Interspecific variation in susceptibility to fungal pathogens in seeds of 10 tree species in the neotropical genus <I>Cecropia</I>

Abstract

1. Species differences in susceptibility to pathogens acting at early life history stages may strongly influence the abundance and distribution of tropical trees. Here, we test the susceptibility of 10 congeners of the pioneer genus Cecropia to fungal seed and seedling pathogens and compare interspecific differences in intrinsic seed defences with survival. 2. Pathogens were experimentally removed through fungicide addition and/or autoclave sterilization of forest soil to determine the relative importance of fungal versus other microbial pathogens. Treatments were applied during a 4-month seed incubation (pre-emergence) phase or during an 8-week germination phase to distinguish between seed and seedling mortality. 3. Overall, seedling emergence after incubation in fungicide-treated, autoclaved soil was twice that in live soil, with significant positive effects of fungicide for six of 10 species. Pathogen infection occurred while seeds were quiescent in soil; fungicide addition during germination had no effect on emergence. Seedling emergence after burial ranged from 6 to 58%, indicating large interspecific variation in the capacity for Cecropia seeds to persist in the seed bank. Neither interspecific variation in survivorship, nor the relative strength of fungicide effects on survivorship was correlated with seed defence traits. 4. For four species, measurements of fungicide effects on emergence were coupled with direct measurements of the fungal and bacterial infection of seeds and seedlings. For two species, fungicide addition resulted in lower fungal infection rates and higher emergence success. However, Cecropia peltata, the species with the highest overall emergence success, also had the highest fungal infection rate. This suggests that either C. peltata was infected by a different suite of fungi than other congeners, or that fungi had low pathogenicity when colonizing this host species. 5. Synthesis. Our study shows strong interspecific variation in seed survival and susceptibility to fungal infection among congeneric tree species with similar life history. These differences are likely to influence recruitment success from the soil seed bank and may play a role in species coexistence.