Characteristics of the forest
A total of 71 types of woody plants were identified, belonging to 34 different families. Among these, 33 species were observed in plantation forests, while 63 species were found in natural forests. Interestingly, 9 species were exclusive to plantation forests, and 38 were exclusive to natural forests. Notably, two non-native species, Cupressus lusitanica and Grevillea robusta, were identified. The diversity of species observed in our study surpasses that documented in other parts of Ethiopia. For instance, previous studies in Chancho and Menagesha (Eshetu Yirdaw, 2001) reported only 20 and 22 species, respectively. Similarly, Abrham Abiyu et al. (2011) discovered only 2 and 4 regenerated species in Cupressus and Eucalyptus plantation forests in the Tehuldere district. However, our study recorded a comparable number of regenerated woody species (33 species) to that reported by Feyera Senbeta et al. (2001) in Menagesha-Suba (37 native woody species) and in Jirren forest (40 species reported by Getachew Tesfaye et al., 2006). Nonetheless, our findings were lower than the 55 species documented in Munessa-Shashemene (Feyera Senbeta et al., 2002). In terms of composition, natural forests consisted of 60% trees, 32% bushes, and 8% climbers, while plantation forests contained 48% trees, 45% shrubs, and 7% climbers. The dominant family in natural forests was Fabaceae (8% of species), followed by Euphorbiaceae and Moraceae (each accounting for 6%). Other families such as Oleaceae, Rubiaceae, Anacardiaceae, and Sapindaceae each represented 3%. In contrast, Oleaceae accounted for 9% of the species in plantation forests, while Fabaceae and Euphorbiaceae each represented 12% of the species.
Biomass and Carbon Stocks of the forest stands
The standing biomass of the trees' above-ground woody portions on two types of forests was determined in the current study. Estimates of the research sites' carbon stock and sequestration potential were also made. Fresh weight and dry weight are the two biomass units employed in investigations of forest biomass (Araujo et al., 1999). (Ketterings et al., 2001; Aboal et al., 2005; Montagu et al, 2005; Saint-Andre et al., 2005). The dry weight is more important for carbon sequestration applications since it contains 50% carbon (Montagnini and Porras, 1998; Losi et al., 2003; Montagu et al., 2005). Because it makes up the bulk of the overall accumulated biomass in the forest ecosystem, above-ground forest biomass is the focus of many biomass assessment research (Brown, S., et al. 1982; Kraenzel et al. 2003; Laclau, 2003; Losi et al. 2003; Aboal et al. 2005; Segura and Kanninen, 2005). Estimating the biomass of trees is the first stage in carbon accounting because biomass is a key indicator of carbon sequestration.
Table 6
; Comparison with other forests in terms of AGC
No
|
Forest name
|
AGC in ton/ha
|
References
|
Way of estimation
|
1
|
African tropical forest
|
183
|
(Sullivan et al.,2017)
|
AE (Chave et al.,2014)
|
2
|
Banja NF (Ethiopia)
|
338.72
|
(Abere et al.,2017)
|
AE (Chave et al.,2014)
|
3
|
Lake Hawassa NF
|
100.45
|
(Wondrade etal., 2015)
|
AE (Chave et al.,2005)
|
4
|
Dry area forest of SMNP
|
270.89
|
(Simegn et al.,2014)
|
AE (Brown etal., 1989
|
5
|
Adaba-Dodola CF
|
278.03
|
(Muluken etal., 2015)
|
AE (Chave et al.,2005
|
8
|
Tara Gedam CF
|
16.7
|
(Sibhatu,2015)
|
AE (Chave et al.,2005
|
9
|
Mahebere Selasie CF
|
9.6
|
(Sibhatu,2015)
|
AE (Chave et al.,2005
|
11
|
Tsahare RNF (Ethiopia)
|
170.1
|
(Gebeyehu etal., 2019
|
AE (Chave et al.,2014)
|
12
|
Kahtasa RNF(Ethiopia)
|
140.8
|
Gebeyehu etal., 2019)
|
AE (Chave et al.,2014)
|
14
|
Akako-TelamoNF
|
201.1
|
(Molla et al.,2017)
|
AE (Brown,1997)
|
15
|
Abo- Bokaso NF
|
240.4
|
(Molla et al.,2017)
|
AE (Brown,1997)
|
16
|
Weiramba NF (Ethiopia)
|
152.33
|
(Teshager etal., 2018)
|
(Chave et al.,2014)
|
17
|
Ambober NF AE
|
8.3
|
(Solomon etal,, 2019)
|
(Chave et al.,2014
|
18
|
Gesha-Sayilem Forest
|
174.95
|
(Admassu etal,, 2019)
|
AE (Chave et al.,2014)
|
19
|
Qimbaba forest (present study)
|
20.34
|
Our Result AE
|
(Chave et al.,2014
|
NB; AE = Allometric Equation, AGC = Above Ground Carbon, BGC = Below Ground Carbon
Based on the findings, the above-ground biomass of a species is influenced by both its age and diameter at breast height (DBH). Older trees with a high DBH tend to exhibit substantial above-ground biomass, and biomass generally increases with tree age (Negash, M., et al., 2007). Comparing our findings with recent investigations, as presented in the table above, aids in evaluating and conducting comparative research in Ethiopia. Our study forest's above-ground carbon (AGC) results surpassed those of Tara Gedam (16.7 tons/ha), Mahebere Selasie (9.6 tons/ha), and Ambober (Sibhatu, 2015). However, despite utilizing comparable allometric conditions for comparison, our AGC result was less impressive than that reported by Sullivan et al. (2017) for tropical forests in South America (140 tons/ha), Tropical Asia (197 tons/ha), and Africa (183 tons/ha). Weiramba NF (152.33 tons/ha) (Teshager et al., 2018), Abo-Bokaso NF (240.4 tons/ha) (Molla et al., 2017), Lake Hawassa NF (100.45 tons/ha) (Wondrade et al., 2015), and Guangua Illala (291.78 tons/ha) represent some of the research sites (Ayen, 2015). Additionally, when compared to Akako-Telamo, Arossa-Garagalo, and Abo-Bokaso forests (Molla et al., 2017), our result was also less favorable. However, these discrepancies may stem from the utilization of different allometric equations (Table 6).
Variation in the Contribution of Species for Biomass and Carbon Stock
Estimating carbon and wood density for tropical forests is crucial for gaining a better understanding of the species present in those forests, for demonstrating their significance in the global carbon cycle budget, and for the production of wood for commercial purposes (Brown, 2002; Woodcock and Shier, 2002; Nogueira et al., 2005).
The concentrations of carbon, both above and below ground, were assessed for the Qimbaba State Forest under field conditions. The study revealed disparities in carbon levels across various forest types and species, likely influenced by differences in wood properties, elevation gradients, forest composition, and growth habits of species. Notably, specific species such as Ficus sycomorus, Sapium ellipticum, Grevillea robusta, Cupressus lusitanica, Syzygium guineense, Croton macrostachyus, Acacia abyssinica, Carissa edulis, and Bersama abyssinica collectively accounted for approximately 93% of aboveground biomass (AGB) and carbon stocks in the plantation forest (Fig. 5). This aligns with prior research by Gebeyehu et al. (2019), which found a similar pattern in Tsahare and Apini forests, where only a few species contributed significantly to AGB and carbon stocks. Additionally, species like Albizia gummifera, Apodytes dimidiata, and Prunus africana played significant roles in AGB and carbon stock in Dabkuli and Bari natural forests, contributing 71.2% and 81.3% respectively. In the natural forest setting, Sapium ellipticum, Ficus ovate Vahl., Ficus vasta, Stereospermum kunthianum, Schrebera alata, and Ficus sycomorus were found to store the highest average carbon stocks, accounting for nearly 69% of the total AGB carbon.
The dominant species within the research forest, characterized by larger basal areas, play a significant role in carbon storage. Plants represented by individuals with larger diameters at breast height (DBH) contribute substantially to the forest's capacity to sequester carbon. Their elimination would significantly affect the biomass dynamics of the forest. Larger trees with greater diameters harbor the most carbon within their biomass, rendering them particularly vulnerable to deforestation and forest degradation, as noted by Gibbs et al. (2007). Likewise, Ekebergia capensis stands out for storing the highest amount of aboveground (ABG) and belowground (BG) carbon per individual tree.