PhD ceremony Mr. X. Liu: Interspecific and intraspecific perspectives of the Janzen-Connell effect. Incorporating phylogenetic and molecular evolutionary approaches

PhD ceremony: Mr. X. Liu, 9.00 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Interspecific and intraspecific perspectives of the Janzen-Connell effect. Incorporating phylogenetic and molecular evolutionary approaches

Promotor(s): prof. R.S. Etienne, prof. S. Yu

Faculty: Mathematics and Natural Sciences

The thesis of Xubing Liu provides a more comprehensive understanding of the Janzen-Connell hypothesis. He argues that the Janzen-Connell effect not only contributes to species diversity maintenance, but also helps to maintain genetic and phylogenetic diversity.

The Janzen-Connell hypothesis describes a density- and/or distance-dependent effect caused by natural enemies (pathogens and herbivores), which can maintain high diversity in tropical and subtropical forests. There have been hundreds of studies showing that many tree species exhibit patterns consistent with Janzen-Connell effects, but interspecific and intraspecific variation has been largely overlooked.

The major purpose of the thesis of Xubing Liu is to investigate the importance of interspecific and intraspecific variation for the Janzen-Connell hypothesis, by phylogenetic and molecular evolutionary approaches. We established large field surveys from 2008 to 2012, to measure the strength of density and/or distance-dependent effects in natural communities. He also conducted a series of shade-house experiments, aimed to investigate interspecific and intraspecific variation of plant-soil microbe interactions.

Xubing Liu clearly demonstrated with observational and experimental data that seedling mortality caused by pathogens decreases with increasing phylogenetic distance of neighboring trees. He also showed that the Janzen-Connell effect experienced by one population’s seedlings surrounded by pathogens from a different conspecific population decreases with decreasing functional trait similarity between the two populations. He then identified fungal pathogens associated with diseased leaves, rotten seeds and infected seedlings, and found that phlogenetically related hosts on the one hand and phylogenetically related pathogens on the other hand coevolved. Although soil pathogens significantly inhibited seedling survival, Arbuscular mycorrhizal fungi (AMF) largely promoted seedling growth. The antagonistic interaction between AMF and pathogens determines seedling performance and consequently species coexistence.