ABSTRAKHutan merupakan aset alam yang melimpah dengan keanekaragaman hayati. Hutan berperan menyediakan hasil kayu dan bahan selain kayu, di antaranya tanaman obat. Keberagaman kondisi lingkungan memberikan kesempatan berbagai jenis tumbuhan mendiami ekosistem, termasuk jenis tumbuhan bawah. Tumbuhan bawah merupakan bagian pokok lapisan tumbuhan di bawah kanopi hutan, selain dari permudaan pohon, yang terdiri atas rumput, herba dan semak belukar. Tanaman ini berfungsi mempertahankan kestabilan ekosistem hutan. Penelitian ini bertujuan memperoleh data potensi keanekaragaman, komposisi dan struktur jenis tumbuhan bawah berkhasiat obat di tiga blok RPTN Coban Trisula, kawasan Taman Nasional Bromo Tengger Semeru (TNBTS) yaitu Blok Pusung Bledok Pasang, Blok Beji, dan Blok Jemplang. Penentuan petak contoh menggunakan teknik purposive sampling dengan karakteristik jenis tumbuhan bawah berkhasiat obat yang ditemukan di lantai hutan, lantai hutan terbuka dan tepi jalan, mulai ketinggian 1500, 2000, dan 2200 mdpl. Metode analisa vegetasi menggunakan teknik jalur berpetak (quadrat line transect). Setiap blok yang ditetapkan terdiri dari 5 petak contoh berukuran 2 x 2 meter. Jarak antar petak contoh 20 meter. Jumlah keseluruhan unit sampel yang digunakan 15 petak contoh dengan luas total 60 m2. Hasil penelitian menunjukkan terdapat 18 jenis tumbuhan bawah berkhasiat obat dari 11 famili yang tersebar di 3 blok penelitian. Dari ketiga blok, famili Asteraceae paling banyak ditemukan. Nilai indeks keanekaragaman jenis tergolong sedang. Blok Pusung Bledok Pasang memiliki nilai 1.01 (sedang), Blok Beji 2 (sedang) dan Blok Jemplang 2.37 (sedang). Nilai indeks kemerataan jenis di Blok Pusung Bledok Pasang 0.34 (tertekan), Blok Beji 0.70 (labil), dan Blok Jemplang 0.84 (stabil).ABSTRACTForests are natural assets that are abundant with biodiversity. Forests play a role in providing wood products and materials other than wood, including medicinal plants. The diversity of environmental conditions provides opportunities for various types of plants to inhabit the ecosystem, including types of undergrowth plants. Undergrowth is the main part of the plant layer under the forest canopy, apart from tree regeneration, which consists of grass, herbs and shrubs. This plant functions to maintain the stability of the forest ecosystem. This research aims to obtain data on the potential diversity, composition and structure of understory plants with medicinal properties in three blocks of the Coban Trisula RPTN, Bromo Tengger Semeru National Park (TNBTS) area, namely the Pusung Bledok Pasang Block, the Beji Block and the Jemplang Block. Determination of sample plots using a purposive sampling technique with the characteristics of understory types of medicinal plants found on the forest floor, open forest floor and roadside, starting at heights of 1500, 2000 and 2200 meters above sea level. The vegetation analysis method uses a quadrat line transect technique. Each designated block consists of 5 sample plots measuring 2 x 2 meters. The distance between sample plots is 20 meters. The total number of sample units used was 15 sample plots with a total area of 60 m2. The research results showed that there were 18 types of medicinal plants from 11 families spread across 3 research blocks. Of the three blocks, the Asteraceae family was most commonly found. The species diversity index value is classified as moderate. The Pusung Bledok Pasang block has a value of 1.01 (medium), the Beji Block 2 (medium) and the Jemplang Block 2.37 (medium). The species evenness index value in the Pusung Bledok Pasang Block is 0.34 (depressed), the Beji Block is 0.70 (unstable), and the Jemplang Block is 0.84 (stable).