The evolution of host specificity in liana-tree interactions
Lianas, woody climbing plants, comprise up to 40% of woody individuals and species in tropical forest. While early studies of liana-host interactions were made by Charles Darwin, the co-evolutionary relationships of lianas and their hosts remains poorly understood. This review synthesizes information from functional anatomy and morphology, ecology, floristics, and the fossil record of woody plants to explore the evolution of interactions between lianas and their hosts. Elwyn Hegarty (1991) suggested that lianas and trees do not engage in species-specific relationships; this review supports this hypothesis and suggests that liana-tree interactions evolved in a diffuse, generalist fashion. Trees served as selective filters, enhancing the effectiveness of anomalous stem anatomy and anchoring mechanisms in lianas, but it is highly unlikely that lianas played much of any role in the maintenance of adaptations in trees to avoid and shed them. This is consistent with the existence of generalist liana-tree interactions in extant forests. In addition, the record of colonization of the terrestrial environment by woody plants suggests that liana-host interactions were always generalized and that any trend to establish species-specific engagement or co-evolution was interrupted by extinctions of lianas and trees. Liana diversity is lower in tropical dry forests, but liana abundance is higher there, suggesting that global change-related intensification of the dry season in many tropical forests may lead many liana species to extinction, and to the proliferation of surviving species.