Afromontane forests with fewer large trees retain bird diversity but show altered species composition

Abstract

Afromontane forest covers a small proportion of the African continent yet supports disproportionately high biodiversity. Forest structural characteristics, particularly large-tree abundance and vertical complexity, are key determinants of habitat suitability for forest birds. We compared forest structure and bird community composition in two Afromontane forests in KwaZulu-Natal, South Africa: Umgano Forest, characterised by a high density of large emergent podocarps, and Karkloof Forest, which contains fewer large trees and a canopy dominated by angiosperms. Umgano had significantly taller trees and greater mean stem diameters, whereas Karkloof exhibited a denser understorey and higher stem density. Despite these structural differences, bird species richness and diversity were similar between forests. However, community composition differed. Umgano supported a greater proportion of canopy-associated and cavity-nesting species, while Karkloof supported more understorey- associated species. Both forests maintained comparable numbers of forest specialist birds. These results indicate that variation in large-tree abundance and vertical structure influences functional and compositional patterns without necessarily altering overall diversity. Our findings suggest that forests with fewer large trees can retain comparable bird diversity while supporting compositionally distinct assemblages. Conserving structurally heterogeneous Afromontane forests may therefore maximise regional avian diversity by maintaining complementary community configurations rather than relying solely on stands dominated by large trees.