Combination of structural and compositional factors for describing forest types using national forest inventory data. Monitoring and indicators of forest biodiversity in Europe: from ideas to operationality

Abstract

For the first time in Portugal, simple variables describing the vertical structure and the
composition of forests on the Portuguese mainland were included in the 2258 sample plots of
the National Forest Inventory (DGF 2001). The vertical forest structure was assessed by
percentage cover of seven height classes and the composition of the different layers was
described using plant species, or groups of plant species, easily identifiable in the field.
Cluster analysis, in particular K-means statistics, was performed using combinations of
vertical structure and compositional data, resulting in ten main natural groups or forest types:
1) Quercus pyrenaica forests; 2) Other deciduous oak forests; 3) Arbutus unedo forests; 4)
Cistus shrubs; 5) Cytisus shrubs; 6) Acacia forests; 7) Quercus suber forests; 8) Pinus
pinaster forests; 9) Eucalyptus forests; and 10) Other forests. The last four groups were
further subdivided according to the vertical structure resulting in twenty final forest types.
The geographical distribution of these forests types and the implications for biodiversity and
other forest issues are presented and discussed.

Full text

Marco Marchetti (ed.)
Monitoring and Indicators of Forest Biodiversity in Europe – From Ideas to Operationality
EFI Proceedings No. 51, 2004
Combination of Structural and Compositional Factors
for Describing Forest Types Using National Forest
Inventory Data
F. Rego1, P. Godinho-Ferreira2, J. S. Uva3 and J. Cunha 4
1CEABN – Centro de Ecologia Aplicada Baeta Neves, Instituto Superior de Agronomia,
Lisbon, Portugal
2EFN – Estação Florestal Nacional, Instituto Nacional de Investigação Agrária e das Pescas,
Lisbon, Portugal
3DGF – Direcção Geral das Florestas, Lisbon, Portugal
4FORESTIS – Associação Florestal de Portugal, Porto, Portugal
Abstract
For the first time in Portugal, simple variables describing the vertical structure and the
composition of forests on the Portuguese mainland were included in the 2258 sample plots of
the National Forest Inventory (DGF 2001). The vertical forest structure was assessed by
percentage cover of seven height classes and the composition of the different layers was
described using plant species, or groups of plant species, easily identifiable in the field.
Cluster analysis, in particular K-means statistics, was performed using combinations of
vertical structure and compositional data, resulting in ten main natural groups or forest types:
1) Quercus pyrenaica forests; 2) Other deciduous oak forests; 3) Arbutus unedo forests; 4)
Cistus shrubs; 5) Cytisus shrubs; 6) Acacia forests; 7) Quercus suber forests; 8) Pinus
pinaster forests; 9) Eucalyptus forests; and 10) Other forests. The last four groups were
further subdivided according to the vertical structure resulting in twenty final forest types.
The geographical distribution of these forests types and the implications for biodiversity and
other forest issues are presented and discussed.
Keywords: national forest inventory, composition, vertical structure, forest types,
biodiversity.

154 Monitoring and Indicators of Forest Biodiversity in Europe – From Ideas to Operationality
1. Introduction
Biological diversity in forests depends on their composition and vertical structure
(Puumalainen 2001) and efforts have been made all over the world to include them in the
criteria and indicators of forest biodiversity (Larson et al. 2001; Stork et al. 1997). The top to
bottom description of forest composition, from tall to smaller trees, shrubs, grasses and other
plants characterise its vertical structure. Among other features, a forest with a more diverse
composition and vertical structure provides more habitats for animals where they can find
food and cover, maintains moderate temperatures by reducing convective and radiative heat
loss, thus providing climatic buffered areas, and has greater aesthetic and recreational value.
The aim of the traditional National Forest Inventory (NFI) was to describe the main
features of Portuguese forests in terms of size, condition and change. But it was more
concerned with their productive features than an extensive description of the forests. The last
revision of the NFI (DGF 2001) aimed to go further, building on the information on
productivity to describe the forests more completely. Despite adopting a simple key based on
the cover of the dominant forest tree species it assessed, for the first time in Portugal, the
cover of other plant species according to height classes. This information provided a useful
tool to better understand our forests in their composition and vertical structure. Adopting a
simple methodology, we obtained a new key of Portuguese forest types based on the vertical
distribution of the forest plant species and a “tri-dimensional” view of our forest areas.
2. Methodology
To describe forest composition and vertical structure together, and to better understand the
combined effect on the diversity of vertical forest structure, the percentage cover of forest
species according to seven height classes are used as variables. These height classes were
defined from ground level to 0.5 m, from 0.5 m to 1.0 m, from 1.0 m to 2.0 m, from 2.0 m to
4.0 m, from 4.0 m to 8.0 m, from 8.0 m to 16.0 m, and over 16.0 metres. The composition of
the different layers was described by plant species, or groups of plant species, easily
identifiable by the NFI team responsible for data collection on the ground.
The measurements were assessed from 2258 sample plots randomly distributed through the
forest area and identified by their geographical coordinates. In every sample plot the
percentage cover of the first three dominant plant species was assessed in decreasing order of
importance (Figure1). The percentage cover of plant species, or groups of plant species, was
assembled in a table in which each species according to height class was considered as
pseudo-species. The connection established between the plant species and its respective layer,
called a pseudo-species, allowed for an estimate of the proportion of cover that each plant
species contributes to a specific height class.
Applying a K-means analysis (SPSS 2003) to the matrix of the pseudo-species percentage
cover, natural groups based on the forest composition and vertical structure similarities were
obtained. Following a sequence of attempts to determine the number of natural groups that better
distributed the 2258 sample plots, ten groups were defined that effectively characterised
Portuguese forests, with each natural group corresponding to a particular forest type. The result
of the K-means analysis was the average percentage cover of plant species according to height
class for each natural group, thus outlining the profiles of resulting forest types.
Once the number and the description of the forest types were determined, they were
mapped by the Thiessen polygons method (Soares 2000) using the geographical coordinates
of the 2258 sample plots.

Combination of Structural and Compositional Factors for Describing Forest Types Using... 155
3. Results and discussion
According to the NFI (DGF 2001), of the total area of the Portuguese mainland (8 879 862
ha), forest constitutes the main land use, occupying 38% of the territory, followed by 33% of
agriculture land and 23% of uncultivated land including shrubs, natural meadows and
abandoned land. The traditional forest area key is described as: maritime pine (31%), cork
oak (22%), eucalyptus (21%), holm oak (14%), other oaks (4%), umbrella pine (3%),
chestnut trees (1%), other broadleaves (3%) and other conifers (1%).
Based on the combination of composition and vertical structure, ten major forest types
were identified: Quercus pyrenaica forest (0.4%), other oak forests (0.2%), Arbutus unedo
forest (0.4%), Cistus shrubs (6.6%), Cytisus shrubs (4.5%), Acacia forest (0.8%), Quercus
suber forest (8.5%), Pinus pinaster forest (29.5%), Eucalyptus forest (18.4%) and other
forests (30.7%). Although the occupation areas of both keys are quite different, the forest
types obtained by the K-means analysis give us complementary information.
Quercus pyrenaica forests
Quercus pyrenaica forests, distributed mainly in the north and the interior centre of Portugal,
represent 0.4% of the Portuguese mainland forest area. They are very dense forests that
occupy all the height classes. The Quercus pyrenaica has a very significant percentage of
cover from the ground to 16 metres. The chestnut trees form, together with the Quercus
pyrenaica some mixed stands where Cytisus spp,Erica spp,Lavandula spp and other groups
of species occupy the understory up to 2 metres (Table 1).
Figure 1. Percentage cover of the first three dominant plant species.

156 Monitoring and Indicators of Forest Biodiversity in Europe – From Ideas to Operationality
Other oak forests
The other oak forests are distributed in three main small areas in the north, the central north
and the centre of the Portuguese mainland. They represent only 0.2% of Portuguese forest
area and are mainly dominated by Quercus faginea. It is a very dense forest of oak from the
ground to 16 metres. In some stands, holm oak is present from 1 to 8 metres. Rubus spp, a
typical climber, is abundant in these forests. The understory is also composed of Cytisus spp
and other plant species (Table 1).
Arbutus unedo forests
These forests are distributed in small areas in the interior north and centre of Portugal but
toward the south, these formations are larger. This forest type represents 0.4% of the
Portuguese forest area. Its vertical structure shows that these formations are lower forests
with a dense cover of Arbutus unedo from the ground to 4 metres, sometimes reaching 8
metres. The understory is also defined by significant amounts of Ulex spp,Erica spp,Cistus
ladanifer, and other plants such as Quercus coccifera,Juniperus spp and Pterospartum
tridentatum. The overstory is mainly composed of cork oak formations, sometimes forming
mixed stands with maritime pine (Table 1).
Cistus shrubs
The Cistus shrubs are distributed in the interior north and centre of the Portuguese mainland
but it is more abundant in the south, increasing from the coast to the interior. They represent
6.6% of the forest area. Although it is considered a forest land use class, it is important to
notice that the most important cover does not exceed the 2–4 metres height class. It is
composed mainly of Cistus ladanifer,Lavandula spp and other understory plants. The
overstory is characterised by open formations of “montados” of cork oak and holm oak and
this is the reason why they are considered forest areas. From our point of view, this forest
type should be considered a shrub formation (Table 1).
Cytisus shrubs
This forest type is mainly distributed in the interior north and centre of the country and a
significant area of Cytisus shrubs exists on the west coast of Portugal. It represents 4.5 % of the
forest area. But, like Cistus shrubs, the most important cover does not exceed the 2-4 metres
height class and is composed mainly by Cytisus spp,Cistus ladanifer, and other understory
plants. The overstory is characterised by very open and tall formations of maritime pine and
Quercus pyrenaica. We could also consider this forest type a shrub formation (Table 1).
Acacia forests
Even if we can find some significant acacia stands on the coast near Lisboa and the southwest
coast of Portugal, this forest type is mostly distributed along the central and northwestern
coasts of Portugal, in some regions reaching the interior north and centre of the country.
Acacia forests represent 0.8% of the forest area. This forest type shows a dense cover from

Combination of Structural and Compositional Factors for Describing Forest Types Using... 157
Table 1. Average percentage cover of plant species, or groups of plant species, according to height class (C1 to C7) of forest types: Quercus pyrenaica forests, other
oak forests, Arbutus unedo forests, Cistus shrubs, Cytisus shrubs and Acacia forests.
Forest type Quercus pyrenaica Other oaks forests Arbutus unedo forests Cistus shrubs Cytisus shrubs Acacia forests
Forests
Plant species
composition
according to
height class
C1 (>16m) 3.3 0.3 0.5 2.8 1.5 0.8 1.8
C2 (8–16m) 10.8 0.9 17.5 3.4 1.5 3.0 1.3 2.6 2.2 1.0 2.0 4.8 4.7 2.1 9.4 0.3
C3 (4–8m) 26.9 1.4 47.5 1.5 8.3 1.8 1.0 8.5 6.5 4.2 3.4 2.5 0.6 6.1 6.6 5.6 2.4 18.5 0.3
C4 (2–4m) 36.7 2.8 0.1 46.3 1.5 6.3 2.0 7.5 2.0 0.3 0.3 0.5 3.9 5.7 4.5 4.1 3.0 2.8 6.4 5.8 2.2 3.5 1.8 25.9 0.6
C5 (1–2m) 42.2 3.9 0.7 0.7 37.5 0.5 8.8 5.0 4.0 26.3 3.5 1.3 1.2 3.2 1.4 2.4 17.1 0.1 5.2 2.7 2.2 22.0 4.9 1.5 2.9 0.6 26.5 0.4
C6 (0.5–1m) 34.7 3.9 2.1 2.3 30.0 15.0 2.5 13.8 2.5 29.0 7.0 2.5 1.5 10.5 0.4 1.5 30.8 1.9 8.2 2.1 1.9 38.1 7.7 1.2 0.6 26.5 3.1
C7 (0–0.5m) 35.6 4.4 4.1 6.7 27.5 16.3 3.8 21.3 30.5 6.0 4.0 4.5 8.0 0.5 1.4 37.3 5.1 12.5 1.8 1.8 42.8 11.2 1.8 26.0 9.5
Quercus pyrenaica
Castanea sativa
Cytisus spp
Others
Quercus spp
Quercus rotundifolia
Rubus spp
Cytisus spp
Others
Quercus suber
Arbutus unedo
Ulex spp
Erica spp
Cistus ladanifer
Others
Quercus suber
Quercus rotundifolia
Cistus ladanifer
Lavandula spp
Others
Pinus pinaster
Quercus pyrenaica
Cytisus spp
Others
Pinus pinaster
Pinus pinea
Eucalyptus spp
Acacia spp
Others

158 Monitoring and Indicators of Forest Biodiversity in Europe – From Ideas to Operationality
the ground level to 16 metres and, as an invader species, it spreads in open and tall maritime
pine, umbrella pine, and eucalyptus stands (Table 1).
The forest types Quercus suber forests, Pinus pinaster forests, Eucalyptus forests and
Other forests reveal a diverse vertical structure in their geographical distribution. To better
characterise these forest types they were subdivided into four different groups according their
vertical structure: open and tall formations, open and low formations, dense and tall
formations, and dense and low formations.
Quercus suber forests
The Quercus suber forests are distributed mainly in the south of Portugal and they represent
8.5% of the forest area. The open formations of Quercus suber, representing 20% of this
major forest type, are “montados” where cork oaks never reach 16 metres and the sparse
understory is never higher than 1 metre, and composed of Cistus salvifolius and Ulex spp
(Figure 2).
Dense formations reveal an overstory composed of cork oak sometimes mixed with
Quercus rotundifolia and Pinus pinea. Dense and tall formations comprise 28% of this forest
type and show an understory composed mainly of Cistus salvifolius,Ulex spp and Erica spp.
Dense and low formations are more abundant, corresponding to 52% of the Quercus suber
forests area; they have a denser understory composed of Cistus salvifolius and Ulex spp.
Cistus ladanifer and Lavandula spp are also significant (Figure 3).
Figure 2. Vertical structure of open formations of Quercus suber forests.
Figure 3. Vertical structure of dense formations of Quercus suber forests.
010203040506070
% o f C o v e r
C7
C6
C5
C4
C3
C2
C1
Op en & Tall Quercus suber Forest
Quercu s sube r Oth e rs
0 10203040506070
% of Cover
C7
C5
C3
C1
Open & Low Quercus suber Forest
Quercus suber Ot hers
01020 3040506070
% of cover
C7
C6
C5
C4
C3
C2
C1
Dens e & Tall Quercus suber Forest
Quercus suber Cist u s sa lv if olius Ot h e rs
01020 3040506070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Dense & Low Quercus suber Forest
Quercus suber Cist us sa lvif o lius Ulex spp Ot he r s

Combination of Structural and Compositional Factors for Describing Forest Types Using... 159
0 10203040506070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Open & Tall Pinus pinaster Forest
Pinus pinaster Others
0 10203040506070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Open & Low Pinus pinaster Forest
Pinus pinaster Erica spp Non representative species Others
Pinus pinaster forests
Open and tall formations occupy 10% of this forest type and they are distributed in large patches
mainly in the centre of Portugal from the coast to the interior. A significant large patch can be
observed south of Lisboa at the Península de Setúbal. Some of these formations are mixed stands
of maritime pine and eucalyptus. The understory is mainly composed of Erica spp and Ulex spp.
Open and low formations represent only 4% of this forest type. They are distributed in the
interior north and centre of the country, and south of the River Tejo with a large stand. The
maritime pine overstory sometimes contains eucalyptus and the understory is composed mainly
of Erica spp,Ulex spp,Pterospartum tridentatum and other plants (Figure 4).
Dense and tall formations of Pinus pinaster represent 32% of this forest type, distributed
mainly in the north and centre, from the coast to the interior, and on the southwest coast of the
country. Stands show a dense overstory of maritime pine, sometimes mixed with eucalyptus
and other oaks. Ulex spp,Erica spp,Cytisus spp,Pterospartum tridentatum and other plants
compose the understory. Dense and low formations characterise 54% of this forest type,
mainly distributed in the north and centre, from the coast to the interior, spreading to the
south of Portugal. The overstory of these stands is composed of maritime pine sometimes
mixed with eucalyptus and cork oak. The dense understory is composed of Erica spp,Ulex
spp,Pterospartum tridentatum and other plants (Figure 5).
Figure 4. Vertical structure of open formations of Pinus pinaster forests.
Figure 5. Vertical structure of dense formations of Pinus pinaster forests.
0 10203040506070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Dense & Tall Pinus pinaster Forest
Pinus pina ster Eucalyptus spp
Ulex spp Erica spp
Non r epresenta tive sp ecies Others
0 10203040506070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Dense & Low Pinus pinaster Forest
Pinus pina ster Ulex spp
Pterospartum tridentatum Erica spp
Non r epresent ative sp ecies Others

160 Monitoring and Indicators of Forest Biodiversity in Europe – From Ideas to Operationality
0 10203040506070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Open & Tall Eucalyptus Forest
Eucalyptus spp Others
0 10203040506070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Open & Low Eucalyptus Forest
Eucalyptus spp Others
0 10203040506070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Dense & Tall Eucalyptus Forest
Eucalyptus spp Pinus pinaster Ulex spp Others
0 10203040506070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Dense & Low Eucalyptus Forest
Eucalyptus spp Pinus pinaster Ulex spp Others
Eucalyptus forests
All the Eucalyptus forest formations follow the same geographical distribution pattern: from
north to south generally near the coast, spreading to the interior in the centre of the country.
Open formations, representing 22% of this forest type, show an overstory mainly composed
of Eucalyptus spp and an understory of Erica spp and Ulex spp (Figure 6).
Dense formations represent 76% of Eucalyptus forests. Both tall and low formations show
an overstory mainly composed of Eucalyptus spp, sometimes mixed with maritime pine. The
tall formation understory is composed of Ulex spp,Erica spp,Pterospartum tridentatum and
other plants. The low formation understory is denser and composed of Ulex spp,Erica spp,
Pterospartum tridentatum,Cistus ladanifer,Cistus salvifolius and other plants (Figure 6).
Other forests
This main forest type covers a variety of situations representing a total of 30.7% of
Portuguese forest. Open and tall formations, representing 25% of this forest type, are open
stands of Quercus suber and Quercus rotundifolia, called “montados de sobro e azinho”,
sometimes mixed with eucalyptus and maritime pine, with an understory mainly composed of
Ulex spp. Open and low formations represent 50% of the Other forests. They are stands of
Quercus rotundifolia, called “montados de azinho”, sometimes mixed with eucalyptus and
maritime pine. The understory is mainly composed of Ulex spp,Cytisus spp and Cistus spp
(Figure 7). Open formations are distributed in the south of Portugal.
Figure 6. Vertical structure of Eucalyptus forests.

Combination of Structural and Compositional Factors for Describing Forest Types Using... 161
Dense and tall formations correspond to 4% of this forest type. They are stands of Pinus
pinea,Castanea sativa and Pinus sylvestris, showing high diversity when mixed with
maritime pine, eucalyptus and oaks. Their understory is composed mainly of Rubus spp,Ulex
spp and other plants. Dense and low formations represent 21% of this forest type. They are
very diverse stands with an overstory mainly composed of maritime pine, umbrella pine,
eucalyptus, cork oak, holm oak and other trees. In the dense and diverse understory mainly
composed of Ulex spp, Rubus spp, Cistus salvifolius, important species of Quercus coccifera
and Cytisus spp can also be found (Figure 8). Dense formations are distributed from north to
south of the country.
4. Conclusions
The map produced according to the forest types is a powerful tool to better understand the
spatial distribution of vegetation structural diversity. The relationships between compositional
Figure 7. Vertical structure of open formations of Other Forests type: “montados de sobro e azinho”
and “montados de azinho”.
Figure 8. Vertical structure of dense formations of Other Forests type: umbrella pine and chestnut
woods, and the very diverse Portuguese forest.
0 10203040506070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Open & Tall Other Forests
Quercus suber Quercus rotundifolia
Eucalyptus spp Others
0 10203040506070
% of Cov er
C7
C6
C5
C4
C3
C2
C1
Open & Low Other Forests
Quercus rotundifolia Others
010 20304050 6070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Dense & Tall Other Forests
Pinus pinea Pi n u s sy l v e st r i s
Castanea sat iva Rub u s sp p
Non representat iv e species Ot h e r s
010203040506070
% of Cover
C7
C6
C5
C4
C3
C2
C1
Dense & Low Other Forests
Pi nus pi nast er Pi nus pi nea Eucalyptus spp
Querc us r ot undi f ol i a Quer cus r obur Quercus suber
Ulex spp Cistus salvifolius Rubus spp
Non repr esent ati ve species Others

162 Monitoring and Indicators of Forest Biodiversity in Europe – From Ideas to Operationality
and vertical structure of forest formations and the vertebrate and invertebrate fauna diversity
can be studied. Research on the influence of these forest types on the distribution of reptile
and amphibian richness in the Portuguese mainland is in progress. The good correlations
obtained by this work suggest how sensitive this tool is. It can be applied to other biodiversity
subjects and forest issues such as the evaluation of above-ground biomass, CO2 sequestration,
fuel loads and fire hazards.
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