The study was conducted in Kewet district situated 213 km northeast of Addis Ababa at the foot of the eastern escarpment of the central Ethiopian highlands. It is located within the coordinates of 9°49′ and the 10°11' latitude N and 39°45′ to 40°6′ longitude E. Administratively, Kewet district lies within the North Shewa Administrative Zone of the Amhara National Regional State (Fig. 1). The altitude ranges from 1080 to 3151 m a.s.l. Based on 10 years (2007–2016) of data from the Shewa Robit meteorology station, the area receives a mean annual rainfall of 1007 mm and a mean annual minimum and maximum temperature of 16.5 and 31°C, respectively. Kewet district lay in Robit Valley which is widely covered by basalt and related pyroclastics rock types of tertiary age (Tefera et al., 1996Tolera, M, Asfaw, Z, Lemenih, M & Karltun, E (2008) Woody species diversity in a changing landscape in the south-central highlands of Ethiopia. Agric Ecosyst Environ, 128(1–2), 52-8.
[http://dx.doi.org/10.1016/j.agee.2008.05.001] ). According to World Reference Base (WRB), Eutric Cambislos and pellic vertisols are the dominant soil groups in the alluvial fan area of the district lies the lower areas are dominantly covered with Calcaric gleysols and Calcic cambisols (Paris, 1986Shimelse, S, Bekele, T & Nemomissa, S (2017) Effect of Exclosure Age on Carbon Sequestration Potential of Restorations in Tigray Region, N. Ethiopia. American Journal of Biological and Environmental Statistics, 3(4), 65-80.
[http://dx.doi.org/10.11648/j.ajbes.20170304.14] ). Over 41 and 26% of the district’s land is covered by cultivated land, and forest and shrubland, respectively (Tesfay et al., 2020Yayneshet, T (2011) Restoration of Degraded Semi-Arid Communal Grazing Land Vegetation Using the Exclosure Model. International Journal of Water Resources and Arid Environment, 1(5), 382-6.). Area exclosure practice is common in the district, which covers 15 km² of the total land coverage (Bizuayehu and Tefera, 2008Chen, Y, Li, Y, Zhao, X, Awada, T, Shang, W & Han, J (2012) Effects of grazing exclusion on soil properties and on ecosystem carbon and nitrogen storage in a sandy rangeland of Inner Mongolia, northern China. Environ Manage, 50(4), 622-32.
[http://dx.doi.org/10.1007/s00267-012-9919-1] [PMID: 22829221] ). According to Ethiopia’s Central Statistical Agency (CSA) population projection for all regions at wereda level from 2014 – 2017, the district has a total population of 147,093 with a population density of 197 persons per km² (CSA, 2013).

Three exclosure sites which are 5, 15, and >20 years old and one adjacent open grazing land were selected purposively which hereafter is referred to as young, middle, and old age exclosure, respectively. Then, three parallel transect lines (which were considered as replicates) across the selected exclosures and open grazing land were laid. Quadrats measuring 20 × 20 m (400 m²) were laid down along the parallel line transects at a regular distance (Kent, 2012Magurran, AE (2004) Measuring Biological Diversity. Blackwell Science Ltd..). Data on woody species type, height, and diameter at breast height (dbh) of individuals with dbh > 2 cm were collected from quadrats along each line transects within each site. The total number of individuals of each wood species seedling and saplings < 1.5 m height was counted and recorded in each quadrat (Kent, 2012Magurran, AE (2004) Measuring Biological Diversity. Blackwell Science Ltd..). Hypsometer and caliper were used to measure the height and dbh of the tree, respectively. A total of 36 sample plots was used to collect data on woody vegetation. The woody species were identified directly in the field using (Hedberg and Edwards, 1989Intergovernmental Panel on Climate Change (IPCC). (2006). IPCC Guidelines for National Greenhouse Gas Inventories The Intergovernmental Panel on Climate Change (IPCC); Edwards et al., 1995Feyisa, K, Beyene, S, Angassa, A, Said, MY & De Leeuw, J (2017) Effects of enclosure management on carbon sequestration, soil properties and vegetation attributes in East African rangelands. Catena, 159, 9-19. [Elsevier.].
[http://dx.doi.org/10.1016/j.catena.2017.08.002] ; Hedberg et al., 2003Kent, M (2012) Vegetation Description and Data Analysis: A Practical Approachhttp://books.google.com/books?id=xHnXf0wHdgQC&pgis=1, 2006Lemenih, M (2006) Expediting ecological restoration with the help of foster tree plantations in Ethiopia. journal of the drylands, 1(1), 72-84.). Geographical information such as location, elevation, slope, and aspect of each plot was recorded using a geographical positioning system (GPS).

Fig. (1) Map of the study site.

Data on vegetation structure, composition, aboveground biomass, and aboveground biomass carbon stock were subjected to a one-way analysis of variance (ANOVA) following a generalized linear model (GLM) of SAS 9.2 software. Post Hoc Test of Tukey’s Honest Significant Difference (HSD) test was used for mean separation if the analysis of variance showed statistically significant differences (p < 0.05).

A total of 21 species that represent 9 families were recorded both in the exclosures and adjacent open grazing land. Woody species richness ranged from 10 species in the open grazing land to 14, 15, and 17 species in the middle, old, and young age exclosures, respectively. This result is comparable to similar studies conducted by Haftom et al., (2019) Hurni, H, Solomon, A, Amare, B, Berhanu, D, Ludi, E, Portner, B, Birru, Y & Gete, Z (2010) Land degradation and sustainable land management in the highlands of Ethiopia.Global change and sustainable develoopment: a synthesis of regional experiences from research partnershipsPerspectives of the Swiss National Centre of Competence in Research (NCCR) North-South, University of Bern 187-207. in Tigray, Northern Ethiopia, and Abiyu et al., (2011)Aerts, R, Nyssen, J & Haile, M (2011) On the difference between “exclosures” and “enclosures” in ecology and the environment. Journal of Arid Environments, 73(8), 762-3. in South Wollo, Northern Ethiopia. which reported 17 and 14 woody species, respectively in area exclosures. However, the result of this study is lower when compared to reports of Mekuria et al., (2018) Negussie, A, Aerts, R, Gebrehiwot, K & Muys, B (2008) Seedling mortality causes recruitment limitation of Boswellia papyrifera in northern Ethiopia, 72, 378-83. which reported 32 woody species in exclosures in the North-Western Ethiopia and Teketay et al., (2018) Witt, GB, Noël, MV, Bird, MI, Beeton, RJS & Menzies, NW (2011) Carbon sequestration and biodiversity restoration potential of semi-arid mulga lands of Australia interpreted from long-term grazing exclosures. Agric Ecosyst Environ, 141(1–2), 108-18. [Elsevier B.V.].
[http://dx.doi.org/10.1016/j.agee.2011.02.020] which recorded 35 woody species in an exclosure in the Mopane woodland in northern Botswana.

Analysis of the Sorensen’s similarity index showed that between all sites there was over 50 and 40% similarity of woody species and family, respectively (Table 1). The woody species in the open grazing land were less similar to the woody species in the area exclosures, whereas the similarity of woody species between exclosure was higher than with the open grazing land. Of the total woody species encountered in the study area, six were common in all the exclosures and open grazing land and 11 species were present only in the exclosures. One woody species (Prosopis juliflora) was only recorded in the open grazing land and three woody species (Balanites aegyptiaca, Berchemia discolor, and Grewia ferruginea) were recorded only in the young age exclosure. The existence of Prosopis juliflora indicated that there is an encroachment of exotic and invasive tree species on the degraded open grazing land. Reports have indicated that certain woodlands in the Eastern Ethiopian highlands are causing some socio-cultural damage (FAO, 2015Giday, K, Eshete, G, Barklund, P, Aertsen, W & Muys, B (2013) Wood biomass functions for Acacia abyssinica trees and shrubs and implications for provision of ecosystem services in a community managed exclosure in Tigray, Ethiopia. J Arid Environ, 94, 80-6. [Elsevier Ltd.].
[http://dx.doi.org/10.1016/j.jaridenv.2013.03.001] ). In the study area and the nearby Afar region, the encroachment of Prosopis juliflora is becoming a great concern to the communal rangeland.

Table 1
Sorensen’s similarity index of woody species and family of exclosures and open grazing land.

Of the total family recorded in the study area, Fabaceae was observed in all the exclosures and open grazing land, and Olacaceae was only recorded in the young age exclosures. Only three family names, i.e., Boraginaceae, Fabaceae, and Tiliaceae were observed in the open grazing land (Tables 4-7). As in many other places of Ethiopia, the Fabaceae family contributed the greatest number of species in the study area (Mekuria and Yami, 2013Newton, A (2007) Forest Ecology and Conservation: A Handbook of Techniques. Oxford University Press.
[http://dx.doi.org/10.1093/acprof:oso/9780198567448.001.0001] ; Mekuria et al., 2015Mengistu, T, Teketay, D, Hulten, H & Yemshaw, Y (2005) The role of enclosures in the recovery of woody vegetation in degraded dryland hillsides of central and northern Ethiopia. J Arid Environ, 60(2), 259-81.
[http://dx.doi.org/10.1016/j.jaridenv.2004.03.014] ; Gebremedihin et al., 2018Haile, M, Herweg, K & Stillhardt, B (2006) Sustainable Land Management – A New Approach to Soil and Water Conservation in Ethiopia. Mekelle University.). Despite its consistency, both family numbers and species numbers increase with an increase in age of exclosure.

Table 2
Woody species Shannon index diversity and Shannon evenness.

The Shannon-Wiener (H') index ranged from 0.74 at the open grazing land to 2.12 at the middle age exclosure (Table 2). According to Magurran, (2004), the Shannon-Wiener (H')diversity index in the exclosures can be rated as moderate whereas the diversity in the open grazing land was very low. Usually, the value of the Shannon-Wiener diversity index ranged from 1.5 to 3.5 and rarely superpassed 4 for a very huge number of species (Magurran, 2004). Shannon-Wiener diversity index in the study area indicated that exclosures can improve the diversity of woody species in the degraded open grazing lands. The changes in species richness were also evident in the Shannon-Wiener. The mean value of the Shannon-Wiener index of this study is in line with other similar studies conducted by Mekuria et al., (2018)Negussie, A, Aerts, R, Gebrehiwot, K & Muys, B (2008) Seedling mortality causes recruitment limitation of Boswellia papyrifera in northern Ethiopia, 72, 378-83. in West Gojam, North-Western Ethiopia, and Gebremedihin et al., (2018)Haile, M, Herweg, K & Stillhardt, B (2006) Sustainable Land Management – A New Approach to Soil and Water Conservation in Ethiopia. Mekelle University. in the highlands of Tigray, Northern Ethiopia.

The Shannon evenness (E) index was also lower (0.32) at the grazing land and it was higher (0.80) in the middle age exclosure (Table 2). The higher the evenness value, the more even the distribution of species is at a site (Kent, 2012Magurran, AE (2004) Measuring Biological Diversity. Blackwell Science Ltd..). Thus, the result of this study showed that there was a better equitable distribution of individual woody species in the exclosure than open grazing land. The high Shannon evenness value in the middle age exclosure might be attributed to the low anthropogenic disturbance in the exclosures since it is excluded from unmanaged access. In line with this study Teketay et al., (2018)Witt, GB, Noël, MV, Bird, MI, Beeton, RJS & Menzies, NW (2011) Carbon sequestration and biodiversity restoration potential of semi-arid mulga lands of Australia interpreted from long-term grazing exclosures. Agric Ecosyst Environ, 141(1–2), 108-18. [Elsevier B.V.].
[http://dx.doi.org/10.1016/j.agee.2011.02.020] reported an evenly distributed individual of woody species in area exclosure than in the open grazing area in Mopane woodland in northern Botswana.

Aboveground biomass and belowground biomass showed statistically significant (p < 0.05) variation between exclosures and open grazing land (Table 8). Aboveground biomass ranged from 12.60 Mg ha^-1 at the open grazing land to 86.61 Mg ha^-1 in the middle age exclosure. Aboveground biomass quantified in this study mirrored the basa area which is significantly higher in the middle and old age exclosures. This might explain the higher AGB in the middle and old age exclosures. In a study undertaken Mekuria et al., (2018)Negussie, A, Aerts, R, Gebrehiwot, K & Muys, B (2008) Seedling mortality causes recruitment limitation of Boswellia papyrifera in northern Ethiopia, 72, 378-83. in North-Western Ethiopia, it was indicated that exclosure land management can improve soil properties, which consequently supports the restoration of native vegetation and the accumulation of aboveground biomass. The result of this study is in line with findings of Yayneshet, (2011) in the Tigray region of northern Ethiopia and Mekuria and Yami, (2013)Newton, A (2007) Forest Ecology and Conservation: A Handbook of Techniques. Oxford University Press.
[http://dx.doi.org/10.1093/acprof:oso/9780198567448.001.0001] in the lowlands of Northern Ethiopia which reported higher aboveground biomass in exclosures than adjacent grazing lands.

Table 8
Mean± S.E values of AGB (Mg ha^-1), BGB (Mg ha^-1), AGB-C stock (Mg ha^-1) and BGB-C stock (Mg ha^-1).

The AGB carbon stock was significantly (p < 0.05) higher in the middle (32 Mg ha^-1) and old (31 Mg ha^-1) age exclosures whereas it was significantly lower in the open grazing land and young age exclosure. The BGB carbon stock followed a similar trend to the AGB carbon stock. The highest AGB carbon stock in the middle and old age exclosure could be associated with the high tree density and tree basal area. Except in the young exclosure, AGB and carbon stock had shown consistent increase from middle age to old exclosure age (Table 8). In line with this, several studies have indicated a significantly higher AGB carbon stock in exclosure than adjacent open grazing lands. For instance, studies conducted by Shimelse et al., (2017)Teketay, D, Kashe, K, Madome, J, Kabelo, M, Neelo, J, Mmusi, M & Masamba, W (2018) Enhancement of diversity, stand structure and regeneration of woody species through area exclosure: The case of a mopane woodland in northern Botswana. Ecol Process, 7(1), 1-15. [Ecological Processes.].
[http://dx.doi.org/10.1186/s13717-018-0116-x] in Tigray and Bikila et al., (2016)Birhane, E, Teketay, D & Barklund, P (2006) Actual and potential contribution of exclosures to enhance biodiversity of woody species in the drylands of Eastern Tigray Actual and potential contribution of exclosures to enhance biodiversity of woody species in the drylands of Eastern Tigray.. Journal of the Drylands, 1(2), 134-47. in Borana, North and Southern Ethiopia, have reported a significantly higher AGB carbon stock in exclosures than open grazing land. Another study by Mekuria and Yami, (2013)Newton, A (2007) Forest Ecology and Conservation: A Handbook of Techniques. Oxford University Press.
[http://dx.doi.org/10.1093/acprof:oso/9780198567448.001.0001] also displayed higher AGB in exclosures when compared to the adjacent communal grazing lands.

More importantly, this study demonstrated that pronounced woody biomass carbon can be stocked in the long-term. Generally, excluding degraded landscapes from free access of domestic animals and humans might bring an overall ecological restoration to the sites. Thus, such a restoration mechanism might contribute to the efforts of mitigating climate change in developing counties. A study by Witt et al., (2011)Yirdaw, E & Monge, A (2018) Reconsidering what enclosure and exclosure mean in restoration ecology, 26(1), 45-7. indicated a pronounced woody biomass carbon stock and significant ecological change following long-term grazing exclusion.