2.1 Sampling area
P. juliflora was sampled from BRW that lies in the south coast of Hambantota District, low country dry zone, Sri Lanka (6008’- 6014’N, 81008’- 81018’E). The total area of BRW is 3,698 ha. The mean annual rainfall in the area ranges from 900 mm to 1300 mm, with two peak rainfall periods during April-May and October-November. The mean annual temperature is about 270C while relative humidity ranges between 76% and 81% [9].
BRW has diverse vegetation, which shows natural succession from low creeping plants to climax forests described as thorny, dry semi-evergreen and dry mixed evergreen. The forest canopy of the tropical semi-deciduous forest area of BRW consists of typical, native forest vegetation M. hexandra [9]. Canopy layers of other areas consist of species such as Salvadora persica, Limonia acidissima, Strychnos potatorum and Drypetes sepiaria. The lower tree layer consists of species such as Allophylus cobbe, Benkara malabarica, Capparis zeylanica, Erythroxylum monogynum, Memecylon umbellatum and Ochna lanceolata [14]. In addition, an array of vegetation occurs in low lying areas along Bundala Lagoon, as aquatic vegetation, salt marshes, mangroves, etc. Presently, the dry mixed evergreen forest area of BRW is dominated by invasive P. juliflora species. This invasion is more abundant along the lagoon as well as inland water bodies [9].
2.2 Selection of sampling sites
Sampling sites were selected using handheld GPS (Garmin eTrex Summit, Taipei/Taiwan).
2.3 Sampling of P.juliflora and L. leucocephala
All the P. juliflora individuals with Diameter Breast Height (DBH) > 5 cm in the sampling sites were considered for the study (Figure 1). Sixty individuals were categorized into different diameter classes; DBH class 01: 10 cm to 19.99 cm; DBH class 02: 20 cm to 29.99 cm and; DBH class 03: 30 cm to 39.99 cm. L. leucocephala is another widely used fuelwood species found in Hambantota District where P. juliflora is abundant. In order to compare the energy potential of P. juliflora with that of L. leucocephala, a site near BRW where L. leucocephala was abundant was selected. Optimum sample size to compare energy characteristics of P. julfiflora with those of L. leucocephala was determined according to Pearson [15]. Altogether 40 individuals of P. juliflora and 20 individuals of L. leucocephala were selected to obtain samples. GPS coordinates of the site were 81°16'41.526"E and 6°12'20.537"N. Study sites of P. juliflora and L. leucocepahala are illustrated in Figure 2.
In order to extract samples to estimate fuelwood properties of P. juliflora, sample wood disks were cut at breast height from the 40 stems found in the study plot. The disks were packed in airtight polyethylene bags. Later, in the laboratory, 5 cm x 5 cm x 5 cm cubes were cut from the disks to test for fuelwood properties such as, moisture content, density, specific gravity, ash content, volatile matter content, fixed carbon content and biomass/ash ratio. An additional set of specimens was prepared from 20 trees of L .leucocephala for comparison of fuelwood characteristics with P. juliflora. All specimens were prepared in triplicate from each individual selected tree for both species.
2.4 Drying profiles of sample specimens
In addition to the previously mentioned disks that were cut from sample trees, another set of samples with dimensions of 18 cm x 2 cm x 2 cm was prepared from both species for the construction of drying profiles. These specimens were prepared within 24 h of sample extractions and each specimen was labeled and packed in airtight bags to avoid moisture loss.
2.5 Measurements and calculations
2.5.1 Moisture content:
The moisture content of P. juliflora was measured following the oven dry method [16].
2.5.2 Wood density:
Densities were determined using the volume and oven dry weight of samples. Volumes of the specimens were determined using the water displacement method as recommended in ASTM D2395-17.
2.5.3 Specific gravity:
Specific gravity was determined using the maximum moisture content method [17].
2.5.4 Ash content and volatile matter:
Ash content of each specimen was determined using the loss of ignition method [18]. The same samples used for moisture content testing were used to determine the percentage of volatile matter [19].
2.5.5 Fixed carbon content:
Fixed carbon content was determined by subtracting the total of volatile matter percentage and ash content percentage from 100. Each data point of every parameter considered here was estimated by taking the average of three replicate samples [16].
2.5.6 Biomass/ash ratio:
The biomass/ash ratio of two species was determined by dividing oven dry weights of the samples by ash weights [20].
2.5.7 Gross calorific values:
Gross calorific value of each sample was determined according to ASTM D 5865 standard procedures.
2.5.8 Fuelwood Value Index:
FVI was calculated for each individual sample using calorific value, wood density, ash content and moisture content and applying Eq. (1) [21]. In the FVI index, ash content and moisture contents were given in ratios (g g-1), density was given in g cm-3 and calorific value in kJ g-1.
Equation (1)
Calorific value data and density data of diesel and furnace oil were provided by the Ceylon Petroleum Corporation [22]. The number of liters of diesel and furnace oil that can be replaced by 1 kg of each fuelwood type was calculated by the authors.
2.6 Statistical analysis
The values for moisture content, density, specific gravity, ash content, volatile matter and fixed carbon content of each DBH class were subjected to one-way Analysis of Variance (ANOVA) using MINITAB® Version 14 statistical software after following the Anderson Darling Normality test. Percentage values of moisture content, ash content, volatile matter content and fixed carbon contents were subjected to arcsine transformation before doing Normality test. Tukey’s pair-wise comparison was carried out to test for significant differences between the three DBH classes for each measured parameter. In addition, comparisons of energy potential of P. juliflora and L. leucocephala were subjected to the same statistical tests.