Short Communication
Forest Cover Changes in the Lungga River Basin of Guadalcanal,Solomon Islands
2025 Volume 31 Pages 38-43
Details
2025 Volume 31 Pages 38-43
Forest monitoring is of great importance for effective forest conservation and management. Changes in forest cover around the Lungga River, Guadalcanal, Solomon Islands, were identified by visual interpretation using the web-mapping service. Four satellite images from Google Earth Pro 7.3 taken in 2014 (April 10), 2016 (September 30), 2018 (July 29), and 2023 (April 22) were used to detect deforestation areas around the river, within a 50 m buffer zone. Deforestation was defined as the conversion of forest land to other land uses. As a result, for the first time, we've been able to show detailed changes in forest cover over 10 years in Guadalcanal, the Solomon Islands, although the area covered is limited. During the period, up to 10% of the forest in the surveyed area had been removed. Although land cover has been gradually recovering with shrubs and grasses within a few years after deforestation, it is a slow process, and the forest will not be able to return to its original pristine state. Increasing population, food security, and economic development are considered the main causes of deforestation on the Solomon Island. Continuous forest monitoring will be necessary in the future for forest conservation and management. Remote sensing and GIS will make a major contribution to this. It would be prudent to prioritize the development of human resources having these skills.
Deforestation is a significant global issue. FAO (2020) reports that 420 million ha of forest have been lost since 1990. Degradation and excessive removal of forests have contributed to water supply depletion, soil erosion, food insecurity, and loss of wildlife habitat (Evans et al., 2018). In particular, tropical countries had the highest net forest loss and deforestation rate, with an average annual deforestation rate of 11.7 million ha/yr over a 30-year period. The earth's tropical rainforests are under the greatest threat, whether from livestock grazing in South America or the expansion of croplands such as oil palm plantations in Asia (FAO, 2022).
Forest monitoring is of great importance for the effective forest conservation and management. Consequently, forest monitoring has been carried out at the global, national, regional, and local levels (Hansen et al., 2013; Goetz et al., 2015; FAO, 2022). However, in some countries and regions, forest monitoring is not adequately conducted due to limitations in human and financial resources (Katovai et al., 2015). The Solomon Islands, the subject of this study, is one such country where forest monitoring is inadequate (Katovai et al., 2015) despite the fact that forests cover 80% of the country's land area (Ministry of Forestry and Research, 2024).
It is helpful that relatively simple and inexpensive monitoring strategies can be implemented in countries with limited financial and human resources to facilitate the accumulation of detailed data on changes that can then be used for forest conservation and management purposes. The visual interpretation of the web-mapping service such as aerial images on Google Earth Pro represents a relatively simple and cost-effective approach to acquiring land cover data (e.g. Taylor and Lovell, 2012; Pulighe and Lupia, 2016; Srivastava and Chinnasamy 2021). Taylor and Lovell (2012) actually succeeded in the detection of land use and land cover changes combining Google Earth Pro and GIS considering the advantages of the manual classification of the high-resolution satellite imagery for a fine-scale study area.
The purpose of this study is to identify forest cover changes around the Lungga River, Guadalcanal, Solomon Islands, using a relatively simple and inexpensive method of visual interpretation using the web-mapping service.
The study area is located on both sides of the Lungga River in Guadalcanal, Solomon Islands (9°37′ S, 160°11′ E). The area within the 50m buffer zones on both sides of the Lungga River in the rectangular area in Figure 1 was surveyed. The Lungga River is the longest river on the Solomon Islands with a catchment area of 377 km2 (FAO, 2016). It starts high in the mountains and empties into Iron Bottom Sound, passing close to the capital of Honiara and Honiara International Airport. The study area was located on the middle reaches of the Lungga River, and the forest type is classified under lowland rainforest below 200 m above sea level (Solomon Islands Government, 2019b). The dominant tree species for timber production in lowland forests are Campnosperma brevipetiolata, Dillenia salomonensis, Endospermum medullosum, Parinari salomonensis, Terminalia calamansanai, Schizomeria serrata, Maranthes corymbosa, Pometia pinnata, Gmelina moluccana, Elaeocarpus sphaericus and Vitex cofasus (Solomon Islands Government, 2024). The annual rainfall of the Solomon Islands is 3,000–5,500 mm (Britannica, 2011; Solomon Islands Government, 2017). According to the Solomon Islands Code of Logging Practice (Solomon Islands Government, 2002; Solomon Islands Government and SPREP, 2021), logging is prohibited in some sensitive areas due to erosion risk or ecological importance (e.g., around rivers, on steep topography, and in higher-elevation forests), around villages, and near important ecological areas (e.g., ocean lagoon, lake, and mangroves). The Solomon Islands are located in one of the rainiest regions in the world (Britannica, 2011; Solomon Islands Government, 2017), and cutting down of trees near rivers has a major impact on downstream areas (Albert et al., 2021). Heavy rainfall causes severe erosion in such areas, resulting in downstream impacts such as siltation, flooding, isolation, and submergence of communities (Pattanayak and Wendland, 2007). Deforestation also affects drinking water quality (Wenger et al., 2018). Therefore, the focus of our analysis was on the forest cover change area around the rivers.
Study site at Guadalcanal, Solomon Islands. It was located in the middle reaches of the Lungga River. The areas within the 50m buffer zones on both sides of the Lungga River within the rectangular area were surveyed.
Four satellite images from Google Earth Pro 7.3 taken in 2014 (April 10), 2016 (September 30), 2018 (July 29), and 2023 (April 22) were used to detect deforestation areas around the river. The study period 2014–2023 was selected, as a clear Google Earth image before 2014 was unavailable.
Visual interpretation for detecting deforestation areasThe area where the water surface and riparian area could be confirmed by visual interpretation was traced and converted into polygons using the polygon creation function in Google Earth Pro 7.3. The water surface and riparian area were recorded as a single unit. Gravelly and sandy sediment piles can be seen on both banks of the river, and they were almost white in color and easily distinguishable from other land cover. Because these areas were different each year, they were identified and recorded using images from each year. These "river polygons" as river were imported into ArcGIS Pro 3.2.2 (ESRI) and 50 m buffers were created around them to focus on forest cover changes around the rivers. The conservation and management of riparian forests is essential to secure the Solomon Islands' water supply (Solomon Islands Government, 2002; Solomon Islands Government and SPREP, 2021).
The buffers were imported into Google Earth Pro 7.3, and the deforestation area within these buffer zones on both sides of the river was visually interpreted and traced as polygons using the polygon creation function of Google Earth Pro 7.3. Generally, brown soil, red-yellow soil, red clay, and brown loam are widely distributed in mountainous areas (Agricultural Development Consultants Association, 1996) in Guadalcanal. Therefore, when the forest cover is removed, reddish-brown soils appear. We defined "deforestation polygons" as areas that were covered with forest and other vegetation in the previous year's image and showed reddish-brown soil in the following year's image. Although the removal of forest cover can be due to anthropogenic such as logging and natural causes such as flooding, it was difficult to distinguish between the two in this study. The FAO also defines deforestation as the conversion of forest land to other land uses, whether or not caused by human activities. These "deforestation polygons" as deforestation areas were imported into ArcGIS Pro 3.2.2 and their areas were calculated.
Figure 2 shows the detected deforestation areas in each year within the 50m buffer around the river. In 2014 and 2016, there was active deforestation on both sides of the river. In 2016, 10% of the forest in the study area was removed. Since 2018, there have been little forest cover changes. In the earlier images, deforestation areas were visible, but in the more recent images, there were areas covered by vegetation with no tree canopy. Although the vegetation has started to recover, as of 2023, these areas still lack large trees. Table 1 lists the river, buffer zone, and deforestation areas for each year. The size of the river area varied significantly by erosion and deposition, decreasing during the study period. It was about 1.6 times smaller in 2023 than in 2014. As a result, the location of the 50m buffer zone that was set based on the river area was also changed for each year. The deforestation area within the 50m buffer has been decreasing since its peak in 2016 (89,351m2) to 1,286m2 by 2023.
Detected deforestation area in the 50 m buffer of Lungga River. Deforested areas between 2014 and 2023 were detected by visual interpretation using satellite images from Google Earth Pro 7.3.
| Year | River area(m2) | 50m buffer zone(m2) | Deforestation area in buffer zone (m2) (Area percentage is in parentheses) |
| 2014 | 1,000,871 | 906,725 | 45,754 (5.1%) |
| 2016 | 872,948 | 838,598 | 89,351 (10.7%) |
| 2018 | 795,457 | 819,555 | 3,236 (0.4%) |
| 2023 | 639,187 | 827,503 | 1,286 (0.2%) |
For the first time, we've been able to show detailed changes in forest cover over 10 years in Guadalcanal, the Solomon Islands, although the area covered is limited to 50 m buffer around the river. The area of deforestation within the area decreased during the study period. Thus, although land cover has been gradually recovering with shrubs and grasses within a few years after deforestation, it is a slow process, and the forest will not be able to return to its original pristine state. In fact, Katovai et al., (2016, 2021) reported that although the trees were covered by a canopy there within 10 years after logging, their original conditions did not recover even after 30 to 50 years in the Solomon Islands in field surveys. Also, considering that reforestation is not common, it may take decades for even secondary forests to become well-established. Tropical rainforests have thin and poor soils, and it is generally difficult to restore natural vegetation after logging (Hayward et al., 2021; Katovai et al., 2015). The width of the river had decreased since the first year of analysis, and the location of the buffer zone changed slightly accordingly. However, since deforestation itself decreased during the study period, and there was almost no deforestation in the surrounding area, the change in the buffer zone is unlikely to have had a significant impact on the extraction of the deforested area.
Solomon Islands Government (2019a) mentioned that increasing population, food security, and economic development are the main causes of deforestation in the Solomon Islands. In fact, the total population of Guadalcanal Island was 109,382 in 1999 and 283,591 in 2019. It has increased 2.6 times in 20 years (Solomon Islands National Statistics Office, 2023). The population is particularly concentrated in the capital, Honiara City of Solomon Islands on Guadalcanal Island, near the coast, where about half of the population (129,569) lived in 2019. The population has increased 2.6 times from 49,107 in 1999. The capital region continues to expand with the construction of industrial, residential, and other important infrastructures into the surrounding forest (Solomon Islands Government, 2019a; Ministry of Lands, Housing and Survey, 2018). Some of the people who have spilled over from the coastal areas of the capital have also been seen building illegal homes in the forests (Ministry of Lands, Housing and Survey, 2018). Excessive commercial logging also exacerbates this problem. The economy of this country has historically been reliant on the export of timber. The government has traditionally relied on the forest industry as a primary source of revenue, with the sector contributing 20% of GDP and 70% of export revenue. The Solomon Islands lost 147,000 ha of primary forest from 2002 to 2023 (Global Forest Watch, 2024). In tandem with this, the log export volume in the Solomon Islands has increased nearly seven-fold from 2001 to 2017 (509,400 m3 to 3,402,339 m3 respectively, Solomon Islands Government, 2019a). Our study site was located in a forest remote from the capital city. Given the absence of subsequent construction, it can be inferred that the deforestation was undertaken for the purpose of timber production. Katovai et al. (2015) highlighted those areas without settlements usually demarcated for logging concession within tribal land.
The Solomon Islands are located in one of the rainiest regions in the world (Britannica, 2011; Solomon Islands Government, 2017). As the study site was located in a 50m buffer zone from the river, it cannot be excluded that the deforestation was caused by flooding. In fact, flooding also occurred in Guadalcanal during our study period, which was in 2014 (World Bank, 2021), 2018 (Radio New Zealand, 2018), and 2023 (National Disaster Council, 2023). The annual precipitation was around 2000mm each year (Australian Government, 2024). However, during the study period, the width of the river decreased, as did the area of new deforestation, suggesting that the floods had little effect on deforestation. Moreover, if the soil is deeply removed, as in a flood, the recovery of vegetation, especially woody species, is very slow due to soil infertility (Flores et al., 2020; Manghwar et al., 2024; Marks et al., 2020). In contrast, in the case of deforestation by logging, shrubs might grow within a year (Magnusson et al., 1999; Fredericksen and Mostacedo, 2000). Shrubs were also observed at the deforestation site in this study area, suggesting deforestation due to logging.
We applied visual interpretation for satellite images on Google Earth Pro 7.3. Because we analyzed a small area in this study, we were able to visually interpret deforested areas with a little effort. However, for frequent monitoring of large forests, it is necessary to automate the process of detecting deforested areas from images such as satellite images taken over a wide area. While this has hitherto required advanced knowledge and expensive tools for image analysis, in recent times, free satellite image analysis tools such as Google Earth Engine have become available to the general public with minimal prior knowledge (Brovelli et al., 2020; Mujetahid et al., 2023). It has been difficult to secure the personnel and tools to use these technologies in the Solomon Islands. However, it is possible that deforestation will persist, and climate change is contributing to the occurrence of frequent heavy rains that result in flooding. It would be prudent to prioritize the development of these human resources.
We would like to express our sincere gratitude to Prof. Ishikawa, whose continuous support was invaluable throughout the course of the study. We also thank the forestry workers who were interviewed and whose comments made an enormous contribution to this work. Finally, we would like to thank our families for their moral support.
