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 12 / 19 PLOS ONE Tropical urban tree selection The overlap of tree species that produce food for humans and food for animals has the potential to cause conflict if not managed properly. This is because all potential species pro- viding food sources for humans also provide food sources for animals, especially primates. Some primates present in the study area that have the potential to cause conflict with humans are long-tailed monkeys (Macaca fascicularis), macaques (Macaca nemestrina), proboscis monkeys (Nasalis larvatus) and orangutans (Pongo pygmaeus). The first three species have natural distribution in the study area, while orangutans have been introduced to Sungai Wain Protection Forest and Gunung Beratus Protection Forest. The prioritization of species to be planted based on the regional spatial plan of IKN [55] must be adjusted to the characteristics and suitability of the growing area. Our study did not include mangrove or wetland habitats, and tree selection was based on the terrestrial charac- teristics of the study area. Emphasis on the right trees for the right place and the right pur- poses is the main underlying challenge that needs to be tackled to make restoration through tree-based interventions successful [56]. Species in priority 1 and 2 can be used for species enrichment or collection gardens (S1 Table) as a demonstration for the use of key species in the IKN region. Due to the complexity of environments and site diversity, testing tree species

under local biotic and management conditions to identify their adaptations to site variables is essential. We recommend that future studies or restoration plans in the region include soil assessments (soil physico-chemical properties) to guide species selection and improve the success of rehabilitation efforts. Seed source availability Seed source availability is essential to developing the forest city [57,58]. There are seven seed sources currently available, which includes the Rubroshorea leprosula seed source at Samboja Research Forest with a total production of 5 ha, plus Dipterocarpus humeratus (5 ha) and Eusideroxylon zwageri (Lauraceae) (30 ha). Other seed sources are available for Aquilaria microcarpa (Thymelaeaceae) (5 ha), Agathis borneensis (5 ha) and Durio kutejensis (Mal- vaceae) (2.5 ha) [59], whilst Ficus variegata (Moraceae) (5 ha), Tengkawang (Rubroshorea stenoptera) (3 ha), Rubroshorea ovalis (1 ha), Tengkawang (Rubroshorea beccariana) (6 ha), Kapur (Dryobalanops beccarii) (5 ha) and Shorea laevis (5 ha) have been growing since 2015 (Nanang Riana, Manager of Samboja Research Forest, pers. comm.). Another approach to obtain seeds is buying from seed industries or local communities’ nurseries. Seeds used for rehabilitation should have good performance, so it is important to understand the seeds’ origin. Indeed, seeds derived from seed industries have to be accompanied by a seed certificate [60]. Furthermore, buying seeds from local communities is another option to supply the seed

requirements for rehabilitation. Some people living in the Samboja area have developed seed nurseries for personal and commercial purposes, and supporting these growers will provide economic benefits to them. This activity can stimulate the local economy by creating job opportunities such as seed collectors and nursery caretakers, increasing communities’ income. For rehabilitation purposes, the government has established a large nursery that produces 15 million seeds per annum in Mentawir, Penajam Paser Utara district. An obstacle emerges to provide seed sources for high-scoring rare species where there is limited information on their ecology and thus silvicultural techniques. There are some efforts to overcome this challenge by natural collection and vegetative propagation. Vegetative propagation, tissue culture and shoot cuttings may be alternatives to supply seeds through generative techniques [61–65]. We therefore suggest that future restoration efforts incorpo- rate propagation research and the development of local nurseries to ensure that the selected species can be effectively propagated and thus out-planted.