Biogeographic variation of distance-dependent effects in an invasive tree species

Plant pathogens and herbivores can maintain forest diversity by reducing survival of tree seedlings close to conspecifics. However, how biogeographic variation in these natural enemies affects such distance-dependent processes is unknown. Because invasive plants escape ecologically important enemies when introduced to a new range, distance-dependent mortality may differ between their native and introduced ranges. Here, we test whether the invasive tree Triadica sebifera escaped distance-dependent mortality when introduced to the United States from China, and examine the roles of natural enemies in native and introduced ranges. In both the United States and China, we performed field surveys along with field and greenhouse experiments with field-collected soils and soil sterilization treatments. In field surveys and the field experiment, insect damage on T. sebifera seedlings decreased with distance to conspecific trees in the native range (China), but damage was low at all distances in the introduced range (United States). In the greenhouse experiment testing the effects of soil pathogens, T. sebifera seedling mortality decreased with soil distance from conspecific trees in both ranges but distance-independent mortality was higher in native range soils. Our findings indicate that both insect herbivores and the soil biota contribute to distance-dependent effects on T. sebifera in its native range. They suggest, however, that plants may more readily escape herbivore than soil biota distance-dependent effects when introduced to a new range and so herbivores, rather than soil pathogens, contribute more strongly to biogeographic variation in distance-dependent effects. These results highlight the importance of considering species biogeographic variation in distance-dependent effects and teasing apart the roles that different natural enemies play when studying species coexistence, community diversity and biological invasions.