Description of the study area
The study area is located in the Southern Nations Nationalities Peoples Regional State (SNNPRS), in Kafa Zone at Gesha and Sayilem districts. It is located between 60 24’ to 70 70’ North and 350 69’ to 36078’ East (Fig. 1). The topography of the landscape is undulating, with valleys and rolling plateaus and some area with flat in the plateaus. The altitude ranges from 1,600m to 3000m [18]. The monthly mean maximum and minimum temperature for Gesha is 29.5 0C and 9.5 0C, respectively. On the other hand, the monthly maximum and minimum temperatures for Sayilem ranges 10oC to 25oC and the annual rainfall for both districts ranges 1853-2004mm.
Data collection and sampling techniques
A stratified random sampling method was used to select tree and shrubs in the study area. For sampling trees, individual plant species were categorized into woody plants who’s Diameter at Breast Height (DBH) is ≥ 5 cm diameter at breast height, shrubs, saplings (height ≥ 1.3 m and DBH 2.5-5 cm following Lamprecht ‘s classification [19, 20]. Based on the density and frequency of the species, a total of 150 individuals of five dominants plant species of Apodytes dimidiata, Ilex mitis, Sapium ellipticum and shrubs (Galiniera saxifraga and Vernonia auriculifera) were selected and 30 individuals from each trees and shrubs were used for the measurements. In order to represent the reasonable size of the diameter distribution and to minimize error of sampling, the trees were classified into five DBH classes and each class having six individuals per DBH class ranging from 10-20, 20.1-30, 30.1-40, 40.1-50, and greater than 50 cm were measured and recorded.
Field measurement
Non-destructive sampling method was used for the measurement of tree biomass, and the trees were divided into separate architectural elements (stem, branches and leaves). Serial measurements of the height and diameter of trunk were done at 2 m intervals by climbing on live trees using the ropes. For the determination of trimmed biomass, four branches whose circumference is less than 10cm were trimmed down from the live tree using the machete [21, 22, 23]. The trimmed branches were separated into leaves and wood and the fresh weight of leaves and wood were recorded Fig. 2.
Laboratory measurement
A three replicates of 1 kg of sample of the wood and leave were weighed and placed in plastic bag, brought to the laboratory and oven dried at 105 °C for 72 hr for wood, and 24 hours for leaves. The total dry weight of each AGB component was calculated using the ratio between the dry and fresh weight of the sub-samples, multiplied by the total fresh weight of the respective components. The basic wood density (gcm−3) of branches of the different sizes of the tree was estimated according to the water displacement method Figure 3. The averaged WD (g/cm3) per sample tree was calculated as oven-dry weight divided by volume at saturation.
For determination of biomass shrubs (Galiniera saxifraga and Vernonia auriculifera), the shrubs were destructively sampled. The following parameters were measured such as stump diameter at 30 cm, DBH at 1.3 m, total height (h). The DBH of the shrubs ranged from 3.8-22.8 cm and 3.0 to 18.3 cm for Galiniera saxifraga and Vernonia auriculifera respectively. The fresh weight of each component was measured using a spring balance. To determine the dry matter content of the woods and leaves all branches from each stem were taken from thickest to the thinnest to make a composite sample and sealed in plastic bags and transported to laboratory. They were then oven-dried at 700 C for 24 hr and samples were weighed and the fresh to oven-dry weight ratios was calculated.
Biomass calculations
The data collected from field and laboratory measurements were organized in excel spread sheet and analyzed using Statistical Package R software [24].
Estimation of Aboveground biomass of tree
The above ground biomass of the tree was calculated by summing up of trimmed dry biomass and the untrimmed dry biomass of the sample trees.
Bdry = Btrimmed dry + Buntrimmed dry……………………………………… (equ.1)
Calculations of trimmed biomass
The trimmed biomass of sample tree was calculated from the fresh biomass Baliquot fresh wood of a wood aliquot and its dry biomass Bdrywoodaliquot, the moisture content was calculated as follow
………………………………………………… (equ.2)
Where is moisture content of the wood, and where Baliquot dry wood, is the oven-dried wood biomass of the aliquot in the sample and where Baliquot fresh wood, is the fresh wood biomass of the branch aliquot in the sample. Similarly, the moisture content of the leaves was calculated from the fresh biomass B fresh leaf aliquot of the leaf aliquot and its dry biomass B dry leaf aliquot as follow.;
………………………………………………….. (equ3)
Trimmed dry biomass was then determined as
Btrimmed dry = B trimmed fresh wood *X wood+ B trimmed fresh leaf* X leaf………… (equ.4)
Where, B trimmed fresh leaf is the fresh biomass of the leaves stripped from the trimmed branches and Btrimmed fresh wood is the fresh biomass of the wood in the trimmed branches.
Calculating untrimmed biomass
Untrimmed biomass was calculated from two parts of the tree still standing (stem and large branches) and the other for small basal branches.
B untrimmed dry = B dry section+ B untrimmed dry branch…………………. (equ5)
Each section i of the stem and the large branches were considered to be a cylinder of volume and volume of stem and large branches were calculated using Smalian’s formula.
Vi= π Li (D2li+D22li) …………………………………………………(equ6)
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Where Vi is the volume of the section i, its length, D21i and D22i are the diameters of the two extremities of section i. The dry biomass of the large branches and stem were being calculated from the product of mean wood density and total volume of the large branches and the stem.
B dry section= ῥ*∑Vi……………………………………………. (equ7.)
Where ῥ the mean wood density was expressed in gcm−3, then volume Vi was expressed in cm3 and the mean wood density was calculated by:
……………………………………………….. (equ.8)
The dry biomass of the untrimmed small branches was then calculated using a model between dry biomass of trimmed branches and its basal diameter. This model is established by following the same procedure as for the development of an allometric model, using a simple linear regression model which is expressed as Bdry branch = a+bDc……………………………………………………………… (equ. 9)
Where a, b and c are model parameters and D branch basal diameter,
Estimation of below ground Biomass (BGB)
The total aboveground biomass of a tree has been good predictors of its belowground biomass. Total root biomass for each of the study trees were calculated following [25]. Thus, a conversion factor of 0.24 for tropical rain forest was used to calculate the below ground biomasses of each of the study trees from their total aboveground biomass.
BGB= AGB × 0.24 …………………………………………………………… (equ. 10)
Data analysis and Model selection
Relationships between basal diameters and dry weight of trimmed branches including twigs and leaves were computed using linear regression models. The assumptions of linear regression model were checked by observing the normal distribution of residuals on P-P plots. Because of the heteroscedasticity nature of biomass data, the data were transformed using a natural logarithm. Furthermore, Pearson correlation analysis was carried out between the response variable (Dry weight of the biomass) and the independent variables (DBH) to examine whether there was the linear relationship between dependent and independent variables (Table 2). In order to identify the multicollinearity with log-transformed models multi collinearity test was carried out using a variance factor [26]. A value greater than 10 (VIF > 10) is an indication of potential multicollinearity among independent variables. Then selection of the best fit model was based on the goodness fit statistics calculated for each species specific equation such as adjusted coefficient of determination (R2 adj), standard error of the mean (SE) and Akaki information criterion (AIC).