3.1 Floristic composition
One hundred and eighty (180) plant species belonging to 145 genera and 69 families were recorded and identified in the sample plots in the Gerba Dima forest. Of these, 52 species (28.9%) were trees, 6 species (3.33%) were Trees/shrubs, 31 species (17.22%) were shrubs, 76 species (42.22%) were herbs, and 15 species (8.33%) were Lianas. Angiosperms were represented by 160 species while the rest 20 species were Pteridophytes. Among Angiosperms, Rubiaceae, Acanthaceae and Asteraceae were the richest family each represented by 11 genera and 11 species (6.11% ), 9 genera and 11 species (6.11% ), 6 genera and 11 species (6.11% ), respectively of total floristic composition, followed by Fabaceae 8 genera and 9 species (5%), Euphorbiaceae 6 genera and 7 species (3.89 %). The remaining families represented less than 3% of species each. Eleven families, 13 genera and 20 species represented pteridophytes. Aspleniaceae, Dryopteridaceae and Pteridaceae were the richest Pteridophytes represented by 6, 3 and 2 species respectively. The genus Vernonia, Ficus, Asparagus, Dracaenawere represented by 5,4,3,3 species respectively and Aframomu, Albizia, Asparagus, Cyperus, Euphorbia, Hippocratea, Hypoestes, Justicia, Maytenus, Olea, Peperomia, Polyscias, Pteris, Rubus, Schefflera, Solanaceo, Solanum, Tacazzea, and Zehneria were represented by 2 species each and the rest genera contained a single species each.
Based on the information available on the published Floras of Ethiopia and in [42] a total of 15 endemic plant species in 11 families were recorded (Table 1), comprising more than 8.33% of the recorded species. Asteraceae was the first family having three endemic species, followed by Acanthaceae and Fabaceae (two species each). The remaining eight families have a single species each in the endemic species list. Among the total endemic species, herb, tree, shrub and liana growth forms were represented by 6,3,4,2 species respectively. Out of the 15 endemic species, Crotalaria rosenii and Polyscias farinosa have been included in the IUCN red data list of Ethiopia and Eritrea [42] qualifying for near threatened and vulnerable category respectively. In the Gerba Dima forest, at 625m2 sample plot, species richness varied from 26 to 59 across the study plots. The Shannon diversity index also varied from 2.92 to 3.83 while evenness ranged from 0.89 to 0.95 in the study plots. The overall mean Shannon diversity index, species richness and evenness of the study area were 3.45, 41 and 0.93 respectively.
Table 1. Endemic species, their habit, IUCN status and geographical distributions
Species
|
Family
|
Habit
|
IUCN category
|
Altitude
(m)
|
Acanthopale ethio germanica
|
Acanthaceae
|
Shrub
|
NE
|
2300_2600
|
Aframomum corrorima
|
Zingiberaceae
|
Herb
|
NE
|
1350_2000
|
Arisaema mooneyanum
|
Araceae
|
Herb
|
NE
|
2000_3450
|
Bothriocline schimperi
|
Asteraceae
|
Shrub
|
LC
|
1300_2820
|
Clematis longicaudata
|
Ranunculaceae
|
Liana
|
LC
|
1350_3300
|
Crotalaria rosenii
|
Fabaceae
|
Herb
|
NT
|
1350_2800
|
Justicia bizuneshiae
|
Acanthaceae
|
Herb
|
NE
|
1200_2100
|
Millettia ferruginea
|
Fabacae
|
Tree
|
LC
|
1000_2500
|
Polyscias farinosa
|
Araliaceae
|
Tree
|
VU
|
1600_2200
|
Scadoxus nutans
|
Amaryllidaceae
|
Herb
|
NE
|
1450_2300
|
Solanecio gigas
|
Asteraceae
|
Shrub
|
LC
|
1750_3350
|
Tiliacora troupinii
|
Menispermaceae
|
Liana
|
NE
|
1500_2100
|
Urtica simensis
|
Urticaceae
|
Herb
|
LC
|
1500_3400
|
Vepris dainellii
|
Rutaceae
|
Ttree
|
LC
|
1750_2500
|
Vernonia rueppellii
|
Asteraceae
|
Shrub
|
LC
|
2150_3000
|
* Source: [25, 26, 27, 28, 29, 30, 31, 32, 33, 42]; *LC, Least Concern; NE, Not evaluated; NT, Near Threatened; VU, Vulnerable.
3.2 Community types and indicator species
Five community types were derived from the hierarchical cluster analysis in combination with Multi-response Permutation Procedures (MRPP) and objective method of the whole data set (Figure 3). From the output of MRPP, the test statistic T value for the five groups was -38.26 (P < 0.001) and the agreement statistic A was 0.13 while the output of objective method revealed a sharp bend at the fifth cluster.
Community 1 (Croton macrostachyus - Bersama abyssinica community) was found in the altitudinal range of 1677-2020 m. a.s.l and slope from flat to 50%. Fourteen plots were associated with the community and has 2 indicator species with significant indicator values (P < 0.05).
Community 2 (Syzygium guineense - Olea capensis community) was distributed from 1699 to 2240 m a.s.l. and slope ranging from flat to 60%. It comprises of 22 plots and twenty species were associated with this community as indicator species where one of the indicator species exhibit significant indicator values (P < 0.05). Community 3 (Dracaena afromontana- Pouteria adolfi-friederici community) was found in the altitudinal range of 1761-2000 m. a.s.l and slope from flat to 25%. Thirteen plots were associated with the community community and seven species were associated with this community as indicator species while two of the indicator species showed significant indicator values (P < 0.05).
Community 4 (Vepris dainellii - Schefflera abyssinica community) was distributed in the altitude range of 1720–2060m a.s.l. and the slope gradient varies flat to 60%%. It comprised of 14 plots, eight species were associated with this community as indicator species, while four of the indicator species exhibited significant indicator values (P < 0.05).
Table 2. Synoptic cover value of plant in Gerba Dima Forest for species reaching ≥ 1% in at least one community.
Cluster number
|
C1
|
C 2
|
C 3
|
C 4
|
C 5
|
Cluster size
|
14
|
22
|
13
|
14
|
27
|
Allophyllus abyssinicus
|
3.50
|
1.05
|
1.15
|
0.86
|
1.26
|
Bersama abyssinica
|
3.71
|
1.73
|
0.08
|
0.64
|
1.22
|
Croton macrostachyus
|
7.50
|
1.77
|
1.54
|
1.79
|
2.48
|
Cordia Africana
|
2.79
|
0.55
|
0.00
|
0.00
|
0.63
|
Olea welwitschii
|
1.43
|
1.27
|
0.15
|
0.86
|
0.56
|
Ehretia cymosa
|
2.36
|
2.55
|
1.92
|
0.79
|
1.63
|
Polyscias fulva
|
1.64
|
1.36
|
1.85
|
0.93
|
1.33
|
Apodytes dimidiate
|
1.43
|
2.41
|
1.54
|
1.64
|
0.93
|
Olea capensis
|
3.14
|
5.59
|
1.54
|
1.93
|
1.04
|
Syzygium guineense
|
0.64
|
5.91
|
1.69
|
2.43
|
1.56
|
Justicia schimperiana
|
1.29
|
1.68
|
0.85
|
0.36
|
0.93
|
Canthium oligocarpum
|
0.29
|
1.05
|
0.62
|
0.50
|
0.52
|
Cassipourea malosana
|
0.79
|
1.55
|
1.08
|
0.43
|
0.56
|
Combretum paniculatum
|
0.86
|
1.14
|
0.54
|
0.43
|
1.04
|
Dracaena steudneri
|
1.29
|
3.09
|
0.92
|
1.50
|
1.22
|
Elaeodendron buchananii
|
0.64
|
1.00
|
0.38
|
0.00
|
0.37
|
Oplismenus hirtellus
|
2.43
|
4.50
|
3.00
|
2.64
|
2.85
|
Rothmannia urcelliformis
|
1.14
|
1.95
|
0.77
|
0.86
|
1.15
|
Sapium ellipticum
|
0.21
|
1.64
|
0.00
|
1.43
|
0.93
|
Tectaria gemmifera
|
0.93
|
1.36
|
1.23
|
1.29
|
0.81
|
Brillantaisia madagascariensis
|
1.43
|
2.73
|
3.00
|
2.79
|
2.48
|
Dracaena afromontana
|
1.00
|
3.05
|
7.69
|
0.86
|
0.78
|
Ficus sur
|
2.50
|
1.68
|
6.77
|
2.07
|
1.37
|
Galiniera saxifrage
|
1.00
|
1.05
|
2.31
|
1.43
|
1.56
|
Hallea rubrostipulata
|
1.07
|
0.00
|
1.31
|
0.00
|
0.00
|
Macaranga capensis
|
1.21
|
1.32
|
2.85
|
0.71
|
0.19
|
Oxyanthus speciosus
|
1.29
|
1.73
|
7.01
|
2.36
|
1.56
|
Pouteria adolfi-friederici
|
2.21
|
3.05
|
7.31
|
2.07
|
1.26
|
Acanthopale ethio-germanica
|
1.36
|
0.77
|
2.08
|
2.43
|
1.96
|
Deinbollia kilimandscharica
|
0.57
|
1.45
|
2.62
|
4.07
|
1.59
|
Ilex mitis
|
0.43
|
0.59
|
1.31
|
4.71
|
0.44
|
Justicia bizuneshiae
|
0.50
|
1.23
|
1.31
|
1.71
|
1.37
|
Landolphia buchananii
|
1.00
|
1.32
|
0.85
|
1.43
|
1.19
|
Piper capense
|
1.00
|
0.55
|
0.46
|
1.43
|
1.07
|
Psychotria orophila
|
0.93
|
1.36
|
0.85
|
1.36
|
0.85
|
Pupalia micrantha
|
0.64
|
1.36
|
0.23
|
1.86
|
0.93
|
Schefflera abyssinica
|
0.50
|
1.73
|
1.38
|
7.29
|
1.33
|
Tiliacora troupinii
|
1.00
|
1.23
|
1.08
|
1.29
|
1.07
|
Vepris dainellii
|
2.21
|
3.36
|
3.00
|
8.43
|
3.59
|
Albizia gummifera
|
3.07
|
2.50
|
2.69
|
2.07
|
8.63
|
Clausena anisate
|
1.79
|
1.86
|
1.31
|
1.43
|
2.11
|
Hippocratea pallens
|
0.64
|
1.91
|
1.23
|
1.50
|
1.93
|
Lepidotrichilia volkensii
|
0.57
|
2.59
|
1.23
|
2.43
|
3.30
|
Maytenus gracilipes
|
2.00
|
1.82
|
1.08
|
2.00
|
2.30
|
Millettia ferruginea
|
2.00
|
2.95
|
2.54
|
2.79
|
7.89
|
(C1= Croton macrostachyus - Bersama abyssinica, C2= Syzygium guineense - Olea capensis, C3 = Dracaena afromontana- Pouteria adolfi-friederici, C4= Vepris dainellii- Schefflera abyssinica C5 = Albizia gummifera - Millettia ferruginea community.
Community 5 (Albizia gummifera - Millettia ferruginea community) was found in the altitudinal range of 1728-2014 m. a.s.l and slope from flat to 50%. Twenty-seven plots were associated to the community. Eight species are associated with this community as indicator species and four of the indicator species exhibited significant indicator values (P < 0.05).
Table 3. Indicator species of clusters in Gerba Dima forest with their significant P-value
Name of indicator species
|
Community type (C)
|
Indicator value
|
P-value
|
Prunus Africana
Rubus apetalus
Flacourtia indica
Pilea rivularis
Elastostema monticolum
Ritchiea albersii
Trema orientalis
Sapium ellipticum
Vernonia hochstetteri
Zehneria scabra
Zehneria minutiflora
Urera hypselodendron
Vernonia wollastonii
|
1
1
2
3
3
4
4
4
4
5
5
5
5
|
0.528
0.516
0.521
0.498
0.467
0.861
0.677
0.636
0.538
0.581
0.552
0.478
0.423
|
0.018 *
0.017 *
0.02 *
0.016 *
0.039 *
0.001 ***
0.001 ***
0.002 **
0.014 *
0.002 **
0.005 **
0.017 *
0.045 *
|
(C1= Croton macrostachyus - Bersama abyssinica, C2= Syzygium guineense - Olea capensis, C3 = Dracaena afromontana- Pouteria adolfi-friederici, C4= Vepris dainellii- Schefflera abyssinica C5 = Albizia gummifera - Millettia ferruginea community.
From computation of vegetation data in the study area Shannon-Weiner diversity and evenness, indices for the five community types showed the output in Table 4.
Table 4. Species richness, evenness and diversity indices of plant community types
Community
|
Species richness
|
Shannon diversity index (H')
|
Shannon Evenness
|
1
2
3
4
5
|
138
144
107
104
140
|
4.40
4.27
3.99
4.05
4.19
|
0.89
0.86
0.85
0.87
0.85
|
3.3 Relationship between community types and environmental factors
Heterogeneity or homogeneity of vegetation data test using DCA resulted in short length (gradient) of DCA first axis i.e., < 3 (2.22) which indicate the presence of lower species turnover or homogeneous vegetation data due to the linear relationship between species and environmental variables. The result of Monte Carlo test showed that out of 14 environmental variables, seven were found to be significant in explaining patterns of plant community distribution. From the seven significant environmental factors, the vif values of sand and silt were higher than 5. Sand and Silt are highly correlated with at least one of the other variables in the model. One solution in dealing with collinearity is to remove some of the violating variables from the model and thus the one with higher vif value (sand) was eliminated. The result of RDA ordination showed that comparatively, the gradient of altitude and potassium was highly correlated on axis one and gradient of disturbance in axis two. The other factors were correlated with the five axes with a different value of correlation. The eigenvalue for axis one, two and three were 10.65, 8.06, and 6.32 respectively. Cumulative proportion variance explained by the first five RDA axis of the joint biplot was 93.9%. The proportion of variation explained by five RDA axis also shows a decline towards the successive higher axis (Table 5).
Table 5. Biplot score for constraining variables and their correlation with the RDA axis, eigenvalues and proportion of variance explained
Environmental variables
|
RDA1
|
RDA2
|
RDA3
|
RDA4
|
RDA5
|
Disturbance
Altitude
SILT
EC
OM
K
Eigenvalue
Proportion Explained
Cumulative Proportion
|
0.089 0.880
0.084 -0.053
-0.094 0.703
10.6445
0.3024
0.3024
|
-0.68 0.42 -0.40
0.31 0.27
-0.27 8.0649
0.2291
0.5315
|
-0.157
-0.054
-0.361
-0.867 -0.208
-0.003 6.3168
0.1794
0.7109
|
0.522
-0.018
-0.323
0.094 -0.357
-0.381 5.0057
0.1422
0.8531
|
0.453
0.218
-0.370
-0.023
0.844
-0.246
3.02318
0.08588
0.93902
|
RDA ordination of the study plots of Gerba Dima forest formed five groups or community based on the species composition. These five community types were segregated following the arrows of the environmental variables. Community two mostly occur at the higher altitude while species in community one are distributed at the lower altitude and higher EC. Silt, Disturbance and potassium axes were strongly influencing the distribution of community five. Organic matter arrow has strongly influenced the distribution of species in community three and four (Figure 4). The ANOVA test indicated that the five community types differ significantly from each other with regard to EC and K. The result of Tukey’s pair-wise comparison test indicates that community 4 and 1 differ significantly with respect to Disturbance and K while community 2 and 3 showed significant differences with respect to EC.