To examine whether the Janzen-Connell mechanism applies to temperate forests, seedling survival and causes of mortality were investigated at two distances (beneath, far) from conspecific adults and at two densities (high, low) at each distance for seedlings (n = 7935) of eight tree species co-occurring in a hardwood forest. Six of the eight species showed distance-and/or density-dependent seedling mortality mainly caused by diseases and rodents. In four of the five species primarily killed by disease (i.e. damping-off, blight, rot, powdery mildew), the infectivity (probability of infection by the disease) and/or the virulence (proportion of seedlings killed to those infected by the disease) were higher beneath than far from conspecific adults. These findings suggest that host specificity and/or spatially heterogeneous activity of natural enemies play an important role in the reciprocal replacement of tree species, maintaining species diversity in temperate forests.
1. In forest communities, the Janzen-Connell (J-C) model proposes that species diversity is maintained by noncompetitive distance-or density-dependent seedling mortality caused by host-specific natural enemies. Host specificity, however, has not been fully elucidated. 2. We conducted a cross-inoculation experiment to evaluate the host specificity of a pathogenic fungus, Colletotrichum anthrisci. The fungus was isolated from seedlings of four tree species (Prunus grayana, Fraxinus lanuginosa, Cornus controversa and Magnolia obovata), all of which were killed by damping-off disease beneath conspecific adults. Each fungal isolate was then inoculated into seedlings of P. grayana and F. lanuginosa. Molecular identification [internal transcribed spacer (ITS) sequences] also confirmed that all isolates (strains) showed 99-100% similarity with C. anthrisci, irrespective of their origin. 3. In both P. grayana and F. lanuginosa seedlings, all isolates of the pathogen caused damage, irrespective of origin, but the damage was more severe with the isolate from conspecifics rather than from any of the three heterospecifics. 4. In response to infection, callose papillae were deposited on the inner side of the leaf cell wall of seedlings in P. grayana; then, circular abscission layers formed between two layers of leaf cells surrounding the locus of infection. The central area of the infection was completely cut off from the rest of the leaf. In F. lanuginosa, infected leaves of seedlings were shed immediately after inoculation. This defensive behaviour, which may prevent further pathogen invasion, was more frequent in seedlings inoculated with isolates from conspecifics than from heterospecifics in both species. 5. Synthesis. Although the pathogenic fungus C. anthrisci is ubiquitous and attacks a wide range of host species, virulence was much stronger for strains derived from conspecifics rather than from heterospecifics, suggesting local adaptation and development of host specificity. If host specificity is common for several pathogens within a microbial community in a given area occupied by an adult, and the trait is also common for multiple tree species co-occurring within a forest community, the J-C model would be applicable to explain tree species diversity.
Bamboo vinegar solutions had pHs of 2.5 to 2.8, and the amounts of organic constituents were estimated to be 2.3 to 4.6% (w/w). Volatile organic compounds (28 components) were detected by GC-MS, and among of these, 11 compounds were common to three samples of bamboo vinegar. Perhaps acetic acid, 3-methyl-1,2-cyclohexadione, guaiacol, p-cresol, and syringol contributed to the characteristic odors (sour, smoky, and medicinal note) in bamboo vinegar.
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