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FORESTRY IDEAS, 2011, vol. 17, No 1 (41)
Maria Rizzo and Patrizia Gasparini
Agricultural Research Council – Forest Monitoring and Management
Research Unit (CRA-MPF), P.za Nicolini 6, 38100 Villazzano, Trento, Italy.
E-mail: maria.rizzo@entecra.it
UDC 630.12 Received: 10 May 2010
Accepted: 15 June 2011
Abstract
The spread of alien plants, intentionally or accidentally introduced by human activity into
areas outside their native ranges, is one of the threats to natural ecosystems. Particularly
the alien plant species defined as “invasive” may alter ecosystem processes and threaten
the survival of native species in natural ecosystems or cause a critical economic impact in
agriculture. The paper presents some data about the occurrence and the distribution of alien
invasive tree species in the Italian forests derived from the Italian national forest inventory.
alien invasive species, biodiversity, forest inventory.
The uncontrolled spread of alien spe-
cies, defined as “plant species whose
presence in an area is due to intention-
al or accidental introduction by man”
(Richardson et al. 2000), represents an
important cause of the loss of biodiver-
sity at the global level.
With reference to their current inva-
sion status, Richardson et al. (2000)
divide the alien species into three cat-
egories: casual alien plants, natural-
ized non-invasive plants and invasive
plants.
This paper focuses on the third cate-
gory, the invasive alien species, defined
as “naturalized plants that produce re-
productive offspring, often in very large
numbers, at considerable distances
from parent plants, and thus have the
potential to spread over a considerable
area” (Richardson et al. 2000).
At the European level, several in-
ventories of invasive alien species have
been compiled. The international project
DAISIE (Delivering Alien Invasive Spe-
cies Inventories for Europe 2008), fund-
ed by the VI Framework Programme of
the European Commission, has provided
a global inventory of alien plant species
in the pan-European region.
Due to its specific climatic condi-
tions, Italy is one of the European coun-
tries most affected by the invasion of
alien species. Its territory has been a
centre of intense exchange and coloni-
zation of non-native biota as a result of
human trade and migrations ever since
ancient times (Blasi et al. 2007).
M. Rizzo and P. Gasparini
98
The project “A survey of the non-
native flora of Italy”, funded by the Ital-
ian Ministry for the Environment in the
years 2005–2008, produced a database
on non-native plant species in Italy. This
inventory listed 1,023 alien species, of
which 154 invasive.
The aim of this paper is to present
some data on the occurrence and the
distribution of alien invasive tree spe-
cies in Italy derived from the analysis of
the national forest inventory data. As a
first step, the list of alien species pub-
lished by Celesti-Grapow et al. (2009)
within the above mentioned project was
compared with the list of tree and shrub
species recorded during the inventory
surveys. The collected data were proc-
essed to estimate the number of stems,
the basal area and the volume per hec-
tare of invasive alien species and to give
some information on their distribution in
the Italian forest stands.
The second Italian National Forest
Inventory (National Inventory of Forests
and forest Carbon pools – INFC) is an
important source of data for forestry
and natural ecosystems management.
It provides reliable and updated infor-
mation, consistent with international
standards, about the forest resources at
national and regional level, including for-
est biodiversity.
The INFC adopted a three-phase
sampling design for stratification
(Gasparini et al. 2010), with three sam-
ples of approximately 300,000, 30,000
and 7,000 samplig units. The first sam-
ple was used to assess the land cover/
land use by photo-interpretation of dig-
ital orthophotos (first inventory phase).
The classification system adopted is
consistent with the first level of the
CORINE Land Cover System (European
Commission 1993) and with the FAO
(2001) – Forest Resources Assessment
(FRA 2000) definition of Forest and
Other Wooded Land (OWL) (UN-ECE/
FAO 1997). The quantitative measure-
ments (counts of stems by species, di-
ameter, tree height, etc.) were taken
in the field during the third phase. The
field plots were randomly selected from
the second phase sample and stratified
by administrative
district (21 re-
gions) and forest
type (23 types).
The forest type
was classified in
the field on the
basis of the pre-
vailing tree spe-
cies in the sample
plot (INFC 2003).
Figure 1 shows
the inventory plot
design. The meas-
urements were
2,000 m
2
530 m
2
N
trees (diameter ≥ 4.5 cm)
trees (diameter ≥ 9.5 cm)
dead wood (diameter ≥ 9.5 cm)
cutted trees (last year stumps)
regeneration and
shrubs (h≥ 50 cm)
forest
health
12 m
2
50 m
2
Occurrence and Distribution of Alien... 99
taken in different sized plots: two cir-
cular concentric plots for dendrometric
measurements (50 m2 and 630 m2),
two small satellite plots for forest re-
generation and shrubs (12 m2), a larger
plot (2000 m2) for the forest health
survey and qualitative measurements
(INFC 2006, Gasparini et al. 2010).
Trees with a diameter at breast height
(DBH) larger than 4.5 cm and 9.5 cm
were measured respectively within the
two concentric plots for dendrometric
measurements; forest regeneration and
shrubs include all stems with a DBH
smaller than 4.5 cm and a height larger
than 50 cm.
The total and per hectare estimates
of the observed or derived variables
(number of stems, basal area, growing
stock, etc.) were obtained using the
estimators proposed by Fattorini et al.
(2006).
For this study, the field data con-
cerning invasive alien species were
processed separately to estimate the
number of stems, the basal area and
the stem volume (total and per hectare
values) of these spe-
cies at national and
regional level and by
forest type. Moreover,
data on regeneration
of invasive alien spe-
cies were used to es-
timate the number of
regeneration stems,
for comparison with
the same estimates on
non-invasive alien spe-
cies. The alien species
classification adopted
is the one proposed by
Celesti-Grapow et al.
(2009).
Comparing the list of Celesti-Grapow et
al. (2009) with the list of the species
recorded during the INFC field surveys,
a noteworthy feature is that 34 alien
species were found, of which 15 natu-
ralized, 10 casual and 9 invasive. Table
1 gives the list of the invasive alien spe-
cies surveyed by INFC.
In 440 of the 6,685 plots surveyed
during the inventory, one or more stems
of invasive alien species (DBH ≥ 4.5 cm)
were counted (Fig. 2). Within “forest”
category of INFC, the total number of al-
ien species stems represented 4.43% of
overall total and 2.7% was the propor-
tion of invasive alien species. In terms
of basal area and volume, the propor-
tion of invasive alien species was 1.9%
and 1.8%, respectively. Focusing on
“tall trees forests”, the most important
inventory category (8,582,968 ha on
8,759,200 ha of the total forest area),
table 2 gives the total and per hectare
estimates of the number of stems, ba-
sal area and volume for invasive alien
Species Common name Habitus
Acacia dealbata Link silver wattle shrub
Acer negundo L. ash leaved maple tree
Ailanthus altissima (Mill.) Swingle tree of heaven tree
Amorpha fruticosa L. false indigo shrub
Opuntia ficus-indica (L.) Mill. prickly-pear cacti shrub
Prunus laurocerasus L. laurel cherry shrub
Prunus serotina Ehrh. wild black cherry tree
Quercus rubra L. red oak tree
Robinia pseudacacia L. black locust tree
M. Rizzo and P. Gasparini
100
tree species, compared with the same
estimates for other species. Figures 3
and 4 show the proportion of invasive
alien tree species respectively by spe-
cies and forest type. The forest type
with the highest value of alien species
was the “other deciduous broadleaved
forests”; one fourth of this forest type
(233,553 ha, 2.7% of tall trees forests)
was represented by invasive alien tree
species dominated mainly by black lo-
cust stands. Coniferous forest types
and cork oak stands were almost not
affected by invasive alien species.
Figures 5 and 6 show the total vol-
ume and the number of stems of inva-
0.1 1.5 0.9
92.7
0.1 1.1 3.0
92.9
3.1 2.0
0
10
20
30
40
50
60
70
80
90
100
Acer neg undo Ailanthus altiss ima Prunus serotina Quercus rubra Robinia
pseudac acia
%
Number of stems Basa l area
1,5
1,1
0,8
0,7
0,6
0,4
0,3
0,2
0,2
0,1
0,0
0,8
1,0
0,5
0,3
0,4
0,3
0,5
0,2
0,2
0,1
0,0
0,7
75,9
7,3
4,4
3,2
3,0
0,0
75,8
5,2
6,1
3,7
4,2
0
10
20
30
40
50
60
70
80
14 9 11 12 1 3 17 10 18 5 15 19 8 6 4 7 20 2
%
Nu mber of s tems Ba sal area
1 Larch and Stone pine
2 Norway spruce
3 Fir
4 Scots pine and Mountain pine
5 Black pines
6 Mediterranean pines
7 Other coniferous forests
8 Beech
9 Temperate oaks
10 Mediterranean oaks
11 Chestnut
12 Hornbeam and Hophornbeam
13 Hygrophilous forests
14 Other deciduous broadleaved forests
15 Holm oak
16 Cork oak
17 Other evergreen broadleaved forests
18 Poplar plantations
19 Other broadleaved plantations
20 Coniferous plantations
Occurrence and Distribution of Alien... 101
sive alien species in the Italian adminis-
trative regions.
As shown in figure 3, the most wide-
spread species, within “tall
trees forest” category, is Ro-
binia pseudacacia L., (more
than 92.7% of the invasive
alien species stems counted
by INFC, 92.9% of the total
invasive alien species basal
area). Prunus serotina Ehrh.,
Ailanthus altissima (Mill.)
Swingle, Quercus rubra L.
and Acer negundo L. are
the other main invasive alien
species.
The highest values for
the number of invasive alien
stems were found in Pie-
monte, Lombardia, Toscana
and Emilia Romagna. These
results are consistent with
those obtained by Celesti-
Grapow et al. (2010).
Finally, figure 7 shows
the percentage of invasive
alien species regeneration
and its distribution by spe-
cies. Invasive alien species
were found in 220 of the
4,850 plots with regen-
eration. The total and per
hectare number of invasive
alien species regeneration
stems compared with the same sta-
tistics for other species are given in
table 3.
Number of stems Basal area Volume
Species
n n.ha
-1
m
2
m
2
.ha
-1
m
3
m
3
.ha
-1
Invasive alien species 314,520,116 36.6 3,343,897 0.4 22,618,478 2.6
Other species 11,556,715,954
1,346.5 173,380,117
20.2 1,233,481,015
143.7
M. Rizzo and P. Gasparini
102
The main statistics concerning in-
vasive tree species produced by the
national forest inventory give a gen-
eral picture of the occurrence and dis-
tribution on these species in Italy. The
percentage standard error of estimates
is generally high (80% or more) as a
consequence of relatively low sampling
intensity, that is suitable for the pur-
poses of the forest inventory and not
specifically for a invasive alien species
inventory.
The data presented in this paper
seem to suggest a limited occurrence
of these species, both in the oversto-
rey and in the regeneration storey. The
data obtained by Celesti-Grapow et al.
(2010) confirm that the number and
density of non-native spe-
cies is highest in artificial
land use types.
Closed-canopy forests
have long been cited as
highly resistant to invasion.
However, while invasion
of forests (in particular by
shade-tolerant exotics) may
be a slower process than
the establishment of exotic
species in disturbed or open
ecosystems, the long-term
effects are likely to be just
as pervasive (Martin et al. 2009).
The level of knowledge on the alien
flora in Italy varies considerably within
regions. In order to identify manage-
ment priorities and gain a better under-
standing of plant invasion processes, it
is crucial to distinguish, among the high
number of plants that make up non-
native floras, those species that pose
a major threat to the environment or
other aspects of human life owing to
their high rate of spread and current or
potential impact. One essential, basic
step in this direction consists of gather-
ing information on the distribution and
invasion status of each species, on the
differences in the land use in which they
occur (e.g. man-made
or natural) and on the
type of impact they
cause (e.g. ecologi-
cal or socioeconomic)
(Celesti-Grapow et al.
2010).
The forest inventory
data give a general pic-
ture of invasive alien
species occurrence and
distribution in Italy.
Further analysis could
4%
3%
6%
2%
Ailanthus altissima
Quercus rubra
Prunus serotina
Robinia pseudacacia
98%
Invasive alien species
Other species
Number of stems
Species
n n.ha
-1
Ailanthus altissima (Mill.) Swingle 58,461,747 6.7
Quercus rubra L. 88,914,371 10.2
Prunus serotina Ehrh. 124,984,909 14.3
Robinia pseudacacia L. 1,808,001,266
206.4
Other species
86,283,872,360
9,850.7
Occurrence and Distribution of Alien... 103
be carried out at plot level to study the
effects of site features and manage-
ment practices.
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