Tree biodiversity in Bornean lowland forest: What are the key species for forest city development in the new capital city of Indonesia?

PLOS ONE | https://doi.org/10.1371/journal.pone.0320489 April 8, 2025 3 / 19 PLOS ONE Tropical urban tree selection [24]. The IKN forest rehabilitation and restoration efforts also support the international agenda to combat forest degradation and deforestation declared at the Convention on Biologi- cal Diversity COP11 in Hyderabad, India, in 2012 [25]. Rehabilitation and restoration efforts require supporting an adequate number of seed sources of species originating from the intact forest and surrounding forest areas [26]. First, we need to consider genetic diversity because it relates to forest city sustainability in the future. Low genetic diversity reduces a forest's ability to cope with diseases and extreme weather [27]. The use of local indigenous species is expected to restore the structure and com- position of IKN forest tree species to mimic their original condition, especially in degraded or restored areas. Moreover, tree species to be planted must be carefully selected to create a forest with a structure and composition close to the natural conditions of Kalimantan’s existing forests. Species selection is, therefore, based on understanding the reference species found in the surrounding forest areas. The condition of some forest areas in and around IKN remains relatively good and many studies have been carried out on their floristic structure and composition, such as the Sun- gai Wain Protection Forest [28], the Samboja Research Forest [19,20,29,30], Bukit Soeharto Grand Forest Park [21], PT. ITCIKU [31], and Bangkirai Hill [32,33].

Based on the above, this research aims to determine the key tree species for the develop- ment of IKN according to the concept of a forest city. The selected tree species are prioritised based on various criteria such as native species, rarity, protected status, animal food, food source, and growth rates. Materials and methods Study sites The research was conducted in forested areas within a radius of 30 km from the centre (zero point) of IKN. A total of eight sampling areas were selected from forests around IKN (Fig 1; Table 1). In each sampling area, a number of square or rectangular plots of 0.04 to 0.1 ha in size were sampled for trees ≥ 10 cm diameter at breast height (dbh) (except Sotek Garden where a single plot large plot was surveyed). Field surveys in forest areas were conducted in the period 2018–2021 in the context of research activities organised by the Ministry of Envi- ronment and Forestry, Indonesia. This area remains forested (S1 Fig), although there was an area of Bangkirai Hill that experienced a fire in 1998 [33]. The current landscape around IKN includes lowland forests, plantation forests, karst, mangrove forests, agriculture, coal mines and settlements. The topography is undulating, with elevations ranging up to 300 meters above sea level. Climatic conditions based on meteoro- logical data collected from Sepinggan Airport, Balikpapan, show that mean annual rainfall (2001–2021) in the study location is 2,820 mm and the temperature ranges from 21.7 ºC to 35.7 ºC. Tree diversity

The flora recorded in the sample plots were woody trees with a diameter of more than 10 cm dbh that were identified at the Herbarium Wanariset (WAN) where needed. Relative fre- quency (RF), relative density (RD), and relative dominancy (Rdom) were calculated and then summed to calculate the importance value index (IVI) [34] (Eqn. 1). The vegetation in Sotek Garden was cultivated trees so, to avoid bias, the data from this location were not included in the IVI calculation; however, they were used in determining priority species. IVI RF RD Rdom = + + (1)