Effects Of A Pathogen On Seedling Recruitment Of Ormosia Glaberrima In A Subtropical Forest

The Janzen-Connell hypothesis provides explanations for species coexistence and predicts that recruitment of juveniles from adult trees at high density is reduced by natural enemies (particular soil-borne pathogens). Soil-borne pathogens possess relatively low dispersal ability, multiply rapidly in response to changes in host density, and possess strong host specialization. The transmission rates of soil pathogens likely depend on the density of host plants, suggesting that pathogens induce density- and distance-dependent mortality of seedlings around parent trees. Indeed, a growing number of papers provide empirical evidence for distance- and density-dependent effects of soil pathogens in tropical and temperate ecosystems. However, effects of specific pathogenic microbes on juveniles of parent trees were often indirectly demonstrated, and only few studies reported isolation and identification of specific pathogenic fungi, such as oomycetes (e.g. Pythium and Phytophthora) causing damping-off disease of juveniles.According to the Janzen-Connell hypothesis, a single adult tree (locally rare species) located at sufficient distance from aggregated conspecific adult trees (locally common species) should display a rare species advantage by ?escaping? from pathogens. In the Heishiding Nature Reserve, Guangdong Province, China, two 1-ha study sites of the subtropical legume tree Ormosia glaberrima were established (site 1 was characterized by a high density of adult O. glaberrima trees, and site 2 was characterized by a single adult O. glaberrima tree). Using a field manipulative experiment combined with a growth-room simulation experiments, we found that the recruitment of juveniles differed.1) Fungicide treatments at site 1 and a growth-room simulation experiment provided evidence that a soil pathogen negatively affected offspring survival (negative density-dependence). Seedlings at site 2, however, did not suffer from this fungus, suggesting that the single tree at this site might have ?escaped? from the pathogen (locally rare species advantage).2) The fungus reduced survival of seedlings depending on the variables“distance from the focal trees”and“seed provenance”. However, planting density at a local scale had no effect when the factors seedling density and distance from focal trees were manipulatively decoupled in situ at site 1 and under simulation conditions in the growth-room.3) The pathogen was identified by PCR (polymerase chain reaction). DNA was isolated from rotten O. glaberrima seeds around the five focal trees at site 1. PCRs were performed with primers ITS1 and ITS4 of the 28S ribosomal RNA gene in order to amplify the fungal internal transcribed spacer region (ITS rDNA). Out of 50 samples, an amplicon of expected size (ca. 600 base pairs) was obtained for 9 PCRs. Nine identical sequences were obtained (GenBank accession No JN002169), which were most similar to ITS rDNA sequences of the Fusarium oxysporum Schlecht. Hence, O. glaberrima seeds around all five focal trees at site 1 were infected by fungus F. oxysporum, whereas no other fungi could be detected by the used PCR approach.4) In total, 18 morphologically different isolates could be distinguished when fungi were isolated from rotten seeds and dying seedlings collected around the five focal trees at site 1. ITS rDNA sequencing for 10 isolates from the morphological group F06 indicated that F. oxysporum was isolated. A series of inoculation experiments with O. glaberrima demonstrated that the fungus could efficiently decrease seedling survival of O. glaberrima, whereas the other fungal isolates did not. Finally, F. oxysporum was successfully re-isolated from rotten seeds and dying seedlings to fulfill Koch?s postulates.5) The F. oxysporum isolate specifically induced disease symptoms on O. glaberrima, but not on the other three tree species Schefflera octophylla (Lour.) Harms, Cryptocarya concinna Hance, and Castanopsis fabri Hance, which co-existed with O. glaberrima in the forest. This finding fulfills a crucial criterion of the Janzen-Connell hypothesis (high host-specificity of natural enemies). Interestingly, the susceptibility of the examined O. glaberrima was also found to be different among the two study sites (likely due to genotype differences).6) High humidity and low light intensity could aggravate effects of soil pathogens on seedling survival of O. glaberrima under experimental conditions. Inoculum concentration and seed provenance were also found to affect the pathogenic interaction. Therefore, these factors should be taken into consideration when studying effects of pathogens on seedling survival under controlled conditions.Taken together, the findings meet the prerequisites of the Janzen-Connell hypothesis, and soil pathogens may promote species coexistence by keeping self-replacement of locally common species in check.