Woody Plant Species Diversity and Carbon Stocks Potential of Homegarden Agroforestry in Ephratana Gidimdistrict, Central Ethiopia

7 Background: Tropical agroforestry systems can contribute incredible benefit for carbon sequestration and plant diversity. This 8 system is one of the common practices in the Central part of Ethiopia. This is because of source of the multifunctional ecosystem 9 services, such as food, feed, biodiversity conservation and carbon storage potential. 10 Methodology: This study was carried out to assess the influence of land size on floristic diversity, richness and biomass carbon 11 stock. The homegardens were classified into small (<0.06 ha), medium (0.06–0.1 ha) and large (>0.1 ha). Biomass of the 12 homegarden was computed using allometric equations. 13 Results: A total of 39 woody species, belonging to 24 families were recorded in all the study homegardens. Tree density 625.8 14 tree ha-1 and basal area 17.3 m2ha-1 were highest for small-size HGs. However, large homegarden had more species richness 15 (Margalef Index) per garden (12.4) compared to medium and small size homegarden. Mean biomass carbon ranged from 9 to 16 89.3 ton ha-1. Mean biomass carbon stock per unit area was higher in small homegarden (49.3 ton ha-1) compared to medium 17 (38.4 ton ha-1) and large (35 ton ha-1). 18 Conclusions: This result implies that homegarden can serve as both for carbon sequestration and conservation of woody species 19 diversity. However, a specific homegarden management plan is necessary to improve the carbon storage and species 20 diversification to the respective area. The results provide a catalyst the implication of the future potential of homegarden 21 management in carbon storage thereby for climate change adaptation and mitigation purpose. 22

2 (Mesele Negash., 2013). Removing atmospheric carbon (C) and storing it within vegetation pool in terrestrial ecosystems is one 30 of the means to mitigate GHG emissions (IPCC, 2013).

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The world needs carbon sequestration techniques that provide social, environmental, and economic benefits while reducing 32 atmospheric CO2 concentration (Kumar and Nair, 2011). Thus, tree-based farming (agroforestry system) is believed to be a major 33 potential for carbon sink and could absorb large quantities of C (Kumar and Nair, 2011). Agroforestry as a land use system is 34 getting wider recognition not only in terms of agricultural sustainability but also in issues related to CC ( Mesele Negash, 2013 ).

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Agroforestry systems maximize carbon stocks in the terrestrial biosphere due to diversity and management for biomass (Henry et

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The precise relation between diversity and sustainability is still heavily debated. However, home-garden agroforestry is 39 ecologically and socio-economically sustainable due to their species diversity. A homegarden agroforestry is defined as an 40 intensive land use system that combine diverse farming components such as annual, perennial crops and livestock that can 41 provide environmental services, employment opportunities and household demands ((Tesfaye Abebe et al., 2005). In addition, it 42 has a potential for C sequestration and thereby maintain a sound and sustainable ecology (Mohan, 2004) mainly for CC 43 mitigation and adaptation under changing environment. This is because of the multifunctional ecosystem services and multiple 44 arrangements of plant and relatively high species diversity compared to other agroforestry practices (Mersha Gebrehiwot, 2013).

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HG agroforestry could also play a significant role in adaptation and mitigation to CC i.e. change the microclimate, provide 46 permanent cover, diversify the agricultural systems, improve resource use efficiency, improve soil fertility, reduce carbon 47 emissions and increase carbon stock in the soil and biomass (Rao et al, 2007). According to studies conducted in Sub Saharan

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Africa homegardens is one of the land use practices suggested for CC adaptation and mitigation more than the monoculture

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Most of the reports which are studied in Amhara region focus on the plant species diversity of the natural/church forests. But the 57 status of woody plant species diversity in homegarden agroforestry and their carbon stock potential is not well studied. Therefore, 58 3 this study designed to show the contribution of traditional homegarden agroforestry of Ephratanagidim district for woody species 59 diversity conservation and the potential role on carbon stock to use as a means for CC mitigation and adaptation strategy. The 60 study can also be used as baseline information to understand the role of diversity on carbon stock and biomass production. The 61 objectives of this study were to assess the current structure and composition of woody plant species and to estimate the biomass 62 carbon stock in homegarden agroforestry.

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Study Site Description

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The study was carried out in Ephratanagidim district, located in the central part of Ethiopia. It is geographically located between 66 9° 45 ꞌ N to 10° 11 ꞌ N latitude and 39° 43 ꞌ E to 40° 06 ꞌ E longitude.

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The study area is categorized under moist tropical climate and receives a mean annual rainfall ranging from 900 -1200 mm with

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Evenness index describes the equality of species abundance in a community. Evenness (E') was calculated as:

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The Simpson's diversity index was derived from probability theory and it is the probability of picking two organisms at random

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Where BGB is the belowground biomass (kg dry matter/ plant) and AGB is aboveground biomass (kg dry matter/plant). For 132 comparisons on unit area basis for each homegarden the values was extrapolated to hectare size.

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Then tree biomass was converted into C by multiplying the total tree/shrub biomass by 0.49 (IPCC, 2006   Margalef's diversity index of species richness was highly significant (P <0.001). The mean number of woody species per hectare 158 (ha) was 147. The maximum diversity of an individual was recorded in a large size HG and the minimum diversity was found in 7 a medium homegarden (Table 1). However, large HG size planting perennial crop and tree components and livestock (Mersha 160 Gebrehiwot, 2013) which maximize the diversity of woody species. In regarding to on species richness HGs are the highest 161 human-made agro ecosystem next to natural forest (Kumar and Nair, 2004). This is due to selective and repeated planting and 162 management of useful woody species from a natural regeneration (Kumar and Nair, 2004).

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The value of described by the Shannon diversity index (H') for woody species was from 1.6-1.9 with the mean value 1.7 (Table 2).  (Table 1). As the size of homegardens increased, woody species richness within homegarden size basis showed increase.

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However, species richness ha -1 was the highest in small sized homegardens followed by, medium sized (Table 1)

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The Important Value Index (IVI) of all woody species in the study area is listed descending order in (

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The IVI is an aggregate index that summarizes the density abundance, and distribution of a species. It measures the overall 183 importance of a species and gives an indication of the ecological success of a species in a particular area (Kent and Coker 1992).

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The IVI values can also be used to prioritize species for conservation, and species with high IVI value need less conservation

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The structural parameters of woody species for each size class are shown in (Table 3)

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The mean C stock of total biomass (above plus below ground biomass) for the 30 sampled HG was 40.93.7 ton C ha -1 , mean 216 SE. The mean AGB carbon was 30.03 ton C ha -1 (73.5%) and the BGB cabon was 10.82 ton C ha -1 (26.5%). Statistically there 217 were no any significance difference among homegarden size (p=0.262), but mean carbon stocks per unit area was slightly higher 218 in the small HG (49.38.1 ton C ha -1 ). The mean carbon stock for medium and large size HG was 38.46.4 ton C ha -1 and 353.3 9 ton C ha -1 , respectively ( Figure 6). The small homegarden relatively higher may be as result of large basal area and tree density 220 (Kumar and Nair, 2004). This result is in contrary to the study of Kumar and Nair (2011). The smaller size HG had higher 221 biomass and carbon stock than medium and large. This may be due to the intensive management of farm plots by the farmers of 222 Yilmo Kebele.

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The total biomass C stocks of the HG agroforestry in Ephratanagidim ranges 35-49.3 ton C ha -1 (Figure 3). And the average 224 aboveground C storage potential of agroforestry systems in semiarid, sub-humid, humid and temperate regions has been 225 estimated to be 9, 21, 50 and 63 ton C ha -1 , respectively (Montagnini and Nair, 2004). The mean carbon stock substantially lower 226 than the range reported from the Bangladesh and Indonesia, which ranges from 6.25-193.83 ton C ha -1 (Jaman et al., 2016) and 227 30-123 ton C ha -1 (Roshetko et al., 2002a). However, the carbon stock of HG of Ephratanagidim is higher than the HG of

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Woleyata 15 ton C ha -1 as reported by (Aklilu Bajigo et al. 2015). The HG from Sri Lanka which is the tropical region is 13 ton C

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Relationship between diversity and carbon stocks 232 A correlation analysis was conducted by using AGB carbon with selected diversity parameters and HG size measures from 30 of 233 HG (Table 4). There were significant correlation between the AGB carbon and the stand characters (i.e., Basal area ha -1 , Trees 234 density ha -1 , Shannon index H' and DBH). Even though statistically not significant, the HG size has a negative correlation in 235 carbon stock. The larger the HG size the lesser the carbon stock per unit area due to small basal area and low stem density (Table   236 4).

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Correlation analysis showed a positive and significant relationship between tree density and carbon stock where (r=0.39; p<0.01) 242 (Table 7). Tree density is an important factor to store carbon as it directly relates to the carbon stock (Roshetko et al., 2002a).

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Considering the relationship between tree density and biomass carbon stock it is indicated that tree density is a strong

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The results suggested that homegarden size was not the factor for AGB carbon stock however, the investigated homegardens in 254 the study area hold a wide range of carbon between 9 to 89.3 ton ha -1 and a mean above-below ground biomass C stock of 41.4   368