2.1. Study Area Description

KAL is a zone that contained two districts, and a city administration. It is found in the South Ethiopia Regional State (Figure 1), between 5°30′0″ and 6°0′0″ N latitude and 37°40′0″ and 38°0′0″ E longitude [16]. It has 35 rural Kebeles (the smallest administrative unit in Ethiopia) and five urban municipalities. Its main town, Kelle, exists 485 km from Addis Ababa, the capital city of Ethiopia. It is bordered by the Burji Zone from the south, Lake Chamo and Nech Sar National Park from the west, and the Oromia National Regional State from the east and north [2].

Topographically, it is characterized by steeply sloping mountains (25%), hills (20%), undulating (25%), and gentle to-plane land features (30%) [15]. Roughly, the soil of KAL texturally described as thin stony soils on hill, predominantly sand or sandy loams and clay along the gradient [15]. In the flood plains of lowland, it is grey to red and sands or dark clays [2]. It has tropical humid, tropical subhumid and semiarid agroecology with their respective shares of 32%, 30%, and 38% of the total area of this specific locality [2, 15]. KAL has a bimodal rainfall distribution, with long rains (belg) occurring between March and June, and short rains (meher) occurring between August and October [15]. The total precipitation ranges from 801 to 1000 mm, and the temperature ranges from 12°C to 25°C [2].

Currently, the study landscape is inhabited with the total population of 253,728. From this, about 127,215 are males, whereas about 126,513 are females. Its population density is 180 people per km² (ADFEDO, 2024 unpublished). Of the total population, 90% live in rural areas based on rain-fed mixed (crop-livestock) agriculture that is supplemented by traditional small-scale irrigation [15]. The rest 10% are urban dweller. Though KAL was mosaic that comprised wider natural vegetation classes, currently it has been degraded due to the encroachment of cultivated land, overgrazing, indiscriminate tree cutting for charcoaling/fuel, and house building [2].

2.2. Sampling Design and Sample Size Determination

KAL was selected purposely because it comprises different land use types that have varied vegetation cover status. Preliminary survey was undertaken to identify the existing land use types across the landscape. Accordingly, five land use types (farmland, grazing land, woodland, forest, and agroforestry) were identified (Table 1, Figure 2). Next, four Kebeles (Jallo, Kelle, Kobo, and Mareta) that are connected from the downstream to the upland were selected since they comprise all the identified five land uses. Then, 105 sample plots were randomly assigned to a stratified different land use types (Table 1, Figure 3). This is due to fragmentation in vegetation cover and intermittent land use types across the studied landscape (Figure 3).

Besides, 10 key informants’ interview and three focus group discussions which contain about eight to 10 members were held in the sampled Kebeles. From these, one focus group discussion was held with Zone and District experts. Key informants and focus group discussants were smallholders and experts who have experience in identifying the local names of woody species, have knowledge about their uses, and smallholders’ preferences for plantation in the KAL. Snowball selection method was used to select key informants. Information obtained from these sources used to support quantitative results from questionnaire-based survey.

2.3. Data Collection Techniques and Materials

All the field data for this study were collected in December as the season was free of precipitation and the leaf of woody species was green. This helped to get the right overview of the vegetation structure for species identification. To achieve accurate and reliable data collection, it is essential to vary sample sizes in vegetation inventories to land use, as vegetation characteristics differ significantly across land uses [18]. Thus, 10 × 20 m (200 m²) for forest [19], 25 × 25 m (625 m²) for agroforestry and grazing land [20], 40 × 40 m (1600 m²) for farmland or field cropping land, and 20 × 20 m (400 m²) for woodland following Endris et al. [3]. Additionally, a 5 × 5 m (25 m²) plot was used for saplings and a 2 × 2 m (4 m²) plot for seedlings [14]. The dimensions of the plots and the sampling size were determined following recommended practices for accuracy, practical considerations of time, available resources, and evidences in the ecological literature [20].

Species richness and abundance parameters were measured by recording the number of species within a community and identifying number of individuals of each species present in the community. The diameter at breast height (DBH) at 1.3-m height above ground was also measured and recorded. As per the recommendation of Singh et al. [14], when trees branched at breast height or below the diameter was measured alone for the branches and averaged, and if the tree trunk was buttressed, the DBH was taken just above the buttresses. Data collection process was guided by species rarefaction curve approach. It shows the expected number of species in a set number of samples, allowing for comparisons between communities with varying sample sizes. The published volumes of the Flora of Ethiopia and the Flora of Ethiopia and Eritrea were used to identify recorded woody species [21–26]. Experienced key informants and experts were used to identify the local names of recorded woody species. For species that were difficult to identify in the field, herbarium specimens were taken to the National Herbarium of Addis Ababa University, Ethiopia.

Individual tree categorization was made at height < 1.0 m and DBH < 3 cm for seedlings [9]. The height was 1.0–3.0 m and DBH 3–10 cm for saplings, and height > 3.0 m and DBH > 10.0 cm for trees [5, 27]. The tree DBH was categorized into five classes such as 1–10, 10.1–20, 20.1–30, 30.1–40, 40.1–50, 50.1–60 cm, and greater than 60 cm [5, 9]. The DBH of trees was measured using a caliper. Diameter tape was used for tree diameters beyond caliper size which measures circumference, then later would convert to diameter. Height was measured using a 6-m graduated pole. All lists of woody species and, the number of matured trees, saplings, and seedlings were recorded to determine the regeneration status. A handheld GPS Garmin Etrex 10 worldwide was used to mark the location of each sampling plot. Information regarding to woody species preferences, seedling demand for plantation, and the reasons for species selection were collected via semistructured questionnaires from 150 smallholders by trained enumerators (Appendix B). Checklist was developed to guide key informants’ interview and focus group discussions (Appendix C).

2.4. Data Analysis

The regeneration status of woody species was analyzed by comparing growth stages of woody individuals. The regeneration status is good if sapling > seedling > matures, and fair if matures > sapling > seedling; if only mature trees are present, it is not regenerating or under risk of extinction, and otherwise poor regeneration [9]. To determine preferences of woody species by smallholders, data collected via questionnaire were analyzed using SPSS 20 version for windows. Data obtained from focus groups and key informants were used to support quantitative findings.

3.1. Woody Species Composition and Diversity in Different Land Use Types of KAL

A total of 73 different woody species belonging to 33 families were identified across all assessed five land use types (Table 2). The Fabaceae family had the highest number of species at 20.5%, followed by Rosaceae (6.8%) and Myrtaceae (5.5%). The lowest proportion of species (0.1%) was recorded from Asclepiadaceae and Bignoniaceae (Table 2). About 53% of woody families consisted of a single species (Table 2). Eleven woody families (33.33%) were common to all land uses, while 10 (30.30%) were found in a single land use (Appendix Table A1). In the KAL, 21.92% of woody species were common to all land use types and 32.88% and 10.96% occurring only in agroforestry and forest, respectively (Appendix Table A1).

In terms of the origin of woody species, 52 (71.23%) were native, and 21 (28.77%) were exotic (Table 3). The forest had the highest number of native woody species, while all reported exotic woody species were from agroforestry (Table 3). Two native woody species (Millettia ferruginea and Erythrina brucei) were found to be endemic to Ethiopia (Appendix Table A1). In the study landscape, agroforestry had the highest fraction of woody species (67.12%) than forest (Table 3).

Compared to other land uses, the highest values of H′ (4.04) and E (0.58) were recorded in agroforestry (Table 4). It implied a woody community with a higher number of different species where the distributions of some species are more abundant than others. Forest had the second highest H′ (3.54) but the lowest value of E (0.52) (Table 3). Agroforestry showed the highest species richness (21.47 ± 38.91), while farmland exhibited the lowest species richness (7.77 ± 5.65) (Table 4).

According to Table 5, the highest similarities (84%) of woody species were found between forest and woodland, and grazing and woodland. The lowest similarity (48%) was observed between agroforestry and grazing land (Table 5).

3.2. Importance of Woody Species

Among the top 10 woody species with the highest IVI: Cordia africana, Acacia tortilis, Erythrina brucei, and Moringa oleifera are listed in Table 6 and play an important ecological role in the farmland of KAL. Conversely, Podocarpus falcatus, Combretum molle, Hagenia abyssinica, and Ficus sur showed lower IVI in farmland (Appendix Table A2). Woody species such as Cordia africana, Acacia tortilis, Croton macrostachyus, and Combretum molle exhibited relatively higher IVI in grazing land. Among them, Acacia tortilis and Cordia africana were the most dominant species (Table 6). Woody species like Gardenia volkensii, Podocarpus falcatus, Rosa abyssinica, and Hagenia abyssinica showed lower IVI in grazing land (Appendix Table A2).

The woody species with the highest IVI and RDo in both woodland and forest were Acacia tortilis, Acacia polyacantha, Juniperus procera, and Cordia africana (Table 7). On the other hand, species such as Calotropis procera, Carissa spinarum, and Ximenia americana exhibited the lowest IVI and RDo in both forests and woodlands (Appendix A2).

In agroforestry, species like Coffea arabica, Grevillea robusta, Persea Americana, Cordia africana, Mangifera indica, and Millettia ferruginea showed the highest IVI (Table 7). Conversely, Polyscias fulva, Podocarpus falcatus, Ehretia cymosa, Olea europaea, and Schinus molle exhibited the lowest IVI and lowest RDo in agroforestry (Appendix Table A2).

3.3. Woody Species Stand Structure

The diameter class distribution of woody species in KAL exhibited an inverse J-shaped pattern. Farmland and agroforestry land uses showed a consistent decrease in the number of woody plants as diameter classes increased (Figure 4). Forest, grazing, and woodlands also showed a decreasing trend, followed by an increase in woody plant frequency in the middle diameter classes, and then a decrease in the larger diameter classes, resulting in an interrupted inverted J-shaped distribution (Figure 4). Across all land use types, the highest numbers of woody plants were found in the lower diameter classes (< 20 cm) (Figure 4). The distribution of the density of trees in different height classes followed a similar pattern to that of diameter classes.

A decline in tree density was observed toward the taller height classes for all land uses (Figure 5). However, farmland forest land had fewer than 50% of trees in the lower height class (1–10 m), while agroforestry (63.10%), grazing land (53.70%), and woodland (53.50%) had a higher frequency of trees in the lower height class (Figure 5).

3.4. Analysis of Regeneration Status

3.5. Preferences and Plantation Purposes of Woody Species by the Smallholders

Woody species presented in Table 10 are the most preferred by smallholder to their various functional and ecological purposes. Indigenous woody species such as Acacia tortilis, Juniperus procera, Cordia Africana, Croton macrostachyus, and Millettia ferruginea were observed in better abundance in the assessed land use types (Appendix Table A2). Among these indigenous woody species, Juniperus procera, Cordia Africana, Croton macrostachyus, and Millettia ferruginea were the most preferred by smallholders (Table 10). These species are highly preferred by the smallholders due to their fast growing, giving shade, income generation, construction wood, fuelwood, live fencing, and soil management purposes (Table 10). The most preferred exotic woody species included Coffea arabica, Mangifera indica, Eucalyptus grandis, Grevillea robusta, Persea americana, and Moringa oleifera (Table 10). Among these woody species Eucalyptus grandis and Grevillea robusta mainly preferred for income generation, construction wood, fuelwood, and live fencing. Whereas Coffea arabica, Mangifera indica, Persea americana, and Moringa oleifera for food, income generation, and shade (Table 10).

4.1. Woody Species Composition and Diversity in Different Land Use Types of KAL

The results of this study showed that the Fabaceae woody family is dominant in the study area. Similar studies conducted elsewhere [5, 9, 33, 34] have also reported the prevalence of Fabaceae woody species. This dominance is likely due to its ability to adapt to various ecological zones in the KAL and elsewhere. However, a study by Tegene et al. [11] and Yahya et al. [35] did not repot Fabaceae among the top families of woody species in Doshke Forest of Chencha, Ethiopia, and Yerer Mountain Forest of Central Highlands, Ethiopia, respectively. As the most dominant families in Doshke Forest of Chencha were Myrsinaceae and Rubiaceae [11], Anacardiaceae and Celastraceae were the dominant families in the Yerer Mountain Forest [35]. The variation in woody family could be attributed to difference in geographical location, level of management by the community, climatic, and anthropogenic factors [5, 34].

The majorities (65.75%) of the identified woody species were in tree form, followed by shrubs in the study area. This could be attributed to the suitability of climatic and soil condition. The dominance of tree form is also reported in different parts of the world [7, 33, 36]. Grazing land had the second lowest amount of woody species next to farmland. Our site observations also confirmed that woody plants were distributed in small patches in grazing land. Species such as Balanites aegyptiaca, Acacia bussei, Dichrostachys cinerea, Dodonaea viscose, Entada abyssinica, and Acacia seyal were frequently found in grazing and woodland. Key informants described these woody species as bush encroachers, invaders, and indicators of land degradation. This suggests that the quality of these land uses is deteriorating due to extensive grazing, which could lead to the loss of some woody species seedlings. This finding is consistent with other studies [9, 10, 33].

The higher H′ value in the agroforestry indicates greater diversity of woody species than others due to its ability to retain native woody species from the nearby natural forests, the plantation of exotic woody species by smallholders, and degradation of the forest. The E value related to the H′ value suggests that there is an uneven distribution of woody species. This means that within a system marked by a high diversity of woody species, certain species are more abundant than others. This finding coincides with the results of Eyasu et al. [10]. Despite anthropogenic disturbances, the forest land reflected relatively better diversity of woody species next to agroforestry. As Mucheye and Yemata [5] reported a larger H′ value from a dry Afromontane forest, Lemi et al. [4] reported a smaller H′ value from Dindin Natural Forest, South East of Ethiopia. This variation could be attributed to differences in the level of anthropogenic disturbances, differences in management, and environmental factors [7, 10, 34, 35]. About 16 woody species that recorded from all the assessed land uses indicated the existence of some multipurpose native woody species in agroforestry and farmland. However, SSI showed a higher dissimilarity in woody species between agroforestry and other land use types. This is due to smallholders introducing exotic woody species (Persea americana, Eucalyptus grandis, Grevillea robusta, Moringa oleifera, Casimiroa edulis, and Mangifera indica) into the system. This finding confirms the results reported by Eyasu et al. [10], who concluded a higher dissimilarity of woody species between homegarden agroforestry and natural forests.

4.2. Importance and Stand Structure of Woody Species

Since woody species that possess a high IVI are considered ecologically more significant than those with a low IVI [10], Cordia africana, Acacia tortilis, and Erica arborea are ecologically important in cultivated lands of the study landscape. These woody species possessed the greatest ecological significance and socioeconomic aspects. Other scholars [4, 7, 10, 34, 35] also reported that woody species that possess a high IVI have the greatest ecological significance and socioeconomic contribution. On the other hand, woody species like Podocarpus falcatus, Acacia nigra, and Acacia abyssinica exhibited the lowest IVI and RDo in the KAL. According to the findings of other scholars [4, 9, 10], this finding implied that these species are threatened and require immediate intervention.

The diameter class distribution of woody species in agroforestry land use showed a consistent decrease in the number of woody individuals as diameter classes increased (Figure 4). This resulted in an inverted J-shape (L-shape) pattern implied the presence of highest density of woody individuals in the lower diameter classes with a gradual decrease as the diameter classes increased. This distribution pattern could be attributed to the selective removal of middle and high-diameter class trees for various purposes by land owners/managers and reflected adequate seedling reproduction, regeneration, and recruitment. This finding is similar to the findings reported by [9, 10] and [14]. On the other hand, an interrupted distribution of woody individuals from lower to higher diameter class in forest, grazing, and woodland may indicate poor reproduction of woody species. The decline in the density of woody individuals within the upper diameter class in forest, wood, and grazing land of the KAL might be linked to illegal logging by the local people. Similar findings were reported by Tesfay et al. [9] from the Gra-Kahsu natural vegetation, southern Tigray.

In the agroforestry, there is a higher frequency of younger or smaller woody plants in the lower height classes. This suggests good regeneration in these areas [5]. In this land use, taller trees are used to provide shade for shorter woody species, and their lateral branch growth is managed to facilitate the growth of smaller plants. Farm management activities and selective woody species plantation in agroforestry facilitate the distribution of tree height classes. Gebeyehu et al. [8] also indicated that the farm management activities and selective woody species plantation in agroforestry affect the height class distribution of trees. In forest land, there is a dominance of middle-height classes of woody individuals, indicating selective removal of mature and larger trees. However, illegal wood chopping and uncontrolled grazing threaten the forest land, leading to poor regeneration. Studies elsewhere have also reported that free grazing and resource exploitation are major challenges for plant regeneration [10, 34].

4.3. Analysis of Regeneration Status

The analysis of woody species growth status (seedling, sapling, and mature) and their densities is important for understanding the regeneration status of a plant community and designing appropriate means of management [21]. In farmland, 19.2% of woody species were not found at the seedling stage. This could be attributed to smallholders managing the land for annual cropping often for seasonal oxen plowing. The absence of seedlings in grazing and woodland for many woody species could be linked with uncontrolled pasturing. According to Gebeyehu et al. [8], this indicates a higher likelihood of discontinuity in their population structures. Notably, woody species such as Acacia nigra, Acacia abyssinica, and Podocarpus falcatus occurred without seedlings and saplings. As suggested by other similar studies elsewhere [9, 10, 35], species with few or no seedlings and saplings are at risk of local extinction. Thus, this finding implies that these woody species should receive priority in conservation management.

At KAL, agroforestry was observed to have a majority of woody species showing good or fair regeneration status. According to Tesfay et al. [9], the regeneration status of natural vegetation is considered good if there are more saplings than seedlings and more seedlings than mature trees. It is considered fair if there are more mature trees than saplings and more saplings than seedlings. In the case of agroforestry, there were more saplings than seedlings, and more seedlings than mature trees. This could be attributed to the presence of mature trees in agroforestry, which contribute to the availability of viable seeds for germination [10, 12]. Woody species (Olea capensis and Acacia brevispica), which were found to have a fair regeneration status in forest, are ecologically less important and invasive in nature. The fair regeneration status of these species could be linked to their unpalatability to herbivores. This finding suggests intervention to control the dominance of woody species with invasive nature in the forest.

4.4. Preferences and Plantation Purposes of Woody Species by the Smallholders

The results from the IVI and regeneration status of woody species suggest a promising prospect for exotic species which used for construction, fuel wood, and fruit production. Mainly smallholders of the studied landscape preferred exotic woody species (Eucalyptus grandis and Grevillea robusta) and grow in agroforestry land use due to their rapid growth, space efficiency, and marketability. These exotic woody species used for income generation, construction wood, fuelwood, and live fencing in KAL. Other fruit trees (Mangifera indica and Persea americana) and stimuli (Coffea Arabica) also planted in agroforestry land use for food, income generation, and shade purposes. This finding is similar with other scholars [9, 10]. Likewise, indigenous woody species such as Juniperus procera, Cordia africana, Croton macrostachyus, and Millettia ferruginea exhibit high IVI, good/fair regeneration status, and possessed higher preference by smallholders mainly for income generation, construction wood, fuelwood, and live fencing. Tesfay et al. [9] and Yakob and Fekadu [34] also reported similar findings.