Alternate successional pathway yields alternate pattern of functional diversity

QUESTION: In Eastern Canada, wildfires turn Picea mariana forests into Kalmia angustifolia dominated heath or P. mariana forest depending on burn severity. These alternate end‐points of succession provide an opportunity to test assumptions concerning alternate successional trajectories dominated by distinct plant functional groups. Disturbance effects on functional diversity (FD) have been studied largely in single post‐disturbance communities, but rarely applied to alternate successional pathways. Do post‐fire Kalmia heaths have lower FD than forest communities, and does heath formation select for a narrow range of traits leading to biotic homogenization? LOCATION: Terra Nova National Park, Newfoundland, Canada. METHODS: Based on five functional traits (specific leaf area, leaf dry matter content, seed mass, height, specific root length), we measured functional trait dispersion within (alpha FD) and between sites (beta FD). We calculated overall FD metrics in nine heath and four forest sites as well as metrics for three functional groups (trees, shrubs, and herbs) in each site. We also performed a complementary taxonomic diversity (TD) analysis to establish links between FD and TD. RESULTS: We could not detect a difference in overall alpha TD between the two successional communities but found significant loss of alpha and beta FD in heaths and within tree and herb groups. Overall beta FD was lower in heaths than forests. It was also lower for trees and herbs, indicating an increase in functional similarity (functional homogenization) of the majority of life forms. CONCLUSIONS: Between the two successional pathways, trait space occupancy was lower in heaths than forests. Heath formation in post‐fire communities consistently restricts functional dispersion (low alpha FD) of tree and herb traits leading to functional homogenization (low beta FD). When a successional trajectory leads to heath formation, it is accompanied by a loss of functional diversity.