Citation: Dharmawan, I.W.S.; Heriyanto, N.M.; Setyawati, T.; Wardani, M.; Susilo, A.; Garsetiasih, R.; Sawitri, R.; D.; Yuskianti, V.; Karlina, E.; et al. The Vegetation Composition and Carbon Stock of Old Shrub Typology to Support the Rehabilitation Program in Sumatra and Kalimantan Islands, Indonesia. Sustainability 2023, 15, 1389. https:// doi.org/10.3390/su15021389 Academic Editors: Kripal Singh, Shalini Dhyani, Debbie Bartlett and Somidh Saha Received: 23 November 2022 Revised: 23 December 2022 Accepted: 9 January 2023 Published: 11 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Article The Vegetation Composition and Carbon Stock of Old Shrub Typology to Support the Rehabilitation Program in Sumatra and Kalimantan Islands, Indonesia I. Wayan Susi Dharmawan *, Nur Muhammad Heriyanto, Titiek Setyawati, Marfuah Wardani , Adi Susilo, Raden Garsetiasih, Reny Sawitri, Denny, Vivi Yuskianti, Endang Karlina, Mariana Takandjandji, Rozza Tri Kwatrina and Zuraida National Research and Innovation Agency, Jl. Raya Jakarta Bogor Km. 46, Bogor 16911, West Java, Indonesia * Correspondence: iway028@brin.go.id Abstract: The typology of certain old shrubs assists with the selection of suitable plant species for rehabilitation. The carbon stock dynamic in old shrubs is fundamental due to their high uptake during the growth process phase. A plot of 100 m × 100 m (1 hectare) was created in each location, referring to the work of Mueller-Dombois and Ellenberg. The plot was further divided into subsquares measuring 20 m × 20 m; in each plot, there were 25 subplots. Research results showed that the diversity index of old shrubs at the tree, pole, and seedling stage is 2.50–2.66, 1.23–1.50, and 0.67–1.11, respectively. For Kalimantan, the diversity index is lower than that on Sumatra Island, which is 1.64–1.80, 1.00–1.02, and 0.52–0.81, respectively. The carbon stock of the old shrub forest in Sumatra has an average of 36.61 Mg C per ha (standard deviation 14.54 Mg C per ha) to 72.50 Mg C per ha (standard deviation 25.61 Mg C per ha), while Kalimantan has an average of 47.94 Mg C per ha (standard deviation 13.30 Mg C per ha) to 144.07 Mg C per ha (standard deviation 54.64 Mg C per ha). The dynamics of the vegetation composition and carbon stock in each old shrub’s typology are considered when choosing a suitable model, including high carbon stock content, to provide optimal results for rehabilitation activities. Keywords: dynamics; diversity index; tree; pole; seedling 1. Introduction Lowland tropical rainforests are generally species rich, especially in primary forests. In general, lowland primary forests in Sumatra and Kalimantan are dominated by plant species from the Dipterocarpaceae family. Primary forest ecosystems can transform into secondary forests and old or young shrubs due to various forest disturbances, including fires, natural disasters, and non-environmentally friendly logging. The old shrub is a small- to large-sized perennial woody plant dominated by small perennial woody plants [1]. Unlike herbaceous plants, old shrubs have persistent woody stems aboveground. Shrubs may be deciduous or evergreen. The old shrub has the advantage of greater tolerance for mechanical disturbances, and they are usually outcompeted by the surrounding trees in terms of light acquisition. In the forest, they are more frequently found in early successional stages [2]. The size of the old shrub area in Indonesia’s dryland is 11,330,200 ha, whereas in Sumatra Island, it covers 2,941,600 ha (the Riau, Jambi, and South Sumatra Provinces comprise 38,700 ha, 775,800 ha, and 275,700 ha, respectively). The shrub area on Kalimantan Island amounts to 4,145,600 ha (that of West Kalimantan and East and North Kalimantan amounts to 334,600 ha and 2,237,200 ha, respectively) [1]. Most of the old shrub areas on the islands of Sumatra and Kalimantan have degraded, primarily as a result of human activities. The rehabilitation program represents the government’s efforts to improve land quality and cover. The level of land degradation is strongly influenced by forest land management, Sustainability 2023, 15, 1389. https://doi.org/10.3390/su15021389 https://www.mdpi.com/journal/sustainability