Host-specific pathogens shape abundances of phylogenetically related tree species

Background/Question/Methods: The factors determining high species diversity in subtropical and tropical forests are poorly understood. According to the Janzen-Connell hypothesis, coexistence of tree species depends on an asymmetric density of host-specific pathogens or herbivores. Phylogenetic analysis and empirical results suggest that closely related species likely share pathogens and herbivores. However, this has been rarely demonstrated with experimental research. Here we tested the effects of different phylogenetically related neighbors on seedling survival in a subtropical forest. Six 1-ha permanent plots were established, and 181 species from 106 genera and 55 families were identified. Two hundred 1×1 m² test quadrates, regularly located within each plot, were defined and all seedlings of woody plants (DBH < 1 cm) were surveyed. The census was repeated twice. Based on these, the phylogenetic structure of subtropical forest tree communities and the effect of different neighboring tree species on seedling survival were analyzed by using Phylomatic program and Webb's net relatedness index (NRI). In a shade-house experiment, eight tree species commonly occurring at the field study site were selected. These species were chosen because their phylogenetic distance to Castanopsis fissa (Fagaceae) increased equidistantly, resulting in a series of species pairs whose evolutionary relatedness gradually decreased. Seedling survival was experimentally investigated with soil collected close to Castanopsis fissa. The treatments were performed by using the following variables: (1) species pairs with different phylogenetic distances (distance between C. fissa and one of the other species), (2) seedling density (four or twelve seedlings were planted into each pot), (3) fungicide treatment (applied to soil material; water was applied as a mock treatment in control pots).

Results/Conclusions: Co-occurring tree species in natural communities were less phylogenetically related than expected. The inhibition effects of neighboring trees on seedling survival decreased with increasing phylogenetic distance between seedling species and tree species in the neighborhood. Experiment result showed that seedling mortality was higher for C. fissa and related species than for more distantly related tree species. The fungicide treatment caused strongest promoting effects on C. fissa and closely related species, whereas only weak effects were found for distantly related species. These indicate negative effects of phylogenetically related neighboring trees on seedling survival and provide evidence that these effects are caused by associated host-specific fungal pathogens in the soil.