil layer increases with the diameter
at breast height of the tree, the threat of a spontaneous black locust stand establishment is
particularly to be expected in areas already sporadically occupied by old black locust trees.
The black locust is responsible for irreversible changes in the physicochemical and biologi-
cal soil properties by N-fixation, leading to the formation of species-poor nitrophilous
weed associations in the herb layer (TOBISCH et al. 2003). Therefore, establishment of the
black locust should be prevented by careful planning of the replacement of Austrian pine
stands by native tree species.
Acknowledgements
Many thanks are due to Erika Cseresnyés-Bózsing for her help in seed germination tests
and for useful suggestions on the manuscript. We thank foresters József Mák and Ferenc
Vadas for their help in data collection concerning Austrian pine stands. We are grateful to
the anonymous referees for their comments.
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