Carbon Storage of Vegetation in the Different Land Uses of Mt. Musuan in Bukidnon, Philippines

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*Corresponding author: [email protected]

Carbon Storage of Vegetation in the Different Land Uses of Mt. Musuan in Bukidnon, Philippines

Joseph C. Paquit¹*, Victor B. Amoroso², and Kleia Deinisa Polinar¹

1Department of Forest Biological Science, College of Forestry and Environmental Science, Central Mindanao University,

Maramag, Bukidnon 8710 Philippines

2Department of Biology, College of Arts and Sciences, Central Mindanao University, Maramag, Bukidnon 8710 Philippines

Mt. Musuan is a mountain ecosystem that is valued for its biodiversity and ecotourism. In order to further enhance the biodiversity and ecotourism activities, the current status of its land uses and the carbon storage of its vegetation was assessed. Findings have shown four major ecosystems in the area – namely, natural forest, plantation forests, grass-shrubland, and agro-ecosystem – which respectively cover 16% (66 ha), 24% (103 ha), 39% (164 ha), and 20% (85 ha) of the mountain’s 425-ha total land area. Trails and physical facilities occupy the remaining 1%. Fifteen (15) land uses were delineated, wherein eight are forest plantations. The largest area was the G. arborea plantation that comprised 37% (38.23 ha), whereas the least was the P. caribaea plantation that only covers 0.7% (0.73 ha). The summit has an area of about 1800 m², from which around 1000 m² can be used by visitors for sightseeing and camping. The designated viewing area at the very top is much smaller and is estimated only at around 100 m². In terms of vegetation carbon storage, the natural forest stores an estimated 208 Mg C ha^–1. For the entire Mt. Musuan, an estimated 25,522 Mg C is stored, 47% (13,657 Mg C) of which is in the natural forest. The data and findings in this study will have potential use in planning.

Based on the spatial pattern of the land uses, planners and implementers will be guided on where to put up development projects and investments related to biodiversity and ecotourism development. Effective management of the natural forest to protect its remaining biodiversity and carbon should be done. Accelerated rehabilitation of the grass-shrubland ecosystem should also be done to further enhance the biodiversity and ecotourism potential of the area, as well as its capacity to store carbon.

Keywords: biodiversity and ecotourism development, land use/ land cover, Mt. Musuan, vegetation carbon

INTRODUCTION

In 2021, the global average carbon dioxide (CO2) was at a new record high of 414.72 ppm (NOAA 2021).

Atmospheric CO2 concentration has continually been on an upward trend and the annual increases

in recent years were also the largest. This is because emissions from various sources are higher than the rate of sequestration and storage in natural systems. Terrestrial ecosystems have the highest atmospheric carbon rate sequestration and storage. Carbon accumulates in vegetation and soil and, thus, can be taken advantage of to reduce atmospheric CO2. However, anthropogenic ISSN 0031 - 7683

Date Received: 12 Dec 2022

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land use activities have disrupted terrestrial ecosystems.

Urbanization and farming have converted previously forested landscapes into other land uses and subsequently altered CO2 exchange between the biosphere and the atmosphere.

Forests are a key component of terrestrial ecosystems.

Forests, especially in the tropics, are rich in biodiversity and are important carbon sinks (Kothandaraman et al.

2020). They hold great stores of carbon and are the most persistent carbon sink compared to other forest biomes (Pan et al. 2011). Yet decades of deforestation have reduced most tropical forests into fragments. These fragments are now patches of once intact forests, which are surrounded by other ecosystems such as forest plantations, grasslands, and agro-ecosystems. This scenario could be observed even in established conservation sites such as protected areas. In most watersheds and mountain ecosystems, some forest patches still exist. However, these patches which are products of fragmentation are more disturbed because of environmental changes brought by the edge effect (Campbell et al. 2017). Because of its small area, the biodiversity of small forests can be highly fragile.

Conserving small forest patches when done collectively can make a difference. Protecting forests also means protecting biodiversity and preserving carbon. In Mt. Musuan for instance, two natural forest patches are present, and the surrounding land uses undergo continued rehabilitation.

As neighboring places undergo heavy infrastructure development, the value of Mt. Musuan for biodiversity conservation and carbon storage will be greater than ever.

Rehabilitation in some parts of Mt. Musuan continues and despite a very slow pace, some areas that were previously dominated by weeds and shrubs are now covered with trees.

In order to track changes, it is necessary to monitor land uses, biodiversity, and carbon storage. This ensures that the rehabilitation efforts are headed in the right direction. This study focuses only on determining the carbon storage of vegetation in the different land uses in Mt. Musuan. Carbon storage in soil and litter was not covered in this study. GIS maps of land uses and carbon storage were also generated using data from the GPS survey and vegetation assessment.

With this, the study provides valuable information about the importance of Mt. Musuan for climate change mitigation and biodiversity conservation, as well as insights into management and planning.

MATERIALS AND METHODS

Study Area

Mt. Musuan is a 425-ha small mountain ecosystem located in Mindanao, Philippines (Figure 1). It is part of the 3,080.80-ha landholding of Central Mindanao University

(CMU). The base is 3 km wide and stands at a summit elevation of 646 m above sea level (masl). Geologically, it is classified as an active volcano. The ecosystems in Mt. Musuan fall into three general types; lowland forest, grass-shrubland, and agro-ecosystem (Marin and Casas 2017). It has become an important area of interest for biodiversity research and conservation because it harbors a rich assemblage of flora and fauna. It is especially rich in trees and the endemism of plants is high at 24% (Amoroso et al. 2002).

Data Collection

Land use/ land cover analysis. The study employed the use of an actual GPS survey in the field to gather data about the biophysical resources in Mt. Musuan. Boundary coordinates of ecosystems and land uses were obtained using a Garmin etrex30 handheld GNSS device. Garmin etrex30 uses both GPS and GLONASS satellites for better positioning and accuracy. It can achieve an accuracy rate of 3 m and even up to 2 m in the open (Weih et al. 2009).

A GPS survey was necessary for the thorough delineation of the land use boundaries. The GPS points were processed in ArcGIS to generate land use polygons and maps. The different land uses were named precisely as they were observed in the field. The area of each land use was also computed. Locations of other biophysical resources and features were also geotagged for inclusion in the mapping.

Vegetation biomass and carbon storage analysis. Prior to sampling, the approximate location of the various 50-m transect lines to be established in each of the 10 land uses have been plotted on Google Earth. The location served as a reference point in establishing the transect line. A 50-m transect line was then laid out inside each ecosystem type except for the agro-ecosystem. For the sampling, three 10 x 10-m plots were set up along the center and at the two opposite ends of the 50-m transect line. A total of 30 sampling plots were used to sample the natural forest, forest plantations, and grass-shrubland ecosystem. There are land uses that have substantially smaller areas and steep terrain. Setting up smaller three 10 x 10-m plots, similar to Ponce-Hernandez and Koohafkan (2004), was found more manageable and practical. For consistency, the same sampling intensity was applied to other land uses. The three plots inside the natural forest were actually subplots of a strategically located 2-ha long-term ecological research (LTER) site from a CHED- funded project. The use of three uniformly sized plots in each of the 10 land uses was aimed at having consistency so that results are easily compared. All trees and shrubs with diameter at breast height (DBH) greater than 5 cm inside the 10 x 10 m plot were measured. A 1 x 1 m subplot was also established inside the 10 x 10 m plots to sample the biomass of herbaceous species and grasses (Ponce- Hernandez and Koohafkan 2004).

Philippine Journal of Science

Vol. 152 No. 3, June 2023 Paquit et al.: Carbon Storage of Vegetation

on Mt. Musuan, Bukidnon, Philippines

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Figure 1. Location map of study and sampling plots: map of Mindanao Island; [B] map of Mt. Musuan.

Data Processing and Analysis

All the acquired geodata were processed and analyzed in ArcGIS. Data from three sampling plots in each land use was summarized. C density was computed and multiplied by the total area of each land use. With input data from processed boundary coordinates obtained from the GPS survey, land use/ land cover polygons were digitized.

Minor positioning errors which created gaps and overlaps in polygons were manually corrected using Google Earth as a reference map. Topological errors must be fixed to ensure that the areas of each land use are precisely computed. Lay outing and generation of maps were done using the same software. The area of each land use type was also computed, which served as input for the carbon mapping. The space at the summit was also computed to obtain the number of visitors it can support for viewing and camping. Tree biomass was analyzed using the updated Brown’s allometric equation (Pearson et al.

2005) as presented below. This is similar to the original equation and had been used in nearby sites already (Paquit and Bulasa 2021). This allows for easy comparison of results from similar studies. Carbon content in biomass was measured using a default value of 45%, which is the average carbon content from tree biomass from natural forests (Lasco and Pulhin 2000).

Aboveground biomass (kg) = Exp(–2.289

+ 2.649 x ln(DBH) – 0.021 x (ln(DBH))))² (1) Belowground biomass (kg) = Exp(–1.0587

+ 0.8836 x ln(ABD)) (2) Carbon content = biomass in kg x 0.45 (3) Biomass of small plants including herbaceous species and grasses were taken from the 1 x 1-m subplots. Oven-drying was done followed by weighing to obtain the biomass.

The carbon content was estimated using default values relative to oven-dried biomass. The values were then expressed as density values and added to the aboveground biomass density as applicable for each land use type. Plot level biomass and carbon density values were evaluated to determine whether or not appropriate generalizations can be applied to the entire land use. The difference in plot-level values for the plantation forests is smaller than for the natural forest. This is because the plantations are even-aged. However, the natural forest is a small patch and a relatively homogenous formation. Effects of climate and elevation to ecosystem productivity were assumed to be constant, hence allowing for generalizations to be made.

The carbon values were joined with the land use data in ArcGIS to generate carbon maps.

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RESULTS AND DISCUSSION

Land Use/ Land Cover of Mt. Musuan

Based on the results, there are four major ecosystems in the area, namely; natural forest, plantation forests, grass-shrubland, and agro-ecosystem (Figure 2), which respectively cover 16% (66 ha), 24% (103 ha), 39% (164 ha), and 20% (85 ha) of mountain’s 425-ha total land area (Figure 3). Fifteen (15) land uses were delineated, wherein eight are forest plantations.

The largest area was the Gmelina arborea plantation comprising 37% (38.23 ha), whereas the least was the Pinus caribaea plantation only covering 0.7% (0.73 ha).

The natural forest carpets most of the northern portion of the mountain. A smaller natural forest patch, which was previously connected with the Taganibong Forest also stands on the western side. In terms of land area, the largest ecosystem in Mt. Musuan is grass-shrubland ecosystem (Figure 3). Moreover, the summit of Mt. Musuan has an area of about 1800 m², from which only around 1000 m² is open space. The designated viewing area that is facing northeast is much smaller, which is estimated only at around 100 m².

Most of the southern part is blanketed with grass, predominantly Imperata cylindrica, an invasive weed species (Kato-Noguchi 2022). The grassland is interspersed with various species of trees and shrubs, which are seen to thrive mostly along ephemeral gullies and in areas adjacent to forest ecosystems. Along its central portion exists an 85-ha sugarcane plantation.

Based on the record, the grassland and sugarcane areas were previously used for livestock grazing and farming because the topography and soil in this portion are suitable for agriculture. However, only rainfed crops are ideal since the area does not have any natural water source and water only drains on ephemeral gullies during rainfall events. Biological and physical resources are also found in Mt. Musuan. Beside the entrance gate lies the Mt.

Musuan Botanical and Zoological Garden (MMBZG).

The management of the natural forest is placed under the MMBZG which is under the bigger umbrella of the Center for Biodiversity Research and Extension in Mindanao (CEBREM), an institutional research center of CMU. CEBREM implements various research on biodiversity and ecotourism most notably the LTER, which established permanent plots inside the natural forest to timely monitor dynamics in biodiversity through space and time (Mohagan et al. 2018). On the summit stands a

Figure 2. Detailed land use/ land cover and resources map of Mt. Musuan.

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few telecommunication towers. The summit has an area that can be used by visitors for sightseeing and camping.

With an elevation of 646 masl, the beautiful view of the surrounding landscape can be seen. There are also other viewing points at the lower portion of the peak.

Carbon Storage of Vegetation in Mt. Musuan

The total vegetation C stock of the entire Mt. Musuan is estimated at 25,522 Mg. 54% (13,657 Mg C), of which is in the natural forest (Figure 4). On a per-hectare basis, the natural forest vegetation stores an estimated 208 Mg C ha^–1 (Table 1). This is lower than the estimate of Lasco et al. (2000) in the secondary forests of Mt.

Makiling, which was approximately 306.55 Mg C ha^–1 when considering only the carbon from vegetation. On a per hectare basis, the P. caribaea plantation obtained the highest vegetation carbon storage which is 282 Mg C ha^–1 (Table 1). Generally, natural forests store more carbon than plantation forests (Waring et al. 2020). However, the higher C is connected with a greater mean DBH, basal area, and stand density P. caribaea plantation as compared with natural forest.

The lower vegetation C stock in the natural forest can be explained by the lower basal area of trees and shrubs (Table 1). This may be attributed to the presence of palms, pandans, and Donax canniformis, which were observed to take up space and reduce tree density and basal area, hence affecting carbon storage in the biomass of trees and shrubs. Reduction of tree density and the basal area had

been found associated with the presence or dominance of other plants such as the dominance of bamboo in Andean forests (Fadrique et al. 2021). The DBH of trees inside the plots in the natural forest also greatly varied from small to very large diameters. This is expected since natural forests exhibit heterogeneity (Heidrich et al. 2020) – for instance, in species, age, diameter, and height. Several trees with a DBH of 65 cm and above were recorded in the natural forest. The largest DBH (70 cm) was recorded from Litsea perrottetii, but some other larger trees were observed outside the plots. Large-diameter trees in forests are important since they store disproportionally massive amounts of carbon and are a major driver of carbon cycle dynamics in forests (Mildrexler et al. 2020).

As observed, numerous pines have already attained a DBH of 40 cm and above and many have a DBH of 30 cm. This may be an indication that the P. caribaea has properly adapted to the site, as evidenced by its growth when compared with other planted tree species in the area.

The species is reported to be adaptable to a wide range of climates and elevations (Oteng-Amoako and Brink 2008).

Most pine species are montane, but P. caribaea thrives well at lower elevations and rarely grows at elevations greater than 700 masl in the native range in Central America. It is one of the most tropically adapted pine species (Burley and Barnes 2004), which could be one of the reasons why it has well-established itself near the summit of Mt. Musuan. The species grows well in shallow and fairly dry soils (Kabogoza 2011). It must be noted, however, that the size of the pine plantation is only around

Figure 3. Proportion of area occupied by major ecosystems and land uses.

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Figure 4. Vegetation C storage in the land uses of Mt. Musuan.

Table 1. Basal area and vegetation C storage characteristics of the land uses.

Land uses Stem density

(/ha) Mean DBH ± SE

(cm) Stand basal

area (m²/ha) Vegetation C density

(Mg C ha^–1) Total C in land use (Mg C)

Natural forest 642 20 ± 1.33 34.91 208 13657

Plantation (Caribbean pine) 1800 21.8 ± 1.84 54.26 282 206

Plantation (Gmelina) 1667 17.31 ± 1.06 28.01 121 4626

Plantation (mahogany) 2067 15.09 ± 1.54 27.19 133 1656

Plantation (Mangium) 3433 12.95 ± 1.27 36.6 178 596

Plantation (mixed species) 5333 8.14 ± 0.35 17.43 52 1598

Plantation (teak) 2500 13.37 ± 0.87 24.21 110 106

Plantation (Thailand shower) 3833 10 ± 0.62 12.33 96 1487

Plantation (rainfo-demo site) 5533 10.04 ± 0.78 26.59 40 60

Grass-shrubland 300 5.14 + 0.65 5.07 6 986

Agroecosystem (sugarcane) N/A N/A N/A 6 513

MMBZG 700 14.71 ± 1.54 12.68 48 30

Trail/road N/A N/A N/A 0 0

Built-up (peak) N/A N/A N/A 0 0

Built-up (entrance area) N/A N/A N/A 0 0

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0.7 ha, which is less significant when considering the ecosystem total. Acacia mangium plantation also recorded a higher vegetation carbon storage per hectare. This could also be attributed to the higher basal area and stem density on the plantation (Table 1). As observed, many other tree species, including native trees such as Shorea contorta and Pterocarpus indicus have now thrived alongside Mangium on the plantation. Some of these native trees have already attained a considerable DBH, which is a good sign of the progress of vegetation succession. The A. mangium plantation is situated in proximity to the natural forest, which could allow for easier dispersal of natural forest trees into the plantation. This could be one of the reasons why there are much more species recorded there than in any of the other plantations. The G. arborea plantation also recorded higher vegetation carbon storage on an ecosystem level, which is 4,626 Mg C. The plantation covers around 38.23 has and is estimated to store 121 Mg C ha^–1 in its vegetation biomass. G. arborea is one of the more favorable choices in many plantation endeavors because it is easy to propagate, highly adaptive, and with known economic value as light construction material.

The least carbon was recorded in the grass-shrubland ecosystem. Grasses and shrubs sequester and store less carbon that trees are highly prone to disturbance such as grass fires, which may trigger a release of carbon into the atmosphere. I. cylindrica dominates this ecosystem with a few scattered trees and shrubs such as G. arborea, Tectona grandis, and Piper aduncum. Cogon is an aggressive species that outcompetes other and forms a relatively single-species ecosystem (Grebner and Boston 2022).

This ecosystem is not favorable for the development of wildlife habitat and biodiversity, which means that afforestation efforts should be strengthened in order to enhance the overall biodiversity value of Mt. Musuan.

Despite more than a third of the area being occupied by grassland-shrubland, trees observed in the plantations have started to colonize more areas. As observed by Aribal et al. (2015), successional forests are developing in Mt.

Musuan and will continue to progress if managed properly.

It can be seen by both actual observation and through satellite imagery; for instance, Google Earth images from 2003 through 2022 show that the afforestation efforts have through time replaced some of the grasslands with trees.

Implications for Management and Planning

A panoramic view of adjacent places can be seen at the summit of Mt. Musuan. Because of this, the area is a favored destination for visitors who wish to experience the scenic view from the top of the mountain. Unlike most other mountain ecosystems, Mt. Musuan is relatively safe and easy to traverse. The summit can be reached in approximately an hour and trekking can be greatly enjoyable with numerous sceneries in view while walking

up the mountain. Aside from the summit, tourists can also visit the MMBZG, wherein numerous species of threatened, endemic, and economically important species are found (Amoroso et al. 2002). Mt. Musuan truly has an enormous biodiversity and ecotourism value. However, there are aspects that should be given careful attention and consideration in planning for the site development.

As a result of intensive afforestation in the late-1990s (Aribal et al. 2015), portions of the grass-shrubland area of Mt. Musuan are already covered with trees. However, grass and shrubs still blanket around 39% of the area, which may imply that afforestation work is far from completion.

The success of recent afforestation and reforestation efforts is also marred by poor site conditions and sporadic fire incidence (Aribal et al. 2015). Nonetheless, the successful establishment of plantation species such as T. grandis, G.

arborea, and P. caribaea proved to be a useful template for future tree-planting efforts. Habitat quality indicators such as shade, soil moisture, and microclimate as observed in the forest plantations are already improved and have allowed a successional forest to develop where native species can thrive (Aribal et al. 2015). With their tried and tested performance in the area, non-native trees can be planted as nurse trees to prepare the area for native trees. Generally, non-native species sequester C faster; hence, they have a role in mitigating climate change (Cunningham et al.

2015). As seen in the Mangium plantation, native trees can eventually flourish when environmental conditions become favorable. Improvement of site conditions and rates of C sequestration using proven non-native trees could be set as short-term goals. The long-term goal will be for a forest dominated by native trees to eventually develop in the grass-shrubland ecosystem. Biodiversity in the area is of paramount importance; other uses should be compatible with biodiversity, and that project should always aim at improving the habitat quality of the area.

Programs and projects to be implemented in the area should always consider biodiversity in such a way that it is not undermined.

CONCLUSION AND RECOMMENDATIONS

Four major ecosystems comprise the 425-ha land area of Mt. Musuan. There are also fifteen specific land uses of which eight are forest plantations. The natural forest stores the highest amount of carbon, which is 18,381 Mg. The forest plantations also store substantial amounts of carbon. On a per hectare basis, P. caribaea plantation obtains the highest carbon storage. Based on observations that the grass-shrubland ecosystem is the largest ecosystem and that there are several afforestation

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species that successfully thrived in the area, then an enhanced afforestation/reforestation program using those species should be implemented. This is to improve the habitat quality and carbon sequestration of the land uses surrounding the natural forest. Tree species such as P.

caribaea, T. grandis, and G. arborea are only some of the potential afforestation species as they are already observed growing across the grass-shrubland ecosystem. These species can potentially improve the microclimate and soil condition in the grassland-shrubland area and promote the establishment of a successional forest (Aribal et al. 2015).

However, non-native trees should only serve as nurse trees to facilitate the establishment of native trees. Silvicultural interventions such as thinning may be employed to make room for the planting of native trees when the growing conditions have already improved. With proper science- based interventions, the perceived negative impacts of using non-native trees in afforestation or reforestation activities may be controlled. This is if the end goal that is set is for native vegetation to flourish. In this manner, it is not only biodiversity conservation that is addressed but climate change mitigation as well, by enhancing carbon sinks using non-native trees.

ACKNOWLEDGMENTS

The authors would like to thank the CMU administration and the CEBREM office for allowing the conduct of fieldwork in Mt. Musuan. Special appreciation is also expressed to PCAARRD (Philippine Council for Agriculture, Aquatic, and Natural Resources Research and Development) for the financial support under its GREAT scholarship program.

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