The continuous deforestation and degradation of forest ecosystems in the tropics is of major concern due to the negative impacts this has on many of the ecosystem services that the forest provides1. Likewise, it is observable phenomena that the forest cover in Ethiopia is declining from time to time as a result of deforestation, in spite of the disagreement on the original forest cover of the country2.
This phenomenon is making the Ethiopian Afromontane forest fragments to be continuously shrinking3 which were ones known to be dense and intact forests in the country4. Studies from pollen analysis provide evidence of decline in forests in response to human impact5,6. Historical evidences from vegetation maps in the country also shows the increase in magnitude of deforestation from time to time7. The continued total deforestation of some areas is causing significant damage preventing the forest from achieving its natural climax state8.
Against this, the Ades dry Afromontane forest, which is characterized by a variety of tree species have gained its cover during the last 10 years due to the concern from the Regional Government of Oromia. The forest comprised both indigenous and exotic species9,10. The protection of the Ades forests results in increasing plant species composition and the floristic diversity. Increase in plant species composition and diversity of the forest increase the capacity of the forests to effectively militate against climate change and again mitigates the biomass and carbon estimates of the woody species in the forests11.
In Ethiopia, studies on biomass and soil carbon stock are inadequate and this is, especially, true for the dry Afromontane forests of south-eastern Ethiopia in general and Ades forest in particular. Above all, such studies have been very important issues for ecological explanations and for effective conservation design. Furthermore, the studies can provide information needed for carbon marketing and the value of forests in carbon sequestration. Given this lack of information, studies that provide woody species biomass and soil carbon stocks in the Ades dry Afromontane forest have become one of the most important issues. This study was therefore aimed to determine the biomass and soil carbon stocks of woody species of the indigenous and exotic species in Ades dry afromontane forest, south-eastern Ethiopia.
MATERIALS AND METHODS
Location of the study area: The study was carried out in Ades dry afromontane forest about 407 km south-east of Addis Ababa, from May 2018 to April 2019. It is located between 1029419 m E-1030017 m E and 746685 m N-746986 m N (Fig. 1).
|Fig. 1:||Map of the study area
||Source: CSA12, field survey 22013/14
|Fig. 2:||Layout of plots and sub-plots
The altitudinal gradients of study area ranges from 2517-2743 m above sea level. The forest coverage of the study site is 618 ha and it contains various species of plants and animals.
Methods of data collection
Plot layout and sampling: The Forest Inventory and Analysis Programme (FIA) developed by USDA13 had been applied for the collection of biomass and soil organic carbon stocks. Circular plots of 0.017 ha were arranged in groups of four in which the central plot was surrounded by three plots forming an equilateral triangle at a distance of 36.6 m out from the central plot (Fig. 2).
A random selection of groups of plots was made and pair of plots was again selected randomly from each group of plots and established for each forest types. The allocation of the total number of plots varies depending on the area of each forest type.
Vegetation measurement: Sampling of the woody vegetation and soils were made from each of the plots selected at random. All woody plants with diameter at breast height (dbh) >5 cm were identified and recorded. The indigenous species were identified using a guide book Flora of Ethiopia and Eritrea, with the assistance of a local Para-taxonomist. For those species difficult to identify in the field, fresh specimens were collected and then pressed properly for identification at the National Herbarium of Addis Ababa University.
Soil sampling: Soil sampling was accomplished by using two soil augers of lengths of 15 and 30 cm, for taking disturbed soil samples at two layers 0-15 and 15-30 cm. The sampled soils were placed separately in each plot and layer in polythene bags which were then tied with cotton strings and taken to the laboratory. Undisturbed soil samples for bulk density analyses were taken using a cylindrical metallic core ring with 5 cm height and 2.5 cm radius. The core ring was driven into the soil surface using a hand sledge and a block of wood. It was then removed by carefully lifting after removal of excess soil around the edges using a flat-bladed knife. The soil samples were pretreated by oven drying at 105°C for 24 h, then crushed and sieved through a 2 mm sieve.
Soil analyses: The soil samples were analyzed for organic carbon content using a Walkley and Black method14 at the Haramaya University Soil laboratory.
Biomass carbon stocks and density analysis: The estimation of Above Ground Biomass (AGB) was done using an allometric function that integrates the DBH and wood density of each species15. Above ground biomass have been shown to be better predicted through the use of diameter15,16. In this study, the allometric function developed for the above ground biomass of dry forests15 has been used estimate the Above Ground Biomass (AGB):
AGB = WD*exp (-0.667+1.784*log (DBH) +0.207*(log (DBH)2 _ 0.0281*( log(DBH))3
where, WD is global wood density, DBH is diameter at breast height, ln is natural logarithm.
The specific wood densities of the different species in the study site were taken from Wood density tables and Global wood density database17,18. In some cases, when the wood density values for some indigenous species were not found in the two tables a value of 0.5 recommended for tropical areas was used15. For those trees of cypress species their wood densities were adapted from FAO19. A shoot: root ratio approach has been applied for the estimation of Below Ground Biomass (BGB)20. In this case, a ratio of 0.25, 0.24 and 0.33 were used for indigenous trees21, Conifers22 and Eucalyptus15 respectively. The root biomass will be then converted in to root carbon stock by taking 50% of the root biomass. The Carbon (C) content of each tree in the plots was calculated as the product of the dry weight biomass and an assumed C content of 50%23 and The tree carbon contents were then summed and divided by the area of the plot to give above ground and below ground C density values for each plot expressed in Mg C ha1 ignoring the analysis of carbon stocks in deadwood and litter15.
Soil Organic Carbon (SOC) density: The Carbon stock of the soil was done using the volume and bulk density of the soil24 as:
where, V is the volume of the soil in the core sampler (cm3), h is the height of the core sampler (cm) and r is the radius of the core sampler (cm)24:
where, BD is the bulk density of the soil sample per, Wav, dry is average air dry weight of soil per the quadrant and V is the volume soil in the soil sampler auger (m3):
where, SOC is the soil organic carbon per unit area (t ha1), BD is the soil bulk density (g cm3), D is the total depth at which the soil was taken (30 cm) and C (%) is the carbon concentration (%). Descriptive statistics were calculated for mean biomass and soil organic carbon densities by forest types. Differences between carbon densities in biomass and soil for each forest type was tested by a two-way ANOVA and Tukey’s multiple comparison using SPSS version 1625.
Estimation of total carbon stock density and its carbon sequestration potential: The total carbon stock density was calculated by summing all the carbon pool stock densities15 using the following Eq:
where, CT is the total carbon stock for all pools (mg ha1).
The total carbon stock was converted to Mg of CO2- equivalent (CO2-e), multiplying it by 44/12, or 3.67 which is the ratio of molecular weights between carbon dioxide and carbon26.
RESULTS AND DISCUSSION
Above ground and below ground biomass carbon densities: A greater amount of carbon stock has been observed in a species which has counted long lived in the study site and also species which are densely populated in the pool. As a result the current study of the site indicates unbalanced tendency of carbon stock input in the above ground. The high biomass rate has been held by species such as Podocarpus falcatus, Juniperus procera, Croton machrostachys and Hagenia abyssinica, on the other hand species such as Rytigynia neglecta, Rhamnus prenoids, Rhus glutinosa and Phytolacca dedocandra had less contribution. There was a significant difference among the mean biomass carbon stocks among the forest types (Table 1). The above ground carbon stock’s potential of the forest ranged from 63.11-5507.73 MgC ha1 with the overall mean above ground carbon stock of 317.55 MgC ha1 for the three forest types was (Table 1).
A two-way analysis of variance revealed significant differences in biomass carbon (C) densities between the three forest types. A significantly higher biomass carbon stock were recorded in the in indigenous forest than the plantation forests.
The minimum (63.11 MgC ha1) and maximum (5507.73 MgC ha1) AGBC from this forest have carbon dioxide (CO2) sequestration potential of 231.61 and 20,213.4 Mg CO2 ha1, respectively. The overall mean AGBC (317 MgC ha1) of the study area sequestered the amount of carbon dioxide equivalent to 1163.39 Mg CO2 ha1. There was variation in terms of AGBC among species of different types in which Podocarpus falcatus, an indigenous species which was dominant in the forest had the highest value of carbon stock of 604 MgC ha1 (Table 2) and this can have the potential to sequester the amount of CO2 equivalent to 2216.68 Mg CO2 ha1 followed by Juniperus procera 403 Mg C ha1 which has potential to sequester CO2 equivalent to 1479 Mg CO2 ha1, Croton macrostachys with 169.16 Mg C ha1 equivalent to CO2 sequestration value of 620.8 Mg CO2 ha1 and Hagenia abyssinica with 143 Mg C ha1 equivalent to sequester 525 Mg CO2 ha1 while the species of Rytigynia neglecta had the lowest mean carbon density of 1.61 Mg C ha1 equivalent to sequester 5.91 Mg CO2 ha1 (Table 2).
It has been reported in some studies, that the carbon stock potential of the forest in given environment can be affected by several factors such as: type of species, age and density of the forest vegetation27. The occurrence species that are characterized by their large number of individuals could have an effect on AGB carbon stock28.
Mean±standard deviation) carbon stocks (MgC ha1) in tree biomass of the three forest types at Ades dry afromontane forest, south-eastern Ethiopia
AGBC: Aboveground biomass carbon, BGBC: Belowground biomass carbon, NB: Means with different letters are significantly different (Tukey’s test, p<0.05)
Above-ground carbon (Mg ha1) density of different species at Ades dry afromontane montane forest, south-eastern Ethiopia
|AGB: Aboveground biomass, BGB: Belowground biomass|
This has been observed in the Ades dry afromontane forest where some tree species like Podocarpus falcatus and Juniperus procera with larger number of individuals in addition to their bigger DBH were dominant in the area.
The mean above ground carbon estimates in Ades forest of 317±75 Mg C ha1 was higher than the above ground carbon densities ranging between 30-46 Mg C ha1 reported from other dry forests28. The values obtained in the Ades dry afromontane forest are an indication of good conservation and management of the forests with resultant gain in carbon stocks. The values of the above ground carbon density in this study are in line with some other results reported from elsewhere29-31. But this value is lower than the mean above ground carbon density of 517 Mg C ha1 reported from Tanzania that ranged between 252 and 581 tC ha1 31. This may be due to differences in geographical variations and the types of the species together with varying degree of exposure to human degradation. It had been reported by various studies that different ecosystems have different biomass and carbon stock densities32.
The biomass of below ground helps to determine the carbon stock of the study area. The minimum and maximum BGC stock’s potential of Ades forest was 12.62 Mg ha1 and 1,101.55, respectively (Table 1). The mean value of BGC stock was 86.2 Mg ha1. The average carbon dioxide sequestration potential of BGBC was found to be about 316.35 Mg CO2 ha1. The total biomass carbon of the three forest types in this study ranged from a minimum of 73.73 Mg ha1 to a maximum of 6,609.28 Mg ha1 with the overall mean total biomass carbon of 402.54 MgC ha1 that can sequester the amount of carbondioxide equivalent to 1,477.32 Mg CO2 ha1.
Carbon stock in soil (SOC): The bulk density of the soil profile found in the study site ranged from a minimum of 0.61 g cm3 to maximum of 1.42 g cm3 with the average value of 0.83 g cm3.
Mean SOC stocks (Mg C ha1) of the three forest types (0-15 and 15-30 cm) at Ades dry afromontane forest South-eastern Ethiopia
NB: Means with different letters are significantly different (Tukey’s test, p<0.05)
Over all SOC stocks (Mg C ha1) of the three forest types (0-30 cm) at Ades dry afromontane forest, South-eastern Ethiopia
NB: Means with different letters are significantly different (Tukey’s test, p<0.05)
The soil organic carbon stocks also showed a similar trend in which the indigenous forest had significantly higher SOC stocks than the exotic plantations of C. lusitanica and Eucalyptus (Fig. 3).
The mean SOC density in the different forest types ranged from 77.52-140.02 Mg C ha1 in the 0-15 cm layer, from 116.57-130.91 Mg C ha1 in the 15-30 cm layer (Fig. 3) and from 205.22-270.93 Mg C ha1 in the total 0-30 cm layer (Fig. 4). In the case of the 0-15 cm layer, the mean SOC density in the indigenous forest was significantly higher than that of the C. lusitanica and Eucalyptus forests. The overall SOC was found to be 234 Mg ha1 that sequestered 858.78 MgCO2 ha1.
A two-way analysis of variance revealed significant differences in Soil Organic Carbon (SOC) densities between the three forests types (p<0.05). The C. lusitanica plantations had higher soil carbon stocks than eucalyptus unlike the biomass carbon stocks which showed higher values in eucalyptus.
The mean soil carbon densities estimates of 234 Mg C ha1 in all three soil depths in this study were higher than some other values for soils in South African savannas32 and Mexican dry forests33. It is also higher than mean soil carbon densities of 6-18 MgC ha1 reported for agricultural soils in Bukoba district34. The value of the SOC from Ades dry forest is in line with soil carbon pools reported from Eastern Arc Afromontane forests that have been observed to be slightly lower than the above ground biomass carbon pools31.
However, the C storage in the soil component in this study is lower than the above ground biomass carbon which is contradictory to the findings reported from Tanzania31 indicating the being there of higher C pools in the soil than the above ground vegetation. The difference is likely to be due to the depth of the soil studied in two forest types. This study used a soil depth of 30 cm for soil analysis while the other studies used soil analysis down to 100 cm depth. There can be considerable amount of soil organic carbon down to up to 200 cm, even though it is usually concentrated in the top 30 cm. The minimum and maximum CO2 sequestration potential of soil carbon in this study was 265.34 and 2005.66 Mg CO2 ha1, respectively.
The total carbon stock of Ades forest was obtained by adding all carbon value of the three pools which revealed (above ground carbon, below ground carbon and soil organic carbon) for the entire sample plots of the study site. As a result of these, the total carbon stock of Ades forest ranged from the minimum value of 146.03 MgC ha1 to the maximum 7155.78 MgC ha1 with the overall mean carbon mass of 637.54 Mg ha1. The difference in Carbon storage between the three forest types can obviously be attributed to the difference in disturbance level, type of forests and its structure. Forests can alter soil carbon sequestration capacity, other soil characteristics. The results showed that the types of forests can impact soil carbon concentration and stock across soil depth and that the Carbon concentrations and stock under indigenous natural forest were higher than plantation forest. The study suggested that using indigenous tree species could have a positive impact on Carbon storage. The increase CO2 in the atmosphere is becoming of global and local concern. The amount of CO2 sequestered in the forests depends on forest type, forest status; dominant tree species and forest stand age31. This therefore requires mapping of carbon distribution in different cover types among others, as a means to establish baseline for Reduction of Emission from Deforestation and Forest Degradation(REDD+).
The greater variation in carbon densities by components in this study clearly show the potential to increase carbon storage within the forest areas by forest management and promoting protection of the Ades forest. It is suggestive that the on-going forest management has increased carbon stocks potential of the forest. There was significant difference among the mean biomass carbon stocks in the three forest types. There was positive relation between the SOC and depth of the soil in all forest types. It was again observed that indigenous forests were found to sequester more CO2 than those of exotic plantations.
The study provides insights into the role of forests in carbon stocks potential income generation from carbon trading that would assist in developing sustainable forest management towards building climate resilient livelihoods in forest areas. It also provides a reliable baseline carbon stocks and sequestration potential of forests. The information will assist managers, planners and policy makers in determining the REDD+ potential of these forests through its sustainable management.
We would like to thank Oda Bultum University for Grant funding of this Research. We appreciate and thank Office of Forestry and Wildlife Enterprise of Oromia National Regional State, Hararghe branch for providing us guidance and logistics during field data collection. We also thank Forestry Experts of Doba District in West Hararghe Zone Administration who were involved in data collection.