Study Area and Sites
The study was conducted on the mangrove forest areas along the Carigara Bay in Leyte Island (Fig. 1). Mangrove forest ecosystems along the bay are represented by stands of fringe and riverine mangrove forests distributed among the five surrounding coastal municipalities (Capoocan, Carigara, Barugo, San Miguel, and Babatngon). However significant areas of mangrove forests along the bay have been lost because of land use conversion mainly due to aquaculture and settlements.
The climate of the study area is characterized as equatorial rainforest-fully humid (Kottek et al 2006). It has no dry season and has more or less evenly distributed rainfall throughout the year. The warmest month is April with a mean annual temperature of 27 ºC and pronounced wetness occurring in the months of November, December, and January with annual total precipitation of 2293 mm (Quiñones and Asio 2015, Marteleira 2019).
The sites considered in the present study included fringe and riverine mangrove forests located along the bay. The fringe mangroves considered were those mangrove forest stands bordering the beach/coastline of the bay. There were two stands of these mangrove forests sampled, one stand is in Barangay Mawodpawod, and the other one is in Barangay Malpag, both located in the municipality of San Miguel. The two stands are separated by a small stream and the sampling location from these stands was 500 m away from each other. The stands of fringe mangroves sampled were about 60–200 m wide from the landward to the seaward zone. The nearest community was about 500 m away, though minor disturbances could be observed in the mangrove areas such as the cutting of branches and harvesting of nipa leaves (Nypa fruticans) leaves and collection of other resources (e.g., mud crabs and edible mollusks).
Similarly, two riverine mangrove stands located near the estuary from two different rivers draining toward Carigara Bay were sampled. The first mangrove stand was in Barangay Bagacay of the municipality of San Miguel. The river is approximately 3.2 km in length and starts flowing from the western side of the Babatngon Range. The sampled mangrove stand was located 200 m from the mouth of the river and was adjacent to a highway. The other riverine mangrove stand was located in Barangay Minuhang of the municipality of Barugo. The mangrove stand was approximately 800 m from the estuary, with settlements on the opposite side of the river. The river is approximately 4.4 km in length and originates from hilly areas located in the southern direction of the river system. Typically, both mangrove stands are characterized by the presence of large-sized mangrove trees (> 100 cm DBH). Minor disturbances were observed from these mangrove stands such as the cutting of small branches or small mangrove trees, as well as harvesting of nipa leaves for making nipa shingles.
Plot Establishment and Sampling
Reconnaissance surveys were conducted first to identify mangrove stands to be sampled. The field data collection was conducted from the months of July to October 2022. The geographic location of each sampling station was determined using a handheld GPS (Model etrex). All the field samplings were carried out between July 2022 to February 2023.
Aboveground Carbon Stocks
Standing Trees
The method proposed by Kauffman et al. (2011) was used to sample live mangrove trees. The method employed the establishment of a 125 m-long transect line parallel to the coastline in each zone (landward, middleward, and seaward) in every site. The transect line in the landward zone was laid 15 m from the adjacent terrestrial forest, as well as transect line at the seaward zone was laid approximately 15 m from the ecotone. Along the transect line, a 7 m radius circular plot with an area of 154 m2 was demarcated at 25 m intervals. In the riverine mangroves, transect lines of the same length were laid at one side of the bank, parallel to the river. Similarly, the riverine mangrove stand was divided into three zones, the landward which is adjacent to the terrestrial forest or ecosystem, middleward or interior, and along the water that is close to the bank. The transect lines were also established at the same distance from the ecotones.
In each mangrove forest type (fringe and riverine), 36 circular plots were established, bringing the total number of plots to 72. All the standing trees with a DBH of ≥ 5 cm inside the plot were counted and measured for DBH and height. The height of the tree was visually determined using a 2-m long calibrated pole (Madeira et al. 2009, Decena et al. 2022). Each tree individual sampled was identified up to the species level using The Field Guide to Philippine Mangroves of Primavera (2009), and Handbook of Mangroves in the Philippines-Panay of Primavera et al. (2004).
To estimate the aboveground tree biomass (Wtop), the general allometric equation developed by Komiyama et al. (2005) for mangrove trees in Southeast Asia was used. The equation is as follows,
W top=0.251ρD2.46
where ρ is the wood density of mangrove tree species and D is the diameter-at-breast height (DBH). The wood density for each of the mangrove tree species was extracted from the wood density database of ICRAF (2023). The individual tree biomass values were computed using the above biomass allometric equation and were summed to give the total tree biomass stock. The biomass stock was then divided by the area sampled (154 m2) to give a value in kg m− 2. This value was converted into Mg ha− 1 by multiplying it by 10. Since the study area belongs to the tropical region, the tree biomass stock was converted to carbon stock density by multiplying with the default carbon value of 0.47 recommended by the Intergovernmental Panel on Climate Change (IPCC) (2006).
Palms
The biomass of the non-woody nipa palms (e.g., N. fruticans) was sampled either by non-destructive or destructive methods. The average mass of the palm leaves was determined by harvesting 22 palm leaves covering varying size distributions. Leaves were cut at ground level and transported to the laboratory for oven drying. However, for the very large nipa leaves (> 6 m in height), the total fresh weight of the whole leaf was determined in the field using a digital weighing scale. Then, only subsamples of two 300 g were taken each from leaflets and rachis for oven-drying. The fresh and oven-dry weights of the sub-samples were subsequently used for computing/adjusting the dry biomass of the whole leaf. Also, all the leaves inside the plot were counted. The mass of the nipa leaves was calculated by multiplying the total number of leaves with the average oven-dried/estimated mass of the leaves. Then, to convert leaf mass to carbon mass, a conversion factor of 0.47 was used (Kauffman et al. 1998).
Shrub Mangroves
The shrub mangroves (< 5 cm DBH) were sampled from the nested 2 m radius circular plots located at the center of the main plot. For each of the individuals, the main stem diameter (30 cm above the ground) and height were measured and later used to calculate aboveground biomass. To calculate the aboveground biomass of shrub mangroves, the allometric equation developed in Puerto Rico by Cintrón and Shaeffer-Novelli (1984) was used. The equation is,
Biomass (g) = 125.9571D302 x H(m)0.8557
where D30 is the diameter 30 cm above the ground and H is the height (m). The biomass of the individuals was computed and summed to give the total shrub biomass stock. The biomass stock was divided by the area sampled (12.57 m2) to arrive at a value in g m− 2, then, the value was converted to Mg ha− 1 by dividing it by 100. Lastly, the biomass stock was converted to carbon stock by multiplying it with the conversion factor of 0.47 (Kauffman et al. 1998).
Standing Dead Trees
Within each main plot, standing dead trees were also recorded. The decay status of each standing dead was further noted as decay status 1, decay status 2, and decay status 3 (Fourqurean et al. 2014). Decay status 1 is when the dead tree still retained small branches and twigs and resembles a live tree except for the absence of leaves. Decay status 2 characterizes the absence of twigs/small branches and may have lost a portion of large branches. In decay status 3, the dead tree has few or no branches, standing stem only, and the main stem may be broken-topped. The biomass of the dead tree (decay status 1) was estimated using the live tree estimations but was subtracted by a constant of 2.5% of the live tree biomass estimate. Likewise, the biomass of a dead tree with decay status 2 was estimated by subtracting 10–20% from the live tree biomass estimate. For dead trees with decay status 3, biomass estimation involves first the determination of volume using an equation for a frustum (truncated cone). To accomplish this, the diameter at the base, DBH, and tree height were determined. Eventually, biomass was derived by multiplying the volume by the wood density. Then the biomass was converted using a conversion factor of 0.50 (Kauffman et al. 1995).
Downed wood
To sample downed wood (≤ 7.6 cm diameter), the planar intercept method (Brown 1974, Waddell 2002) was employed by establishing four 12 m transects extending from the center of the circular main plot, oriented at 45o along the transect. The downed wood with a diameter ≤ 7.6 cm was tallied according to size classes along the subsections of the sampling plane: 0.6–2.5 cm, 3 m plane; 2.5–7.6 cm, 10 m plane. Whereas downed wood with a diameter of > 7.6 cm was measured in actual diameter (cm) along a 12 m sampling plane and further noted in terms of decay status whether sound or rotten. The smallest class of downed wood (0–0.6 cm) was rather collected in litter sampling (Kauffman and Donato 2012). Representative samples of each size class were collected and measured for their diameter. As a requirement for computing downed wood volume or carbon stock, quadratic mean diameters (QMD) (Brown and Roussopolous 1974) of the collected samples were determined using the following equation,
$$\text{Q}\text{M}\text{D}=\sqrt{\frac{\left({\Sigma }{\text{d}}_{\text{i}}^{2}\right)}{\text{n}}}$$
where di is the diameter of each sampled piece of wood in the size class and n is the total number of pieces sampled. Also, the wood specific gravity (g cm− 3) of the collected samples was determined through oven drying (105 oC) and then using the water displacement method. Now, the downed wood volume of all the size classes was determined using the equations of van Wagner (1968) and Brown (1971). For fine, small, and medium wood size classes, the equation is,
$$\text{V}\text{o}\text{l}\text{u}\text{m}\text{e} \left({\text{m}}^{3}{\text{h}\text{a}}^{-1}\right)={\pi }^{2}*\left(\frac{{\text{N}}_{\text{i}}*{\text{Q}\text{M}\text{D}\text{I}}_{\text{i}}^{2}}{8\text{*}\text{L}}\right)$$
where Ni is the count of intersecting woody debris pieces in size class i, QMDi is the quadratic mean diameter of size class i (cm) and L is the transect length (m). For the large (> 7.6 cm diameter) downed wood, the equation is,
$$\text{V}\text{o}\text{l}\text{u}\text{m}\text{e} \left({\text{m}}^{3}{\text{h}\text{a}}^{-1}\right)={\pi }^{2}\left(\frac{{\text{d}}_{1}^{2}+{\text{d}}_{2}^{2}+{\text{d}}_{3}^{2}+...+{\text{d}}_{\text{n}}^{2}}{8\text{*}\text{L}}\right)$$
where d1, d2, etc. is the diameters of intersecting pieces of large dead wood (cm), L is the length of the transect line for large size class (m). Then, the downed wood biomass was derived by multiplying the volume by wood density. Lastly, wood biomass is converted to carbon mass using an acceptable default value of 0.50 based on carbon concentrations of dead wood in tropical forests.
Litter
The litter layer was sampled using two microplots with a dimension of 0.50 m x 0.50 m laid 2 m away from the center of the plot. All the litter materials inside the microplot such as fallen leaves, fruits, flowers, seeds, bark fragments, and small woods (< 0.6 cm in diameter) were collected and placed in properly labeled ziplock bags. The samples were transported to the laboratory and oven-dried at a temperature of 80°C until the weights of the samples became constant. The oven-dried litter biomass was divided by the area sampled to get a value in g m− 2, then, the value was divided by 100 to get a value in Mg ha− 1. Finally, the litter biomass was multiplied with the recommended conversion factor of 0.45 (Kauffman and Donato 2012) to obtain carbon content.
Belowground Carbon Stocks
Tree Roots
To estimate the root biomass (WR) of mangrove trees, still, the allometric equation developed by Komiyama et al. (2005) was used. The equation is as follows,
WR=0.199ρ0.899D2.22
Similarly, to the computation of the aboveground biomass, the belowground biomass for each mangrove tree was computed using the above allometric equation and then summed up to give the total belowground biomass. The same extrapolations were also performed to derive a value in Mg ha− 1. Finally, to derive the belowground root carbon stock, the biomass stock was multiplied by a factor of 0.39 (Kauffman and Donato 2012).
Data Analysis
The Generalised Linear Models (GLMs) were performed to examine the influence of mangrove forest types (fringe and riverine) and zones (landward, middleward, and seaward/along water) on aboveground carbon stock components (standing trees, palm, shrubs, standing dead tree, litter, downed wood, and total aboveground), belowground (roots), and total carbon stocks. The GLMs analyses used gamma distribution with a log link function as the analysis involved continuous data. Post-hoc tests were performed whenever there were significant variations at α = 0.05, using pairwise comparisons. The statistical analysis was performed using SPSS 20 for Windows.