Graduation Year

2023

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Biology (Integrative Biology)

Major Professor

Paul Camilo Zalamea, Ph.D.

Committee Member

Marc Lajeunesse, Ph.D.

Committee Member

Camila Pizano, Ph.D.

Keywords

germination, Lowland tropical forest, seed pathogens, seed-associated fungi

Abstract

Negative density-dependent regulation is the process by which host-specific natural enemies reduce the survival of seeds and seedlings of conspecifics near adult trees, favoring heterospecific recruitment, and ultimately increasing local diversity. Negative density-dependent regulation by fungal pathogens has been consistently reported to promote plant diversity in the tropics, even though they tend to be multi-host and host-specificity is rarely reported. Nevertheless, evidence suggests that host-generalist fungi can have host-specific effects that produce disproportionate negative effects on some plant species but not others, promoting regional diversity. However, such studies have not tested for both host-specificity and distance-dependent effects in fully replicated experiments; furthermore, they have been conducted using culture-based approaches neglecting non-culturable fungi.

We conducted a common garden experiment to evaluate the impact of burial duration, distance from adult hosts, and host relatedness on seed fate and the structure of seed-associated fungal communities. We buried freshly collected seeds of four species of tropical pioneer trees (Jacaranda copaia, Cecropia insignis, peltata, and longipes) below and away from the crown of adult trees of insignis and J. copaia in Barro Colorado Island, Panama. We retrieved seeds 3 and 12 months after burial and evaluated both seed germination and fungal infections characterized via culture-based and culture-independent approaches.

Germination decreased as seeds spent more time in the soil, varying across species. Simultaneously, for all species, isolation frequency, and fungal richness increased once the seeds were buried. Furthermore, fungal richness consistently increased over time in all species. Seed-associated fungal communities were mainly structured by plant species, with little contribution from burial duration, seed viability, and distance from the host. Finally, the taxonomic composition of the seed-associated fungal communities was dominated by fungi belonging to the phylum Ascomycota, and the relative abundance of different classes and orders changed over burial duration. Overall, the detrimental effects that we found on seed performance provide supporting evidence that fungal infections can cause important changes in plants’ demographics that could promote local diversity.

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