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Abstract

Information on forest structure is fundamentally important to track successional vegetation dynamics for efficient forest management. This study reports on vegetation characteristics, dominance patterns and species height growth in a northern mistbelt forest type in South Africa. Common alpha-diversity indices (species richness and Shannon–Weiner diversity), structural vegetation parameters (tree density and basal area), and species importance value index were used. Size class distribution and height–diameter allometry were further examined for the overall stand and most important species. Stem densities (472.0 ± 43.5 and 605.3 ± 28.1 trees ha-1 for >5cm to <10 cm and > 10 cm dbh (diameter at breast height) classes, respectively) and basal area values (1.99 ± 0.19 and 48.07 ± 3.46 m2 ha-1, respectively) are comparable to other Afromontane forests in East Africa. The overall stand showed an inverted-J shaped distribution pattern which is a typical feature of stand size class distribution in most natural forests. Most ecologically important species also exhibited an inverted-J shaped distribution pattern, suggesting good regeneration and recruitment potential. There were significant differences in species on height, reflecting species-specific height growth patterns, possibly a result of intrinsic growth potential and competitive interactions. The present study suggests that conservation and management policies, including protection of surrounding land uses against fire, contribute to maintaining a successful recovery of these forests. However, it should be noted that these forests may be experiencing relatively slow dynamic flux as a result of the overmature state of some trees with several years under relatively strict protection.
ORIGINAL PAPER
Vegetation structure, dominance patterns and height growth
in an Afromontane forest, Southern Africa
Sylvanus Mensah
1,2
Anthony Egeru
2
Achille Ephrem Assogbadjo
1,3
Romain Gle
`le
`Kakaı
¨
1
Received: 26 January 2018 / Accepted: 28 March 2018
Northeast Forestry University and Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract Information on forest structure is fundamentally
important to track successional vegetation dynamics for
efficient forest management. This study reports on vege-
tation characteristics, dominance patterns and species
height growth in a northern mistbelt forest type in South
Africa. Common alpha-diversity indices (species richness
and Shannon–Weiner diversity), structural vegetation
parameters (tree density and basal area), and species
importance value index were used. Size class distribution
and height–diameter allometry were further examined for
the overall stand and most important species. Stem densi-
ties (472.0 ±43.5 and 605.3 ±28.1 trees ha
-1
for C5
cm to \10 cm and C10 cm dbh (diameter at breast
height) classes, respectively) and basal area values
(1.99 ±0.19 and 48.07 ±3.46 m
2
ha
-1
, respectively) are
comparable to other Afromontane forests in East Africa.
The overall stand showed an inverted-J shaped distribution
pattern which is a typical feature of stand size class dis-
tribution in most natural forests. Most ecologically
important species also exhibited an inverted-J shaped dis-
tribution pattern, suggesting good regeneration and
recruitment potential. There were significant differences in
species on height, reflecting species-specific height growth
patterns, possibly a result of intrinsic growth potential and
competitive interactions. The present study suggests that
conservation and management policies, including protec-
tion of surrounding land uses against fire, contribute to
maintaining a successful recovery of these forests. How-
ever, it should be noted that these forests may be experi-
encing relatively slow dynamic flux as a result of the over-
mature state of some trees with several years under rela-
tively strict protection.
Keywords Diversity Population structure Species
composition Size class distribution
Introduction
Over the last century, tropical forests suffered severely
from natural disturbances (fires, winds, floods) and timber
harvesting. Logging and forest clearing for subsistence
agriculture increase loss of biodiversity, which affects
ecosystem integrity, functions and services (Foley et al.
2007). South Africa has not been immune to such impacts
(King 1941; Cooper 1985), as the country’s forests have
been exposed to fire and unregulated harvesting. Never-
theless, by 1939, illegal logging was prohibited, and this,
added to the efforts to control fire, has contributed to the
restoration of natural forest vegetation in many degraded
areas (Geldenhuys 2002).
Project funding: This work was supported by the African Forestry
Forum and the National Research Foundation of South Africa through
the ‘‘Catchman Letaba’’ project.
The online version is available at http://www.springerlink.com
Corresponding editor: Zhu Hong.
&Sylvanus Mensah
sylvanus.m89@gmail.com
1
Laboratoire de Biomathe
´matiques et d’Estimations
Forestie
`res, Faculte
´des Sciences Agronomiques, Universite
´
d’Abomey-Calavi, 04 BP 1525 Cotonou, Benin
2
Regional Universities Forum for Capacity Building in
Agriculture, Makerere University,
P.O Box 16811, Wandegeya, Kampala, Uganda
3
Laboratoire d’Ecologie Applique
´e, Faculte
´des Sciences
Agronomiques, Universite
´d’Abomey-Calavi,
03 BP 1974 Cotonou, Benin
123
J. For. Res.
https://doi.org/10.1007/s11676-018-0801-8
Afromontane mistbelt forests are one of the few natural
forest ecosystems in South Africa which, due to the mod-
ification of the fire regime, have developed in areas that
were not previously covered in forests (Geldenhuys 2000;
Geldenhuys and Venter 2002). These forests persisted in a
fire-prone environment and then expanded into suitable ar-
eas with fire protection for timber plantations and intensive
agriculture. They consist of many small, fragmented and
widely distributed patches. Two mistbelt forests form part
of the eight natural forest groups in South Africa: the
northern mistbelt forests and the southern mistbelt forests
(Mucina and Rutherford 2006). One of the most striking
characteristics in the northern mistbelt forests is the tall
moist evergreen vegetation at altitudes up to 1800 m.
These forests serve as habitats for insect pollinators
(Mensah et al. 2017b), but also play important socio-eco-
nomic and ecological roles that support human well-being.
Among other benefits, they provide forage resources to
domestic animals and insect pollinators (contributing to
crop pollination in marginal agricultural areas), fuel wood,
food, medicinal resources, help to control biological inva-
sions, and alleviate the effects of climate change by storing
atmospheric carbon (Geldenhuys 2002; Rasethe et al. 2013;
Mensah et al. 2016a,2017a).
Information on current floristic composition and species
dominance are important to track successional dynamics of
the forest vegetation and its response to environmental
variations and human interventions. Most previous research
in the northern mistbelt forests assessed the potential for
regeneration and traditional plantation forestry of native
species (Geldenhuys and von dem Bussche 1997; Gelden-
huys 1997), growth and mortality of indigenous species
(Geldenhuys 2000), and biodiversity and floristic composi-
tion for management options (Geldenhuys and Venter 2002).
Most of these studies were conducted more than a decade ago
and although diversity and vegetation patterns are known to
be responsive to environmental conditions, species distri-
bution patterns, recovery and growth rates can also shift on a
temporal scale in response to the nature and intensity of
surrounding land uses, and to management practices (Ken-
nard 2002; Sampaio et al. 2007). Information on current
forest structure (horizontal and vertical) are crucial to char-
acterize the vegetation, and describe and interpret their
dynamics. This information is also relevant to appreciate the
effectiveness of management interventions in surrounding
land uses as well as conservation decisions.
In this study, recently collected inventory data from the
Woodbush–De Hoek natural forest in Limpopo province,
South Africa was used to explore patterns of species
diversity, vegetation structure and dominance. The specific
objective was to assess (i) overall vegetation patterns (in-
cluding dominance), focusing on taxonomic diversity
metrics (species richness and Shannon–Weiner diversity
index), and (ii) common vegetation structural parameters
such as regeneration density, tree density, basal area,
Importance Value Index, and size class distribution.
Materials and methods
Study area and data
The Woodbush–De Hoek natural forest is situated at
23500S and 29590E and is part of the northern mistbelt
forest group. The vegetation is dominated by tall evergreen
species at altitudes up to 1800 m (Geldenhuys 2002). The
area is surrounded by pine plantations managed by the
Komatiland Forests Company in Limpopo. The mean
annual rainfall is 1200 mm, with the highest peak during
summer. Additional information about the study area and
Woodbush–De Hoek natural forest are available in our
previous studies (Mensah et al. 2018a,b).
Floristic and structural data were collected by means
of forest inventories. Thirty circular plots of 500 m
2
each
were randomly laid out and thirty additional circul ar subplots
(250 m
2
) were laid out in each 500 m
2
plot. Inside the sub-
plots, diameter at breast height (dbh) was measured for all
stems C5cmto\10 cm while in the 500 m
2
plots, the
same measurements were taken for all trees with dbh C10
cm. Both dbh sizes were recorded to gain more insight into
regeneration and adult trees. Species names were recorded
using taxonomic keys of the revised and updated version of
the book, Trees of Southern Africa (Coates-Palgrave 2003).
Data analysis
Taxonomic diversity and floristic composition were anal-
ysed for all woody plants having dbh C5 cm. Taxonomic
diversity, species richness and Shannon–Weiner diversity
index were computed following Magurran (1988). Struc-
tural variables such as stem density (N, trees ha
-1
) and
basal area (G,m
2
ha
-1
) were computed and combined to
provide relative species frequencies to determine the
importance value index (IVI; Curtis and McIntosh 1951)as
an indicator of a species relative ecological importance.
Tree density was calculated as the average number of trees
per hectare, and basal area as the sum of the cross-sectional
area 1.3 m above the ground using the following equation:
G¼p
4sX
n
i¼1
0:0001 d2
ið1Þ
where Gis basal area; d
i
is diameter in cm of the i-th tree in
the plot and sthe unit area of the plot. IVI was calculated
by summing up the species relative frequency, relative
density and relative basal area as:
S. Mensah et al.
123
IVI ¼ni
Ps
i¼1ni
þfi
Ps
i¼1fi
þci
Ps
i¼1ci
ð2Þ
where IVI is the importance value index; n
i
,f
i
and c
i
are
respectively the density, frequency and basal area of the i-
th species. Size class distribution and height growth models
are important in understanding the dynamics of forest
stands. Stem diameter structures were established for the
overall stand and the ten most important species as
revealed by IVI. To better depict the size class distribution
patterns, we used the method of Condit et al. (1998). For
each species, we performed a least square linear regression
analysis using the number of individuals as dependent
variable and the midpoint of each size class as independent
variable. Each size class had 10 cm width, starting from
5 cm dbh. Because some size classes had no recording, the
number of individual species in each class was transformed
by ln (1 ?number of individuals), as suggested by Obiri
et al. (2002). The slope from each species-specific size
class regression was used as an indicator of the level of
recruitment and population structure (Lykke 1998). Nega-
tive slope values are indicative of a reverse-J SCD (size
class distribution) curve and of good recruitment, with
more individuals in smaller size classes and fewer in larger
classes (Obiri et al. 2002; Martins and Shackleton 2017).
Positive slope values indicate populations with a unimodal
SCD curve and limited recruitment, with more individuals
in larger size classes and fewer in smaller size classes.
For height growth, seven out of the ten most important
species were considered due to the limitation of height
data. The allometric relationship between tree diameter and
total height for these species were examined using the
linearized form of the power function (Mensah et al.
2016a,2017c). We tested whether height–diameter rela-
tionship varied among species by adding the species as a
categorical factor and performing an analysis of covari-
ance. Individual and interaction term effects were com-
puted from the fitted model:
ln hðÞ¼ln aðÞþbln dbh
ðÞþcspecies þe0ð3Þ
where, ln (a) is the intercept, bthe proportional height
growth rate induced by tree diameter, and c
species
the pro-
portional growth rate due to the species; ln (d
bh
) is the
natural logarithm of tree diameter and e0the additive error.
Results
Diversity, structures and dominance patterns
Fifty woody plant species belonging to 46 genera and 33
families were recorded. The overall Shannon–Wiener
diversity was 2.84. Species richness was slightly lower
for C5cmto\10 cm dbh class (35 species), compared
to the C10 cm dbh class (45 species). However, further
analyses showed that the number of species per plot was
significantly higher for smaller trees (\30 cm dbh) and
lowest for larger trees ([60 cm) (F = 115.19; P\0.001;
Fig. 1). The most diversified families were the Rutaceae
and Rubiaceae, with five and four species, respectively.
In terms of relative basal area, the most dominant
families were the Myrtaceae (12.7%) and Monimiaceae
(8.3%) (Table 1). These two families also had the highest
number of individuals, although both were represented by
only three species (Xymalos monospora (Harv.) Baill,
Syzygium gerrardii (Harv. ex Hook. f.) Burtt Davy and
Eugenia natalitia Sond.). Stem densities of the C5to\
10 cm and C10 cm dbh classes were 472.0 ±43.5 and
605.3 ±28.1 trees ha
-1
, respectively. Xymalos mono-
spora,Syzygium gerrardii,Combretum kraussii Hochst.,
Cussonia sphaerocephala Strey, and Cassipourea mal-
osana (Baker) Alston were the most abundant species in
the C10 cm dbh class, with 139.3, 119.3, 43.3, 30.7 and
24.7 stems ha
-1
, respectively. For the C5to\10 cm dbh
class, Cassipourea malosana (68.0 stems ha
-1
), Kraussia
floribunda Harv. (56.0 stems ha
-1
), Xymalos monospora
(50.7 stems ha
-1
), Syzygium gerrardii (44.0 stems ha
-1
),
Ochna arborea Burch. ex DC. (36.0 stems ha
-1
) and Ri-
norea angustifolia (Thouars) Baill. (34.7 stems ha
-1
) had
the highest densities, accounting for 61.3% of the density
in this class.
The basal areas for the C5to\10 cm and C10 cm
dbh classes were 1.99 ±0.19 and 48.07 ±3.46 m
2
ha
-1
,
respectively. Out of the 50 species, 42 had basal areas less
than 1.0 m
2
ha
-1
, contributing less than 20.0% of the stand
basal area; five species had basal areas between 1.0 and
3.0 m
2
ha
-1
(20.0% of the stand basal area). The greatest
contribution to stand basal area ([25%) was made only
the forest waterberry (Syzygium gerrardii (Fig. 2). The ten
0
5
10
Smalle r t rees Me d iu m tre es Larger t rees
Size
Species richness
Fig. 1 Variation in plot species richness among larger, medium-sized
and smaller trees
Vegetation structure, dominance patterns and height growth in an Afromontane forest, Southern
123
most important species based on the Importance Value
Index were, in order of ecological importance: X. mono-
spora,S. gerrardii,C. sphaerocephala,C. kraussii,C.
malosana,Trichilia dregeana Sond., Nuxia congesta R. Br.
ex Fresen., Kraussia floribunda, Croton sylvaticus Hochst.,
and O. arborea (Table 2).
Size class distribution
The overall size class distribution showed an inverted-J
shaped curve with several gaps in diameter classes beyond
100 cm dbh (Fig. 3). Overall, most trees were in the lower
diameter classes (\20 cm), and shows a drastic decline in
density in the higher classes. Diameter class structures
were further established for the ten most important species
Table 1 Number of species,
individuals and basal area per
taxonomic family
Family Number of species family Number of individuals Basal area (m
2
ha
-1
)
Rutaceae 5 23 0.5552
Rubiaceae 4 100 0.7278
Stilbaceae 3 63 3.6875
Celastraceae 3 38 0.3959
Myrtaceae 2 220 12.718
Lauraceae 2 28 1.5664
Achariaceae 2 25 0.8746
Ochnaceae 2 64 0.6915
Podocarpaceae 2 16 0.6155
Oleaceae 2 3 0.0175
Monimiaceae 1 247 8.3173
Araliaceae 1 47 6.0547
Meliaceae 1 31 3.7079
Combretaceae 1 82 2.7897
Euphorbiaceae 1 14 2.0073
Asteraceae 1 9 0.9156
Rosaceae 1 7 0.8973
Rhizophoraceae 1 88 0.8889
Curtisiaceae 1 12 0.6821
Moraceae 1 9 0.4275
Aphloiaceae 1 18 0.3847
Violaceae 1 49 0.3602
Putranjivaceae 1 23 0.2630
Thymelaeaceae 1 22 0.1180
Myrsinaceae 1 2 0.0854
Sapotaceae 1 2 0.0742
Cannabaceae 1 1 0.0634
Aquifoliaceae 1 2 0.0600
Malvaceae 1 4 0.0334
Melianthaceae 1 4 0.0293
Salicaceae 1 4 0.0269
Ebenaceae 1 2 0.0158
Fabaceae 1 3 0.0153
The top five most dominant families are bold
0
5
10
15
20
25
30
35
40
45
0
5
10
15
20
25
30
< 1 1–3 3–5 5–7 7–9 > 9
Contribution to basal area (%)
Number of species
Basal area (m2ha-1)
Fig. 2 Variation in number of species (solid line) according to their
contribution to the stand basal area
S. Mensah et al.
123
Table 2 Species names and importance value index (IVI)
Species RF RD RG IVI
Xymalos monospora (Harv.) Baill. 9.62 20.83 17.41 47.87
Syzygium gerrardii (Harv.) ex Hook. f.) Burtt Davy 7.45 15.85 23.75 47.06
Cussonia sphaerocephala Strey 5.62 3.61 9.68 18.91
Combretum kraussii Hochst. 4.17 6.10 5.97 16.25
Cassipourea malosana (Baker) Alston 5.08 6.32 1.41 12.82
Trichilia dregeana Sond. 2.56 3.15 6.97 12.69
Nuxia congesta R. Br. ex Fresen. 3.03 2.38 3.95 9.36
Kraussia floribunda Harv. 4.85 3.83 0.49 9.17
Croton sylvaticus Hochst. 2.35 1.78 5.02 9.16
Ochna arborea Burch. ex DC. 4.62 3.50 0.95 9.07
Cryptocarya transvaalensis Burtt Davy 3.29 1.79 3.20 8.28
Nuxia floribunda Benth. 1.97 2.05 3.40 7.43
Rinorea angustifolia (Thouars) Baill. 1.66 3.73 1.07 6.46
Oxyanthus speciosus DC 3.14 2.70 0.56 6.39
Aphloia theiformis (Vahl) Benn. 2.06 1.32 1.29 4.66
Curtisia dentata (Burm. f.) C.A. Sm. 1.87 1.08 1.67 4.62
Ochna holstii Engl. 2.60 1.60 0.39 4.60
Kiggelaria africana L. 1.76 1.09 1.69 4.54
Oricia bachmannii (Engl.) I. Verd. 2.71 1.26 0.39 4.35
Drypetes gerrardii Hutch. 1.63 1.81 0.85 4.30
Brachylaena transvaalensis Phill. & Schweick. p.p. 1.12 0.65 2.46 4.24
Peddiea africana Hook. 2.35 1.59 0.20 4.15
Podocarpus latifolius (Thunb.) R. Br. ex Mirb. 2.49 0.71 0.82 4.02
Rothmania capensis Thunb. 2.43 1.26 0.27 3.96
Prunus africana (Hook. f.) Kalkman 1.56 0.60 1.65 3.81
Ficus craterostoma Warb. ex Mildbr. & Burret 2.01 0.85 0.89 3.75
Pterocelastrus rostratus Walp. 2.02 1.34 0.18 3.54
Maytenus spp 1.09 1.58 0.63 3.30
Rawsonia lucida Harv. & Sond. 1.15 1.00 0.32 2.48
Ocotea kenyensis Robyns & R. Wilczek 1.05 0.34 0.33 1.73
Trimeria grandifolia (Hochst.) Warb. 1.28 0.35 0.07 1.71
Afrocarpus falcatus (Thunb.) R. Br. ex Mirb. 1.00 0.40 0.21 1.60
Bersama tysoniana Oliv. 1.09 0.29 0.06 1.45
Eugenia natalitia Sond. 0.74 0.52 0.11 1.38
Calpurnia aurea Baker 0.67 0.53 0.05 1.24
Calodendrum capense Thunb. 0.56 0.19 0.38 1.13
Ilex mitis (L.) Radlk. 0.70 0.18 0.11 1.00
Dombeya burgessiae Gerrard ex Harv. & Sond. 0.42 0.26 0.18 0.86
Mimusops obovata Pierre ex Engl. 0.33 0.15 0.34 0.82
Halleria lucida L. 0.56 0.17 0.09 0.81
Zanthoxylum davyi (I. Verd.) P.G. Waterman 0.44 0.16 0.18 0.78
Rapanea melanophloeos (L.) Mez 0.49 0.15 0.04 0.68
Celtis africana Burm. f. 0.42 0.09 0.15 0.65
Olea capensis L. 0.28 0.18 0.03 0.48
Diospyros whyteana (Hiern) F. White 0.28 0.16 0.03 0.46
Zanthoxylum capense (Thunb.) Harv. 0.33 0.11 0.01 0.46
Clausena anisata (Willd.) Hook. f. ex Benth. 0.28 0.16 0.02 0.46
Pavetta galpinii Bremek. 0.33 0.10 0.02 0.45
Pleurostylia capensis Oliv. 0.28 0.09 0.03 0.39
Chionanthus peglerae (C.H. Wright) Stearn 0.19 0.06 0.01 0.25
The top ten most important species are bold
RF relative frequency, RD relative density, RG relative dominance. Species order based on decreasing IVI
Vegetation structure, dominance patterns and height growth in an Afromontane forest, Southern
123
(Fig. 4). All species exhibited a tendency to the inverted
J-shape distribution, indicating higher frequency in the
lowest diameter classes and a gradual decrease from lower
to higher size classes. The J-shape distribution was more
pronounced for X. monospora,S. gerrardii,C. kraussii,C.
malosana, O. arborea and K. floribunda.C. sphaero-
cephala, T. dregeana,N. congesta and C. sylvaticus
showed a size class distribution with several gaps in
intermediate size classes. From the results of regression
analyses of SCD curves, these species had negative and
steeper slopes (from -0.09 to -0.04; Table 3), as com-
pared to N. congesta (slope = -0.03), T. dregeana
(slope = -0.01), C. sphaerocephala (slope = -0.01) and
C. sylvaticus (slope = -0.009). Size class distribution
slopes for both O. arborea and K. floribunda were not
significant due to the low number of size classes (Table 3).
These slope values suggest that species such as X.
Diameter class (cm)
Frequency
0 50 100 150 200 250
0 100 200 300 400 500
Fig. 3 Size class distribution for the overall stand
Fig. 4 Size class distribution
for the ten most important
species
S. Mensah et al.
123
monospora, S. gerrardii,C. kraussii,C. malosana had a
higher number of individuals in smaller size classes.
Meanwhile, species such as C. sphaerocephala,S. ger-
rardii and C. kraussii were still present in the higher
diameter classes ([80 cm).
Dominant species height–diameter allometry
The results of the analysis of covariance show significant
effects of diameter and species on height growth (Table 4).
There was also significant interaction between diameter
and species (F statistic = 4.34; p\0.001; Table 4), indi-
cating that height growth varied among dominant species.
As expected, height increased with increasing diameter for
the seven dominant species (Table 5). For a given dbh, C.
malosana and C. sylvaticus had slopes that were respec-
tively 0.27 ±0.10 and 0.13 ±0.06 significantly higher
than the ones for K. floribunda, S. gerrardii,T. dregeana,
X. monospora and C. kraussii, which was used here as a
reference. These results indicate that, for the same dbh, C.
malosana and C. sylvaticus had significantly higher aver-
age heights at tree level, as compared to K. floribunda, C.
kraussii, S. gerrardii, X. monospora and T. dregeana
(Fig. 5).
Discussion
When compared with Afromontane forests in Ethiopia
(Kebede et al. 2013; Tadele et al. 2014), our results show,
despite the limited spatial coverage, that the northern
mistbelt forest harbours considerable floristic diversity in
terms of both tree species and taxonomic families. More
species were expected to be found in the regeneration class,
as general floristic patterns in multispecies natural forests.
Out of the 50 species enumerated, 35 were in the C5
to \10 cm dbh class and 45 in C10 cm dbh class. The
slightly lower species richness in the 5-0 cm class could be
due to the 5-cm threshold considered, which excludes
additional species \5 cm. Our results further reveal that
Table 3 Slope of species-specific size class distribution
Species Slope SE R square pvalue
X. monospora -0.06 0.009 0.89 \0.001
S. gerrardii -0.05 0.008 0.83 \0.001
C. sphaerocephala -0.01 0.002 0.63 \0.001
C. kraussii -0.04 0.004 0.93 \0.001
C. malosana -0.09 0.02 0.75 0.037
T. dregeana -0.01 0.004 0.48 0.004
N. congesta -0.03 0.006 0.61 0.005
C. sylvaticus -0.009 0.006 0.11 0.183
O. arborea -0.13 0.01 0.99 0.053
K. floribunda ––– –
SE standard error; pvalue indicates the level of significance for each
slope value; these values were not calculated for K. floribunda due to
a limited number of size classes
Table 4 Results of the analysis of covariance on the effects of spe-
cies and diameter on height
Source of variation Df Sum Sq Mean Sq Fvalue Pr([F)
Log (dbh) 1 49.20 49.20 2544.00 \0.001
Species 6 3.57 0.60 30.80 \0.001
Log (dbh): species 6 0.50 0.08 4.34 \0.001
Residuals 263 5.09 0.02
dbh is diameter at breast height, Df is degrees of freedom, Sum Sq is
sum of squares, Mean Sq is mean square
Table 5 Results of the linear
models of individual and
interaction effects of species
and diameter on height
Terms Independent variable Estimate SE t value Pr([|t|)
Individual terms (Intercept) 0.96 0.09 10.96 \0.001
Log (dbh) 0.53 0.03 18.67 \0.001
C. malosana -0.49 0.22 -2.20 0.029
C. sylvaticus -0.27 0.18 -1.49 0.137
K. floribunda 0.08 0.22 0.35 0.729
S. gerrardii 0.18 0.11 1.69 0.093
T. dregeana 0.13 0.14 0.87 0.384
X. monospora 0.05 0.12 0.40 0.690
Interaction terms Log (dbh): C. malosana 0.27 0.10 2.85 0.005
Log (dbh): C. sylvaticus 0.13 0.06 2.25 0.026
Log (dbh): K. floribunda -0.15 0.11 -1.41 0.159
Log (dbh): S. gerrardii -0.04 0.03 -1.29 0.198
Log (dbh): T. dregeana 0.01 0.05 0.14 0.891
Log (dbh): X. monospora -0.06 0.04 -1.59 0.112
dbh is diameter at breast height
Vegetation structure, dominance patterns and height growth in an Afromontane forest, Southern
123
species richness decreased with increasing size class
(Fig. 1). Other studies have also demonstrated that species
richness declines from lower to upper diameter classes
(Lulekal et al. 2008; Kebede et al. 2013). Our findings are
also in agreement with Guilherme et al. (2004) who
reported higher floristic richness and diversity in lower/
intermediate layers than in upper layers of tropical forests.
The diversity patterns observed in regards to diameter
classes may be the results of species interactions (espe-
cially competition for light), which also determines the
maximum height.
Tree density and basal area were respectively,
472.0 ±43.5 stems/ha and 1.99 ±0.19 m
2
ha
-1
for the
C5to\10 cm dbh class, and 605.3 ±28.1 stems/ha and
48.07 ±3.46 m
2
/ha for the C10 cm class, suggesting that
these forests have great timber potential, comparable to
other Afromontane forests in East Africa (Lulekal et al.
2008; Fisaha et al. 2013; Kebede et al. 2013). For example,
with 75 (20 m 920 m) quadrats recording diameters and
heights greater than 2 cm and 2 m, respectively, Kebede
et al. (2013) reported lower values of tree density (379.3
stems/ha) and basal area (31.4 m
2
ha
-1
) in a remnant moist
Afromontane forest of Wondo Genet in south central
Ethiopia. Yirdaw et al. (2015) reported similar density
values (537.0–622.0 stems ha
-1
) and relatively lower basal
area (29.4–36.2 m
2
/ha) in Afromontane cloud forests in the
Bale Mountains of southeast Ethiopia. Knowing that these
structural features are key indicators of forest biomass
potential, our results partly confirm the remarkable biomass
and carbon potential of these mistbelt forests (Mensah et al.
2016a). Over past decades, wild fires and forest fragmen-
tation took significant toll on these indigenous forests, now
confined to small, scattered patches. Given the historical
disturbances, it is important to note that conservation
policies and recent management interventions, including
the protection of surrounding land uses against fire, have
contributed to successful development of these forests. X.
monospora and S. gerrardii were the most abundant and
dominant species in the inventoried forest area. These two
species contributed more than 40% of the stand basal area.
Past studies in the Limpopo mistbelt forests also showed
similar dominance patterns (Geldenhuys 2002; Geldenhuys
and Venter 2002).
The overall size class distribution was an inverted
J-shape with a sharp decline in tree density towards the
higher classes, a typical feature of diameter class distri-
butions in most Southern Africa’s forests (Geldenhuys
1993,1996; Mujuru and Kundhlande 2007; Kebede et al.
2013). Few individual trees were over 80 cm, as also
shown in previous reports by Geldenhuys (2000) and
Geldenhuys and Venter (2002). The overall distribution is
characteristic of multi-species forest stands with sufficient
regeneration. An inverted J-shape without gaps is also
indicative of a species successful reproduction and suc-
cession in diameter classes. Inverted J-shape trends were
observed for X. monospora,S. gerrardii,C. kraussii,C.
malosana,O. arborea, and K. floribunda, indicating that
their regeneration pool would be sufficient for maintaining
their populations. However, given the 5-cm diameter
threshold considered in this study, more in-depth analyses
on stability across diameter classes for these species would
provide insights into their ecological status and dynamic
patterns. Species such as C. sphaerocephala, T. dregeana,
N. congesta and C. sylvaticus exhibited several gaps in
lower and intermediate size classes, which may be partially
due to natural events such as tree fall. For instance, the
gaps in C. sphaerocephala diameter class distribution is
possibly the result of frequent wind throw on steep slopes
and/or trees dying. Gaps in lower and intermediate size
classes may also be attributed to irregular and sporadic
seedling establishment as a result of competition for
nutrients and light (Geldenhuys 1993; Syampungani et al.
2015). When examining the relationship of species to
height, it was interesting to see that species with relatively
lower wood density such as C. sylvaticus (Mensah et al.
2016b), tend to have taller individuals. This is partially
attributable to the fact that species with lower wood density
grow faster than those with higher wood density (Mensah
et al. 2016a,b). The variation in height growth between
dominant species is also indicative of species-specific
performance resulting from differential resource acquisi-
tion, competition and biomass production.
Fig. 5 Height–diameter allometries for important species. Scatter
plots are based on 277 trees from seven dominant species
S. Mensah et al.
123
Limitations and conclusion
We assessed the patterns of species diversity, vegetation
structures and dominance in a northern mistbelt forest type
in South Africa. Vegetation observational studies often
benefit from larger sample and plot size. The sample size
used in this study was slightly lower than what Yirdaw
et al. (2015) (n = 36; 400 m
2
) used in studying floristic
diversity and structure in Afromontane cloud forests in
Ethiopia. Thus, our sampling design might have not cap-
tured the overall variation in these forests composition.
Despite the limited spatial coverage of our data, the results
in terms of flora and structure were comparable to previous
reports in other Afromontane forests. The overall stand
showed an inverted-J shaped diameter distribution pattern,
a typical feature of size class distribution in natural forests;
and similar diameter structures were observed for most
dominant species. We also found significant effects of
species on tree height, suggesting differential height
growth among dominant species, probably as result of
different wood density and competitive interactions.
Overall, this study suggests that the long period of rel-
atively strict conservation and the management policies,
including protection of surrounding agricultural and for-
estry land uses against fire, have contributed to maintain a
successful recovery of the forests. Considering adverse
climate and land use change effects, our results indicate
that diversity and structure in the study area will generally
benefit current from legal protection. However, these for-
ests might be experiencing a relatively slow dynamic flux
as a result of minimal anthropogenic disturbance and the
over-mature state of larger trees. Institutional and man-
agerial policies permitting few extractive human uses
would contribute to preserve a healthy ecosystem, which is
in line with the intermediate disturbance hypothesis and the
general idea that anthropogenic disturbance cannot be
abandoned as a regulatory force in species structuring
(Huston 2014).
Acknowledgements The data used in this study were collected
during the first author’s doctoral research field work, co-financed by
the African Forestry Forum and the National Research Foundation of
South Africa through the ‘‘Catchman Letaba’’ project. The authors are
grateful to the anonymous reviewers for the comments on the first
version of this paper.
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... In both periods of assessments, the overall diameter and height class distribution of woody species revealed that a positively skewed distribution was prevalent in which more individual species were found in the lower diameter and height classes. It is a general pattern of healthy population structure and good recruitment potential of vegetation (Zegeye et al., 2006;Lulekal et al., 2008;Goncalves et al., 2018;Mensah et al., 2018). ...
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