Forests play an important role in the ecosystem and in humanity’s survival. They support watersheds, provide habitat to millions of animals, control atmospheric temperature by a process called evapotranspiration, act as sink to atmospheric carbon dioxide, prevent floods, and provide timber and wood. However, due to clearing for human settlements, agriculture, logging, and other human activities the world’s forest cover has been disappearing with the rate of disappearance accelerating in the last 50 years (Brooks, Pimm, & Collar, 1997). Forest denudation can also be both a cause and effect of global warming. The clearing of forest introduces more greenhouse gases in the atmosphere. At the same time, less forest diminishes terrestrial land’s ability to absorb greenhouse gases. Climate change due to global warming also contributes to forest denudation introducing positive feedback to the greenhouse effect (Trenberth, et al., 2007; Meehl, et al., 2007; Christensen, et al., 2007).
Central Visayas has total forest cover of 62,065 ha out of the total land area of 459,270 ha, which is classified as alienable and disposable land and forestland (Forest Management Bureau, 2012). Based from the 2010 satellite images, there are more open forest areas (35,798 ha) as compared to the closed forest area (11,464 ha) as shown in Figure 5.1 (Forest Management Bureau, 2012). The classified forestlands covered 459,270 ha is further classified into 6 areas, namely: established timberland (390,151 ha), established forest reserves (49,619 ha), national parks, game refuge and bird sanctuaries and wilderness areas (15,054), military and naval reservations (4 ha), civil reservation (114 ha) and fishponds (4,328). The unclassified forestland of 65,638 ha comprised of alienable and disposable lands (964,169 ha). Moreover, there are 7 proclaimed watershed forest reserves in the region comprising 104,381 ha where Bohol (75,556 ha) has a bigger reserve as compared to Cebu (28,825 ha). The diversity of available indicators in the forestland suggests that conservation in the area shows sign of success and thus deserves greater attention (Posa, Diesmos, Soohi, & Brooks, 2008). A forest map derived from SPOT-vegetation satellite imagery shows the approximate percentages and distribution of forest cover remaining on the major islands in the Philippines (Stibig, Achard, & Fritz, 2004).
Global trend of forest denudation is also operating in the Philippine landscape (Kummer & Turner II, 1994; Lasco & Pulhin, 2003). Some researchers claimed that deforestation is most severe in Cebu than any other islands in the Philippines (SSC, 1988). In fact majority of the dipterocarp forest had been damaged (Colina & Jumalon, 1974) due to rampant practice of swidden agriculture (Cadiz and Buot. 2009). This perception has far reaching implications because a significant number of families in Region 7 depend on forestry and more than half of the families used wood as fuel (Lasco, et al., 2006). An opposite narrative has also been enunciated that, contrary to this perception, demand for woodfuel and other wood products is one of the main factors that drive reforestation efforts in the island province (Bensel T. G., 2008). Natural disasters such as the intense earthquake in Bohol (EMI 2014) also have great adverse impacts to the vegetative structure of the forest.
This section aims to evaluate the forest resources in Region 7 (Central Visayas), Philippines. As part of USC Phil LiDAR 2 project in this region, the study shall provide a reference for ongoing ecological research and conservation efforts in the forest of Central Visayas.
Secondary Data Collection and Initial Ocular Survey
Data on forest resources in Central Visayas, Philippines were secured from various government and non-government agencies in Cebu. Papers on forest resources in Central Visayas were also searched and retrieved from various databases such as library collections, electronic journals and from the internet. GIS maps were generated from the various literatures and data gathered in the field during the reconnaissance survey of the Mananga Watershed and Central Cebu Protected Landscape (CCPL) conducted by the USC Phil LiDAR Team on May 7, 2014.
Classification of Philippine Forest
There are various basis of classifying forest land in tropical countries. The forest type, degradation level (unlogged/little logging, logged and burned) and accessibility are basis for forest inventory (Kronseder, Ballhorn, Bohm, & Siegert, 2012). In the Philippines, forest lands are classified as open and closed (Forest Management Bureau, 2011) and unclassified or classified, where classified lands are sub classified farther into established for reserve, established timberland, national parks, military and naval reserves, civil reservation and fish pond (Forest Management Bureau, 2012). The open forest includes the old growth forest that had been logged with tree canopy cover of less than 40% (Forest Management Bureau, 2009). Different types of patches (Bagarinao, 2011) and fragments (Cadiz & Buot, 2009) also serve as basis in identifying forest land in Central Cebu Protected Landscape (CCPL) and Cantipla, Cebu, Philippines, respectively.
Forest resources in the Philippines are also divided into 4 major types, namely: timber, minor resources, recreation, wildlife and water resources, and range resources (Forest Management Bureau, 2011). Figure 5.1 shows land use classification portraying forest types in Central Visayas. It can be observed that only few areas in Central Visayas, in particular only in Negros Oriental, are considered closed natural forest. Most forest areas are secondary and plantation forests, which are broadly classified as open forest according to the classification scheme cited above. Figure 5.2 shows the protected landscapes (established for reserve) and seascapes in Central Visayas.
Forest vegetation studies are generally focused on the taxonomy of vascular plant species in Philippine rainforest (Hamann, Barbon, Curio, & Madulid, 1999; Bramley, 2013; Langenberger, Martin, & Saverborn, 2006). Hence, forest vegetation in Cebu is being surveyed, particularly in Lusaran, Balamban, Kawasan, Tabunan, Talamban, Mananga Watershed and in Bohol (see Figure 5.1 and Figure 5.2). Based from our ocular survey, forest vegetation in Cebu and in Bohol are distributed in patches and are mixed with agricultural crops and shrubs (see Figure 5.3 and Figure 5.4). Very few published studies relate the species diversity in Central Visayas forest with some physico-chemical factors, such as soil water-holding capacity (Cadiz & Buot, 2009).
There are various methods in determining biomass in a forest tree. Laumonier et al. (Laumonier, Eden, Kanninen, & Munandar, 2010) estimated the above-ground biomass (AGBM) of a tree using the formula AGBM = exp (-2.289 + 2.649 x In DBH-0.028 x In DBH2). Kronseder et al. (Kronseder, Ballhorn, Bohm, & Siegert, 2012) calculated AGBM from Chave et al. (2005) using the diameter breast height (DBH) and wood density or canopy height, leaf area index, leaf mass area, and chlorophyll content (Maire, et al., 2008). However, no studies were published on biomass in the forest of Central Visayas.
Carbon Stock Estimate
There are already studies conducted on carbon stock estimate using LiDAR (Stephens, Watt, Loubser, Haywood, & Kimberly, 2007; Asner, et al., 2012). Ecosystem carbon content can be derived from forest biomass (Mohren, Hasenauer, Kohl, & Nabuurs, 2012). However, such methods are mostly studied in temperate forests (Liski, Korotkov, Prins, Karjalainen, Victor, & Kauppi, 2003; Thurner, et al., 2014) but no such thing in the Philippines.
Detailed Field Survey of the Forests of Cebu
The reconnaissance field work conducted on May 7, 2014 in Central Cebu Protected Landscape (CCPL) gives a gross idea of the available forest resources in that area (see Figure 5.3 and Figure 5.4). Since CCPL and other forests in Cebu are vital component of the hydrologic cycle in the province and the fact that Metro Cebu is suffering from acute water supply, the USC Phil LiDAR 2 team decided to conduct a detailed field survey of the forests in Cebu. In addition, CCPL and some forested areas in the south are not covered by the LiDAR surveys because of bad weather during the data acquisition mission in Cebu. To prepare the FRExLS Component for this gargantuan task, Dr. Luisito Barbaran, Senior Forest Management Specialist of DENR 7 was requested to conduct training on forest resources assessment. Dr. Barbaran gave a lecture on forest assessment on June 20, 2014 and subsequently conducted hands-on training for our research associates.
MATERIALS AND METHODS
Field Data Collection
Succession classification of the type of forest was determined using the historical census from Department of Environment and Natural Resources (DENR 7).
Density, Frequency and Abundance
The important quantitative descriptions such as density, frequency, and abundance of the biotic components were determined according to Curtis & Mc Intosh (1950).
Density is an expression of the numerical strength of a species where the total number of individuals of each species in all the quadrats is divided by the total number of quadrats studied. Density is calculated by the equation:
This term refers to the degree of dispersion of individual species in an area and usually expressed in terms of percentage occurrence. It was studied by sampling the study area at several places at random and recorded the name of the species that occurred in each sampling units. It is calculated by the equation:
It is the study of the number of individuals of different species in the community per unit area. By quadrats method, samplings are made at random at several places and the number of individuals of each species was summed up for all the quadrats divided by the total number of quadrats in which the species occurred. It is represented by the equation:
Importance Value Index
This index determines the overall importance of each species in the community structure. In calculating this index, the percentage values of the relative frequency, relative density and relative dominance are summed up together and this value is designated as the Importance Value Index or IVI of the species (Curtis 1959). This index represents the contribution of a particular species to the biomass and productivity of the given area.
Species richness, diversity and dominance indices
The species richness of the vascular plants will be calculated by using Margalef’s index of richness (Dmg) (Magurran 1988).
Species diversity and dominance will be evaluated by using the following indices. Shannon’s diversity index and Simpson’s index of dominance
Estimation of the Carbon Biomass
Trunk diameters of every tree that belongs to the 20×20 quadrats were measured. Trunk diameter were measured 1.30 m (diameter at breast height) aboveground for trees that have no buttress roots. For trees having buttress roots, measurement was at 0.30 meter from the end of the buttress roots (Djomo et al. 2011). Trees were grouped in diameter classes with width interval of 10cm. The first class (1) consists of trees with diameter at breast height (dbh) between 5cm and 10 cm. Class 2 consists of tress with dbh from 10cm to 20 cm. The succeeding class numbers were assigned accordingly based on the 10-cm interval. The last class (11) consists of trees with dbh of more than 100 cm if the maximum tree DBH in the area is 100 cm.
Aboveground biomass (AGB) of each tree stem was estimated using the allometric equation for moist tropical forest from Chave et al. (2005):
Root biomass was estimated with Eq. (2) (Cairns et al., 1997).
Where ln (AGB) is the natural logarithm of above-ground biomass.
A factor of 0.45 was used to convert above-ground and root biomass to carbon (Brown and Lugo, 1982; Brown, 2002; this paper). The estimation to root biomass from DBH will be taken from Niiyama et al. (2010) for primary forest (R2 = 0.98) and Kenzo et al. (2009) for secondary forest (R2 = 0.94). Calculations confidence intervals for both AGB and coarse root biomass will use 1000 bootstraps over 20 x 20 m quadrats, to provide information on uncertainty related to spatial variation in biomass within the study area (Ngo et al., 2013).
Fallen woody debris was measured along 4 lines of 25 m length, applying the line-intersect methodology (Brown and Roussopoulos, 1974; Valentine et al., 2008) and recorded in 4 separate decomposition classes: fresh with leaves attached, without leaves and bark firmly attached to branch, bark falls off after hitting and branch falls apart after hitting. Dead woody debris in this study comprises the sum of dead standing trees and fallen woody debris. Samples will be collected in each class of fallen woody debris to determine relative density and C-content in the laboratory.
Woody debris volume per area will be calculated from transect data using V=Л2/8L X Σd2i, where V is the volume per area (m3/m2), L is the total length of the transect (m) and d the diameter (m) of the piece of woody debris encountered (Ngo et al., 2013). Total mass of woody debris was calculated using M= Л/2L x Σ Ci where M is total mass per area (kg m-2), L is total transect length and C is the cross-section mass (kg m-1), i.e., dry mass per unit length of the fallen log (Larjavaara and Muller-Landau, 2011). Mean woody debris values will be calculated by treating 20 m sections of all transects as replicates. Confidence intervals will be calculated by 1000 bootstraps over 20 m sections. Soil carbon and fine root biomass per unit volume, and per unit ground area, will be calculated for each sample using bulk density values. To convert aboveground biomass, coarse root, fine root, and coarse woody debris dry mass values to carbon stocks, we assumed that 50% of the dry mass was carbon (Kenzo et al., 2010; Ngo et al., 2013).
Non-tree biomass was harvested randomly in the 20 x 20 quadrat. Fresh weight will be determined in the field and dry weight in the laboratory after drying for 72 h at 70 ◦C. Carbon content of leaves, fruits, flowers, and branches will be measured in the laboratory according to standard procedures (IPCC, 2003). Litter will be collected in 4 circles of 0.07 m2 each, separated in three layers: freshly fallen, dry litter and humus. All litter material will be oven dried for 72 h at 70 ◦C. C-content will be determined in the laboratory of samples of the collected material.
For soil sampling, soil samples was collected in each quadrat randomly at depth layers of 0–15cm and 15–30 cm, systematically 25m from the centre of the plot (Usuga et al., 2010; Djomo et al., 2011). To evaluate bulk density and soil organic carbon, three soil samples will be collected at each layer (0–15cm and 15–30 cm) with a cylinder of 5 cm of diameter and 5 cm of length. Soil samples taken from same areas were mixed and air dried for a week. After air-drying, the soil samples will be pulverized with a wooden gavel and will be sieved using a 2mm metal screen mesh sieve before actual testing.
Total SOC was determined by wet digestion with 133 mmol L-1 K2Cr2O7 and concentrated H2SO4 at 170– 180 °C. Total N (TN) will be measured using a semi-micro-Kjeldahl method. Soil bulk density will be calculated after determining the soil water content of the bulk density samples. The water content of each sample will be used to calculate the bulk density based on the volume and total oven-dry weight of the soil within each soil corer.
The particle-size distribution of soil samples was determined by wet-sieving and sedimentation using the pipette method after the pre-treatment of the sample with H2O2 (15%) and Na4P2O7 (0.1 mol L-1). All of the above-mentioned methods are described in Lu (2000).
The SOC storage in each soil layer was calculated according to the following formula (Guo and Gifford, 2002):
where SOC is the soil organic C concentration (g kg_1), BD is the bulk density of the soil layer (Mg m-3), and th is the thickness of the soil layer (cm).
Tabunan Forest, Central Cebu
Tabunan is an ecologically viable remnant secondary forest of Central Cebu. Forest cover has a total area of about 144 ha and some of which was classified into closed-canopy forest and restoration area. Maximum elevation will reach to about 800m from the sea level with varying slopes of no less than 300. It contains the critically endangered plant and bird species which is endemic to Cebu such as the Cebu Black shama (Copsychus cebuensis), Cebu Flowerpecker (Dicaeum quadricolor), Cebu cinnamon (Cinnamomum cebuense), Polycias cenabrei and Cynometra cebuense. Tabunan forest is also a vital watershed area in inland Cebu and has been the subject for several conservation studies. It exhibits the entire range of human-nature conflict scenarios as depicted by deforestation in selective areas. A summary of its generalized attributes is presented in Table 5 1. The ten tree species with the highest importance values (IV) are presented in Table 5 2. It is interesting to note that an introduced invasive species, Acacia mangium, is one of the tree species that contributes greatly to the biomass and productivity in the total area. This could be an indication of invasive species starting to change the forest ecosystem of Tabunan. Figure 5.5 to Figure 5.8 show the distribution of the forest attributes per quadrat sampled.
Asturias is located in the northwestern part of Cebu and is approximately 81km from Cebu City. It has a total land area of 12,173 ha, 9,000 ha of which are classified as disposable and alienable land, 15 ha are forest reserve/timberland area and the remaining 175 ha remain unclassified. Topography is generally categorized into rolling and ridges and the municipality experienced the Type IV climate. Soil types were Hydrosol, Baguio Clay Loam, Faraon Clay.
Forest patches are situated in midnorth area specifically to Barangay Baye, Lunas, Tagbobonga, Tag-amakan and Bairan. Forest covers come into patches in area that is hardly convertible to agriculture because of large stone boulders. Forest area in Barangay Bairan is characterized as secondary forest with trees growing up to 30m. Copsychus cebuensis, Intsia bijuga and Cynometra cebuense are valuable and critically endangered species observed in the area.
Attributes of the sampled forest areas in Asturias are presented in Table 5 3 with a list of tree species with high IV in Table 5 4. Apparently, even with some disturbance, the main contributors to the productivity in the area are all native trees. Figure 5.9 and Figure 5.10 portray the distribution of the attributes among the seven quadrats sampled. Quadrat 7 has the greatest tree biomass due to the several large Ficus benjamina trees (Dakit). These trees were not cut down or disturbed in any way due to the superstitious belief of the people.
Borbon is located 82.7 kilometers on the northeastern side of Cebu. It has a total land area of 8,111 ha., 3,820 ha. of which is categorized as unclassified forest/wilderness. However, the municipality’s area for the alienable and disposable land and classified forest/reservation/timberland are not yet determined. Topography of the area is generally flat and slightly rolling. Climate is both wet and dry. Its soil type is Faraon clay, Bolinao clay, and Lugo clay. Forest cover extends from barangay Bongdo Gua, Bongdo Proper, up to barangay Kaorasan in Tuburan. Forest cover comes in patches and has an open canopy forest. Bird species like brown shrikes, Lanius cristatus, were seen in the area.
Table 5 5 presents the summary of the forest features in the areas that were studied. Species richness is relatively high for the area and that in turn also gives a relatively high species diversity. The forest appears to be heterogeneous as evidenced by similarity indices that correspond to almost none to very low similarity. The ten main contributors to the productivity of the area are all indigenous trees as presented in Table 5 6. Figure 5.11 and Figure 5.12 show the varying values of the forest features measured and computed for which again leads to the heterogeneity of the area.
Carmen lies at the Northeastern side of Cebu Province and is approximately 38.6 kilometers away from Cebu City. It is bounded by the Municipality of Catmon on the North and the City of Danao on the South, and the Municipality of Tuburan in the West. Its land area covers a total of 6,640 ha which 2,500 ha of is certified as alienable and disposable land; however its forest cover/timberland has not yet been determined.
The topography of the municipality is characterized by coastal flood plains with 0-35 in slope extending one to three kilometres inland. It has slightly rolling hills with interspread flat lands with slopes 3-155 rise parallel to the plains and its highland mountainous ridges slopes 18-505 and above. Carmen experiences a Type III climate. Its soil type is Hydrosol Mandaue clay, loam Baguio clay loam Faraon clay loam Faraon clay steep phase. Carmen has a rich source of water which is why commercial water companies have factories in the area. Banana plantation, ecoparks, zoos, and a religious monastery are some of the developments that can be found in municipality nearing forest areas.
Carmen has the smallest number of species in all the areas studied which lead to a relatively low species richness and diversity presented in Table 5 7 and Figure 5.13 The total number of trees, number of species, species richness, and biomass in the forest of Carmen, Northern Cebu.. One introduced, invasive species, Gliricidia sepium, contributes greatly to the productivity of the area as presented in Table 5 8. Most trees are relatively small as presented in Figure 5.14.
Sogod is located in the Northeastern side of Cebu Province. It is about 60.5 kilometers from Cebu City and is bordered on the North by Borbon. It has a total land area of approximately 7,833 ha; approximately 51.36 ha of which is classified as forest feserve/timberland, approximately 68.48 ha is categorized under wildlife and military, and approximately 198.02 ha is categorized under unclassified forest Land. The terrain is generally rolling and becoming hilly up to the mountain side.
Some of the areas of the mountain barangays are rocky, eroded and with too poor agricultural cultivation. Sogod has a tropical rainforest climate with no marked long dry season to occur in a year. The municipality experiences heavy rains during the months of May, June, and October reaching to a level of approx. 300mm, while the minimal levels occur in the months of December to January. It has a soil type of Faraon clay Faraon clay steep phase Lugo clay, and Baguio clay.
Forest patches cover barangay Pansoy where military detachments have been in place for a long time. This might be one of the reasons why this area has a relatively high species richness and diversity as presented in Table 5 9. Native trees dominate the area with Ficus species as the main contributors to the productivity of the area as shown in Table 5 10. Some areas have a lot of trees but of small stature while other areas have fewer but larger trees giving a less heterogenous distribution as evidenced in the similarity indices among quadrats and as shown in Figure 5.15 and Figure 5.16.
Tabuelan is located in the Northwestern side of Cebu Island. Tabuelan is still on the process of demarking their political boundaries with the neighboring municipalities. Almost half of the land area of Tabuelan was already declared a timberland area. However, these forest reserves were claimed and occupied by few claimants and were converted to agricultural areas. These scenario leads to the problem of zoning and delineating boundaries which is still on the process.
Forest cover in Tabuelan is an open canopy forest. Though the area has more species compared to Carmen, the number of individuals in the area are greater which lead to a higher species richness and diversity index value as shown in Table 5 11 and Figure 5.17. Most of these species are Neonauclea calycina and Oncosperma horridum as presented in Table 5 12. Alstonia macrophylla dominated Quadrat 7 in the area. However, with trees growing mostly up to 5 meters only (Figure 5.18), biomass is relatively small (Table 5 11 and Figure 5.17).
Tuburan is located in the northwestern part of Cebu Island and is approximately 97km from Cebu City. It has a total land area of 24,648 ha and has no prescribe classification on alienable and disposable land and timberland area because the comprehensive land use plan (CLUP) is still on the process. Flat land is approximately 2,185 ha or 7.45%; nearly rolling land is approximately 11,522 ha or 39.30%. Very highly rolling lands is approximately 15,609 ha or 53.24% and is located at the midnorth part near the boundary of Danao City and Municipality of Asturias. Typical climate of the area is Type III and soil classifications are Bolinao clay, Mandaue Salt loam, Hydrosol Faraon clay Bolinao clay steep phase, Faraon clay steep phase. Stones and gravels are apparent in the understory of the forest.
Forest cover comes in patches which is located in Barangay Can-an, Kaorasan and Marmol. Species richness and diversity are moderate relative to the other sites shown in Table 5 13 and most of the dominant trees are native ones even in the more disturbed forest in Brgy. Can-an shown in Table 5 14. This difference in terms of level of disturbance is clearly shown in Figure 5.19 and Figure 5.20 as Quadrat 1 to 5 are located in Brgy. Can-an while the last two are in Brgy. Marmol with tree heights reaching up to 15 m that makes up the open canopy cover of the area. Forest conservation was carried out based on the anecdotal myth of “Maria Tang-an” in Barangay Marmol.
Alcoy is located approximately 93km from Cebu City. It has a land area of approximately 4,981 ha with 4, 219 ha forest reserve/timberland area. Most of the forest cover is located in Barangay Nug-as which has a forest area of about 600 ha. Terrain is generally rugged with extensive plain only in some part of Nug-as and Sitio Datag, Dannglungsod. Highest elevation is approximately 800m from the sea level. Due to generally steep slope, about 78% of the municipality is flood-prone area. Soil type is generally Faraon clay with tropical rain forest type of climate.
Forest cover can be classified as open-canopy forest with densely growing native tree species but height of no taller than 20m. Nug-as forest is a home of Cebu endemic bird species, the Cebu Black Shama (Copsychus cebuensis) and Cebu Flowerpecker (Dicaeum quadricolor) (LGU, CLUP; LGU, MENRO).
As the second largest remaining forest patch of Cebu, Nug-as Forest boast a high species richness and diversity, as well as in its other attributes (Table 5 15). Although most of the dominant tree species are native, Sweitenia macrophylla, an economically important but introduced species, is showing signs of invasiveness as evidenced in Table 5 16. This tree is starting to dominate in Quadrat 7 as shown in Figure 5.21 and Figure 5.22 where there are only very few trees but with large DBH and height. Even though the diversity is high, the biomass is low which indicates a recovering forest.
Barili is located in the southwestern part of Cebu and is about 61km from Cebu City. Alienable and disposable land has a total area of approximately 12,262 ha with only 3,000 ha as forest reserve/timberland. Topography is from undulating to rolling with a type III climate. Soil classifications were Faraon Clay, Faraon Clay steep phase, Lugo Clay, Mandaue Clay, and Beach Sand. Forest covers are in patches and are situated in areas that are hardly convertible to agriculture. It has a remarkable forest fragment that serves a watershed umbrella in Mantayupan Falls, a valuable tourist destination of the municipality (LGU,CLUP).
The forest fragments in Barili are found in two separate barangays namely Brgy. Campangga and Brgy. Mayana. This explains the low similarity index value between the two areas but high similarity among the quadrats within a barangay (Table 5 17). The forest fragment in Brgy. Campangga adjacent to the Mantayupan falls is actually a reforested area which also explains why Sweitenia macrophylla is the dominant tree species as presented in Table 5 18. In Figure 5.23, Quadrat 1 to 4 represents the reforestation area showing high biomass whereas the Quadrat 5 t o7 represents the disturbed forest thereby the greater number of trees but low biomass. Figure 5.24 also shows the same trend in the mean DBH and heights.
Carcar is located approximately 40 kilometres south of Cebu City. It is bounded on the north by San Fernando; on the south by Sibonga; on the west by Barili and Aloguinsan; and on the east by the Cebu Strait. It has a land area of 11,678 hectares. The land is generally level with less than 18% slope composing 78.7% of the total land area. Areas with slopes ranging from 18 to 50% cover 19.3% of the total land area and those over 50% slopes comprise approximately 1.9%. The highest recorded elevation is a little over 660 metres above sea level, located within the barangay of Napo. The wet season occurs during the months of May to October and the dry season, from January to May. Natural forest cover is located in barangay Napo with trees growing no more than 20m. Remarkable anecdotal evidence documented is the presence of the Cebu Flowerpecker (Dicaeum quadricolor) (LGU, CLUP).
Apart from Tabunan, this is the only forest within a city but unlike Tabunan, it only has a moderate species richness and diversity (Table 5 19). Most of the trees with high IV are actually fuelwood and charcoal species (Table 5 20). Even though the quadrats were close, they are more heterogeneous in terms of the number of species, individuals and biomass as shown in Figure 5.25 but more homogeneous in terms of DBH and height as shown in Figure 5.26 showing varying levels of disturbance.
Dalaguete is located in the southeastern part of Cebu Island and is approximately 85km from Cebu City. It has a total land area of about 13, 500 ha, 4,275 of which is alienable and disposable land and the remaining 8,775 ha is considered as forest reserved/timberland area. Topography of the area is typically undulating with several sloppy mountain ranges. The highest peak of Cebu island is in Mantalongon, Dalaguete which is approximately 1000m from the sea level. Forest reserve and protected areas are situated in lowly elevated area in between ranges. Remarkably, critically protected area is Barangay Malones, where flying foxes and variety of fruit bat species are located. Tall trees usually thrive in cleft and in riparian zone of Mantalongon river. Forest patches are a home of protected bats, one of which is considered as the largest in the world and also a refuge of cebu Black Shama and Cebu Flowerpecker. Valuable tree species observed in the area was Ipil (Intsia bijuga).
Dalaguete boasts of three high peaks with dense vegetation (FLUP). Although its species richness and diversity are only moderate (Table 5 21), it holds the highest amount of biomass, second only to Tabunan Forest. “Matamsi” and “Ipil”, Instia bijuga, are two native trees that contribute mostly to the productivity and biomass in the area. Figure 5.27 shows that varying degrees of number of tree species, number of individuals and biomass that affects the heterogeneity of the area with special emphasis on the biomass. Although the mean DBH and heights are mostly below 20 and 15 respectively (Figure 5.28), there are some individuals that have very large DBH and heights contributing to the high biomass in the area.
Dumanjug is located in the Southwestern part of Cebu Island and is about 73km from Cebu City. Most of the areas were classified into alienable and disposable land with only the barangay of Matalao and Manlapay as timberland area. Forest patches are situated in areas that are hardly convertible to agriculture. Forest cover was characterized as open-canopy with trees growing of no more than 20m. Soil is derived from the weathered limestone and some of which are apparent in highly elevated areas. Some important tree species that can be seen in the area are the naturally growing Molave (Vitex parviflora) and Tindalo (Afzelia rhomboidea) (LGU, CLUP, 1983).
Although Dumanjug has a very high biomass, all other aspects of its forest attributes are just moderate (Table 5 23). The dominant species is a palm, Oncosperma horridum (Table 5 24). The major contributor to the biomass as shown in Figure 5.29 in Quadrats 1 and 2 are the giant “Gumihan” and “Bogo” trees that are found in the area along with other large tree species. This is further evidenced by the high maximum DBH and heights presented in Figure 5.30.
Summary and Conclusion
Table 5 25 presents the summary of all forest attributes of all the areas covered with emphasis on the top three highest values after Tabunan Forest. Tabunan Forest has the most number of tree species, number of individual trees and total tree biomass since this site has the greatest area covered. Tabunan also remains to be the highest in terms of species richness and diversity though it is safe to compare this to the other sites since these indices already incorporate species richness, evenness and dominance per unit area as shown in Table 5 1. Furthermore, two areas in Northern Cebu namely Borbon and Sogod, have higher species richness and diversity than the other sites. Only Dalaguete in Southern Cebu showed relatively higher values of the attributes.
Table 5 25 Over-all summary of the forest attributes of all sampling sites. Bold items indicate the next highest values after Tabunan.
South Cebu generally has a greater number of individual trees compared to North Cebu. This may be due to intense logging for fuelwood and charcoal production. Tabuelan and Tuburan were two of the top municipalities in terms of fuelwood and charcoal production in Cebu (Bensel & Remedio, 1993). However, Asturias has a relatively high amount of tree biomass since the forests there have private claimants that control the amount of disturbance and logging. That led to fewer but larger trees therefore the higher tree biomass.
Among the different areas, Tabunan and Tuburan are the only areas with sampled quadrats having a Similarity Index of 0. This means that at least two of these quadrats within these areas are completely dissimilar to each other. For Tabunan, this indicates the diversity of forest types within the entire forest. However, this is not the case for Tuburan. One quadrat is composed almost exclusively of Alstonia macrophylla (Matamban) and presents a natural monoculture of trees. When this quadrat is compared to another with no A. macrophylla, this would give a zero to very low similarity index.
Table 5 26 presents the over-all similarity of the sites to one another. Neighboring municipalities are expected to be more similar to each other. However, this does not seem to be the case. Dumanjug and Dalaguete are the most similar with a similarity index of 0.707 followed by Tuburan and Dalaguete and Borbon and Tuburan. The first pair of municipalities is on the western and eastern side of South Cebu and the third pair is on the western and eastern side of North Cebu but both pairs are not adjacent to each other. The second pair shows municipalities on the North and South side of Cebu. Tabunan apparently is the most dissimilar to the other sites with 3 out of the top five lowest similarity indices.