COS 76-1 - Conspecific tree-soil feedbacks reduce survivorship and growth of tropical tree seedlings

Wednesday, August 5, 2009: 1:30 PM
Grand Pavillion I, Hyatt
Sarah McCarthy-Neumann, Department of Forestry, Michigan State University, East Lansing, MI and Richard K. Kobe, Department of Forestry and Grad Program in Ecology, Evolutionary Biology and Behavior, Michigan State University, East Lansing, MI
Background/Question/Methods

The Janzen-Connell (J-C) Model proposes that host-specific enemies could maintain high tree species diversity by reducing seedling performance near conspecific adults, promoting replacement by heterospecific seedlings. Support for the J-C Model often comes from decreased seedling performance for a focal species at near versus far distances from conspecific adults. The relative success of (con- versus hetero- specific) seedlings recruiting under a given tree species -- analogous to species contesting a canopy slot -- is a critical component of J-C, but has not been tested.  In a shade-house experiment, we tested plant-soil feedbacks as a J-C mechanism in six tropical tree species (Apeiba membranacea, Colubrina spinosa, Pentaclethra macroloba, Prestoea decurrens, Iriartea deltoidea and Virola koschnyi) by assessing effects of con-  versus hetero-specific cultured soil extracts on seedling performance. By sterilizing extracts, we tested whether soil microbes were the mechanism creating these plant-soil feedbacks. We also tested whether low light availability increased species vulnerability to pathogens.

Results/Conclusions

Supporting the J-C Model, three of the six species had decreased seedling performance [(mean total mass x mean lifespan) / (days of experiment)] when grown with extract cultured by con- vs. all hetero-specific individuals; however, we do not know if this result would be robust to inclusion of additional hetersospecifics. I. deltoidea had mixed results, performing better in two and worse in one hetero-versus con-specific cultured soil. C. spinosa performed better in con- vs. one of the hetero-specific soils. Only A. membranacea’s seedling performance was not significantly affected by extract source. From the perspective of canopy tree replacement in a given soil extract, con- versus hetero-specific seedlings were disadvantaged in 15 and favored in 6 out of 30 cases. Species pair-wise interactions of soil modification / seedling performance were chemically mediated, occurring regardless of sterilization. Microbes lacked host-specificity and reduced performance regardless of extract source, and light availability. These results, along with parallel prior research in temperate forests, suggest that plant-soil feedbacks are an important component of seedling dynamics in both temperate and tropical forests. However, even though seedling performance for a given species may have been lower in con- versus hetero-specific soil extracts, within a given soil extract conspecific seedlings were not always at a disadvantage, hampering the ability for negative plant-soil feedbacks to facilitate species coexistence via Janzen-Connell processes.

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