Effects of pathogenic and mycorrhizal fungi on regeneration of two tropical tree species
This thesis tests the hypothesis that the influence of fungal pathogens and arbuscular mycorrhizal (AM) fungi varies in response to proximity to parent trees and light environment to affect seedling establishment in two tree species, Milicia regia and Antiaris toxicaria, in Ghanaian tropical rain forest. Both species are from the same family (Moraceae) yet they differed markedly in their degree of susceptibility to disease and AM infection. Milicia regia seedlings were highly vulnerable to disease (caused mainly by Oomycetes, particularly Phytophthora species) and exhibited high levels of mycorrhizal colonisation, whereas Antiaris toxicaria seedlings suffered no mortality due to disease and had very low levels of AM colonisation in all forest situations. This species was, however, more susceptible to predation, the intensity of which depended on proximity to conspecifics. Spatial patterns of disease were evident in Milicia regia: seedlings had a higher probability of incurring pathogen-induced mortality close to female trees than under male trees or at a distance from parents, and were less susceptible to disease in light gap conditions. It is possible that spatial patterns in seedling disease were due to negative feedback resulting in higher inoculum loads of soil-borne Oomycetes under female trees. Differences in disease occurrence in contrasting light conditions were due to increased resistance of the host in gaps as opposed to differential pathogen activity. AM colonisation also varied spatially, with seedlings displaying host-specific inoculum preference. Moreover, AM colonisation also varied in response to light environment, being higher in gap conditions. It is proposed that a continuum of mycorrhizal benefit exists for Milicia regia - from improved nutrition to protection against pathogens, with the type of benefit being highly dependent on light environment. Milicia regia regeneration is thus subject to complex interactions between pathogens, mycorrhizal fungi and light environment which vary spatially around conspecifics, whereas these processes have limited influence on Antiaris toxicaria seedlings. Implications for patterns of seedling regeneration and the maintenance of tropical forest species diversity are discussed.