Plant diversity patterns across karst and non-karst substrates in old-growth coastal temperate rainforests of Vancouver Island, Canada

Karst landscapes, which cover ∼10 % of British Columbia, are globally significant for their hydrology and biodiversity, yet comparative studies of vascular plant diversity on karst versus non-karst substrates remain scarce in Canada and worldwide. We evaluated tree, shrub, and herb communities in 90 plots and 360 nested subplots across three paired old-growth sites on Vancouver Island, assessing species richness, Shannon diversity (alpha), compositional dissimilarity (beta), and regional heterogeneity (gamma). Karst forests were consistently species-poor at local scales, with markedly low shrub and herb diversity, yet supported compositionally distinct assemblages that contributed disproportionately to beta diversity. Non-karst sites, in contrast, maintained significantly higher richness, alpha, and gamma diversity ( p < 0.001) across all vegetation layers, with indicator species including mountain hemlock ( Tsuga mertensiana ), yellow cedar ( Callitropsis nootkatensis ), and false azalea ( Menziesia ferruginea ). These results underscore the role of bedrock geology in structuring biodiversity and provide rare empirical data from temperate karst ecosystems: karst forests enhance landscape-scale heterogeneity through distinct assemblages, whereas non-karst forests sustain greater taxonomic and structural diversity. Recognizing these complementary roles is essential for developing conservation strategies that maintain biodiversity, ecosystem function, and long-term resilience in coastal temperate rainforests. • Non-karst old-growth forests had higher vascular plant richness and diversity. • Tree, shrub, and herb composition differed by bedrock substrate. • Bray-Curtis dissimilarity revealed substantial beta diversity in community composition. • Indicator species identified for non-karst old-growth forest communities.