Ecology and systematics of ectomycorrhizal fungi associated with Dicymbe (Caesalpiniaceae) in Guyana.

摘要

This dissertation addresses the ecology and systematics of ectomycorrhizal (EM) fungi in recently discovered EM forests of Guyana. Ectomycorrhizal associations are poorly known in neotropical rainforests. Forests dominated by ectomycorrhizal host trees in Guyana bear a large assemblage of EM fungi, the majority of which are undescribed species. This has necessitated extensive systematic work on these ecologically important fungi. Following a series of collecting expeditions to Guyana's Pakaraima Mountains, alpha-taxonomic methods were used to determine and describe species of EM Boletaceae and Russulaceae, and molecular methods were employed to assess their phylogeny and mycorrhizal status. These EM fungi are symbiotically associated with the legume genus Dicymbe Spruce ex Benth., which locally dominates forests in the central Pakaraima Mountains. The sharp demarcation of EM Dicymbe stands with adjacent, mixed rainforests lacking EM trees raises important questions regarding the role of Dicymbe forests in harboring EM fungi, and their contribution to regional macrofungal diversity. Transect studies tested this relationship in the Upper Ireng River Basin of Guyana. Field observations suggested that Dicymbe species may achieve single-species dominance of forests, a phenomenon poorly known in the neotropics. Monodominance and regeneration levels of Dicymbe corymbosa were quantified in a series of 1-hectare forest inventory plots comparing Dicymbe-dominated and adjacent mixed forests.; Major conclusions of the study are: (1) EM Dicymbe species are highly clumped and dominant at specific sites in the Pakaraima Mountains; these Dicymbe stands harbor a diverse EM fungal guild largely absent from the anectotrophic regional forest matrix; (2) Dicymbe corymbosa dominance ranges from moderate to extremely high in the Upper Ireng and Upper Potaro Basins; these forests occur on a diversity of soil types and therefore lack edaphic specificity, and trees co-occurring with D. corymbosa are from the same species pool as the adjacent mixed forest. These factors suggest that biological mechanisms maintain dominance by D. corymbosa. Seedling and sapling regeneration and coppicing of adult individuals was high for D. corymbosa, indicating persistence of Dicymbe-dominated forests through time; (3) Dicymbe-associated EM fungi present a significant addition to the known neotropical EM mycota; and (4) traditional morpho-generic concepts in EM fungi based on temperate taxa are not consistently applicable when dealing with tropical taxa, here exemplified by Tylopilus species and pleurotoid Russulaceae.