Investigation of Woody Species Structure and Regeneration Status in the Central Rift Valley, Sidama Regional State, Ethiopia

Assefa, Assegid; Bekele, Tamrat; Demissew, Sebsebe; Abebe, Tesfaye

doi:https://doi.org/10.1155/2022/7021477

International Journal of Forestry Research

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Data Availability Ethical Approval Consent Conflicts of Interest Authors’ Contributions Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2022 | Article ID 7021477 | https://doi.org/10.1155/2022/7021477

Investigation of Woody Species Structure and Regeneration Status in the Central Rift Valley, Sidama Regional State, Ethiopia

Assegid Assefa,^1,2Tamrat Bekele,²Sebsebe Demissew,²and Tesfaye Abebe³

Academic Editor: Nikolaos D. Hasanagas

Received06 May 2022

Accepted29 Aug 2022

Published28 Oct 2022

Abstract

Protected areas are the most commonly used tool for in situ conservation of biodiversity. Selective removal of species proposed by the local communities living surrounding the national park and grazing pressure negatively affect the composition, structure, and regeneration of woody species. Assessment of vegetation structure and regeneration status of woody species is essential for orienting management activities. The purpose of this study was to investigate the floristic composition, population structure, and regeneration status of woody species in the Loka Abaya National Park, to design conservation strategies. A total of 99, 20 m × 20 m quadrats were systematically laid along an established line transect to collect a list of woody species, abundance, height, and diameter at breast height (DBH), while five 3 m × 3 m subquadrats within the main quadrats were established to assess the regeneration status of woody species. In each quadrat, all woody species were identified, counted, and recorded. In each quadrat, all tree and shrub species higher than ≥2 m in height and ≥2 cm in diameter at breast height were measured by a calibrated wooden stick and by a caliper, respectively. Density, frequency, basal area, importance value index (IVI), height, and diameter at breast height (DBH) were used for description of vegetation structure, while the density of mature trees, saplings, and seedlings was used for assessment of regeneration status of woody species. A total of 101 woody plant species representing 40 families in 69 genera were collected, identified, and documented. Fabaceae was the most diverse family representing 16 (15.84%) species, followed by Euphorbiaceae 9 species (8.91%) and Anacardiaceae with 6 species (5.94%). Four families including Combretaceae, Moraceae, Olacaceae, and Tiliaceae were represented by 4 species each. 4 families were also represented by 3 species each, 12 families were represented by two species each, and 18 families were represented by one species. The density of trees was 831.31 individuals ha⁻¹, while the total basal area was 73.18 m²·ha⁻¹. D. angustifolia, C. molle, E. schimperi, R. natalensis, O. europaea L. subsp. cuspidataD. cinerea, A. brevispica, I. mitis, and E. tirucalli were ecologically important woody species. The majority (75%) of woody plant species had a less than 5% importance value index (IVI). The diameter class distribution of selected tree species demonstrated various patterns of population structure, implying the existence of different population dynamics among ecologically important tree species. The regeneration assessment results demonstrate that 32.35% had poor regeneration, 19.12% had good regeneration, 16.17% had fair regeneration, 8.82% lacked regeneration, and 14.08% appeared as newly regenerated species in the national park. The majority of woody species had a small population size, and some of them were found in specific habitats which need attention for conservation, and those woody species lack regeneration study soil seed bank and propagation methods for sustainable conservation.

1. Introduction

Ethiopia has a wide range of altitudes, varying between 125 m below sea level at Danakil depression (Afar) and 4533 m.a.s.l at Ras Dashen (Gondar) that have resulted in a wider range of climatic conditions and a multitude of agroecological zones. This varied ecological setting has enhanced the evolution of various life forms including about 6027 species of vascular plants out of which about 10% are believed to be endemic [1]. The total number of woody plants is estimated to be about 1000, about 300 of which are estimated to be trees [2, 3]. According to Kent and Coker [4], vegetations are a crucial part of the earth and an integral part of an ecosystem that provides essential services to human society. The high floristic and ecological diversity of Ethiopian vegetation is a source of wild and domesticated plants. It is also a source for at least 197 species of crops including grains, pulses, vegetables, tubers, fruits, spices, stimulants, fibers, dye, and medicine [5].

Records on conservation efforts in Ethiopia dated to the days of Emperor Zera Yakob (1434–1468) who brought juniper seedlings from Wof washa of North Shewa and planted them in the Managesha Suba area [6]. Modern conservation was started by Emperor Menelik II in 1895 by introducing exotic species mainly Australia Eucalyptus in 1895 to solve the shortage of fuel wood and construction materials and for conservation of indigenous tree species. This conservation initiative eventually evolved into the formulation of protected areas in the 1960s [6]. Today, the country has established several protected areas which include 21 national parks and 58 national forest priority areas [7]. These areas are critical habitats for the most endangered wildlife forests and biodiversity in the country and the foundation for national conservation strategies. Millennium Ecosystem Assessment [8] reports indicated that significant amounts of forests have come under protected status over the past several decades.

The vegetation of Ethiopia has been modified by anthropogenic activities for long periods. This strong and prolonged human interference is believed to have degraded a range of vegetation types to a badly eroded landscape with very little differentiation of the vegetation left [9]. In Ethiopia, environmental degradation and deforestation have been taking place for hundreds of years; trees have been used and still are cleared for agriculture and fuel wood [10]. The forest cover of Ethiopia was about 16% of the land area in the early 1950s and rapidly declined to 3.6% in the early 1980s and 2.7% in 1989 [11]. According to the FAO, 2007, [12] reported deforestation in Ethiopia as 1410000 ha per year, that is, 0.93% between 1900 and 2000 which had an increase to 1.04% between 2000 and 2005. A study on forest cover change and socioeconomic drivers of southwest Ethiopia estimated a forest decline of 2.1% per year in four districts between 1973 and 2005 [13], and Getaneh [14] reported deforestation rate of 1.35% per year for the Yayu forest in southeast Ethiopia. This is higher than the deforestation rate of tropical forests in Asia, at a range of 0.4–0.9% per year. Thus, in our world, extensive biodiversity loss, including loss of genetic, species, and habitat diversity, has been one result of the shrinking of the world’s forests [15].

In Ethiopia, the increasing population and demand for more agricultural land and forest products resulted in the destruction of natural vegetation. In addition, Ethiopia also ranks first in Africa with a livestock population that exerts heavy grazing pressure and degradation of natural vegetation. This horizontal expansion of cultivated land at the expense of forest and other natural vegetation is as old as crop culture [16]. According to the IBC report [7], most of the national parks in Ethiopia are found in the Acacia–Commiphora woodland ecosystem, which is currently under strong environmental stress, such as extraction of fuel wood and charcoal, and major towns in the country have increased the rate of deforestation and natural resource depletion. The settlement, charcoal production, and grazing are common in all Ethiopian national parks. The destruction of vegetation in protected areas, national forest priority areas, and elsewhere has continued alarmingly. Even though no concrete evidence is available, species unknown to science and/or those species restricted to areas where anthropogenic impacts are hostile may have gone extinct [2].

Loka Abaya is one of the national parks of Ethiopia found in the Loka Abaya district, Sidama Regional State, in the central rift valley of Ethiopia. The park was formally established in 2009, in an area of 500 km². Before delineation as a park, the land served as a community-free grazing area. The park accommodates a mosaic forest, woodland, bushland, riverine vegetation, and Lake Abaya-associated wetland vegetation. Additionally, the park shares some portion of the water body from Lake Abaya. Twenty-seven large mammal species were found, including the IUCN Red listed African wild dog which is endangered [17]. Seasonal settlement within the national park, grazing, selective removal of species for firewood, charcoal production, and construction are currently the prevailing problems of the national park. The selective removal of species may have different effects on the establishment of seedlings and saplings [18]. It could create favorable conditions for the regeneration of some species and disfavor others by reducing the abundance of mother trees. Selective removal of such important species leads to higher dominance of low-quality species [3]. Knowing the abundance, relative density, frequency, and even regeneration of woody species would provide the essential foundations for the formulation of strategies for in situ and ex situ conservation of the species [19]. Tesfaye et al. [20] added natural regeneration is important to identify plant species for conservation priority. The national park has a recent establishment history, so its floristic composition, vegetation structure, and regeneration status of woody species were not investigated. For effective management and conservation of this area, there is a need to develop management plans, and this, in turn, requires detailed information on the floristic composition, vegetation structure, and regeneration status of woody species. Therefore, this study aimed to assess the floristic composition, population structure, and regeneration status of woody species in the vegetation of Loka Abaya National Park.

2. Materials and Methods

2.1. Description of the Study Area

The study was conducted in the Loka Abaya National Park which is one of the national parks found in the Sidama Regional State of Ethiopia (Figure 1). It is located about 345 km away from Addis Ababa and 72 km from the regional capital, Hawassa, and is managed by the regional government. The altitude ranges from 1178 m.a.s.l Shall–Oda at the bottom of Lake Abaya to 1650 m.a.s.l. at Gedano hill.

2.1.1. Vegetation and Wild Life

The vegetation of the study area is dominantly woodland, wooded grassland, forestland, and vegetation along the seasonal and permanent riversides, Lake Abaya, and associated wetland vegetation. The local community practices traditional home garden agroforestry. The dominant plant species in the traditional agroforestry practice are E. ventricosum, Z. mays, C. arabica, C. edulis, S. officinarum, P. vulgaris, S. bicolor, and C. cajan which are some of the cultivated crops in the system mainly for home consumption. C. macrostachyus, C. africana, A. schimperiana, and B. aegyptiaca are the dominant woody species in the surrounding land use system. Additionally, the national park shares some portion of the water body from Lake Abaya. The national park also harbors a significant variety of large and medium-sized mammals in different habitats. According to the survey report [17]), the conspicuous and observed mammals in the park include savanna, baboon (Papio cynocephalus), buhshbuck (Tragelaphus scriptus), lion (Panthera leo), and common duiker (Sylvicapra grimmia). The park also contains different bird species such as Helimmeted gunieafowl (Numida meleagris), bee-eater (Merops nubicus), long-crested eagle (Lophaetus occipitalis), and white-headed vulture (Aegypius occipitalis) which are some of commonly found bird species.

2.1.2. Climate

The meteorological data collected from Billate Meteorological Station, which is found at an altitude of 1361 m.a.s.l and a distance of 2 km away from Loka Abaya National Park, indicate that the area receives bimodal rainfall; the first peak starts from mid-March to the end of April and the second peak from July to mid-October. The mean annual rainfall of the range of the study area is 857.86 mm, with a mean monthly maximum rainfall of 125.38 mm in April and a mean monthly minimum rainfall of 13.36 mm recorded in December (National Meteorological Service Agency Hawassa Branch). The mean annual minimum temperature of the area is 14.1°C in November, while the mean annual maximum temperature recorded in February is 33.8°C (Figure 2).

2.1.3. Geology and Soil

The geology of the Sidama regional state is one of the parts of the basement complex and the younger formations that were deposited in the basement which contains the oldest rock in the country [21]. The Precambrian rocks with ages of over 600 million years form the foundation of all rocks. It is exposed in areas where the younger cover rocks have been eroded. The physical and chemical compositions of soils are very important in determining the occurrence, growth, diversity, and distribution of plant species [16]. The soil type of the study area is dominantly Eutric Fluvisols [21]. This soil is derived from materials transported from the draining area of the river, including topsoil from high land.

2.2. Reconnaissance Survey and Delineation of the Study Site

Before starting the sample collection, a reconnaissance survey was carried out to identify the study site. The reconnaissance survey was made across the Loka Abaya National Park in the 3^rd and 4^th weeks of February 2017 to obtain an impression of the conditions of the vegetation, collect information on accessibility, identify sampling sites, and calculate the sample size. Then, the transect directions of the vegetation data collection were determined.

2.3. Sampling Design

The systematic sampling design was used to locate the sample quadrats to assess woody species composition in the Loka Abaya National Park following the [22] method. Quadrats were laid systematically at every 200 m along transect lines and 800 m apart between the consecutive transect lines. To eliminate any influence of the road effects on the species of the use of vegetation national park, all quadrats were laid at least 50 m away from the nearest roads.

2.4. Sampling Techniques

Sampling quadrats of 20 m × 20 m were established systematically at every 200 m interval along the transect lines which are 800 m apart. The systematic sampling method was used to take vegetation samples. Vegetation composition and related data were collected from the Loka Abaya National Park by using transect lines laid parallel to each other. Accordingly, a total of 99 quadrats in the 17 transect lines were used for vegetation data collection.

2.5. Methods of Data Collection

2.5.1. Structural Data Collection

In all plots, all mature trees ≥2 m in height and ≥2 cm diameter at breast height were recorded, and diameter at breast height (DBH) was measured at a height of 1.3 m using a diameter caliper, and the height of individual trees was measured by a 6 meter calibrated wooden stick and above it was visually estimated. In cases where a tree or shrub bole branched at breast height or below, the diameters were measured separately for the branches, and each diameter was squared and put under square root; the square root of the sum of all squared stems averaged as one DBH [23]:where b1 represents the branch one and b2 represents the branch two. Maximums of up to four branches were measured. Finally, woody species near a 10-meter radius but absent in the sample quadrats were noted for floristic completion.

2.5.2. Natural Regeneration Status Data Collection

Regeneration of woody plants was assessed with five 3 m × 3 m subplots at four corners and one at the center of main plots (totally 45 m²) was used to count saplings (individuals height > 0.5 m and dbh < 2 cm) and seedlings (with two normal leaves above the cotyledons and with height less than 0.5 m). The regeneration status of the species was based on the population size of seedlings and saplings [25]. Good regeneration, if seedlings > saplings > adults; fair regeneration, if seedlings > or ≤ saplings ≤ adults; poor regeneration, if the species survives only in the sapling stage, but not seedlings (saplings may be or = adults). If a species is present only in an adult form, it is considered as not regenerating.

2.6. Plant Identification

Finally, plant specimens were collected during the interview and brought to the National Herbarium (ETH) of Addis Ababa University for identification. The specimens were properly dried and identified based on the published volumes of flora of Ethiopia and Eritrea [26–32]. The specimens identified were compared with the properly identified specimens. Botanical names and authorities were verified using the species and family lists in Volume 8 of the Flora of Ethiopia and Eritrea [26]. All plant specimens were properly labeled and deposited at the National Herbarium (ETH) of Addis Ababa University.

2.7. Data Analysis

2.7.1. Structural Analysis

The diameter at breast height (DBH), basal area, tree density, height, frequency, and important value index were used for the description of vegetation structure [22] as follows:

Frequency (%): this term refers to the degree of dispersion of individual species in an area and is usually expressed in terms of percentage occurrence. It is calculated by the following equation.

Relative frequency (RF): the degree of dispersion of individual species in an area in relation to the number of all species that occurred.

Density: it is an expression of the numerical strength of a species where the total number of individuals of each species in all quadrats is divided by the total number of quadrats studied. Density is calculated by the following equation.

Relative density: it is the study of the numerical strength of a species in relation to the total number of individuals of all species and can be calculated as

The density of individuals with DBH 10–20 cm and DBH > 20 cm was computed, and the ratio of these two was taken as a measure of the proportion of small and large-sized individuals following [33].

Basal area (m²·ha⁻¹): it is measured as the cross-section area of a tree at breast height, computed from the measurement of DBH as follows.where d represents the diameter at breast height (m), and π = 3.14. However, since DBH was measured in centimeters, the formula was modified in such a way that the basal area was in square meters. Thus, Ba = πd2/40,000 or 0.0000785d2, where d is the DBH in centimeters. The mean basal area of all investigated plots was converted to the mean basal area per hectare. The basal area provides a better measure of the relative importance of tree species than simple stem counts [34].

Relative dominance: relative dominance is the coverage value of a species with respect to the sum of coverage of the rest of the species in the area.

2.7.2. Population Structure of Woody Species

Analyses of the population structure of woody species in the study area were performed by categorizing trees and shrubs into nine height and twelve DBH classes following [35, 36]. Diameter at breast height (DBH) was classified into twelve classes: C₁ = <2 cm, C₂ = 2 cm–5 cm, C₃ = 5.1 cm–8 cm, C₄ = 8.1 cm–11 cm, C₅ = 11.1 cm–14 cm, C₆ = 14.1 cm–17 cm, C₇ = 17.1 cm–20 cm, C₈ = 20.1 cm–23 cm C₉ = 23.1 cm–26 cm C₁₀ 26.1–29 cm, C₁₁ 29.1 cm–32 cm, and C₁₂ = >32 cm. Height was classified into nine classes: C₁ = <2 m, C₂ = 2 m–5 m, C₃ = 5.1 m–8 m, C₄ = 8.1 m–11 m, C₅ = 11.1–14 m, C₆ = 14.1 m–17 m, C₇ = 17.1 m–20 m, C₈ = 20.1 m–23 m, and C₉>23 m. Histograms were constructed by using the density of individuals of each species (Y-axis) and categorized by diameter and height class (X-axis); then, based on the profile depicted in the population structures, the regeneration status of each woody species was determined. The patterns of species population structure were established based on the relative density of species in different DBH classes and interpreted as an indication of variation in population dynamics following [33, 37].

2.7.3. Regeneration Status Analysis

Regeneration status was estimated based on the composition and density of seedlings and saplings of all woody species recorded in each plot [25]. The density of seedlings and saplings was calculated per hectare. The regeneration status of the forest was examined by computing and comparing the present tree populations (large trees) with the regenerating populations (seedlings and saplings) of tree species according to [38, 39].

3. Results and Discussion

3.1. Floristic Composition of Woody Species

A total of 101 woody plant species that belong to 40 families in 69 genera were collected, identified, and documented from the study of national park (Table 1). Among these, 79 (86.29%) were recorded from 99 sample plots laid in the national park, while 22 (13.71%) species were recorded outside the quadrats but inside the national park for floristic compilation. Trees were the dominant growth forms in the study site representing 39.74% followed by shrubs (21.79%). E. camaldulensis, J. acerifolia, and M. azedarach were the only three exotic tree species recorded in the natural vegetation. The recorded woody plant species in the current study area were higher than 70 species of Kafta Sheraro National Park [40], while less than 118 trees and shrubs were recorded from Nechisar National Park [41]. The highest number of woody species in this study was partly explained by the entire vegetation of the national park which is dominated by woodland and forestland. Whitmore [42] noted that species richness in tropical forests varies greatly from site to site due to variations in habitat, level of ecological disturbance, and total area sampled. The floristic composition plays a crucial role in assessing the health of forests [43] and helps to design conservation and management plans. Of all families investigated in the study area, Fabaceae was the most diverse family representing 16 (15.84%) species, followed by Euphorbiaceae, 9 species (8.91%), and Anacardiaceae with 6 species (5.94%). Four families including Combretaceae, Moraceae, Olacaceae, and Tiliaceae were represented by 4 species each. Four families were also represented by 3 species, 12 families were represented by two species, and 18 families were represented by one species. The dominancy of Fabaceae was reported in several studies conducted in Ethiopia [1, 40, 41]. Fabaceae is also known to have the highest number of species, more than any other plant family in the world [44]. Woldearegay et al. [45] noted that the dominancy of the family Fabaceae could also be attributed to its efficient and successful dispersal strategies as well as better adaptation to a wide range of ecological conditions. In addition, in the tropics, N-fixation occurs primarily in Fabaceae.

3.2. Structure of Woody Species in the Loka Abaya National Park

3.2.1. Density and Frequency of Woody Species

Stem density is the result of recruitment, growth, and mortality that is potentially influenced by a wide variety of factors operating at a range of spatial and temporal scales with varying effects on different size classes [46]. In the current study, a total of 3292 individuals of woody plants (831 individuals per hectare) were recorded in the study site (Table 2). The stem density of the current study area was low compared to other studies in different forest vegetation of Ethiopia, Adelle (898 individuals ha⁻¹) [47] and Nechisar National Park, 867 stem ha⁻¹ [41]. While its density is higher than Boditi (498 individuals ha⁻¹) [47] and woodlands (376.86 individuals ha⁻¹) [47] in Ethiopia. The variation in stem density directly correlates with topographic factors such as elevation, slope, aspect, and richness [48] and habitat quality linked to ecological requirements of component tree and shrub species in the respective forests [41] and age structure [49]. According to Richards [50], tree abundance can likewise be affected by natural and anthropogenic disturbances or soil conditions. In the vegetation of Loka Abaya National Park, 54.46% of the density of woody species was contributed by 8 abundant species. D. angustifolia had 108.33 individual ha⁻¹, C. molle 27.5 (69.44 individual ha⁻¹), E. schimperi 243 (61.36 individual ha⁻¹), R. natalensis 217 (54.75 individual ha⁻¹), O. europaea L. subsp. cuspidata 216 (54.54 individual ha⁻¹), D. cinerea 193 (48.23 individual ha⁻¹), A. brevispica 152 (38.38 individual ha⁻¹), I. mitis 148 (37.37 individual ha⁻¹), and E. tirucalli 138 (34.84% individual ha⁻¹) were the most abundant woody species. The majority of woody species in the study area including M. kummel, S. birrea, F. rochetiana, E. cymosa, C. sinensis, C. africana, A. polyacantha, Z. chalybeum, D. abyssinica, V. apiculata, C. malosana, and C. africana were found rare. Thus, those rare species might be important components of the ecological system, even more than the species having higher relative abundance. In highly diverse ecosystems such as tropical forests, rare species are an important component of biological diversity, generally comprising the majority of species, and they are also particularly vulnerable to extinction [51, 52], so they need attention for conservation.

A comparison of tree density with DBH between 10 and 20 cm (a) and tree density with DBH >20 cm (b) and their ratios was present (Supplementary file 1). The ratio of density >10 cm to density >20 cm is taken as a measure of the distribution of the size classes as described by [33]. The density of woody species with DBH >2 cm was 466.48 individuals ha⁻¹ and the density of woody species with DBH >10 cm was 239.69 individuals per ha, whereas species with DBH >20 cm was 93.64 individual ha⁻¹. About 58.32% of the total density of woody species was contributed by species with DBH >2 cm, whereas those woody species with DBH >10 cm and DBH >20 cm contributed 29.96% and 11.71%, respectively, to the total density (Additional File 1). The two species I. mitis and C. molle had the highest contribution for both DBH above 10 cm and 20 cm, while D. cinerea, O. europaea L. subsp. cuspidata, D. angustifolia, and E. schimperi had a higher contribution at lower DBH classes. The analysis of the ratio of the density of trees and shrubs of DBH greater than 10 cm to DBH greater than 20 cm for trees and shrubs was found to be 2.55 indicating that there is a predominance of small-sized trees and shrubs in the study site. This value was found to be higher than (1.85) Donkoro forest [53], (1.67) Nechisar National Park [41], and (2.09) Mena-Angut [54]. The dominance of small individuals in the vegetation of the current study area could be related to the recent establishment of the national park, before which the site was an open-access and unprotected area from communities that extracted wood and other products, which lead to the reduction of big trees and shrubs. Tamrat Bekele [35] concluded that if the forest has developed under natural conditions and without major disturbances, no differences were observed between the two size classes.

Frequency is the number of sample plots in which a given species occurred in the study area, expressed in percentage of the total number of sample plots. The most frequently found woody species in the vegetation of the national park were R. natalensis (57.57%), C. molle (38.38%), D. angustifolia (38.38%), I. mitis (37.77%), X. americana (37.37%), D. cinerea (36.36%), O. europaea L. subsp. cuspidata (35.35%), and A. brevispica (32.32%). In general, only nine species occurred in above 30% of all quadrats sampled, which means a higher percentage of woody species was found at lower frequency classes. 63.93% of species had a frequency value of less than 10%, indicating a relatively good floristic heterogeneity in the study area (Table 2). According to Rey et al. [55], variation in the frequency of species might be attributed to habitat preferences among species, species characteristics for adaptation, degree of disturbance, and availability of suitable conditions for regeneration. In the current study area, the pattern of floristic heterogeneity may be influenced by D. cinerea, the species encroached into different habitats. A similar observation was reported from Nechisar National Park [41]. According to Bekele [56], the species produces many pods, and each coiled pod contains four seeds, and the pods fall on the ground and rot to set free seeds that are prolifically and germinate easily. This may be the reason for the aggressive encroachment of the species in the national park. Some species including A. polyacantha, Cassipourea malosana, S. persica, E. cymosa, Z. chalybeum, and M. kummel were not only frequent but also habitat-specific as well as low population size in the national park. According to Tetetla-Rangel et al. [57], species having a small population size, high habitat specificity, and small geographic range were termed as the highest level of rarity. In the current study, those species qualified for at least a small population size and high habitat specificity are termed as an intermediate level of rarity that demands conservation attention.

3.2.2. Basal Area and Importance Value Index (IVI) of Woody Species

The total basal area of woody species in Loka Abaya National Park was 73.18 m²·ha⁻¹ (Table 2). The trees belonging to higher DBH classes are few, but their contribution to the total basal area is high. The result shows that three species I. mitis (23.92%), C. molle (12.31%), and Olea europaea L. subsp. cuspidata (9.02%) contribute about 45.25% of the total basal area which shows their dominancy in the vegetation of the national park. The relative importance of tree species in a forest can better be depicted from measurements of the basal area than stem counts [58]. The basal area of Loka Abaya National Park is less than that reported from different study areas in Ethiopia; the basal area of Belete Forest (103.5 m²·ha⁻¹) [59], Mana-Angetu (94 m²·ha⁻¹) [54], Berbere Forest (87.49 m²·ha⁻¹) [60], Kafta Sheraro National Park (79.3 m²·ha⁻¹) [41], and Alemsaga Forest (75.37 m²·ha⁻¹) [61] was higher than the basal area of woodlands of Metema (42.54 m²·ha⁻¹) [62] and Munessa–Shashmene degraded secondary forest of (9.5 m²·ha⁻¹) [62].

In terms of individual species, the highest basal area in the study was recorded by Ilex mitis (23.93 m²) followed by C. molle (12.31 m²), O. europaea L. subsp. cuspidata (9 m²), E. schimperi (6.29 m²), and E. tirucalli (5.93 m²). Those five species contributed 78.51% of the total basal area of the woody vegetation in the national park. Analysis of the relationship between basal area, stem density, and mean stem DBH indicated that stem DBH had a stronger influence on standing basal area than stem density; D. angustifolia, D. cinerea, and X. americana had relatively high stem density, but their contribution to basal area was small due to smaller DBH [35]. Bekele noted that species with the highest basal area/ha do not always have the highest density, indicating size differences between the species. In other cases, differences in growth forms could be important; shrubby tree species such as D. angustifolia, X. americana, and S. persica and their contribution to the basal area were minimum. Basal area provides a measure of the relative importance of the species rather than a simple stem count [63]. In this study, basal area analysis across individual species revealed that there was high domination by very few or small woody species. According to Cain and Castro [58], basal area measurements are used to indicate the relative ecological significance and/or dominance of woody species in a forest ecosystem. In the current study, I. mitis, C. molle, O. europaea L. subsp. cuspidata, E. schimperi, and E. tirucalli were some of the most important species in terms of their basal area, and thus, they were the most ecologically important species in the national park.

(1) Importance Value Index of Woody Plants. The relative ecological significance and/or dominance of tree species in a forest ecosystem could best be unraveled from the analysis of IVI values [64]. Cain and Castro [58] and Uhl and Murphy [65] noted that few species have been reported to have high IVI in tropical and subtropical forests. In the study area, eight species had more than 10 values of IVI, and these include I. mitis, C. molle, O. europaea L. subsp. cuspidata, D. cinerea, D. angustifolia, E. schimperi, E. tirucalli, and R. natalensis had high IVI values (Table 2). These top eight woody species contributed about 59.66% of the total IVI of woody species. These species are dominant and more frequent in the national park and they have higher IVI values. On the other side, 22 woody species had less than one IVI value; the species include A. asak, A. polyacantha, A. sieberiana, B. rotundifolia, C. spinarum, C. malosana, C. africana, C. sinensis, D. abyssinica, E. cymosa, D. mespiliformis, G. villosa, K. squerosa, F. rochetiana, F. indica, M. arbutifolia, M. kummel, V. apiculata, P. viridiflorum, P. thonningii, and Z. chalybeum (Table 2). According to Kent and Coker [4], the important value index (IVI) is a good index for summarizing vegetation characteristics and ranking species for conservation practices. The species with the highest IVI are the most ecologically important species because those species have high relative dominance in the vegetation, which needs attention for monitoring and management. Those species that had low IVI values in the vegetation of the national park are rare and warrant high conservation efforts.

3.3. Overall Population Structure

3.3.1. Vertical Height (H) and Horizontal (Diameter at Breast Height) Distribution

Vertical structure (height) of the trees and shrubs indicated that the majority of individual species’ height below 8 m indicated the vegetation was dominated by small size trees and shrubs. Few individuals were recorded above the height of 25 meters, and those individual species including A. albida, F. sur, and I. mitis were associated with along Billate riverside vegetation. This larger growth in terms of height may be associated with the high underground water table and soil nutrient inputs from the upper portion of the water shade especially in a dry land. The overall population patterns of the trees and shrubs in terms of height class revealed that the populations were in healthy status (inverted J-shaped) (Figure 3).

Similarly, the horizontal structure of the overall population pattern of trees and shrubs in terms of DBH class distribution also has shown an inverted J shape structure (Figure 4). Most of the individuals in the vegetation stand were found in the lowest DBH classes, which are reflected by the highest density of individuals in the lower height classes. According to Neelo et al. [36], an inverted J shape population structure is attributed to the prevalence of shrubs, pioneers, and other woody species naturally with small diameters rather than the normal regeneration of climax species. In current, the overall population structure in terms of DBH and height classes revealed stable regeneration and recruitment; however, an inverted J shape population structure is commonly attributed to the prevalence of shrub species naturally small in terms of growth height including X. americana, T. nobilis, M. arbutifolia, S. persica, and C. aurea, and pioneers such as D. angustifolia and E. schimperi had a large contribution to an inverted J shape population structure in terms of height and DBH rather than health population structure of the vegetation in the national park.

3.3.2. Species Population Structure

Examination of patterns of species-level population structures could provide valuable information about their regeneration and/or recruitment status as well as the viability status of individual tree species, which could further be employed for devising evidence-based conservation and management strategies [36]. Various patterns of species-level population structures have been reported for different species in forests of the country and outside the country [35, 36, 54]. In this study, twenty-five woody species were investigated for population structure in national parks. The results indicated five different species-level population structure were identified. The first group was composed of species that exhibited a higher number of individuals at the lowest diameter class and progressively declining numbers with increasing diameter classes. An inverted J-shaped distribution was represented by D. cinerea (Figure 5(a)). This group includes E. schimperi, O. europaea L. subsp. cuspidata, C. collinum, and S. alata; tropical forests show inverted J curve structure [65] and regeneration was successful for those species. The second pattern is represented by B. aegyptiaca (Figure 5(b)) in this category species including B. rotundifolia and O. insignis; this pattern indicates good reproduction but discontinuous recruitment. The lack of a middle class in the population indicates that there was a selective removal of individuals of preferred size for a particular use by the local communities living surrounding the national park.

(a)

(b)

(c)

(d)

(e)

Figure 5

Population patterns of representative individual tree species: C₁ = <2 cm, C₂ = 2 cm–5 cm, C₃ = 5.1 cm–8 cm, C₄ = 8.1 cm–11 cm, C₅ = 11.1 cm–14 cm, C₆ = 14.1 cm–17 cm, C₇ = 17.1 cm–20 cm, C₈ = 20.1 cm–23 cm, C₉ = 23.1 cm–26 cm, C₁₀ = 26.1–29 cm, C₁₁ = 29.1 cm–32 cm, and C₁₂ = >32 cm. (a) Dichrostachys cinerea. (b) Balanites aegyptiaca. (c) Combretum molle. (d) Acacia brevispica. (e) Acacia seyal.

Third type of species shows the pattern where the frequency is high at lower DBH classes but becomes irregulartowards higher classes. This pattern indicates good reproduction but discontinuous recruitment. These species that showed such population patterns were represented by C. molle (Figure 5(c). Other species including I. mitis, A. schimperi, and A. tortilissubsp. spirocarpa had such a pattern because the species were used by the local community for more than four use categories, so different size class individuals were used for different use categories. The Fourth group is represented by A. brevispica figure 5(d); this group consisted of species that showed higher individuals at the first class and second class and no individuals at the higher diameter class. The absence of those species at the higher DBH classes was due to the growth nature of the species. This group was represented by species A. brevispica, X. americana, G. bicolor, M. arbutifolia, R. natalensis, and C. aurea. It suggests good reproduction. According to [5], the forest and woodlands of Ethiopia are generally characterized by poor regeneration and recruitment due to heavy and continued anthropogenic factors. The fifth group was represented by species that have no seedlings and saplings represented by A. seyal (Figure 5(e)); in this group, A. lahai, A. sieberiana, L. triphylla, D. abyssinica, and A. polyacantha were some of the species that showed such regeneration pattern.

3.4. Regeneration Status of Woody Species in the Loka Abaya National Park

The composition, distribution, and density of seedlings and saplings indicate the future appearance of a particular forest [66]. Thus, the regeneration or recruitment potential of plants is one of the major factors that are useful to see their conservation status. In the current study, the regeneration status of 70 woody species was investigated based on the total count of seedlings and saplings of each species across all quadrats. A total of 1792.72 seedlings ha⁻¹, 817.10 saplings ha⁻¹, and 831.31 ha⁻¹ matured tree and shrub species were recorded, identified, and documented (Supplementary file 2). The ratio of seedlings and saplings to the matured woody plants was 2.19, 2.15, and 0.98, respectively. The ratio between the seedlings, saplings and mature trees can provide information regarding the distribution of mature trees, saplings, and seedlings in the national park. The large number of seedlings ha⁻¹ as compared to the matured trees has shown that the vegetation seems under normal regeneration status, but this higher number of seedlings was contributed by a few individual species.

According to Dhaulkhandi et al. [38], the density values of seedlings and saplings are considered the regeneration potential of the species. Concerning the individual species, the highest seedling density in the study national park was recorded by D. angustifolia 446.69 seedling ha⁻¹ followed by D. cinerea 222.22 seedling ha ⁻¹ and E. schimperi 172.84 seedling ha⁻¹. The sapling layers were also dominated by D. angustifolia 253.64 sapling ha⁻¹ followed by A. brevispica 120.20 sapling ha⁻¹ and E. schimperi 80.81 sapling ha ⁻¹ (Supplementary file 2). The regeneration pattern of the species varies depending on environmental variables including altitude, slope, aspect, canopy light, edaphic conditions, and the intrinsic and adaptation behavior of the species [5]. The regeneration status of the individual tree species categories as good, bad, poor, fair, and newly emerging species following [38, 39]. Out of 70 wood species, 17 (24.28%) tree species achieved good regeneration, and 21 species (30%) had fair regeneration (Figure 6). 9 species (20%) had no seedlings and only saplings and adults. Some disturbances influence seedling survival more than seedling emergences; the effect is also species-specific [5], for example, grazing and herbivores impose species-specific effects on seedling emergence, so grazing pressure can create a severe threat to plant biodiversity and species composition.

14 species (20%) including A. asak, C. malosana, C. sinensis, D. abyssinica, E. cymosa, F. speciosa, F. indica, G. tenax, G. villosa, B. rotundifolia, M. kummel, V. apiculata, S. birrea, and L. schimperi recorded only in an adult form which are not regenerating. All of them are multipurpose species and economically important species, for example, C. sinensis, D. abyssinica, E. cymosa, F. indica, G. tenax, G. villosa, B. rotundifolia, M. kummel, V. apiculata, S. birrea, and L. schimperi are edible fruit-bearing species as reported from ethnobotanical research conducted in Ethiopian and other parts of the worlds [68–70]; for those species, it is important to investigate soil seed bank analysis and study propagation methods for conservation of those valuable species. Seven species (10.29%) of new emerging species including S. guineese, S. septemtrionalis, M. azedarach, J. acerifolia, F. thonningii, E. camaldulensis, and E. capensis only found at sapling and seedling stage along Billate riverside vegetation were appearing as new species regenerating in the natural vegetation (Figure 4). This revealed that water is an important road for the introduction of invasive alien species to native natural vegetation and one of the great rift valley lakes Abaya, so it needs monitoring.

4. Conclusions

In this study, 101 woody species were recorded which are an essential component of forest vegetation. Those species play a crucial role in the national carbon stock of the country, so it is essential to investigate the carbon-storing potential of the national park. The study also confirms that the majority of woody species had a small population size, and some of them were found in a specific habitat. Therefore, those rare species need high conservation priority. In addition, our results indicated that the majority of woody species have low-frequency values indicating a relatively good floristic heterogeneity in the study of national park, but species like D. cinerea have encroached into different habitats which may significantly influence the pattern of species heterogeneity, diversity, structure, and soil nutrient dynamics, such change in vegetation may also associate with a change in carbon stocks, so the species needs monitoring to overcome further expansion and homogenization of the vegetation of the park. Furthermore, among a major plant invader species, A. drepanolobium was also confined to specific habitats in the national park which is a potential threat to the biodiversity in the area, so the vegetation needs monitoring. The diameter class distribution of selected tree species demonstrated various patterns of population structure, implying the existence of different population dynamics among ecologically important tree species. Morphological differences in species, pioneer nature of some species, selective removal of woody species as evidenced by the existence of stubs and illegal charcoal production sites/pits in the national park for livelihood diversification by local communities, grazing, and removal of preferred size for particular uses partly contributed to the existence of different population dynamics. Regeneration assessment result also confirms that some woody species occurred only in an adult form which was not regenerating and may become extinct. Thus, there is selective removal of species, although logging or grazing leads to higher dominancy of unpalatable and low-quality species. This negatively affects the structure, diversity, and regeneration of woody species and potential threats to the conservation of biodiversity at the local level. Therefore, based on our findings, the following recommendations were drowned for conservation of woody species in the study area. For those species with low population size and habitat-specific species, produce seedlings in the nursery for enrichment planting in the national park. Those species that lack regeneration ability study the soil seed banks to identify specific problems that make them unable to regenerate under the natural environment and investigate the coppicing ability of selectively removed species to design conservation strategies.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Ethical Approval

This study was approved by Addis Ababa University, Department of Plant Biology and Biodiversity Management, and Southern Agricultural Research Institute. The research was conducted inside the national park with the permission of the National Park Management Bureau under the Regional Bureau of Culture and Tourism.

Not applicable.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Authors’ Contributions

All authors played a vital role to accomplish this manuscript. Assegid Assefa developed the idea of the research, designed the research method, identified the plant, performed statistical analysis, and wrote the manuscript. Professor Tamrat Bekele, Professor Sebsebe Demissew, and Professor Tesfaye Abebe contributed significant input to the successful completion of the manuscript by supervising the study, consistent and inspiring guidance, valuable suggestions, constructive comments, and reviews on the manuscript preparation.

Acknowledgments

We are grateful to the Loka- Abaya national park office for facilitation of research work. Special thanks go to South Agricultural Research Institute and Addis Ababa University for its grant of research funds and all staffs of National Herbarium Addis Ababa University acknowledged for their support for species identification.

Supplementary Materials

Supplementary File 1. The density of trees and shrubs with DBH greater than 2 cm, 10 cm, and 20 cm in Loka Abaya National Park. Supplementary file 2. The regeneration status of woody species in the Loka Abaya National Park: density of matured trees and shrubs and density of saplings and seedlings in the national park. (Supplementary Materials)

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Copyright © 2022 Assegid Assefa et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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