| Description |
In Panama, a high level of spatial turnover in tree species is correlated with a rainfall gradient. Seasonal drought is known to exclude tree species typical of wetter forests from drier forests; however, the factors contributing to the converse are ambiguous. This dissertation research experimentally tested the hypothesis that pathogens attacking seedlings contribute to the exclusion of dry-forest tree species from wetter forests. We tested two related hypotheses: that the phytopathogens attacking seedlings are (i) geographically widespread and (ii) host generalists. To test if pathogens exclude dry-forest species from wetter forests, wet- and dry-forest tree species were planted in wetter and drier forests in Panama and monitored for pathogen-caused damage and mortality. Seedlings suffered more pathogen-caused damage and mortality in the wetter forest, while dry-forest tree species suffered a greater impact from pathogen attack than wet-forest species. Together, these results support our hypothesis. Next, fungi isolated from symptomatic seedlings were identified using molecular techniques and phylogenetic analyses. We observed 28 fungal species and found that, while diversity was greater in the wetter forest, one-third of the observed fungal species were found in both the wetter and drier forests. This suggests that some phytopathogens are geographically widespread and that the elevated impact from pathogens in wetter forests may not be the result of different pathogen communities. Finally, we surveyed the tree species in which potential phytopathogens occurred and used inoculation experiments to assess the pathogenicity and host ranges of the potential phytopathogens. Most of the potential phytopathogens were isolated from multiple, phylogenetically distant families of trees. Similarly, in the experiments, phytopathogens were able to attack phylogenetically distant tree species. Tree species were differentially vulnerable to attack, suggesting that these generalist phytopathogens can influence plant community composition. While specialist phytopathogens have received considerable attention for their role in the maintenance of local diversity, this work highlights the underappreciated effect of generalist pathogens on regional diversity, represents one of the few studies to experimentally assess the host ranges of seedling pathogens in the tropics, and provides the first estimate of the taxonomy, diversity, and spatial structure of tropical phytopathogens at the landscape scale. |
