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

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.