Ecological and Floristic Study of Hirmi Forest, Tigray Region, Northern Ethiopia

Full of author information is available the end of the article. Abstract Background : The Dryland area in Ethiopia contributes large vegetation resources. However, the landmass has received less attention even if it has high ecological, environmental and economic importance. The present study was conducted in Hirmi forest, which is one of the dry forest in Northwestern, Ethiopia; to study the floristic composition, plant community types, community-environment relation, vegetation structure and regeneration status of the forest. Method : Vegetation and environmental data were collected from 80 sampling plots having equal size of 25m×25m were designated as the main plots. Within the main plot, five 2mx2m subplots were laid to record seedlings, saplings, and herbaceous species. Furthermore, within each subplot, soil samples were collected to analyze the relationship of edaphic parameters with the plant community. DBH, height, BA, density, vertical structure and frequency were computed. Floristic diversity and evenness were computed using Shannon diversity and Evenness indices. The plant community types and vegetation-environment relationships were analyzed using classification and ordination tools in R package (ver. 3.6.1), respectively. Result : A total of 171 vascular plant species belonging to 135 genera and 56 families were recorded. About 5.85% of the species were endemic/near endemic to Ethiopia. Woody species (48%) were dominant over herbs (45.6%) and liana (6.4%). The highest number of species was recorded in plant families of Fabaceae (16.37%) and Poaceae (11.7%) followed by Asteraceae (7.02%) and Combretaceae (3.5%). Five plant communities were identified and positively correlated with altitude, slope, sand, silt, soil organic matter, total Nitrogen and disturbance. Vegetation structure reveals that, large number individual species were categorized in the lower class of DBH, BA, frequency and height. The highest Shannon diversity index and evenness values of the study area were 4.21 and 0.95, respectively. Anogeissus leiocarpa, Combretum hartmannianum, Ziziphus mucronata, Terminalia macroptera, Acacia polyacantha were the species with high important value. Conclusion : the study shows Hirmi forest has high species diversity, richness, and endemism. However, due to various disturbances, it is under poor regeneration status that requires urgent conservation measures. Integrated forest management, monitoring, applying restoration techniques by considering the significant environmental factors associated with the species diversity, IUCN threat and regeneration status of the species is recommended to preserve the study forest. as Terminalia abyssinica, and Diospyros mespiliformis are species with high basal area 50% of the total of the study forest. value in lower frequency classes of the study the existence of a high degree of floristic heterogeneity in the forest. The result the assessment for the regeneration status of the study forest shows, the population of sapling was higher than seedling and mature trees/shrubs. This indicates the forest is under poor stage.


Background
Terrestrial biodiversity is highest in the tropics and near to the equator (Gaston 2000), which seems to be the result of the warm climate, high primary productivity and diverse topography (Field et al. 2009). About 40% of the global tropical forest area is covered by the dryland ecosystem (Mayaux et al. 2005) and 14% of the total African land surface that contribute 25% of the natural vegetation (Eshete et al. 2011). Though the dryland area has high ecological, environmental and economic importance, currently the vegetation resources inside the landmass are facing serious problems because of human intervention and less attention than moist forests (Muys et al., 2006). Deforestation and habitat degradation on that area bring several socioeconomic and environmental challenges that have strongly affect the capacity of forests to provide ecosystem services (Nyssen et al. 2007). Recently conservation of biodiversity in the dryland ecosystems has been receiving greater international attention (CBD 2006).
Environmental determinants such as soil properties, elevation, habitat suitability and diversity, species interaction, disturbance are critical and need to be considered in the design of hotspot biodiversity conservation programs. The hotspot concept is based on species richness, endemism, and threat (Reid 1998).
Ethiopia is a tropical country that shares more than 50% of the Afromontane regions land areas above 1500m of Africa (Bekele 1994) as well as extensive dryland landmass (Teketay 2001).
According to a study conducted by Teketay (2001), about 65% of the Ethiopia landmass is covered by dryland ecosystems, where 12% of human and 20% of livestock populations reside. It is the center for biodiversity, endemism and grazing (Mengistu et al. 2005).
The wide range of agro-ecologies in northern Ethiopia covered dryland with woody dry forests (Georgis 2010). Following the long history of land degradation, many land rehabilitation and conservation programs have been carried out in northern Ethiopia in general, Tigray region in particular. However, inadequate investigation of ecological baseline information makes the effectiveness of biodiversity conservation policy and strategy to fall in a challenge for its implementation as it intends (EFAP 1994;Teketay 2001).
Hirmi forest is one of the dryland forest with remnant climax vegetation found in North Ethiopia, Tigray region, north to Tekeze rift valley (Hagos et al., 1999). An assessment conducted by Ethiopian REDD+ (MEF 2015) secretariat reveals Hirmi forest is identified as one of the potential areas that require forest ecosystem restoration and conservation by the participation of the local community and other relevant approaches. However, ecological conservation is efficient and feasible when the plant community in relation to environmental and anthropogenic drivers are well investigated (Pimm and Raven 2000;Brooks et al. 2002). The plant diversity, composition, structure, threats and regeneration status of Hirmi forest is not studied yet. Therefore, this study was conducted to fill the existing gap for conservation and sustainable utilization of the forest and might also be applied in other parts of the country.
The objectives of the research were to: 1. Examine the floristic composition and plant community types of Hirmi forest.
2. Assess the population structure and regeneration status of the study forest.
3. Examine the relationship between selected environmental determinants with the patterns of community formation in Hirmi forest. 4. Identify threats associated with the vegetation in the study area. 5. Suggest appropriate methods of ecological conservation and management for sustainable utilization of the study forest.

Vegetation data
Preferential sampling design technique with a flexible systematic model was used for data collection following Mueller-Dombois and Ellenberg (1974) and Smartt (1978). A flexible systematic transect was laid within distance from 300-500m apart each other along with altitudinal drop. Vegetation data were gathered from eighty (80) plots 50 m far each and ten (10) transects. For the collection of woody species plots of 25 m x 25 m (625 m 2 ) were laid.
Herbaceous species, seedlings and saplings of all woody plants (shrubs and trees) were recorded in a 2 m x 2 m (4 m 2 ) subplot inside each main plot, four from each corner and one at the center following Bekele (1993), Woldemariam (2003) and Kidane (2015). Plants with a height of ≥ 2m and DBH ≥ 2cm was considered as shrub or tree whereas plants less than 2 m long were considered as a sapling or seedlings (Senbeta and Teketay 2001;Tuxill and Nabhan 2001;Tadesse 2015). Vegetation cover-abundance was estimated based on the Van der Maarel scale (1979).
Plant specimens were collected, pressed, dried properly during the data collection period at the field site and brought to the National Herbarium (ETH), Addis Ababa University for identification and verification. An attempt was made to record the vernacular names of species encountered in the study during collection whenever possible.

Environmental data collection
Terrain variables such as altitude, aspect, slope and geographical coordinates were taken and recorded for each sample plot. Altitude and geographical coordinates were measured using GPS and the slope of each plot was measured using Clinometer. Soil samples were taken from five points (four from each corner and one from the center) measuring 15 x 15cm, at a depth of 0-30cm (Tenkir 2006;Tadesse et al. 2008) of the main plot laid for vegetation data collection.
These samples were mixed to form a composite sample and about a kilo of the sample was taken for analysis. The soil samples were air-dried by spreading on plastic trays crushed and sieved with a mesh size of 2 mm to remove root particles and other organic debris before chemical analyses. Finally, measurements and analysis for soil texture (sand, silt, and clay), pH, Electrical conductivity (EC), Cation Exchange Capacity (CEC), soil organic matter (SOM), total Nitrogen (N) and available phosphorus (P) were done at Shire agricultural research center, soil laboratory Department following the standard procedures outlined in Landon (1991); Sertsu and Bekele (2000).
Ecological disturbances were noticed and recorded as present or absent in the sampled plots. The magnitude of disturbance was rated from 0 to 4 following Yeshitla and Bekele (2002) and Senbeta et al. (2007) with slight modification; based on visible signs of forest disturbance parameters such as tree cutting, debarking, grazing, firing and charcoal production signs. Values were coded as follows: 0: no disturbance; 1: if any one of the above-mentioned factors was present (slightly disturbed); 2: if any two of the above-mentioned factors were present (moderately disturbed); 3: if any three of the above-mentioned factors were present (highly disturbed); 4: if all of the above-mentioned and other associated factors such as tree stumps due to natural factors and landslide/degradation, were present (extreme disturbed).

Multivariate and diversity analysis
Agglomerative cluster analysis methods were used to group the collected species data into a sort of group (community types) based on their similarity using free packages R programs, Version 3.6.1 (R Development Core Team 2019). The plant community types from the cluster analysis were further analyzed in a synoptic table based on cover-abundance values (van der Maarel et al. 1987). The community was named after the dominant tree or shrub species in that specific community with higher synoptic values. In each plant community type the significance of synoptic value of each species were further tested in order to find significant indicator value of the species in the identified community types performed in R using package labdsv.CCA was used to correlate the identified plant community types with selected environmental and disturbance factors (Woldu 2016;Kent 2012). Shannon-Wiener index was used to compute species diversity, richness and evenness of Hirmi forest using the free statistical software R

Vegetation structure
Diameter at Breast Height (DBH), vertical structure, basal area, density, frequency, height and important value index was used for the description of vegetation structure. These parameters were calculated to determine the vegetation structure as described below: Diameter at Breast Height (DBH): DBH measurement was taken at about 1.3 m from the ground using a measuring tape for those woody plants having circumferences greater than 6 cm.
For tree/shrub species that branch at breast height or below, DBH of each branch was measured separately and the average was taken and then treated as a single individual for the basal area calculation. There is a direct relationship between DBH and basal area (Kent 2012).
Vertical structure: The vertical structure of the woody species was analyzed using the International Union for Forestry Research Organization (IUFRO) classification scheme (Lamprecht 1989 = ( ) where; π=3.14 First, all measurements were in a square centimeter, latter it was converted into m 2 per hectare.
Calculating of basal area of species to sampled area gives dominance of species.
Frequency: is defined as the probability or chance of finding a species in a given sample plot. It is dependent on quadrat size, plant size and patterning in the vegetation (Kent 2012). Frequency is calculated with this formula: Frequency = x 100 Density: is the count of individuals per unit area and calculated using the following formula: Density = Total number of stems of a given species sample size in hectare X 100 Importance Value Index (IVI): is useful to compare the ecological significance of species and to take a measurement on conservation priority (Kent 2012). The Importance Value Index is the cumulative value of the three parameters (i.e. relative frequency, relative density and relative dominance), which was calculated using the following formula:

IVI = Relative frequency(RF) + Relative dominance(RDO) + Relative density (RD)
Where, Relative Frequency = Frequency of single species Frequency of single species x 100 Relative density (RD) = density of a single species total density of all species X 100 Relative dominance = Basal area of a single species Total basal area of all species X100

Regeneration status:
Regeneration status was estimated based on the composition and density of seedling and sapling of all woody species recorded in each sample plot (Khan et al. 1987). The density of seedlings and saplings were calculated using the following formula: Density of sapling/ seedling = the number of sapling/seedling area of sample in a hectare Woody species of the study forest were grouped into four categories, based on the density of seedlings. These are category I: species with the no seedlings, category II: species with no saplings, category III: species with no saplings and seedling; category IV: species with high saplings and seedlings density.

Plant community types
Five plant community types were identified from the agglomerative hierarchical cluster analysis based on the abundance data of the species on the study plots (Fig. 2). Two species with relatively high synoptic values in the group were used to name the corresponding communities (Table 1). In each plant community type the significance of synoptic value of each species were further tested in order to find significant indicator value of the species in the identified community types performed in R using package labdsv.

Ziziphus mucronata-Acacia polyacantha community type
The altitudinal ranges of this community were from 1170-1565 m.a.s.l. At the edge and lower altitude of the forest, high disturbance such as cutting, debarking, grazing was docummented. In contrast to other community types, this community is situated at a moderate slope, which ranges from 2-35%. More than 60% of the total species were found within < 10% slope. Totally, 19 plots and 64 species were associated in this community. Ziziphus mucronata, Acacia polyacantha, Ziziphus spina.christi, Flueggea virosa, and Chamaecrista mimosoides were the most icon and high significant values (Table 1). grazing, firing and charcoal production sign) were among the selected environmental variables used to investigate their association with plant community types in this study. Total of nitrogen, disturbances and sand were associated with community type one and two. Community type three was found in scattered form and affected by different factors. However, slope, soil organic matter and silt were strongly associated with the plant community. Community four occurred in high altitude, slope, soil organic matter and silt; while community five is highly associated with silt and disturbance (Fig. 3).

Species richness, evenness and diversity of the five community types
The Shannon-Wiener diversity result for the identified five community types are shown in Table   2. In terms of the actual number of species per community; community III was the most speciesrich followed by community V, IV, and I respectively. Community II was the most species poor community. Similarly, community V had the highest species diversity and community II had the lowest. In terms of evenness community, V had the highest evenness index (0.95) and community II and III had the highest similar species or lowest evenness of species (0.90 each) relatively with the rest community types.   Vertical structure: The top height for trees in the study forest was 25 m. According to the IUFRO classification scheme classification 65.2% of the specis were in the lower storey (< 8.3 m). The cumulative vertical structure of Hirmi forest was an inverted J-shape (Fig. 6)

Density of trees and shrubs
A total of 2642 species/5 hactare were recorded from 80 plots of Hirmi forest. As indicated in table 3, only 10 species accounted for 40.4% of the total density. On the other hand, the remaining 70 woody species altogether accounted for 59.6% of the total density in the forest.

Terminalia catappa, Diospyros abyssinica, Ficus ingens, Ozoroa insignis and Diospyros mespiliformis.
Frequency is obtained by dividing the number of plots in which the species occurred by the total number of the plots from which all the species sampled in the area under the study. The computed frequency values of species were classified into five frequency classes as: 1=0-10; 2=10.1-20; 3=20.1-30; 4=30.1-40; 5>40 (Fig. 7). The result showed that 59 % of the total woody species were distributed in frequency class one. The grand frequency distribution of woody species in Hirmi forest showed that higher proportions of species were found to be distributed in the first frequency classes followed by a rapid decline in the distribution of species across successive higher frequency classes. Anogeissus leiocarpa was the most highly frequent species (40.5%), followed by Terminalia macroptera (

Importance Value Indices (IVI) of the three plant communities
The first top ten woody species which contributed 37.78% of the IVI in decreasing order were;

Regeneration status of Hirmi forest
Regeneration status of Hirmi forest was characterized by recording and counting of seedlings, saplings and adults of eighty (80) trees/shrubs. The total density of seedlings, saplings and mature tree and shrub species were 2750 ha -1 , 3025 ha -1 and 528.4 ha -1 respectively, (Fig. 8). For the sake of conservation priority, the regeneration status of woody species recorded in the study area was grouped into four categories based on the density of seedling, sapling and mature of trees and shrubs. These are category I: species with the no seedlings, category II: species with no saplings, category III: species with no saplings seedling and category IV: species with high saplings and seedlings density (Table 5).

DISCUSSION Floristic composition and diversity of Hirmi forest
The forest was composed of Acacia-Commiphora woodland, Combretum-Terminalia woodland and dry evergreen Afromontane forest species within 1098m-2002m of elevation. The existence of high species richness in the study site was due to the fact of heterogeneity within the forests community (Murphy and Lugo 1986), altitude difference (Bekele 1993), anthropogenic impacts (disturbance) (Lyaruu et al. 2000) and soil properties (Kooch et al. 2009). The species richness of Hirmi forest is higher than some dryland forest studied in Ethiopia such as in Metema with 87 species (Adamu et al. 2012), Kimphe Lafa (West Arsi) with 130 species (Aliyi et al. 2015) and in Grat-Kahsu 64 species (Atsbha et al. 2019) but less than the study conducted in Benshangul Gumuz regional state with 1102 species (Awas 2007).
High richness of herbaceous species may be attributed due to the open canopy cover of the forest patch enables to grow freely and diversely in the study area. This was in agreement with Murphy and Lugo (1986), which reported that an abundance of herbs has an inverse relation with canopy cover. Anthropogenic impacts such as exploiting plant products and selective cutting of trees Yineger et al. 2008) and shrub species in dryland forest (Lemenih and Bongers 2011) increased the growth of herbaceous species. More than half of the total sampled (54.5%) of the plant family was represented by one species indicates the richness of families and great diversity in the area. The highest number of species recorded from Hirmi forest was Fabaceae and Poaceae followed by Asteraceae and Lamiaceae proves the fact that they are species-rich families in the flora Ethiopia and Eritrea (Tadess 2004). Out of the recorded 10 endemic species; Acacia venosa and Lippia adoensis have been included in the IUCN red data list of Ethiopia and Eritrea (Vivero, et al. 2005). This was in association with existing significant disturbance in the study forest. The threat status of the endemic spices in Hirmi forest reveals that, the need for conservation prioritization. The dryland forest type in Ethiopia is currently under strong environmental stress (Friis et al. 2010). The ecosystem approach of biodiversity conservation and participatory forest management provides an opportunity to conserve large numbers of species that are rare and endemic (Aynekulu 2011;Lemenih and Bongers 2011).
The Shannon diversity index of the study forest was rage from 3.25 to 4.21 (with mean value 3.73) and evenness was from 0.90-95 (with mean value 0.93). The higher value of Shannon diversity index and evenness indicates that the study forest has high species diversity with more evenly distributed of the species within the study plots. Community type V had the highest species diversity and evenness whereas community II was with the lowest species diversity and evenness. The variation in species diversity is associated with environmental and disturbance factors. This was in line with a study conducted by Suratman (2012) reported, the difference in terrain, gradient and slope direction causes differences in the soils, water, and microclimate conditions which cause differences in species diversity and adaptability.

Plant community and Environmental variables relationship (Ordination)
The relationship between plant communities and environmental variables is important in understanding the plant communities in a given ecology (McCune and Grace 2002). Patterns of species community is associated with environmental factors such as climate, topography, and soil as well as anthropogenic and livestock disturbances (Whittaker et al. 2003). Ecological study conducted in Ethiopia by Woldu et al. (1989), Friis (1992), Bekele (1994), Aynekulu (2011), Girma (2011) andGebrehiwot et al. (2019) reported importance of environmental variables (altitude, slope, landscape, disturbances, and soil physical and chemical properties) in separation of forest communities.
CCA analysis shows that environmental variables such as altitude, slope, silt, sand, soil organic matter, total Nitrogen and disturbances were associated with species compositions of plant communities in Hirmi forest. However, all those environmental determinants have different effects for the five identified communities from the Agglomerative clustering algorithm.
Community one (Ziziphus mucronata-Acacia polyacantha) and community two (Combretum hartmannianum-Terminalia macroptera) of the present study area were strongly influenced by sand and total Nitrogen. Nitrogen content of a soil is suitable for species growth and diversity (Brown 2003). Similarly, disturbances (cutting, debarking and grazing) were common and significantly associated in both communities. This can be plots of these communities were from a small difference of environmental gradients and near human settlement which is easily accessible by local people (edge effect). A study by Condit et al. (2002) reports that there is a high probability species composition similarity in a community with small environmental determinants differences and taken from adjacent plots. Disturbance such as selective cutting and grazing is high at the edge of the forest ecosystem (Daniel et.al. 2006) is also agreed with the present result.
Least disturbance and high species composition in community three (Anogeissus leiocarpa-Ozoroa insignis) was due to most plots were sampled from the center or far from the edge of the forest. This community was positively correlated with soil organic matter, silt and slope. Soils having high organic matter and moisture holding capacity of the silt loam of soil are suitable for species growth and diversity (Brown 2003;O'Geen 2006). Furtherly, species found in strong slope are less disturbed and would be rich in species composition (Tilahun et al. 2011;Gebrehiwot et al., 2019). The longest arrow of altitude in the ordination axis was positively correlated with community type four (Euclea racemosa -Acacia abyssinica) than the other community types. Longest arrows environmental determinant in the ordination diagram indicates its strong influence on species composition of plant community diversity (ter Braak 1987). The highest elevation difference  was recorded in this community. Altitudinal difference did not bring only species richness and diversity, but also it affects the soil texture, atmospheric pressure, moisture and temperature that directly affect species distribution and composition (Bekele 1994;Lovett and Wasser 1993). The existence of high species richness as well as high Generally, the identified community types in the study forest have either positive or negative with the selected environmental and disturbance factors. Forest community has its distribution area with a specific combination of environmental variables (Brinkmann et al. 2009). Hence, conservation techniques should implement based on the species composition and diversity of the community types in relation to the associated environmental gradients.

Vegetation structure of Hirmi Forest
Analysis of vegetation structure was implemented by considering species; DBH class, Vertical structure, density, basal area, frequency, height and Important Value index.
DBH: the overall height and DBH class distribution of all individuals in different size showed an inverted J-shape distribution. In the lower class of DBH (DBH=2-20cm), high number of species were existed. The existences of high number of species at the lower the DBH with gradual decrease towards high DBH indicats, healthy regeneration of the forest (Silvertown and Doust 1993). Similar result was reported in Abergele dry woodlands (Eshete et al. 2011), Dawsura -Tembien woodlands  and Ilu Gelan of Combretum-Terminalia and Acacia vegetation (Tadesse et.al. 2018). In addition the study forest was dominated by small-  (Kent, 2012). More than 53.7% of woody species of Hirmi forest comprises shrubs layer.
According to White (1983), shrubby layer forest is mostly dominated by lower undifferentiated woody species. Similar result was also reported in another dryland forest of Ethiopia by Aliyi et al. (2015). Besides a study in Metema and Abergele by Eshete et al. (2011) reports that the replacement of large size (height) of woody species by small-sized species (shrubs) was associated with the intensity of human/livestock intervention in the natural vegetations and the regeneration of secondary young plants.
Density: the overall density of woody species in Hirmi forest was 528.4 individuals per hectare.
This was greater than density reported from dryland forest studied in Metema (18.8 stems/ha, less than the study in other dryland forests in Abobo-Gog forest (1579 stems/ha, Bareke 2018) and Shai Hills dryland in Ghana (959/ha, Swaine et.al 1990). Such variation could be attributed to variations in topographic gradients and habitat preferences of species forming the forest, and the degree of anthropogenic disturbances (Whittaker et al. 2003).

Basal area:
The total basal area of Hirmi natural forest for plant species with DBH >2 cm was 14m 2 /ha. This result was higher than the study conducted in Grat-Kahsu (5.32 m 2 /ha; Atsbha et al. 2019) and Taltalle woodland area (0.44 m 2 /ha; Lemessa 2009). However, it was smaller than the other dry woodland forest studied in Abobo-Gog forest (Gambella Regional State) (29.3 m 2/ ha; Bareke 2018) and the normal basal area value for virgin tropical forests in Africa is (23-37 m 2 /ha; Lamprecht 1989). More than 50% of the total basal area of Hirmi natural forest was contributed by large large-sized tree species such as Anogeissus leiocarpa, Acacia polyacantha, Combretum hartmannianum, Terminalia macroptera, Ziziphus mucronata, Terminalia catappa, Diospyros abyssinica, Ficus ingens, Ozoroa insignis and Diospyros mespiliformis. However, the number and diversity of the above tree species were far less than the shrub species in Hirmi forest. Small DBH of plant species in a forest covered by the shrub layer would have a small total basal area in contrast to tree species (Murphy and Lugo 1986;Lemenih and Bongers 2011).
The lower and higher DBH class contributed to the total basal area was also supported by other studies such as Bekele (1993) and Daniel et.al. (2006). Frequency: frequency is an indicator for the homogeneity and heterogeneity of given vegetation (Kent, 2012). The number of species decreased as the rate of frequency class was increases.
Based on the Lamprecht (1989)  High frequency of a species always depends on factors that relate to habitat preferences, adaptation, degree of exploitation and availability of suitable environmental conditions for regeneration (Rey et al. 2000). According to (Friis et al. 2010 (Lamprecht, 1989). Species with high IVI in the present study were different from other dryland studied in Kimphe Lafa Forest (Aliyi et al. 2015), Abobo-Gog forest (Bareke 2018); but there was small similarity with a study done in Benshangul Gumuz regional by Awas (2007) and Dawsura-Temben by Woldu et al. (2019). Species IVI of the study forest varies from 0.35 to 20.55. This in turn implies the importance of conservation priority for the existing species in the dry forest land according the species frequency, density and dominace status. IVI is a good index for summarizing vegetation characteristics and ranking species for management and conservation practices (Kent 2012).

Regeneration status of Hirmi forest
Density of sapling was higher than seedling and mature trees/shrubs. According to Khan et al. (1987) regeneration analysis, Hirmi forest is categorized under poor status. The ordination result shows disturbance was significantly associated with the vegetation diversity of Hirmi forest. Dry forests of Ethiopia are characterized by poor and fluctuating patterns of regeneration status due to periodic disturbances (Yebeyen 2006;Tesfaye 2008;Lemenih and Bongers 2011). A study conducted by Kelbessa and Girma (2011) reported that human coused disturbances sch as intensive grazing, firing and removal of trees are the common threats for the dryland forest regeneration. Frequent sign of disturbances were recorded in palnt communities near to the settlement and edge of the study forest. A study conducted by Girma (2014); Gebrelibanos and Assen (2015) reveals, previously human settlements around the study area were restricted to afrmoutains landmasses. However, rapid human population growth imposes to settle in lowland areas for the demading of large farmland as well as plant and plant product. Thus, a human and livestock intervention in that area leads to be poor in the regenerating of species. As a result, there were plant species either with no seedling, sapling or both. Hence, this study suggests that high conservation priory is required to the species either with no both seedling and sapling (considered as priority one) and species either with sapling or seedling (considered ad priority two). Besides recoding and studding these species will help to rescue from extinction.

CONCLUSION
The result of this study shows, Hirmi dryland forest is one of the remnant forest in Northern Ethiopia with high floristic composition and diversity. The largest part the forest was covered with undifferentiated floristic composition belongs to Acacia-Commiphora and Combretum-Terminalia forest type. At the elevated part of the forest, floristic composition similar to the DAF species was documented. The variation in diversity and floristic composition of the study forest structure was associated with the topographic variation and heterogeneity of the forest community. The presence of endemic and near endemic species (some of them categorized in IUCN threatened list) in the study forest indicates the area's potential of plant diversity and conservation priority.
The multivariate analysis result revealed that, the identified five plant communities in the study were influenced by; altitude, slope, total Nitrogen (N), soil organic matter, sand, silt and disturbance. Community 1 and 2 were strongly influenced by similar affinity of sand, total Nitrogen and disturbances. Community three was the most species-rich community associated with high content of soil organic matter and silt loam. The location of this community was in a strong slope and less disturbed site also contributes to the species richness. The longest arrow of altitude in the ordination axis was positively correlated with community type four indicate as it strongly affects the community and found at high elevations isolated from the others. The existence of high Shanon diversity and evenness (J=0.93), in community was associated with the availability of high silty loam soil in the community which is sutabel for the growth of all species.
The vegetation structure of Hirmi forest reveals that, large number of individual species was categorized in the lower class of DBH. The hight class of the forest has an inverted J-shape indicates that, the decline in density of woody plants with increasing height classes. High density of species at the lower DBH class and hight class is due to the dominance shrubs or/and developments secondary vegetation species in the forest. Dodonaea angustifolia, Anogeissus leiocarpa, Flueggea virosa and Ziziphus mucronata were among the species with high density whereas species such as Anogeissus leiocarpa, Acacia polyacantha, Combretum hartmannianum, Terminalia macroptera, Ziziphus mucronata, Diospyros abyssinica, Ficus ingens, Ozoroa insignis and Diospyros mespiliformis are species with high basal area which accounts more than 50% of the total BA of the study forest. High value in lower frequency classes of the study forest indicates that, the existence of a high degree of floristic heterogeneity in the forest. The result form the assessment for the regeneration status of the study forest shows, the population of sapling was higher than seedling and mature trees/shrubs. This indicates the forest is under poor regeneration stage. This was associated with disturbance events such as selective harvesting/cutting, debarking, grazing and charcoal production. As a result of these threats, there were plant species either with no seedlings, saplings or both seedling and sapling.
Generally, the study concludes conservation measurement in the study forest should consider the significant environmental factors associated with the species diversity, IUCN threat status and regeneration status of the species.

Acknowledgments
First more, the first author thanks for the Ministry of mining and petroleum to resume the Ph.D. study.The financial support from the Rufford foundation and Addis Ababa University is highly acknowledged. We are grateful to all staff Shire agricultural research center. Last, but not the least, my special thanks go to the local community for their kind respect and cooperation while collecting data.