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Plant Biosystems - An International Journal Dealing with
all Aspects of Plant Biology
Official Journal of the Societa Botanica Italiana
ISSN: (Print) (Online) Journal homepage: www.tandfonline.com/journals/tplb20
Exploring the distribution pattern of native and
alien forests and their woody species diversity in a
small Mediterranean city
Marco Varricchione, M. Laura Carranza, Chiara D’Angeli, M. Carla de
Francesco, Michele Innangi, Lucia A. Santoianni & Angela Stanisci
To cite this article: Marco Varricchione, M. Laura Carranza, Chiara D’Angeli, M. Carla de
Francesco, Michele Innangi, Lucia A. Santoianni & Angela Stanisci (2024) Exploring the
distribution pattern of native and alien forests and their woody species diversity in a small
Mediterranean city, Plant Biosystems - An International Journal Dealing with all Aspects of
Plant Biology, 158:6, 1335-1346, DOI: 10.1080/11263504.2024.2415613
To link to this article: https://doi.org/10.1080/11263504.2024.2415613
© 2024 The Author(s). Published by Informa
UK Limited, trading as Taylor & Francis
Group
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Published online: 26 Oct 2024.
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PLANT BIOSYSTEMS AN INTERNATIONAL JOURNAL DEALING WITH ALL ASPECTS OF PLANT BIOLOGY
2024, VOL. 158, NO. 6, 1335–1346
Exploring the distribution pattern of native and alien forests and their woody
species diversity in a small Mediterranean city
Marco Varricchionea,b , M. Laura Carranzaa,b , Chiara D’Angelia,c , M. Carla de Francescoa,b ,
Michele Innangia , Lucia A. Santoiannia and Angela Staniscia,b
aEnviXLab, Department of Biosciences and Territory, University of Molise, Pesche, IS, Italy; bNational Biodiversity Future Center (NBFC), Palermo,
PA, Italy; cItalian Institute for Environmental Protection and Research, ISPRA, Roma, RM, Italy
ABSTRACT
Recent studies have explored the ecological relationship between native urban forests and self-sown
non-native forests in large cities and metropolises but further research efforts dedicated to analyzing
this relationship in small cities are still needed. To improve our understanding of the ecology of urban
native and alien forests in Mediterranean small cities, we analyzed the woody species richness, the
community-weight mean of moisture and nitrogen ecological indicators, and soil disturbance indicators
in the main urban wood types of the city of Campobasso (Italy), as well as their spatial distribution
pattern across a gradient of cover and aggregation of green areas. The study showed that both native
oak forests and Robinia pseudoacacia forests contributed to the maintenance of woody species richness.
If we exclude the riparian environment, R. pseudoacacia forests occupied small marginal patches,
tolerating soil disturbance and a high disturbance frequency, thus occupying habitats where the native
oak forests could not grow. Conversely, R. pseudoacacia forests shared the ecological niche with the
native riparian forests, which calls for action to prevent the spread of this alien species along river
banks. Our results highlighted that urban remnant forests should be considered important assets for the
planning and upkeep of urban green areas.
1. Introduction
Currently, over 50% of the world’s human population lives in
urban and peri-urban areas and more than two-thirds are
expected to live in cities by 2050 (UN-Habitat 2022). Often
the remnant patches of semi-natural vegetation embedded
in the urban fabric represent the only contact with nature of
people living in cities (Endlicher 2012).
Urban green spaces, which encompass a variety of urban
element types including public parks and gardens, green
infrastructures, residual urban forests, and adjacent rural
areas, have a significant impact on the livability of cities.
The primary function of urban green spaces is to reduce
noise and purify the air by absorbing pollutants such as
ozone (O3), sulfur dioxide (SO2), nitrogen dioxide (NO2), car-
bon monoxide (CO), and particulate matter less than 10 m
(PM10) (Nowak et al. 2006; Tallis et al. 2011; Islam et al.
2012; Livesley et al. 2016). Moreover, biodiversity contrib-
utes to improving the life conditions of the citizens, provid-
ing vital recreational spaces that support the mental,
physical, and social well-being of residents (Niemelä et al.
2010; Gómez-Baggethun and Barton 2013; Wolch et al.
2014; Carrus et al. 2015; Nesbitt et al. 2017; Spano et al.
2020). The role of nature in urban areas is also related to a
buffer function mitigating the effects of climate change
(Lafortezza and Sanesi 2019; Manoli et al. 2019). Green
spaces also contribute to regulating the local climate, reduc-
ing urban heat islands, and mitigating the runoff of storm
water in flood events (Ballinas and Barradas 2016;
Jaganmohan et al. 2016; Livesley et al. 2016). In addition,
woody species and, in general, urban forests, being sinks of
CO2 storing carbon during photosynthesis in a proportional
way to the biomass of the tree play a key role in climate
change adaptations (Rathore and Jasrai 2013).
However, cities may constitute inhospitable environments
for native plants (Cadotte et al. 2017), especially the most
specialized species, while may be particularly receptive to
opportunistic and non-native invasive species, often conform-
ing to real hotspots of biological invasion (Cadotte et al.
2017). Cities, being particularly anthropized, are characterized
by a consistent disturbance pressure that reduces native flora
and fauna diversity and leaves empty niches to be occupied
by non-native species that are often well adapted to recur-
rent disturbances (Pinto and Ortega 2016; Aryal et al. 2022).
In cities, the introduction and spread of alien propagules are
favored (Lockwood et al. 2005; Malavasi et al. 2014), and
© 2024 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group
CONTACT Maria C. de Francesco maria.defrancesco@unimol.it EnviXLab, Department of Biosciences and Territory, University of Molise, Contrada Fonte
Lappone, I-86090 Pesche, IS, Italy.
Supplemental data for this article can be accessed online at https://doi.org/10.1080/11263504.2024.2415613.
https://doi.org/10.1080/11263504.2024.2415613
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/),
which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any
way. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent.
ARTICLE HISTORY
Received 23 May 2024
Accepted 30 September
2024
KEYWORDS
Remnant native forest;
Robinia pseudoacacia;
urban green areas; patch
quality indicator;
ecological indicators;
disturbance indicators
Published online 26 Oct 2024
1336 M. VARRICCHIONE ETAL.
non-native plants benefit from urban habitat heterogeneity
(Deutschewitz et al. 2003), higher temperatures (Lososová
et al. 2012) and disturbance pressure (Chytrý et al. 2008).
Cities may become key hubs for introducing non-native
species, potentially leading to conflicts between urban man-
agement practices and efforts to conserve biodiversity and
ecosystem services not only within the cities but also on sur-
rounding landscapes (Gaertner et al. 2017; Shackleton et al.
2019; Kowarik et al. 2021). For instance, the spread of
self-sown alien trees across cities is a growing challenge
because of the adverse effects of invasions on human health,
such as those derived from allergic reactions (Nentwig and
Mebs 2017) or on biodiversity and ecosystem functioning,
due to the homogenization of forest structure and composi-
tion (McKinney 2006; Trentanovi et al. 2013). At the same
time, non-native species may play a positive role in support-
ing ecosystem services (e.g. cultural, and climate regulation)
comparable to that of native species (Kowarik 2011; Schlaepfer
et al. 2020). Furthermore, in certain circumstances, planting
non-native species may be the only viable option for estab-
lishing urban green infrastructures (Petřík et al. 2019;
Schlaepfer et al. 2020) especially in highly altered and dis-
turbed areas where native species may not thrive (Sjöman
et al. 2016).
In this context, the development of effective and sustain-
able management strategies for alien forests in urban areas
requires further research efforts aimed at enhancing the cur-
rent knowledge of the ecological behavior of non-native spe-
cies within urban settlements (Brundu and Richardson 2016).
Additionally, extra research is needed to explore their potential
benefits to society and their negative impacts on urban biodi-
versity (van Wilgen et al. 2012; Dickie et al. 2014; Potgieter
et al. 2019; Shackleton et al. 2020). For instance, pioneering
studies have delved into the ecological relationship between
native urban forests and self-sown non-native forests in large
cities and metropolises (Kowarik et al. 2019; Potgieter et al.
2022). To the best of our knowledge, there is currently a lack
of studies on this topic for small or medium-sized cities.
One tool with great potential for studying the ecology of
native and alien species in urban forests is the use of
Ecological Indicator Values for Europe (EIVE) (Dengler et al.
2023) and Disturbance Indicator Values (DIV) (Midolo et al.
2023). The usefulness of EIVE and DIV to indirectly evaluate
the ecological features of natural forests, in terms of light,
temperature, and nitrogen preferences of the plant commu-
nity, and tolerance to disturbance types and intensity have
been widely demonstrated (Bita-Nicolae 2023; Mastrogianni
et al. 2023; Seliger et al. 2023; Bricca et al. 2024). Recently,
the implementation of EIVE and DIV has been extended to
urban forests in large cities confirming their effectiveness
(Dyderski and Jagodziński 2019; De Pauw et al. 2024). To our
knowledge, the application of these indicators to explore the
ecology of urban forests and the relationship between native
and non-native forests in small cities has not yet been tested.
Besides, the spatial distribution analysis of urban forest
patches, through the application of landscape metrics, can
help to understand their contribution to biodiversity conser-
vation (Kowarik et al. 2019). Although this analysis was
already applied to woody vegetation patches in large cities
(Uuemaa et al. 2013; Grigorescu and Geacu 2017; Salvati
et al. 2017), only a few studies concerned small-medium cit-
ies (Sitzia et al. 2016).
To improve our understanding of the ecology of urban
native and alien forests in small cities and to provide new
insights for optimizing their management and conservation,
we focused on a small Mediterranean city.
Accordingly, we investigated the diversity pattern of
woody plant species, and the ecological and disturbance fea-
tures of urban forests, as well as their spatial distribution pat-
tern across a gradient of cover and aggregation of green
areas in the city of Campobasso (Italy).
At the community scale, we analyzed the woody species
richness, the community weight mean of moisture and nitro-
gen ecological indicators (Dengler et al. 2023), and soil dis-
turbance indicators (Midolo et al. 2023) in the main urban
forests types, according to the vegetation map of the city
(D’Angeli et al. 2024).
At the landscape scale, we quantified the size and shape
of sampled forest patches across the gradient of green areas
cover and fragmentation.
In detail, we addressed the following research questions:
1. Do urban forests that are dominated by native vs
alien/introduced woody species dier in terms of
woody species richness?
2. Do urban forests that are dominated by native vs
alien/introduced woody species dier in terms of eco-
logical features and disturbance regimes?
3. Do patches of native and alien-dominated forests dif-
fer in size and shape along the gradient of green
areas cover and fragmentation?
4. Is the occurrence of urban self-sown alien forests
context-dependent?
Small cities may be often considered disadvantaged in
terms of access to facilities, services, and socio-economic
opportunities (Nesticò et al. 2020), but they are places where
nature is often most present, even if not consistently taken
into account. Still, the majority of citizens could benefit from
ecosystem services provided by nature in small cities, unlike
what happens in metropolises (Pukowiec-Kurda 2022). Our
results can provide new insights that may be useful to opti-
mize the management and the conservation of native urban
forests, as they should be considered important assets for the
planning and upkeep of urban green areas.
2. Data and methods
2.1. Study area
We selected Campobasso, a small Mediterranean city in
Southern Italy, located 70 km from the Adriatic coast.
The Functional Urban Area (FUA) (i.e. cities with the
respective commuting zone) of Campobasso covers 1,028 km2,
and has approximately a population of 100,000 inhabitants
with a population density of 97.3 inhabitants/km2 (ISTAT
2022). It is the regional capital of Molise (one of the 20 Italian
PLANT BIOSYSTEMS AN INTERNATIONAL JOURNAL DEALING WITH ALL ASPECTS OF PLANT BIOLOGY 1337
administrative regions) and it is located in Southern Italy at a
mean elevation of 701 m a.s.l.
The site is characterized by a temperate sub-mediterranean
climate (Pesaresi etal. 2017), with average annual temperature
of 13.28 °C (period 1991 – 2021), average temperature of the
coldest month between −1 °C and 4 °C, two months with aver-
age > 20 °C and average total annual precipitation of 806 mm
(period 1991 – 2021) (Martinelli and Matzarakis 2017).
Forty-one percent of the area is covered by agricultural
land, 35% by natural and semi-natural habitats, and 24% by
artificial areas (D’Angeli etal. 2024).
According to the Map of Nature of the region (Ceralli
et al. 2021; D’Angeli et al. 2024), the main forest types refer
to six different EUNIS habitat types: Southern Italic Quercus
cerris forests, Southern Italic Quercus frainetto forest,
Italo-Sicilian Quercus pubescens forest, Southern Mediterranean
riparian Populus alba forest, Deciduous self-sown forest of
non-native trees, and Coniferous forest (Table 1). Within the
natural and semi-natural environments, Quercus cerris L. for-
ests are the most widespread and account for about 17%,
followed by forests of Quercus frainetto Ten. (10%) and
Quercus pubescens Willd. subsp. pubescens (5%), and riparian
forests (6%).
The habitats of the three native oak forests are different in
terms of geomorphological and pedological features (Biondi
et al. 2014). Quercus cerris forests prevail on moderate slopes
and N-NW aspects, on fairly evolved soils of alluvial deposits
and clayey-pelitic complexes (Taffetani and Biondi 1995). Q.
frainetto forests grow on evolved, leached, acidic, and sub-
acid soils, well-drained, in flat environments or on slightly
steep slopes (5-20°) with various aspects (Abbate et al. 1990;
Blasi and Paura 1995). Finally, Q. pubescens forests develop on
shallow soils characterized by clayey deposits, calcareous
marls, or evaporites, on sunny slopes with medium acclivity
(20-35°) (Biondi et al. 2014).
Populus alba L. and Salix alba L. forests refer to the ripar-
ian forest of base-rich soils submitted to seasonal prolonged
inundation with slow drainage (Biondi et al. 2014).
In the study area, two types of non-native forests also
occur, Robinia pseudoacacia L. forests and Pinus nigra
J.F.Arnold plantations, which account for about 7% and 1% of
natural and semi-natural environments, respectively.
Robinia pseudoacacia is an invasive alien, light-demanding
pioneer species, which forms woods along roads and railways
(Celesti-Grapow etal. 2009) in urban areas (Sitzia etal. 2018),
and in alluvial floodplains with low-slopes morphology and
on humid soils that are rich in organic matter (Vítková et al.
2015; Sitzia et al. 2016; Viciani et al. 2020).
Pinus nigra forests are old coniferous plantations included in
man-made vegetated habitats. The process of naturalization
leads to the growth of native woody species in the under-
growth (Biondi et al. 2014).
2.2. Vegetation sampling
Vegetation sampling was carried out following a random
stratified approach, according to the Italian National
Biodiversity Future Center (NBFC) methodological protocol for
studying the urban biodiversity in Italian cities (Sabelli 2023).
Inside this research center an interdisciplinary group was
implemented (Spoke 5: Urban Biodiversity) aimed at improv-
ing the current knowledge on biodiversity in Italian cities and
providing new insights to protect and improve nature in
built-up areas (https://www.nbfc.it/en/environments). In this
framework, the FUA (Functional Urban Area sensu Dijkstra
et al. (2019)) of each Italian city was divided into grid cells of
1 km2 and classified according to the extent and the number
of patches of green areas as reported on the Urban Atlas
maps, freely downloadable form Copernicus platform (https://
land.copernicus.eu/en/products/urban-atlas). The grid cell
classification ranges from low cover of green areas with high
fragmentation (distributed on several small patches) to high
cover of green areas with low fragmentation (distributed on
few large patches); in the city of Campobasso, 10 cell grid
types were found (Supplementary Appendix 1; Supplementary
Appendix 2).
Vegetation plots inside the grid cells were placed follow-
ing a random stratified approach using the six main forest
types mapped by D’Angeli et al. (2024) as strata (Table 1). We
selected 74 plots (size 10 × 10 m) and for each one, we
recorded the complete list of woody plant species, their per-
centage cover, and vegetation structure based on forest lay-
ers cover, including shrubs (0.5-5 m), canopy (>5 m), and
lianas. Species nomenclature followed the updated checklist
of “Flora d’Italia” (Pignatti et al. 2017). For alien taxa status,
we followed Galasso et al. (2018).
2.3. Data analysis
First, we briefly described the sampled EUNIS Habitats by the
number of woody species, the number of woody alien
Table 1. List of EUNIS habitats (reported in map of nature of the region (Ceralli et al. 2021; D’Angeli et al. 2024) associated with the sampled urban forests of
Campobasso city, along with the code, name, abbreviation, number of plots (N plots), total number of woody species (N species), total number of alien woody
species (N alien), and mean cover of each layer (tree, shrub and liana).
EUNIS Habitat Mean Cover Layer (%)
Code Name Abbreviation N plots N species
N
alien Tree Shrub Liana
T1932 Italo-Sicilian Quercus pubescens forests Quercus pubescens 17 54 3 78 57 30
T19511 Southern Italic Quercus cerris forests Quercus cerris 12 42 3 90 45 35
T19512 Southern Italic Quercus frainetto forests Quercus frainetto 17 43 1 83 47 40
T142 Mediterranean riparian Populus forest Salix alba 6 30 0 60 40 8
T1J Deciduous self sown forest of non site-native trees Robinia pseudoacacia 12 35 3 81 63 27
T3 Coniferous forest Pinus nigra 10 45 4 67 31 13
1338 M. VARRICCHIONE ETAL.
species, the mean cover of each layer (tree, shrub, and liana),
and woody species richness.
To assess the ecological characteristics of the six forest
types we used the species Ecological Indicator Values for
Europe (EIVE) (Dengler et al. 2023), which indicate species
preferences for moisture (EIVE M) and nitrogen (EIVE N). Then,
to indirectly assess the disturbance regime of each forest
type, we used the recently developed Disturbance Indicator
Values (Midolo et al. 2023). Specifically, we focused on indi-
cators of Soil disturbance (Proportional increase in cover of
bare ground by furrowing or soil turning) and Disturbance
frequency (Mean frequency of disturbance events) (Midolo
et al. 2023).
Moreover, we explored forest type’s ecological characteris-
tics by calculating the CWM (Community Weighted Mean)
EIVEs and CWM Disturbance indicator Values and they were
compared by the Mann-Whitney post-hoc test and repre-
sented by boxplots.
In addition, we performed a landscape-scale analysis to
investigate the spatial pattern of the six urban forest types.
First, we calculated, for each forest patch, the size (area in
m2), and the perimeter-to-area ratio (McGarigal et al. 2002)
and they were compared by the Mann-Whitney post-hoc test
and represented by boxplots. Then, to further explore the
association between forest types and the cell grid types, we
developed a patch quality index made up of four categories
of cell grid types: Low Quality (cells with high fragmentation
and low cover of green areas), Medium-Low Quality (cells
with low fragmentation and low cover of green areas),
Medium-High Quality (cells with high fragmentation and
high cover of green areas), and High Quality (cells with low
fragmentation and high cover of green areas). Accordingly,
we fitted a Poisson model (estimated using ML) to predict
plot counts within the interaction between wood type and
the Quality Index.
Statistical analyses were performed in the R statistical
computing program (R Core Team 2020).
3. Results
3.1. Woody species composition, structure, and richness
of native and alien urban forests
In the 74 sampled plots of the urban area of Campobasso,
we recorded 90 species of trees, shrubs, and liana plants, and
native species reached 91% of the total recorded species
(Supplementary Appendix 3). Among these, the most abun-
dant species (occurring in 50-70% of total sampling plots)
were Quercus cerris, Acer campestre L., Crataegus monogyna
Jacq., Cornus sanguinea L. and Prunus spinosa L. subsp. spi-
nosa as trees, Rubus ulmifolius Schott and Ligustrum vulgare L.
as common shrubs, and Hedera helix L. subsp. helix, Lonicera
caprifolium L., Clematis vitalba L. and Dioscorea communis (L.)
Caddisk & Wilkin, among lianas (Supplementary Appendix 4).
Quercus pubescens forests had the highest total number of
species while the riparian forest had the lowest one (Table 1).
Eight species out of the total woody species pool con-
sisted of alien species, and of these only five species were
self-sown: Robinia pseudoacacia, Ailanthus altissima (Mill.)
Swingle, Ligustrum japonicum Thunb., Hesperocyparis arizonica
(Greene) Bartel and Trachycarpus fortunei (Hook.) H.Wendl.
(Supplementary Appendix 3). The other alien species were
planted in areas reforested with Pinus nigra. The most fre-
quent were Robinia pseudoacacia (25.7%) and Ailanthus altis-
sima (6.7%) (Supplementary Appendix 3).
The incidence of invasive alien species in the investigated
native forests was low (Table 1); in fact, the only alien species
that was sporadically observed was R. pseudoacacia in Q.
frainetto forests (Supplementary Appendix 3).
Conversely, Ailanthus altissima and Trachycarpus fortunei
were found, albeit sporadically, in Q. pubescens forests. While
in the Q. cerris forest, always sporadically, in addition to A.
altissima and R. pseudoacacia, Hesperocyparis arizonica has
been found (Supplementary Appendix 3). The number of
woody alien species remained low also in R. pseudoacacia
forests and in the coniferous reforestations (Table 1).
The cover of the tree layer varied depending on the forest
type. It was highest in Q. cerris forests, reaching up to 90%,
and subordinately in the other oak forests, as well as in R.
pseudoacacia forests. On the other hand, it was lowest in S.
alba forests, at around 60%, and in reforested areas with P.
nigra, at approximately 67% (Table 1). The shrub layer had
the greatest cover in R. pseudoacacia forests and was primar-
ily composed of Rubus ulmifolius and R. hirtus Waldst. & Kit.
Meanwhile, the liana layer was more abundant in Q. frainetto
forests, mainly represented by Hedera helix subsp. helix,
Lonicera caprifolium, and Dioscorea communis (Table 1 and
Supplementary Appendix 4).
Overall, the Robinia pseudoacacia forests had the highest
total cover between the tree layer and the shrub layer
(Table 1).
As far as the woody species richness is concerned. The
box plots showed that Q. pubescens and Q. frainetto forests
were significantly richer than R. pseudoacacia and S. alba for-
ests (Figure 1).
3.2. Ecological and disturbance indicators
Concerning the ecological indicators, the analysis of the CWM
indicated that the oak forests and pine reforestation had sig-
nificantly different values compared to the R. pseudoacacia,
Figure 1. Boxplot comparing woody species richness per plot in each Forest
type. Letters indicate signicant dierences according to the Mann-Whitney
post-hoc test.
PLANT BIOSYSTEMS AN INTERNATIONAL JOURNAL DEALING WITH ALL ASPECTS OF PLANT BIOLOGY 1339
and Salix alba forests (Figure 2). The latter two had much
higher values for the Nitrogen (N) and the Moisture (M) indi-
cators, even if M was significantly highest in S. alba forests.
Regarding the Soil Disturbance indicator (SD), the Q.
pubescens forests and Q. frainetto forests differed significantly
from the pine forest, and S. alba and R. pseudoacacia forests,
which showed the highest values for this indicator (Figure 3).
For the Disturbance Frequency Indicator (DF), the Q. pubes-
cens forests was significantly different from Q. frainetto
forests, which had the lowest value. Additionally, S. alba and
R. pseudoacacia forests had the highest values for this indica-
tor (Figure 3).
3.3. Landscape-scale analysis
The landscape-scale analysis of the wooded patches high-
lighted that the oak-dominated forests developed on larger
patches and were therefore significantly different from the R.
pseudoacacia forests, which occupied smaller patches.
Moreover, in terms of shape index the patches of forests
dominated by oaks were significantly different from those
dominated by S. alba and R. pseudoacacia, which had the
highest values (Figure 4).
The results of the Poisson model to explore the associa-
tion between forest types and patch quality index can be
seen in Supplementary Appendix 5. The model showed that
some forest types were strongly and significantly associated
with the patch quality index, as shown in Figure 5.
In detail, oak forests were associated with higher values of
the patch quality index. However, only Q. frainetto forests
were exclusively found in the highest level of patch quality,
as Q. pubescens and Q. cerris forests were also significantly
associated with medium to high values of the index.
S. alba forests, on the other hand, was indifferent to the
gradient of patch quality, and Robinia pseudoacacia forests
were only weakly associated with high values of the gradient.
Lastly, the pine forest was associated with medium to high
index values.
Figure 2. Boxplot comparing the CWM values of EIVEs values of nitrogen (N) and moisture (M) in each Forest type. Letters indicate signicant dierences accord-
ing to the Mann-Whitney post-hoc test.
Figure 3. Boxplot comparing the CWM values of disturbance indicator values of soil disturbance (SD) and disturbance frequency (DF) in each Forest type. Letters
indicate signicant dierences according to the Mann-Whitney post-hoc test.
1340 M. VARRICCHIONE ETAL.
4. Discussion
This study has shed light on the ecological and spatial distri-
bution characteristics of remaining native forests in a small
Mediterranean city, to enhance their conservation and man-
agement. Additionally, we have gained a better understand-
ing of the ecology and invasive behavior of urban R.
pseudoacacia forests, indicating that this invasive alien spe-
cies only poses a threat to riparian habitats in the studied
urban context.
We found that the woody species pool occurring in the
city of Campobasso was mainly composed of native species
(91%) belonging to the forest types already recorded in the
region by previous studies (Abbate et al. 1990; Paura et al.
2010; D’Angeli et al. 2024). Among the most frequent woody
native species, we found several deciduous and evergreen
shrub and liana species commonly found in Q. pubescens,
Q. cerris, and Q. frainetto oak forests, which represent the
Potential Natural Vegetation of the sampled urban area (Blasi
et al. 2014). The woody flora census revealed a high natural-
ness of the sampled oak forests, even though they were
located in an urban context and on limited surfaces. These
data were consistent with observations made in other small
cities (Forman 2019) and differed greatly from what has been
recorded in large cities where aliens in the urban floras make
up 40% of the total number of taxa (Pyšek 1998; Ricotta
et al. 2009; Lososová etal. 2012). The native oak forests may
also host several herbaceous endemic species (Selvi et al.
2023) and thus are of relevant conservation concern.
Out of all the woody species recorded in the studied
urban area, R. pseudoacacia was the most common alien spe-
cies, even if its occurrence was low. R. pseudoacacia is native
to mixed deciduous forests of the warm temperate climate of
the southeastern United States (Kleinbauer et al. 2010; Sitzia
et al. 2016) and therefore it comes from warmer climates
than that of the studied area. It is consistent with the finding
that urban aliens often come from warmer climates than that
of the invaded cities, allowing them to have a pre-adaptation
to climate change (Walther et al. 2009; Géron et al. 2021).
Ailanthus altissima is less common and more heliophilous
than R. pseudoacacia (Pepe etal. 2020) and it was only found
in a few open stands with deeply disturbed soil. Also, this
species originates from Asian areas with warmer climates
than the study area (Sádlo et al. 2017; Vítková et al. 2020).
The incidence of alien species in Campobasso was low
compared to other medium-small urban areas, such as Padua
(northern Italy), where 38 aliens were recorded out of 106
species (Sitzia etal. 2016). This result was likely influenced by
reduced propagule pressure due to medium-low anthropo-
genic pressure, resulting from the small size of the city, lim-
ited industrial and commercial areas, and the hilly landscape
that characterizes the investigated urban context. Indeed, it
is well-known that the density of urban settlements and the
Figure 4. Boxplot comparing the patch areas and the Perimeter-Area ratio in each sampled Forest type. Letters indicate signicant dierences according to the
Mann-Whitney post-hoc test.
Figure 5. Graph showing counts of the interaction among the dierent Forest
types and the patch quality index. Asterisks represent signicance and are
derived from the results in Table S3 (** p < 0.01, *** p < 0.001).
PLANT BIOSYSTEMS AN INTERNATIONAL JOURNAL DEALING WITH ALL ASPECTS OF PLANT BIOLOGY 1341
intensity of land consumption are directly proportional to
the spread of invasive alien species (Jogan et al. 2022).
Moreover, urban areas surrounded by hills and landscapes
with articulated morphology often contain unused land
where native vegetation can thrive (Farinha-Marques et al.
2011). The low incidence of aliens was due to the occur-
rence of the good naturalness of the studied area, which
still preserved oak woods distributed in numerous, albeit
medium-small patches.
The high frequency of oak forests patches prevents the
occurrence of alien woody species. Additionally, the investi-
gated oak forests had a well-structured stratification and high
vegetation cover, and these ecological features are known to
provide resistance to invasive processes (Essl et al. 2012;
Wagner etal. 2017; Slabejová et al. 2023).
On the other hand, in disturbed habitats and at the edge
of roads or agricultural areas, the R. pseudoacacia forests also
showed high levels of cover in the tree and shrub layer,
exploiting with efficiency the spatial niche they occupy, as
observed in previous studies (Benesperi et al. 2012; Sádlo
et al. 2017; Vítková et al. 2020).
Among the six forest types analyzed, Q. pubescens and Q.
frainetto forests were significantly richer in woody species
than S. alba and R. pseudoacacia forests, and alien species
sporadically occurred (Supplementary Appendix 3). Oak for-
ests hosted several sclerophyllous species (Read and Sanson
2003), which tolerate periods of aridity and can absorb mois-
ture from the air even in the absence of rain (Bussotti et al.
2014) and semi-deciduous species that were able to exploit
mild autumn periods (late deciduousness) (Gratani et al.
2013). These eco-morphological features might enable these
species to buffer the climate change effects (Bussotti et al.
2014; Sferlazza et al. 2017) in the studied Mediterranean city.
The survival of these urban remnant forests was also linked
to the long life cycle of the dominant woody species, so the
effects of the constraints imposed by the urban environment
and climate change might become apparent later (Le Roux
et al. 2014; Colangelo et al. 2021). New research is necessary
to predict the composition, structure, and distribution of
urban vegetation in the coming decades to identify suitable
native tree and shrub species for urban afforestation projects
(Doody etal. 2010; Oldfield etal. 2013; Romanelli etal. 2024).
The richness of woody species in riparian forests was sig-
nificantly lower compared to other native forests. This obser-
vation is consistent with findings in scientific literature, which
attribute this pattern to the specific environmental conditions
found in riparian strips, where only a few woody species are
adapted to withstand frequent flooding (Giupponi etal. 2022;
Chiaffarelli and Vagge 2023).
Species richness was also significantly low in R. pseudoaca-
cia forests due to tree high cover and substantial soil alter-
ations caused by the dominant species (Sitzia et al. 2016;
Vítková et al. 2017). Literature confirms that R. pseudoacacia
stands are on average poorer in species than native forests in
the same biogeographic context, and particularly poor in
native species (Trentanovi et al. 2013; Kowarik et al. 2019).
However, it was interesting to note that despite the number
of native species not being equal to that of native forests,
some native shrub and tree species grew in the undergrowth
of investigated R. pseudoacacia forests, as also was observed
in other urban contexts (Sitzia etal. 2012; Vítková etal. 2020).
Moreover, as observed by Kowarik et al. (2019), in Berlin’s
invaded forests, a contingent of native species remained and
might be able to allow the regrowth of the native forests if
the disturbance ceases and when R. pseudoacacia loses com-
petitive ability in later successional stages. Indeed, this was
reported where stands have been left unmanaged for a suf-
ficient time (Motta et al. 2009; Staska et al. 2014).
In the investigated area, R. pseudoacacia forests, together
with the riparian forest of S. alba, were associated with sig-
nificantly high values for the Nitrogen indicator. Robinia pseu-
doacacia, given its ability to fix atmospheric nitrogen,
increases the availability of N in the soil and often develops
in areas close to ditches and edges of agricultural areas
where the nitrogen input in the soil is high (Vítková et al.
2015). This determined the presence of a plant community
rich in nitrophilous and ruderal species, as seen in other
urban areas as well (Slabejová etal. 2023). The Moisture indi-
cator was also significantly high in R. pseudoacacia forests.
However, as expected, the highest value was found in the
riparian forest of S. alba, rich in hygrophilous species.
Additionally, the CWM values of soil disturbance and distur-
bance frequency indicators were also significantly high in
both the above-mentioned forests. According to previous
studies, the ecological characteristics of R. pseudoacacia for-
ests closely resembled those of the riparian forests (Allegrezza
et al. 2019; Vítková et al. 2020). Indeed, R. pseudoacacia for-
ests can tolerate the disturbance that comes from occasional
flooding along ditches and streams and this allows it to
invade the areas close to river banks (Crosti etal. 2016). The
rapid growth and suckering ability of R. pseudoacacia enable
its survival and regeneration after a disturbance event (Carl
et al. 2018). However, the tolerance of R. pseudoacacia to
flooding is limited (Nadal-Sala etal. 2019) and the amount of
deadwood produced after alluvial events, can cause obstruc-
tions to normal water flow along rivers and flooding in agri-
cultural and inhabited areas (Lazzaro et al. 2020).
R. pseudoacacia forests are also frequent in newly aban-
doned urban land or vacant lots, where disturbance is recur-
rent and a ruderal succession has taken place (Vítková et al.
2020). In these habitats, native tree species cannot settle
unless the disturbance is significantly reduced. The speed of
growth and the high photosynthetic capacity make this alien
species highly performing in subtracting CO2 and pollutants
in both air and soil in disturbed terrain (Riley etal. 2018) and
fulfilling ecosystem services in harsh urban environments
(Sjöman et al. 2016).
In the P. nigra reforested area, we observed a high value
of soil disturbance indicator, as it grows in city parks where
trampling and topsoil processing occur periodically. In this
case, the disturbance-tolerant species were both native and
alien woody species, which together contribute to providing
green urban areas for recreational and leisure activities
(Caneva et al. 2020; Güner et al. 2021).
The analysis applied at the landscape scale provided fur-
ther issues in understanding the spatial configuration of the
investigated urban forests. The patches of the oak forest
types were larger and more regular in shape than R.
1342 M. VARRICCHIONE ETAL.
pseudoacacia forest, and this supports the opportunistic char-
acter of urban alien forests. Indeed, as observed by Wagner
et al. (2017), alien-dominated forests were enhanced by hab-
itat fragmentation, along with disturbance, alien propagule
pressure, and high soil nutrient levels, and it was also
observed in other large cities (Alston and Richardson 2006;
Celesti-Grapow et al. 2006; Referowska-Chodak 2019).
Pine forests were usually found in large and regular
patches, as they often were the main part of urban parks. On
the other hand, riparian forests tend to be smaller and nar-
rower, as they naturally occupy patches of elongated shape,
along the water streams (Brice et al. 2016).
As regards the results obtained through the patch quality
indicator, it is worth noting that the oak forests were signifi-
cantly different from the other analyzed forests. Q. frainetto
forest was exclusively associated with the highest value of the
patch quality index, meaning that it has been found where
more extensive green areas and low fragmentation occurred.
This oak forest was the wildest forest type found to date in
Campobasso; it was found in a well-preserved state likely
because it has experienced less forestry use after the 1960s
(Mazzoleni et al. 2004), as shown by its specific composition
and structure (Table 1; Supplementary Appendix 3 and
Supplementary Appendix 4). This finding was consistent with
what was reported by Salvati et al. (2017), who assessed that
forest patch size increases in medium-density settlements.
Q. pubescens and Q. cerris forests were also associated with
high and medium-high quality index values. These oak for-
ests developed in urban contexts with greater land consump-
tion (areas with the presence of roads, and housing) and on
smaller surfaces. These native forests have also historically
undergone more felling due to their use for firewood and
joinery (Paletto etal. 2012), and the current remnant patches
often have a simplified structure. Thus, conservation and
management policies for the enhancement of urban native
oak forests should focus on the importance of maintaining
adequate wood patch size and avoiding forest patch frag-
mentation and soil disturbance.
Pine forests were associated with medium-high values of
the index, as they were usually present in large urban parks
as previously reported (Bartoli et al. 2021). Conversely, the
riparian forest was indifferent to the patch quality gradient
because it was only linked to the presence of water streams
with natural banks (Brice et al. 2016; Pérez-Silos et al. 2019).
As far as R. pseudoacacia forests are concerned, they were
only weakly associated with high values of the patch quality
index, highlighting that its presence was only slightly linked
with large and not fragmented green areas, and that it grew
even in more urbanized sites. The high richness of native for-
ests patches is a strong detractor for the spread of invaders
as few spaces (spatial and ecological niches) are left to the
most invasive alien species (Gaertner et al. 2017; Štajerová
et al. 2017; Aryal et al. 2022). Our results thus confirmed the
necessity of a complex management strategy for Robinia
urban stands that takes into account the invaded habitat, as
well as environmental risks, and economic, cultural, and bio-
diversity aspects (Brundu and Richardson 2016; Sádlo et al.
2017). Nonetheless, R. pseudoacacia is often appreciated by
citizens (Van Herzele and Wiedemann 2003; Giannico et al.
2021) as it has been long cultivated for multiple purposes
(e.g. timber and energy production, amelioration, reclamation
of disturbed sites, honey production, forage and for orna-
mental purposes) (Vítková et al. 2020).
5. Conclusion
The study showed that both native oak forests and R. pseu-
doacacia forests contributed to the maintenance of woody
species diversity in the analyzed Mediterranean city. If we
exclude the riparian environment along ditches and streams,
urban alien forests dominated by R. pseudoacacia occupied
marginal areas, tolerating soil disturbance and a high fre-
quency of disturbance, thus occupying habitats where
the native oak forests could not grow. Native oak forests
were only found in large and medium-large patches while
urban alien forests filled up the vacant lots even in small
patches, showing therefore a context-dependent distribu-
tion. The situation was different for the riparian habitat,
where R. pseudoacacia forests shared the ecological niche
with the native forests of P. alba and S. alba, which calls for
action to prevent the spread of this alien species along
river banks.
We singled out further issues for implementing urban for-
ests conservation and management. The conservation of the
wildest oak forest (Q. frainetto forest) is possible only in large
patches where low fragmentation and high cover of green
area occur. The other oak forests (dominated by Q. pubescens
or by Q. cerris) are more tolerant, as they were also found in
the cells with high fragmentation and high cover of green
areas. The identified ecological needs of the analyzed native
forests should be taken into account for maintaining woody
plant diversity in Mediterranean cities and for preserving
green areas where citizens and tourists may live an interac-
tion with nature.
In disturbed sites, urban forest ecosystems comprising
both native forests and self-sown alien-dominated forests
may contribute to providing a range of social, cultural,
and ecological services and their integration into urban
green areas planning and upkeep is a key challenge for
the quality of life of citizens and for mitigating climate
change effects.
Acknowledgements
We are grateful to the anonymous reviewers and the editors for their
valuable comments, which helped us to improve the manuscript.
Authors’ contributions
Study conception and design were performed by A. Stanisci, M.L.
Carranza, M. Innangi, M. Varricchione. Material preparation and data col-
lection were carried out by M. Varricchione, M.C. de Francesco, L.A.
Santoianni, C. D’Angeli, M. Innangi, A. Stanisci. Data analysis was per-
formed by M. Varricchione, M. Innangi, C. D’Angeli, M.L. Carranza, L.A.
Santoianni. The rst draft of the manuscript was written by A. Stanisci,
M. Varricchione, L.A. Santoianni and all authors commented on previous
versions of the manuscript. All authors read and approved the nal
manuscript.
PLANT BIOSYSTEMS AN INTERNATIONAL JOURNAL DEALING WITH ALL ASPECTS OF PLANT BIOLOGY 1343
Disclosure statement
No potential conict of interest was reported by the author(s).
Funding
Project funded under the National Recovery and Resilience Plan (NRRP),
Mission 4 Component 2 Investment 1.4 - Call for tender No. 3138 of 16
December 2021, rectied by Decree n.3175 of 18 December 2021 of
Italian Ministry of University and Research funded by the European Union
– NextGenerationEU; Project code CN_00000033, Concession Decree No.
1034 of 17 June 2022 adopted by the Italian Ministry of University and
Research, CUP H73C22000300001, Hub: Biodiversity, Spoke 5: Urban bio-
diversity, Project title “National Biodiversity Future Center - NBFC”.
ORCID
Marco Varricchione http://orcid.org/0000-0003-4716-6609
M. Laura Carranza http://orcid.org/0000-0001-5753-890X
Chiara D’Angeli http://orcid.org/0009-0006-8983-4280
M. Carla de Francesco http://orcid.org/0000-0002-5238-1154
Michele Innangi http://orcid.org/0000-0003-2362-6025
Lucia A. Santoianni http://orcid.org/0009-0008-3486-0769
Angela Stanisci http://orcid.org/0000-0002-5302-0932
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