Editor’s Choice 100:4
A major aim in ecology is to search for the determinants of biological diversity and species coexistence. There is increasing evidence that soil micro-organisms play a key role in regulating plant diversity, and that plant-soil feedback contributes to plant abundance and coexistence. The very nice study by Yu Liu, Shixiao Yu, Zhi-Ping Xie and Christian Staehelin of Sun Yat-sen University, PRC in the latest issue of the Journal of Ecology (issue 100: 4, 2012), demonstrates that a pathogenic soil fungus, Fusarium oxysporum is responsible for density dependent decline of a common leguminous tree species, Ormosia glaberrima, in tropical forest in Heishiding Nature Reserve, Guangdong Province, China.
The work shows that the negative effects of the fungus are greatest near adult trees, that they decline with distance from the parent tree and that the fungus is only active as a pathogen at a site where the host tree is abundant. The study provides a text book example of the so called “Janzen-Connell” hypothesis which predicts that recruitment of seedlings near adult parent trees can be reduced by a host-specific enemy, in this case a soil fungus.
Importantly, the authors not only assessed the Janzen-Connell effect in the field, which has been done by others in the past, but they also fulfilled Koch’s postulates by isolating and characterizing the pathogen responsible for decline in seedling recruitment close to adult trees, and by showing that the isolated pathogen indeed is responsible for suppression of the tree under controlled conditions in a growth chamber. Interestingly, further experiments by the authors showed that the isolated fungus is host specific. Thus, by suppressing tree seedling establishment and by preventing the tree from becoming more abundant, the fungus is likely to contribute to greater tree diversity in these forests.
Marcel van der Heijden
Associate Editor, Journal of Ecology