<![CDATA[Mengilan Forest Reserve, a class I protected forest in Southern Sabah adjacent to Kalimantan, was previously logged and is now managed for biodiversity under the Heart of Borneo Initiative. Despite its strategic location within a transboundary conservation landscape, information on dipterocarp diversity and regeneration remains limited. This study established a baseline assessment of dipterocarp diversity, species composition, and regeneration patterns following historical logging across an elevational gradient. A rapid survey of dipterocarp trees and seedlings was conducted along three transects spanning upland mixed dipterocarp and lower montane kerangas forests across an elevational gradient. Adult dipterocarps were inventoried along line transects, and seedling regeneration was quantified using systematically distributed 1 m × 1 m quadrats. Species diversity and community composition were evaluated using diversity indices, Jaccard dissimilarity, ANOSIM, and NMDS. A total of 31 dipterocarp species representing six genera and 41 adult individuals were recorded. Of these, 19 species (61.30%) were globally threatened, and 14 species (45.20%) were endemic, including the Sabah endemic Shorea symingtonii. Species richness and diversity were highest in the lower-elevation transect (15 species; H' = 2.69; 1-D = 0.93) and lowest in the upper transect (5 species; H' = 1.61; 1-D = 0.80). Species composition differed significantly among transects (ANOSIM R = 0.72, p < 0.01), with high turnover (Jaccard dissimilarity = 0.89-0.95). A total of 131 seedlings representing 14 species were recorded, with regeneration highest in upland mixed dipterocarp forest and lowest in lower montane kerangas forest. These findings demonstrate the lasting influence of historical logging and elevation on dipterocarp communities. Mengilan FR therefore represents an important refuge for threatened and endemic dipterocarps while providing a baseline for long-term monitoring and transboundary conservation management within the Heart of Borneo landscape. Keywords: dipterocarp diversity, Heart of Borneo, IUCN Red List, regeneration, Sabah–Kalimantan border]]>
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