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Journal of Resources and Ecology >

2023 , Vol. 14 >Issue 1: 35 - 45

DOI: https://doi.org/10.5814/j.issn.1674-764x.2023.01.004

Plant and Animal Ecology

Effects of Prosopis juliflora Invasion on Native Species Diversity and Woody Species Regenerations in Rangelands of Afar National Regional State, Northeast Ethiopia

  • Wakshum SHIFERAW , 1, * ,
  • Sebsebe DEMISSEW 2 ,
  • Tamrat BEKELE 2 ,
  • Ermias AYNEKULU 3
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  • 1. College of Agricultural Sciences, Department of Natural Resources Management, Arba Minch University, Arba Minch 21, Ethiopia
  • 2. College of Natural and Computational Sciences, Department of Plant Biology and Biodiversity Management, Addis Ababa University, Addis Ababa 3434, Ethiopia
  • 3. World Agroforestry (ICRAF), Nairobi 30677, Kenya
*Wakshum SHIFERAW, E-mail:

Received date: 2021-07-25

  Accepted date: 2022-06-29

  Online published: 2023-01-31

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Abstract

Investigation of the invasion of Prosopis juliflora and its effects on indigenous plant species are important for the control of the species. The study aimed to assess: (1) the effects of Prosopis juliflora invasion on the diversity of plant species in Awash Fentale and Amibara Woredas; (2) the effects of Prosopis juliflora invasion on the regeneration potential of native woody species. Sample collection was performed in habitats of Prosopis juliflora thicket, Prosopis juliflora mixed with native species stands, non-invaded woodlands, and open grazing lands. The vegetation was stratified into invasion levels of Prosopis juliflora and then a random sampling technique for data collection. Among species of plants, the highest proportion of species, 75 (47.8%), was recorded under non-invaded woodlands, but the lowest proportion of species, 22 (14%), was recorded under open grazing lands. The invasion of Prosopis juliflora reduced the Shannon diversity index. The mean values of the Shannon diversity index and species richness under Prosopis juliflora mixed with native species (H´=2.22, R=14) and non-invaded woodlands (H´=2.23, R=13) were significantly higher than Prosopis juliflora thicket (H´=1.96, R=12) and open grazing lands (H´=1.84, R=10). The highest total density (358 stems ha‒1) of seedlings was recorded under Prosopis juliflora mixed with native species. But, the lowest total density (153 stems ha‒1) of seedlings was recorded under Prosopis juliflora thickets. Moreover, 102 trees ha‒1 native woody species were recorded under Prosopis juliflora thicket, but 1252 trees ha‒1 native species were recorded under non-invaded woodlands. If the invasion of Prosopis juliflora and its effects on native species diversity continue coupled with a drier climate, plant diversity of the Afar flora region will be highly affected and its ecosystem services will be under question. Thus, the participation of all stakeholders and multidisciplinary research approaches should be designed for the management of the species and rehabilitation of the rangelands in the region.

Key words： composition; diversity indices; habitat; invasion level, pastoralists; Prosopis juliflora

Cite this article

Wakshum SHIFERAW , Sebsebe DEMISSEW , Tamrat BEKELE , Ermias AYNEKULU . Effects of Prosopis juliflora Invasion on Native Species Diversity and Woody Species Regenerations in Rangelands of Afar National Regional State, Northeast Ethiopia[J]. Journal of Resources and Ecology, 2023 , 14(1) : 35 -45 . DOI: 10.5814/j.issn.1674-764x.2023.01.004

1 Introduction

A species is considered an invasive alien species when it spreads beyond its native range in distribution (Shine et al., 2009). Prosopis juliflora (hereafter P. juliflora) is among the invasive plant species native to South America, the Caribbean, Mexico, and Central America (Pasiecznik et al., 2001). P. juliflora has been introduced intentionally to Ethiopia in the Afar region in the late 1970s for the reclamation of degraded areas (Abebe, 2012; Haji and Mohammed, 2013; Ayanu et al., 2015). Although introduced P. juliflora has been providing a few uses, for instance, fuelwood and dry season fodder in rural areas, the threat posed by it in terms of invasion of fertile agricultural lands and loss of bio-diversity is looming enormously (Kathiresan, 2006). In the lowlands of Ethiopia, rangelands are subjected to different human and natural impacts. These have facilitated the invasion of weeds in rangelands thereby threatening pastoral production systems (Dalle et al., 2006). Among woody invasions, P. juliflora is imposing the most hazards to lowlands in the Eastern and Northeastern Ethiopia particularly in Afar region (Shiferaw et al., 2004; Abebe, 2012; Shiferaw et al., 2018).
Land use and land cover changes, abiotic and biotic competitions, overgrazing, and climate changes are aggravating the probability of invasion of this alien plant species (Pasiecznik et al., 2001; Shiferaw et al., 2019c). Using of woody species for fuelwood and construction purposes are also caused the rangelands susceptible to invasion (Tsegaye et al., 2010; Islam et al., 2018). Seed dispersal mechanism of P. juliflora was also the most facilitating process for its invasiveness. Endozoochory has been recognized as the most important dispersal mechanism in the invasion P. juliflora, because their sugary and tasty pods attract animals. Some of their seeds remain intact after passing through some animals’ digestive systems (Abbas et al., 2018).
A study by Shiferaw et al. (2019a) reveals that 1.2 million ha of land which had been invaded by P. juliflora constituted 12.3% of the surface area in Afar region. Due to dispersal agents of P. juliflora, roadsides, river courses, farmlands, irrigation canals, and wetlands are the most severely invaded habitats in the region (Abbas et al., 2019). P. juliflora is also dominating large areas of prime grazing land in the region. Nutrient-rich grasses which are the main feed sources for grazers are progressively outcompeted (Ayanu et al., 2015).
There are plenty of literatures showing both the negative as well as the positive environmental effects and use values of P. juliflora. However, in East Africa and the Afar region of Ethiopia in particular, the problems of P. juliflora are outweighing positive ones in the context of ecological, socio-economic, and health aspects (Abebe, 2012; Shiferaw et al., 2018). It has been noted that P. juliflora has a depressing effect on annual compared with perennial plants, especially on grasses in arid lands (El-Keblawy, 2012). The study by Kahi et al. (2004) reveals that standing biomass, frequency, and the cover of understorey plant species were more significantly higher in the open area than under the canopies of P. juliflora. Effects of P. juliflora on diversity studies in the Afar region have been discussed by many researchers (Kebede, 2009; Getachew et al., 2012; Koyira, 2015; Shiferaw et al., 2019a). Social aspects about community perceptions and P. juliflora control options in Afar region were reported by Shiferaw et al. (2022), relations of P. juliflora with livelihoods of the community by Shiferaw et al. (2020) and Land-use/ cover dynamics by Tsegaye et al. (2010) and Shiferaw et al. (2019a). However, several of these studies didn’t consider the effects of the invasion levels of P. juliflora on native species diversity and regeneration of woody species. In addition, P. juliflora was not excluded from data analysis rather most of the findings were for the whole woodlands not categorized into habitats. Our study tried to show the progress of P. juliflora invasion into other land uses and the directions were from wetlands, homesteads, roadsides, settlement areas towards woodlands and grazing lands exiting in hilly areas. Furthermore, the current study shows the effects of P. juliflora on selected native woody species in different habitats of the study woredas of Awash Fenatle and Amibara. Even though the species progressed towards other land uses, income from P. juliflora for households shows insignificant (0.9%) in comparison to other income activities in Awash Fentale and Amibara Woredas (Shiferaw et al., 2020) and it was less utilized by the local communities in Afar region. Traditionally, the agro-pastoralists and pastoralists in Afar region were not using fire for rangeland management. Therefore, the species was easily invading the rangelands and progress into other land uses and land covers. Little is known about the extent of the effects of P. juliflora on plant diversity, composition, and regeneration of woody species. In this study, we tried to assess the effects of P. juliflora invasion levels on (i) the floristic composition and diversity of plant species and (ii) regeneration potential of native woody species in selected P. juliflora invaded areas of Amibara and Awash Fentale Woredas. We tested the following hypotheses: 1) P. juliflora invasion levels didn’t affect native species diversity and compositions and 2) P. juliflora invasion levels didn’t affect regeneration patterns of woody species.

2 Materials and methods

2.1 Description of the study area

For data collection, two P. juliflora invaded districts from the Southern Afar region namely Awash Fentale Woreda and Amibara Woreda were selected. The aforementioned woredas were selected based on invasion of P. juliflora and existence of non-invaded land use and land covers (P. juliflora invasion levels) for comparisons. Four sites of Diduba and Kebena from Awash Fentale Woreda and Kurkura and Andido from Amibara Woreda were selected. Amibara Woreda is located in between 741-746 m asl and 9°19°44N and 40°10°52E, whereas Awash Fentale is located at 700-1000 m asl and 9°10°00N and 40°03°33E (Fig. 1).
Fig. 1 Location of the study area (Afar region) in Ethiopia (a) and location of Amibara Woreda and Awash Fentale Woreda in Afar region (b)
An annual precipitation in Awash Fentale Woreda was 490 mm and Amibara Woreda was 416 mm. The mean annual temperature for Awash Fentale Woreda was 27.0 ℃, the mean minimum annual temperature was 16.7 ℃, and the mean maximum annual temperature was 37.8 ℃ (Fig. 2a). The mean annual temperature of Amibara Woreda was 26.8 ℃, the mean minimum annual temperature was 13.8 ℃, and the mean maximum annual temperature was 38.2 ℃ (Shiferaw et al., 2020) (Fig. 2b).
Fig. 2 Thirty-one-year (1986-2017) climate diagram for Awash Fentale Woreda (a) and fifteen-year (1986-2001) climate diagram for Amibara Woreda (b)

Note: Data source: Shiferaw et al. (2020).

The texture of the soil was usually sandy and originated from Jurassic and Cretaceous limestone and other sedimentary rocks (Friis et al., 2010). According to FAO soil classification and ISRIC-world soil information, the soils of the Afar Floristic region were Lithic and Eutric leptosols, and Eutric fluvisols. Acacia-Commiphora woodland and bushlands were among the vegetation types in Ethiopia which were characterizing the Floristic region (Friis et al., 2010). A total population of 83851, and 40901 numbers were living in Amibara and Awash Fentale Woreda, respectively (CSA, 2013). About 90% of Afar people were pastoralists, while the rest 10% were considered agro-pastoralists (Wakie et al., 2014).

2.2 Sampling design

During the preliminary reconnaissance survey, P. juliflora invaded patches and non-invaded habitats were selected for vegetation inventory. The sites were selected based on the severity of the invasion of P. juliflora and the existence of non-invaded adjacent sites. Moreover, the study sites were stratified into approximately homogeneous units based on the following parameters: Such as invasion levels of P. juliflora (invasion levels were quantified on number stems of P. juliflora in habitats), land uses and land covers, and physiography (topography) such as slope and altitude of the sites. Thus, vegetation data were collected from: 1) P. juliflora thicket >75% stems of which contained purely P. juliflora stems in the patch, 2) Mixed P. juliflora with native species in the patch (50%- 75% stems of P. juliflora), and 3) Non-invaded woodland and open grazing lands as control which contained no stems of P. juliflora in those habitats.
Quadrants were laid at different invasion levels to collect vegetation and other environmental variables (Kent, 2011). A stratified random sampling technique was used. Those quadrats for habitat categories were selected to assess the variations of P. juliflora invasion levels and other environmental factors to relate with vegetation patterns, abundances, and regeneration potentials of woody species. Modified methods of habitat selection were followed (Gairola et al., 2012; Muturi et al., 2013). A total of 64 quadrats from the study sites i.e. 16 quadrats from each of the four P. juliflora invasion levels were sampled (Fig. 3).
Fig. 3 Plates showing P. juliflora invasion levels or habitats in Afar Floristic Region

Note: Plate (a) indicates an area invaded purely by P. juliflora and plate (b) P. juliflora mixed with native species, however, plate (c) is non-invaded woodlands without P. juliflora invasion and plate (d) is only grassland area not invaded by P. juliflora or encroached by other native woody species.

Data collection was carried out immediately after rainy season (end of August to end of November 2017). Vegetation data were collected under different P. juliflora invasion levels using quadrat sizes of 20 m×20 m (400 m2) for P. juliflora thicket, P. juliflora with native species, and non-invaded woodland and open grazing lands. The first quadrat was started randomly, and then the successive quadrats were established using preferential sampling for the three patches of P. juliflora thicket, P. juliflora with native species, and non-invaded woodlands, and other open grazing lands adjacent to each patch.

2.3 Data collection

Individual woody categorizations were made at a height of less than 1 m and diameter at stump height (DSH) less than 1 cm for seedlings. The height 1-2 m and diameter at breast height (DBH) or diameter at stump height 1-5 cm for saplings and height greater than 2 m and DBH/DSH greater than 2 cm for tree/shrub species were measured. For regenerated seedlings (height less than 1 m), only their number was counted and recorded. In each quadrant, all trees or shrubs were measured and counted. Based on DBH size classes, size class structures were made for each woody species (Fig. 3). Cover abundance was also identified and estimated for all growth forms using Maarel (1979). Diameter at breath height (DBH) for trees or diameter at the stump height (DSH) for shrubs was measured in the quadrats to capture all multi-stem in each individual of the shrub. Caliper and hypsometer were used to measure tree/shrub diameter and height respectively.
If a tree was branched at breast height or below, the diameter was measured separately for the branch and then averaged. Any individual with its above-ground stem growing in a cluster for woody plants (shrubs) was counted and measured as a single individual for basal area calculation following the fixed-area micro-plot method (Chojnacky and Milton, 2008). Quadrats of 1 m×1 m (1 m2), 10 m×10 m (100 m2), and 20 m×20 m (400 m2) were used for recording seedlings/herbaceous, saplings, and tree/shrub species, respectively. That was 80 sample quadrats of 1 m×1 m (1 m2), 16 quadrats of 10 m×10 m (100 m2), and 16 quadrats of 20 m×20 m (400 m2) were laid in each invasion habitats (levels).
The presence or absence of plant species was registered by direct counting. Percentage cover (ground cover) of herbaceous plants was estimated from the five subplots of 1 m2 (at four corners and one in the center of the main plot) and then the mean estimates were taken (Maarel, 1979). That is, for identification of all grasses and herbaceous within the marked area of the 1 m2 were estimated, recorded, and collected. All quadrats were geo-referenced using GPS for location (coordinates) and altitudes; and a clinometer for slopes, hypsometer for the height of trees. Plant specimens were brought and stored in Addis Ababa University’s national herbarium for further identification. Plant identification and naming were followed by published flora books of Ethiopia and Eritrea (Volume 1-7).

2.4 Data analysis

The cover/abundance values of all plant species in each plot were visually estimated using a 1-9 modified scale (Maarel, 1979). The diversity of plant species were analyzed using R-software version 3.5.2 for vegetation patterns. Plant diversity could be measured using Shannon-Weiner diversity index and species richness using Equation (1) (Taylor et al., 2018). Evenness (E°) was calculated from the ratio of observed diversity to maximum diversity following Pielou (1966) using Equation (2).
$H’=-\sum^{k}_{i=1}p_{i}log_{2}(P_{i})$
where pi denoting the proportion in group k.
$E’=\frac{H’}{H_{max}}=\frac{H’}{InS}$
where H° is index of species diversity; Hmax = the maximum level of diversity possible within a given population, given S species.
Data series were tested for normality and homogeneity of variance. The regeneration status of woody species was determined based on the population size of seedlings, saplings, and adults. Then, all obtained vegetation patterns in terms of diversity indices and regeneration of woody species in different P. juliflora invasion levels were subjected to analysis of variance using the General Linear Model of SAS (version 9.0) software (SAS, 2002). Significant differences among means were separated using Duncan’s multiple range tests for effects of P. juliflora invasion levels on vegetation patterns and regeneration potential of woody species. The drawing of Fig. 1 and Fig. 2 are drawn using ArcGIS and Microsoft excel. The analysis of vegetation patterns were organized by recording and arranging the data on the Excel Microsoft data sheet. All vegetation data were verified and analyzed data analysis in base R Software programs to compute the diversity (Woldu, 2017). In addition, the regeneration patterns of the species were determined based upon the population size of seedlings, saplings and trees. A histogram in Fig. 4, Fig. 5, and Appendix 2 (a-j) were drawn using Microsoft Excel Software.

3 Results

3.1 Invasive effects of Prosopis juliflora on floristic composition and structure

In total 153 plant species belonging to 38 families were recorded in the study woodland. Number of species distribution among the different families showed high variability. Plant families are the taxonomic categories that were assessed in different habitats selected in the study areas; 29 (18.5%) species belonged to Poaceae, 13 (8.3%) belonged to Fabaceae, and 11 (7%) species each belonged to the families Acanthaceae and Malvaceae (Appendix 1). Most species 72 (45.9%) were forbs, and only 9 (5.8%) of species were trees (Fig. 4).
Fig. 4 Proportion (%) of plant species in the study woredas
Among species recorded, 47.8% were under NIWLs, but 14% were under OGLs. The dominant growth form was forbs under NIWL, which accounted for 22.9% of all growth form of species. Results also show that 11.5% grass species were recorded under NIWLs and 6.4% under OGL. In addition, 1.9% tree species each in PJT and NIWLs. However, 1.3% trees were recorded under PJM and 0.6% in OGLs (Fig. 5).
Fig. 5 Proportion (%) of plant species in each habitat

Note: PJT is Prosopis juliflora thicket; PJM is Prosopis juliflora mixed with native species; NIWL is non-invaded woodland; OGL is open grazing land. P. juliflora invasions levels didn’t show significant variations in growth forms (P > 0.05).

3.2 Invasion of Prosopis juliflora and plant species diversity

As depicted in Table 1, P. juliflora invasion levels significantly affected the Shannon diversity index and species richness (P < 0.05). However, the P. juliflora invasion levels did not affect Shannon evenness and species diversity indices (Shannon diversity index, species richness, and Shannon evenness) did not show significant difference between districts at P > 0.05 (Table 1).
Table 1 GLM showing effects location and P. juliflora invasion levels on species diversity in Amibara and Awash Fentale Woredas, Ethiopia
Explanatory variable ×Response variable Source df Sum of squares Mean square F-value P-value (Pr >F)
Model 3 1.86 0.62 3.74 0.016
Habitat ×H° Error 60 9.92 0.17 - -
Corrected total 63 11.77 - - -
Model 3 191.42 63.81 3.52 0.020
Habitat × R Error 60 1086.81 18.11 - -
Corrected total 63 1278.23 - - -
Model 3 0.01997 0.0067 2.41 0.076
Habitat × E° Error 60 0.166 0.0028 - -
Corrected total 63 0.186 - - -
Model 1 0.068 0.068 0.26 0.61
District × H° Error 62 15.94 0.26
Corrected total 63 16.006
Model 1 0.0 0.0 0.0 1.0
District × R Error 62 4716.0 76.06
Corrected total 63 4716.0
Model 1 0.005 0.0049 0.33 0.57
District × E° Error 62 0.948 0.0153
Corrected total 63 0.9527

Note: Significant at α = 0.05, H° is Shannon diversity, R is species richness, E° is Shannon evenness, Pr > F is the P-value associated with the F statistic of a given effect and test statistic. The null hypothesis that a given predictor has no effect on either of the outcomes is evaluated with regard to this P-value.

The average value of Shannon diversity index in Table 2 under P. juliflora mixed with native species (2.22) and non-invaded woodlands (2.23) were significantly higher than P. juliflora thicket (1.96) and open grazing lands (1.84). Moreover, the average values of species richness in Table 2 under P. juliflora mixed with native species (13.94) showed significantly higher than open grazing lands (9.56). Also, the Shannon evenness of non-invaded woodlands (0.87) was significantly higher than P. juliflora thicket (0.83) and open grazing lands (0.83) (Table 2).
Table 2 Mean values of vegetation patterns for P. juliflora invasion levels in Awash Fentale and Amibara Woredas
Vegetation patterns Prosopis invasion levels
PJT (N=16) PJM (N=16) NIWL (N=16) OGL (N=16)
Shannon diversity (H°) 1.96ab 2.22a 2.23a 1.84b
Richness (R) 11.50ab 13.94a 13.44a 9.56b
Shannon evenness (E´) 0.83a 0.86a 0.87a 0.83a
Vegetation patterns District
Awash Fentale (N= 32) Amibara (N=32)
Shannon diversity (H°) 1.96a 1.90a
Richness (R) 58a 58a
Shannon evenness (E°) 0.49a 0.47a

Note: PJT is P. juliflora thicket; PJM is mixed (P. juliflora + native woody species); NIWL is non-invaded woodland; OGL is open grazing land; N= Number of individuals per 400 m2. Values with similar letters are insignificant (P >0.05), but different letters are significantly different (P <0.05).

3.3 Invasion of Prosopis juliflora and size class structures of woody species

As can be seen from the result in Table 4, tree, saplings and seedlings showed to be statistically not affected by the invasion of P. juliflora at the different invaded habitats (P > 0.05). The lowest total densities of trees were recorded under P. juliflora thicket (102 stems ha‒1). But, the highest total densities of trees recorded under non-invaded woodlands (1252 stems ha‒1) and the highest total density of seedlings recorded under P. juliflora mixed with native species (358 stems ha‒1). However, the lowest total densities of seedlings were recorded under P. juliflora thickets (153 stems ha‒1) (Table 4).
Table 3 General linear model showing the effects of P. juliflora invasion levels on regeneration status of woody species in Awash Fentale and Amibara Woredas
Response variables Source df Sum of squares Mean square F-value P-value (Pr> F)
Model 2 36907.7 18454 0.94 0.41
Habitat × Trees Error 21 410318.9 19539 - -
Corrected total 23 447226.7
Model 2 32.66 16.33 0.71 0.49
Habitat × Saplings Error 1592 36640.01 23.01 - -
Corrected total 1594 36672.67 - - -
Model 2 69.28 34.64 1.06 0.35
Habitat × Seedlings Error 1592 51796.23 32.53 - -
Corrected total 1594 51865.51 - - -

Note: α = 0.05, Pr > F is the P-value associated with the F statistic of a given effect and test statistic. The null hypothesis that a given predictor has no effect on either of the outcomes is evaluated with regard to this P-value.

Table 4 Mean values of the regeneration status for P. juliflora invaded and non-invaded habitats at Amibara and Awash Fentale districts, Ethiopia (Unit: stems ha-1)
Growth Stages Habitat
PJT PJM NIWLs
Trees 102a 585b 1252b
Saplings 151a 334b 324c
Seedlings 153a 358b 242c

Note: PJT is P. juliflora thicket; PJM is P. juliflora mixed with native species; NIWLs is non-invaded woodlands.

3.4 Invasion of Prosopis juliflora and size class structures of a selected population of woody species

Results showed that the pattern of population structure for a given species can be roughly grouped in one of four basic types (Shiferaw et al., 2019b): I (inverted J-shape), II (bell-shaped type), III (J-shape), and IV (U-shaped). Type I is a pattern in which a diameter size class distribution displays a greater number of smaller trees than large ones. Type II is characteristic of species that show discontinuous, irregular, and or periodic recruitment. Type III reflects a species whose regeneration is severely limited. Type IV is characteristic of species that show a lower number of saplings or mid-class distribution than seedlings and trees.
The results in Appendix (2b and f) show that the population of P. juliflora and Acacia tortilis (Frossk.) Hayne both under P. juliflora mixed with native species, their regeneration patterns had U-shaped (Type IV) type. Meanwhile, the population of Cordia sinensis Lam under non-invaded woodlands; Cadaba rotundifolia Forssk and Grewia tenax (Forssk.) Fiori under P. juliflora mixed with native species showed that regeneration patterns bell-shaped type (Type II) (Appendix 2i and j). Furthermore, in Appendix (2d and j) the population of C. rotundifolia, under non-invaded woodlands, and Salvadora persica L. under P. juliflora mixed with native species showed the regeneration patterns of inverted J-shape (Type I). On the other hand, the population of A. mellifera, S. persica and A. tortlis under NIWL; and P. juliflora, G. tenax, A. oerfota, A. senegal, A. mellifera and C. rotundifolia under P. juliflora thickets; and A. nilotica under non-invaded wood lands and woody species like A. mellifera, A. oerfota, A. tortilis, G. tenax, S. persica, C. rotundifolia under open grazing lands resembled the limited regeneration patterns of J-shape (Type III) in the study areas (Appendix 2b, d, g, h, i and j).

4 Discussion

4.1 Prosopis juliflora and floristic composition

The invasion of P. juliflora in Awash Fentale and Amibara Woredas changed the diversity, and abundances of native woody species. The number of families identified was comparable with research reported by Aggarwal (2012) from the pre-urban region of India and findings by Demissie (2009) in Awash National Park in Ethiopia. Other findings by Mukherjee et al. (2017) show that both the number of species and families were lower than the number of families recorded in the present study.
The species richness under the canopy of P. juliflora, in this study, was lower compared with those reported by Patel et al. (2012) in Delhi, Gujarat, and Rajasthan of India. Our study also reveals that the number of species under P. juliflora thicket was higher than that of the number of species under the P. juliflora thicket on Delhi ridge of India (Naudiyal et al., 2017). Similar trends on the number of associated species were also observed in sites with a lower density of P. juliflora under P. juliflora with native species and outside (non-invaded woodlands) canopies in UAE (El-Wahab et al., 2018).
The presence of more climbers under P. juliflora thicket compared to other habitats might be due to the presence of woody species that help the climbers as support. The trees in the habitats also serve as safety factors to resist a range of natural factors, particularly high winds (Wainwright et al., 1976).
A higher number of forb species were also observed under P. juliflora thicket than other habitats. The availability of forbs contributes to suitable resource islands which out-competed other species under the thicket of P. juliflora canopies (Tiessen et al., 2003; Kahi et al., 2009). On the other hand, grass species were higher under P. juliflora with native species than other habitats. In these particular cases, P. juliflora invasion might be affected the growth of grasses due to the inhibition of light penetration and allelopathic chemicals under the thicket of P. juliflora (El-Keblawy and Abdelfatah, 2014).

4.2 Prosopis juliflora invasion and species diversity

Species diversity has been recognized as an important component of the sustainable development of P. juliflora with native species (Wakie et al., 2014). This is because of ecosystems with diverse species resilient for the competition of resources with P. juliflora. Species diversity was lower both under open grazing lands (1.84) and P. juliflora thicket (1.96) compared to P. juliflora mixed with native species (2.22) and non-invaded woodlands (2.23) habitats. The reason might be disturbance intensities under the open grazing lands and thus affected canopy P. juliflora which had been dominated other native species. The species diversity under non-invaded woodlands and P. juliflora mixed with native species stands were comparable to the report made by Zerga (2015) in Awash National Park of Ethiopia and by Aggarwal (2012) in Peri-urban Region of India.
Other findings such as those of Kumar and Mathur (2014) indicate higher species diversity than that of the present study. The reason might due to grazing impacts and moisture stress in the study areas. In the present study, the average values of species richness ranged from open grazing lands (9.6) to P. juliflora with native species (13.9). The higher the species richness under P. juliflora with native species and non-invaded woodlands contributes to anthropogenic impacts, grazing, and disturbance intensities, and the decline of P. juliflora invasions and its canopy effects which P. juliflora is an aggressive invader, canopy effects were consistently and strongly negative on species richness (Kaur et al., 2012). The mean values of species richness were similar under P. juliflora with native species to P. juliflora invaded forest and arid grazing lands at the Gujarat state of India, but the mean values of species richness in our study were higher under P. juliflora with native species and P. juliflora thicket habitat than P. juliflora invaded areas of Reserve Vidis and open grazing of arid grazing lands at Gujarat state of India (Kumar and Mathur, 2014). In all of the habitats of this study on the contrary, we found lower species richness than Aggarwal (2012) in a Peri-urban Region of India during rainy in winter and summer seasons.

4.3 Prosopis juliflora invasion and regeneration of woody species

Quantitative analysis of the regeneration status of woody species recorded in this study may provide baseline information to design and formulate conservation and management strategies for P. juliflora dominated woodlands. The regeneration status of woody species of any vegetation is determined based on densities of seedlings and saplings (Singh et al., 2016). The ratio of various age groups in a population determines the reproductive status of the population and indicates the future course (Singh et al., 2014).
In this study, the highest density of trees, saplings, and seedlings were recorded under P. juliflora thicket, P. juliflora with native species stands, and under non-invaded woodlands in that order. Besides, the increase in the density of individual stems in the form of the tree and seedling stages under non-invaded woodlands and P. juliflora mixed with native species stands might be due to its allopathic substances that would inhibit the growth of associated native species (Pasiecznik et al., 2001; El-Keblawy and Al-Rawai, 2007; Getachew et al., 2012).
Due to illegal cutting, the abundance of tree species under non-invaded woodlands and P. juliflora with native species stands were similar to the research findings by Patel et al. (2012) in western Kachchousehold of Gujarat in India. However, our findings were in contrary to results reported by Muturi et al. (2013) in which the distribution of trees under P. juliflora with native species stand was higher than non-invaded woodlands and P. juliflora thicket in the Turkwel riverine forest of Kenya. The reason could be the variations in the management of vegetation types and age of P. juliflora variations which might not also affect native species under the P. juliflora with native species stands in Kenya.

4.4 Prosopis juliflora invasion levels and size class structures for selected woody species

Regeneration is a crucial phase of vegetation management as it maintains the desired species composition and stocking and can be predicted by the structure of the population (Manral et al., 2018). Different categories of regeneration status were designated following Pokhriyal et al. (2010).
A. mellifera under P. juliflora thicket and P. juliflora mixed with native species stands, A. oerfota under P. juliflora thicket and non-invaded wood lands, G. tenax, A. senegal, A. nilotica and C. rotundifolia under open grazing lands, and P. juliflora under non-invaded woodlands and open grazing lands, and G. tenax under non-invaded woodlands showed new regeneration categories. Whereas C. rotundifolia and P. juliflora under P. juliflora thicket, A. mellifera and A. senegal under P. juliflora mixed with native species stands, and A. senegal, S. persica under non-invaded woodlands, C. rotundifolia and G. tenax under P. juliflora thicket showed fair regeneration categories.
Furthermore, S. persica and C. rotundifolia under non-invaded woodlands showed good regeneration in the study areas. In contrast to our findings, the findings of Ahmed et al. (2017) showed that P. juliflora under P. juliflora mixed with native species stands and A. mellifera under non-invaded woodlands showed good regeneration profiles in the Hallideghie wildlife reserve, Northeast Ethiopia. Furthermore, the density of seedlings for P. juliflora and A. tortilis under P. juliflora mixed with native species stands, non-invaded woodlands, and P. juliflora thicket also showed inconsistent patterns of densities in comparison to reports by Muturi et al. (2013) that show higher densities in comparison to trees and saplings.

5 Conclusions

It has been demonstrated in this study that species diversity decreased under grazed lands and P. juliflora thicket. P. juliflora invasion thus remains a threat to the overall native species diversity and eventually harming the rangelands and livestock production. Under Prosopis juliflora thicket, higher DBH classes of native woody species were dominated by P. juliflora. But, lower DBH size classes (seedlings) were recorded under P. juliflora mixed with native species stands implied that the effects of P. juliflora on the individuals declined. Thus, appropriate silvicultural techniques (e.g. thinning) of P. juliflora should be practiced to lessen the invasiveness of the species. Further studies about the phenology of P. juliflora are vital to know the seasons of seed dispersal to manage its invasiveness. Furthermore, the regional natural resource office should provide alternative energy sources such as solar radiation and P. juliflora based biogas plants to alleviate the devastation of other indigenous woody species in the region. Thus, more researches will be needed in collaboration with other stakeholders such as governmental organizations, NGOs, and policymakers focusing on management and control methods to reverse the situations in the invaded regions of the country.
Table 6 Appendix 1 Plant species list in Awash Fentale and Amibara Woredas
Scientific name Family Life form Vernacular name* SCN
Abutilon anglosomaliae Cufod. Malvaceae Forb Hambukto WSC143
Abutilon figarianum Webb Malvaceae Forb Hedayito WSC154
Abutilon fruticosum Guill. & Perr. Malvaceae Shrub Hambukto WSC005
Abutilon ramosum Guill. & Perr. Malvaceae Forb Hambukto WSC144
Acacia mellifera (Vahl) Benth. Fabaceae Shrub Makharto WSC007
Acacia nilotica (L.) Willd. ex. Del. Fabaceae Tree Kasalto WSC145
Acacia oerfota (Forssk) Schweinf. Fabaceae Tree Gerento WSC115
Acacia senegal (L.) Wild. Fabaceae Tree Adado WSC104
Acacia tortilis (Frossk.) Hayne Fabaceae Tree E’ebto WSC003
Acalypha acrogyna Pax Euphorbiaceae Forb Berbere WSC063
Achyranthes aspera L. var. pubeseens Moq. Amaranthaceae Forb Murit tabiri WSC125
Aerva javanica (Burm.f.) Schultes Amaranthaceae Shrub Olyato WSC018
Alternanthera pungens Kunth. Amaranthaceae Forb Ferengisisib WSC040
Amaranthus dubius Thell. Amaranthaceae Forb Bonket/Bunkete WSC146
Amaranthus thunbergii Moq. Amaranthaceae Forb Aburi WSC107
Asparagus africanus Lam. Asparagaceae Forb ‒ WSC147
Balanites aegyptiaca (L.) Del. Balanitaceae Tree Udda WSC044
Barleria acanthoides Vahl Acanthaceae Forb Ganzalto WSC004
Berchemia discolor (Klotzsch) Hemsl. Rhamnaceae Tree Jajaba'ito WSC122
Blepharis maderaspatensis (L.) Roth Acanthaceae Forb Yamarukta WSC118
Boerhavia coccinea Mill. Nyctaginaceae Forb Asara WSC167
Boerhavia repens L. Nyctaginaceae Forb Asara WSC119
Brachiaria ramosa (L.) Stapf Poaceae Herb (A) Gewita WSC075
Cadaba glandulosa Forssk. Capparidaceae Shrub Ududoyta WSC139
Cadaba longifolia (R. Br.) DC. Capparidaceae Shrub Dunbiya WSC110
Cadaba rotundifolia Forssk. Capparidaceae Tree Haragali WSC184
Calotropis procera (Ait.) Ait. f. Asclepiadaceae Forb Gelato/garbaadiyita WSC059
Capparis tomentosa Lam. Capparidaceae Climber Dangayito WSC169
Ceinfugosia somaliana Fryx Malvaceae Shrub Hameresa WSC117
Cenchrus ciliaris L. Poaceae Herb (P) Serdoyita WSC024
Centrostachys aquatica (R.Br.) Wall. ex Moq. Amaranthaceae Forb Bete WSC038
Chenopodium album L. Chenopodiaceae Forb Riba WSC073
Chrysopogon plumulosus Hochst. Poaceae Herb (P) Durfu WSC079
Cissus quadrangularis L. Vitaceae Climber Ala’e WSC149
Cissus rotundifolia (Forssk.) Vahl Vitaceae Climber Al’ie WSC106
Clematis hirsuta Perr. & Guill. Ranunculaceae Climber Adayito WSC056
Clematis longicauda Steud. ex A. Rich. Ranunculaceae Climber Matamato WSC172
Cocculus pendulus (J.R. & G. Forst) Menispermaceae Climber Hayikto WSC131
Commelina diffusa Burm.f Commelinaceae Forb Mutuki WSC070
Commicarpus plumbagineus (Cav.) Standl. Nyctaginaceae Climber Yejib chama WSC162
Commiphora coronillifolia Chiov. Burseraceae Shrub File neme’a WSC033
Cordia sinensis Lam. Boraginaceae Shrub Maderto WSC002
Cryptostegia grandiflora Roxb. Ex R. Br. Asclepiadaceae Climber Halimaro WSC046
Cucumis dipsaceus Ehrnb. ex spach. Cucurbitaceae Climber Seroyita WSC051
Cucumis prophetarum L. Cucurbitaceae Forb Garaun WSC089
Cyathula cylindrica Moq. Amaranthaceae Forb Kandadaf WSC077
Cycnium erectum Rendle Sclrofloriaceae Forb Andoliva WSC151
Cymbopogon commutatus (Steud.) Stapf Poaceae Herb (P) Haragali WSC083
Cymbopogon pospischilii (K. Schum.) C.E. Hubb. Poaceae Herb ‒ WSC170
Cynodon dactylon (L.) Pers. Poaceae Herb (P) Rareta WSC041
Cynodon nlemfuensis Vanderyst Poaceae Herb (P) Rareta WSC037
Dactyloctenium scindicum Boiss. Poaceae Herb (P) Afara mole WSC026
Delonix elata L. Gamble Fabaceae Tree Amayito WSC134
Diceratella elliptica Guill. & Perr. Brassicaceae Herb Moroe’i WSC080
Diceratella incana Balf.f. Brassicaceae Forb Atikhara WSC103
Digitaria velutina (Forssk.) P. Beauv. Poaceae Herb (A) Santkefa WSC097
Dobera glabra (Forssk.) Poir. Salvadoraceae Shrub Gersayiato WSC154
Dregea rubicunda K. Schum. Asclepiadaceae Forb Golfaha WSC064
Ecbolium viride (Forssk.) Alston Acanthaceae Forb Kinoyita WSC120
Echinochloa colona (L.) Link Poaceae Herb (A) ‒ WSC163
Eleusine africana Kenn.‒O’Byrne Poaceae Herb (A) ‒ WSC110
Eragrostis aethiopica Chiov. Poaceae Herb (A) Ayti‒adoyta WSC074
Eragrostis biflora Hack. Poaceae Herb (A) ‒ WSC155
Eragrostis cilianensis (All.) Vign. ex Janchen Poaceae Herb ‒ WSC118
Eragrostis cylindriflora Hochst. Poaceae Herb (A) Dankit/feresgera WSC156
Eragrostis papposa (Roem. & Schult.) Steud. Poaceae Herb (P) Bekelayso WSC078
Eriochloa fatmensis (Hochst. & Steud.) Clayton Poaceae Herb (A) Bohale WSC019
Erucastrum arabicum Fisch. & Mey. Brassicaceae Forb Ferate WSC026
Eurphorbia longituberculosa Boiss. Euphorbiaceae Forb Haliforesi WSC034
Fagonia schweinfurthii (Hadidi) Hadidi. Zagophyllaceae Shrub Adihara WSC129
Fuirena leptostachya Oliv. Cyperaceae Herb Ka’ato WSC158
Glossonema revoilii Franch. Asclepiadaceae Forb Sanga hayu WSC081
Grewia flavescens Juss. Tiliaceae Shrub Menangure/Garuwayito WSC035
Grewia schweinfurthii Burret Tiliaceae Shrub Manangurita/AdePJTo WSC127
Grewia tenax (Forssk.) Fiori Tiliaceae Forb Hedayito WSC032
Grewia villosa Willd. Tiliaceae Shrub Garawayito/legida WSC035
Heliotropium longiflorum (A.DC. in DC.) Jaub. & Spach Boraginaceae Forb Kinbira hara WSC092
Hermannia paniculata Franch. Sterculiaceae Shrub ‒ WSC043
Hibiscus calyphyllus Cavan. Malvaceae Shrub Hambukito WSC091
Hibiscus crassinervius Hochst. ex A. Rich. Malvaceae Forb Akulito WSC106
Hibiscus dongolensis Del. Malvaceae Forb Walayneba WSC028
Hibiscus micranthus L. f. Malvaceae Forb Okelehina WSC164
Huernia somalica N.E. Br. Asclepiadaceae Forb ‒ WSC200
Hyparrhenia rufa (Nees) Stapf Poaceae Herb (P) Isesu WSC163
Hypertelis bowkeriana Sond. Molluginaceae Forb Asara WSC073
Hypoestes forskaolii (Vahl) R. Br. Acanthaceae Forb Harawayitu WSC006
Hypoestes triflora (Forssk.) Roem. & Schult. Acanthaceae Forb Harawayitu WSC180
Hyptis pectinata (L.) Poit. Lamiaceae Forb Amada WSC165
Hyptis spicigera Lam. Lamiaceae Forb Amado WSC054
Indigofera coerulea Roxb. Fabaceae Forb Dunwayito WSC126
Indigofera hochstetteri Bak. Fabaceae Forb Aro WSC060
Ipomoea cairica (L.) Sweet Convolvulaceae Climber ‒ WSC166
Jatropha ellenbeckii Pax Euphorbiaceae Forb ‒ WSC036
Jatropha glauca Vahl Euphorbiaceae Shrub Halfolisi WSC022
Kleinia odora (Forssk.) DC. Astreraceae Climber Bisilto WSC133
Leptadenia hastata (Pers.) Decne. Asclepiadaceae Climber Hayikto WSC012
Leucas inflata Benth Lamiaceae Forb Bunketi WSC025
Leucas martinicensis (Jacq.) R. Br. Lamiaceae Forb Kurufule WSC160
Linum strictum L. Linaceae Forb ‒ WSC090
Linum volkensii Engl. Linaceae Forb Susui WSC140
Lipocarpha hemisphaericus (Roth) Goetgh. Cyperaceae Herb Gerandoyta/Gedoyta WSC153
Lipocarpha rehmannii (Ridl.) Goetgh. Cyperaceae Herb Abuu WSC111
Lonchocarpus laxiflorus Guill. & Perr. Fabaceae Forb Halemagira WSC143
Maerua angolensis DC. Capparidaceae Shrub Dunbayito WSC144
Megalochlamys ogadenensis Vollesen Acanthaceae Shrub Gelsanto WSC116
Megalochlamys violacea (Vahl) Vollesen Acanthaceae Forb Uraurto WSC052
Otostegia fruticosa (Forssk.) Schweinf.ex Penzig Acanthaceae Forb Atihara/michi WSC169
Monothecium glandulosum Hochst. Acanthaceae Forb Kulumintili WSC069
Nicotiana glauca R. Grah. Solanaceae Forb Adihara WSC170
Ocimum circinatum A.J Paton Lamiaceae Forb Harawayitu WSC106
Ocimum forskolei Benth Lamiaceae Forb Kayidiriya WSC085
Ocimum jamesii Sebald Lamiaceae Herb ‒ WSC174
Ocimum americanum L. Lamiaceae Forb Kaqtuhara WSC016
Orthosiphon pallidus Royle ex Benth. Lamiaceae Forb Yengula hebaki WSC067
Otostegia fruticosa (Forssk.) Schweinf. ex Penzig Lamiaceae Shrub ‒ WSC183
Otostegia modesta S. Moore Lamiaceae Forb ‒ WSC121
Echinochloa colona (L.) Link Poaceae Herb (A) Baruri WSC034
Panicum subalbidum Kunth Poaceae Herb (A) Heniso WSC124
Parkinsonia scioana (Chiov). Brenan Fabaceae Shrub Sosokite WSC171
Parthenium hysterophorus L. Astreraceae Forb Hari WSC001
Paspalidium desertorum (A. Rich.) Stapf Poaceae Herb (P) ‒ WSC019
Pavonia arabica Hochst. & Steud. ex Boiss. Malvaceae Forb ‒ WSC091
Pergularia daemia (Forssk.) Chiov. Asclepiadaceae climber Gime’ito WSC076
Peristrophe paniculata (Forssk.) Brummitt Acanthaceae Forb Aburi WSC136
Pisonia aculeata L. Nyctaginaceae Climber Dambikiso WSC042
Portulaca quadrifida L. Portulacaceae Forb Hatihara WSC175
Prosopis juliflora (Sw.) DC. Fabaceae Shrub Dargi‒haraa WSC010
Pupalia lappacea (L.) A. Juss. Amaranthaceae Forb Sarotkefa WSC024
Salvadora persica L. Salvadoraceae Shrub Adaito WSC056
Sansevieria ehrenbergii Schweinf. ex Baker. Dracaenaceae Forb Ya’ato WSC176
Sansevieria forskaoliana (Schult. f.) Hepper & Wood Dracaenaceae Forb Ya’a WSC123
Schoenoplectus maritimus (L.) Lye Cyperaceae Herb Ka’ato WSC053
Seddera bagshawei Rendle. Convolvulaceae Forb Riba WSC023
Seddera latifolia Hochst. & Steud. Convolvulaceae Forb Adiriba WSC114
Senna italica Mill. Fabaceae Shrub Oklehina WSC027
Senna obtusifolia (L.) Irwin & Barneby Fabaceae Shrub Salilimeki WSC109
Setaria verticillata (L.) P. Beauv. Poaceae Herb (A) Delayta WSC177
Solanum coagulans Forssk. Solanaceae Forb Alulis WSC178
Solanum cordatum Forssk. Solanaceae Forb Ubabulto WSC050
Solanum incanum L. Solanaceae Forb Kurara’i WSC066
Solanum schimperianum Hochst. ex A. Rich. Solanaceae Forb Bobao WSC015
Sporobolus agrostoides Chiov. Poaceae Herb (P) ‒ WSS171
Sporobolus consimilis Fresen. Poaceae Herb (P) Denekto WSC180
Sporobolus panicoides A.Rich. Poaceae Herb (A) Gewita WSC009
Sporobolus pellucidus Hoehst. Poaceae Herb (P) Sosokete WSC181
Sporobolus spicatus (Vahl) Kunth Poaceae Herb (P) Edolatyansi WSC039
Tetrapogon cenchriformis (A.Rich.) Clayton Poaceae Herb (A) Sabrisi WSC045
Thunbergia ruspolii Lindau Acanthaceae Forb Harawayito WSC086
Tragus racemosus (L.) All. Poaceae Herb (A) Bekelayso WSC078
Tribulus parvispinus Presl. Zygophyllaceae Forb Bunket WSC061
Tribulus terrestris L. Zygophyllaceae Forb Bunket WSC182
Verbesina encelioides (Cav.) A. Gray Asteraceae Forb Surimia WSC062
Wissadula rostrata (Schumach. & Thonn.) Hook.f. Malvaceae Shrub Delgida WSC142
Xanthium strumarium L. Asteraceae Forb Bangi WSC030
Ziziphus spina‒christi (L.) Desf. var. mitissima Chiov. Rhamnaceae Tree Kusirayito WSC132
Zygophyllum simplex L. Zygophllaceae Forb Mutiki WSC034

Note: Vernacular name* is plant name in Afar language. SCN is species collection name; A is annual grasses; P is perennial grasses.

Fig 6 Appendix 2 a‒j. Size class structures of ten species population in each habitat

Note: PT = P. juliflora thicket stands; PJM = P. juliflora with native species stands; NIWL = Non-invaded woodland; OGL = Open grazing land.

Acknowledgments

The authors are thankful to Arba Minch and Addis Ababa Universities for financing the project. The first author special thanks Afar pastoral communities for their cooperation and assistance during data collection. Department of natural resources management in pastoral and agro-pastoral districts of the Amibara and Awash Fentale districts are acknowledged for material and human resources assistance during site selection and data collection. All the members of the National Herbarium of Addis Ababa University are also appreciated for their facilitation of materials in the herbarium for plant identification.

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