Tropical rainforests are distributed in Asia, New Guinea, Africa, Madagascar, and the Neotropics. Asian tropical rainforests are uniquely dominated by trees from the family Dipterocarpaceae (Corlett, 2007), where Vatica mangachapoi is a keystone species for tropical rainforest in Hainan Island, China (Ding et al., 2006). This species accounts for 20–50% of forest basal area and > 50% of the tree canopy cover (Corlett and Primack, 2006; Corlett, 2011; Ghazoul, 2016). It is native to Thailand, Brunei, Malaysia, Philippines, Indonesia, and the Hainan Island of China (Supplementary Figure 1). These trees can grow up to 20 m tall, and mostly grow on hill- and mountain-slopes below an altitude of 700 meters (De Guzman et al., 1986). It has two recognized subspecies, V. mangachapoi subsp. obtusifolia (found only in Philippines and Malaysia) and V. mangachapoi subsp. mangachapoi (found in rest of the world, except in China), which grows in deep soils, mixed peat swamp forests, hills and coastal areas (Soepadmo et al., 2004).

Vatica mangachapoi had been commonly found in its natural habitats, but in the last three decades (or any specific year), its population has been declining substantially (Pooma et al., 2017). For instance, in Hainan Island, its population has had a reduction of 35% between 1991 and 2008, something which is of major concern for the conservation of this tree species (Zhang et al., 2010). Due to such large decline in Chinese V. mangachapoi has led to its listing as “vulnerable” in the international union for conservation of nature (IUCN) red list of threatened species (Pooma et al., 2017). After such an IUCN assessment, constant monitoring of V. mangachapoi’s population decline was warranted (Pooma et al., 2017). Yet, an accurate and up-to-date scientific report on the conditions (e.g., fitness and health) in which it is found is lacking. We have therefore addressed this challenge in the current investigation by surveying all places where V. mangachapoi population exists in the Hainan Island, southeast China (Supplementary Figure 1).

Vatica mangachapoi-mixed forests have dominated the old-growth tropical evergreen monsoon rainforests on Hainan Island of China (Ding et al., 2006; Dai et al., 2018). However, the conversion of these natural mixed forests into rubber plantations is the main driver of large decline in V. mangachapoi population (Zhang et al., 2010; Pooma et al., 2017; Guo et al., 2018), which are now dispersed across the whole islands. Therefore, we first surveyed its population density (%) in all the V. mangachapoi pockets to determine its relative abundance and spatial distribution patterns. Moreover, the conversion of tropical lowland rainforest into rubber plantation has resulted in a large decrease in tree diversity (Ahrends et al., 2015; Warren-Thomas et al., 2020). Replanting of mono-cultured V. mangachapoi in these locations raises the threat of diseases and other biotic challenges. Validation of these possibilities require evaluation of tree diversity (tree species richness) and assessment of V. mangachapoi health. Additionally, When asking and hearing from local people, indicates that several protected areas have been planted with monocultures of seedlings of V. mangachapoi to increase its density in Hainan Island. This can be confirmed by tracing the land use history and the age of the oldest V. mangachapoi at various locations. For example, remote sensing (RS) and geographic information systems (GIS) are suitable tools for detecting and evaluating long-term changes in habitat cover and quality of threatened species management (Zou et al., 2022). Moreover, δ¹⁴C estimates can also help estimate tree age (Ichie et al., 2022).

In this investigation, we used the currently available remote sensing images of the highest resolution to trace the changes in historical land use. We also used δ¹⁴C estimation methods to measure the age of the oldest V. mangachapoi trees to determine whether they have been artificially planted or naturally regenerated.

We found that V. mangachapoi populations in China are dispersed across 23 protected areas (Figure 1A). Among them, a costal V. mangachapoi forest accounts for 96.84% of all of its existing population, whereas the remaining 22 protected areas account for merely 0.01–1.25% at each site (Figure 1A). This coastal forest has very low tree species richness (only 44), and an extremely high relative abundance (91%) of V. mangachapoi (Figure 1B). In contrast, the other 13 protected areas have much higher (152–451) tree species richness, and consequently, a much lower (6.1–25%) relative abundance of V. mangachapoi (Figure 1B).

Our analysis of remote sensing data clearly revealed the artificially plantations of V. mangachapoi in this unique costal forest. From 1988 to 1993, the total area of this forest had decreased from 900,900 to 622,800 m², but it quickly recovered to 1,097,500 m² between 1993 and 1998 (Figure 2), whereas the total forest area in 2021 was estimated to be 1,016,039 m² (Figure 2). Moreover, δ¹⁴C measurements of the age of the oldest V. mangachapoi trees, in the protected coastal forest, indicate that they could have been planted or regenerated only after 1950 (age of the oldest trees is around 70 years; Supplementary Table 1). Whereas, the age of the oldest V. mangachapoi trees in the other 22 protected area range from 160 to 460 years (Table 1).

When investigating the occurrence of diseases, we found that 89.9% of V. mangachapoi in the coastal forests of Hainan have been infected by a fungal stem rot disease (Figures 3, 4), which is caused by the fungus Amauroderma perplexum (ITS sequencing; Supplementary Figures 2, 3). It is alarming that the 14.9% of V. mangachapoi trees have already died due to A. perplexum-induced stem rot (Figures 3, 4). An additional 75% of the infected V. mangachapoi trees are at an extreme risk of dying soon. Furthermore, 3.6% of V. mangachapoi have died due to vine strangulations (Figures 3, 4). On the other hand, V. mangachapoi in the other 13 protected areas did not suffer from Amauroderma infections and vine strangulation (Figure 3).

After the listing of Chinese V. mangachapoi in the IUCN red list of threatened species (Pooma et al., 2017), it is absolutely necessary to monitor the accurate status of its population. Our results provide the first evidence that V. mangachapoi is now sparingly distributed in 14 protected areas, of which a coastal forest in Hainan contains 96.84% of all of its Chinese populations. Therefore, this coastal forest becomes a unique site for conservation of V. mangachapoi as it would directly determine the threat that this species faces.

Unfortunately, the current situation of V. mangachapoi in this coastal forest is extremely bad. Since 1988, nearly half of this coastal forest has been destroyed by construction work. Further, the relative abundance of V. mangachapoi in this forest is now extremely high (90.8%), whereas plant diversity is very low. In contrast, the other 13 protected areas have much higher tree species richness (152–451), and lower (6.1–25%) relative abundance for V. mangachapoi (Figure 2). Tree communities with low diversity are at a high risk of epidemic diseases (Chen and Zhou, 2015; Liu et al., 2016, 2020), and of vine invasions (Livingstone et al., 2020; Carboni et al., 2021). Indeed, a total of 18.5% of V. mangachapoi trees in the coastal forests of Hainan have died due to vine strangulations or an infection of a fungal stem rot disease, which is caused by Amauroderma perplexum (Figures 3, 4 and Supplementary Figures 2, 3). Additional 75% of the V. mangachapoi trees are already infected by stem rot and are at an extreme risk of dying soon. On the other hand, V. mangachapoi the other 13 protected areas neither suffer from any vine strangulation nor any fungal diseases. It is evident that a low species diversity and an extremely high relative abundance of V. mangachapoi in this coastal forest are subjecting it to a threat of extinction.

Analysis of remote sensing data, δ¹⁴C measurements of the age of the oldest trees, and local provided insights into such a low tree diversity and an extremely high relative abundance of V. mangachapoi in the coastal forest. Since 1960, all old and mature V. mangachapoi trees of this forest have been cut down for building fishing boats and supporting the local steel industry. In Circular of the people’s Government of Hainan Province (1988), the government of the People’s Republic of China established a “provincial natural reserve” (PNR) for a high-level protection of this forest. Local surveys revealed that since 1993, PNR has created monocultures of the new V. mangachapoi saplings after cutting down other native tree species that originally coexisted with V. mangachapoi. This has transformed an original mixed-forest into a cultivated monoculture of V. mangachapoi in an agricultural setup. This was evident in our analysis of remote sensing data (1988–2021; Supplementary Figure 3). Further, δ¹⁴C measurements showed that the oldest V. mangachapoi tree in this forest, were planted/regenerated only after 1950 (age of the oldest trees is around 70 years; Supplementary Table 1). Whereas, the age of the oldest V. mangachapoi in the other 13 protected area range from 160 to 460 years (Supplementary Table 1).

Taken together, our results show that V. mangachapoi is under a threat of extinction in China. This warrants an immediate revision of the threat status of this species, from vulnerable to endangered or extremely endangered, in IUCN and in China. Moreover, the PNR needs a paradigm shift in their strategy to conserve this unique costal forest. This should involve measures to increase biodiversity by planting native species in the understory of the V. mangachapoi or at the sites where dead V. mangachapoi trees are standing. It would gradually restore the current monoculture plantations into a mix-V. mangachapoi forest, and would also enhance the ecosystem functioning and curtail biotic challenges. In addition, liana removal should also be performed to aid V. mangachapoi to grow better. Similar to China, large areas of tropical rainforest has been destroyed by rubber plantations in Thailand, Brunei, Malaysia, Philippines, and in Indonesia (Ahrends et al., 2015). Thus, an accurate survey of the current status of V. mangachapoi is also needed in these countries to ascertain the status of this species and prevent it from a possible acute threat of extinction.

The original contributions presented in this study are included in this article/Supplementary materials, further inquiries can be directed to the corresponding author.

HY: Conceptualization, Data curation, Investigation, Methodology, Writing – review & editing. XP: Conceptualization, Investigation, Writing – original draft. HZ: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Investigation, Methodology, Resources, Writing – original draft, Writing – review & editing.