There has been tremendous interest in understanding the mechanisms of plant species co-existence and diversity. Fungal pathogens and herbivores are known to play a key role in maintaining forest diversity by reducing tree seedling survival in a density- or distance-dependent manner. However, it’s unknown how biogeographic variation in these natural enemies affects such density- or distance-dependent processes.    

In this study, using Triadica sebifera as a model species, the authors tested whether this invasive tree could escape distance dependent effects when introduced to US from China, and examined how much different classes of enemies contributed to this phenomenon.    

In both US and China, the authors performed field surveys along with field and greenhouse experiments with field collected soils and soil sterilization treatments.  

In field surveys and the field common garden experiment, they found that insect herbivore damage on Triadica seedlings decreased with distance to conspecific trees in the native range (China) but damage was low at all distances in the introduced range (US). In the greenhouse experiment, Triadica seedling survival decreased with soil collection distance from conspecific adult trees in both ranges but distance independent mortality was higher in native soils. There were no effects of soil collection distance to adult Triadica tree in sterilized soils.   

The work including both soil pathogens and aboveground herbivory enemies is the first study examining the distance/ density dependent effect from a biogeographical perspective.  

The findings indicate that invasive trees can escape distance-dependent mortality, primarily those from herbivory. They also have important implications for better understanding the roles of different types of enemies in distance/ density dependent effects, particularly for predicting species community composition and invasion success.    

The results suggest the importance of considering species introduction history and biogeographic variation in distance-dependent effects when studying species coexistence, community diversity, and biological invasions.   

The work, entitled “Biogeographic variation of distance-dependent effect of an invasive tree species”, was published in Functional Ecology with Dr. YANG Qiang from Wuhan Botanical Garden (now working at Lanzhou University) as the first author and Dr. DING Jianqing as corresponding author. Dr. Evan Siemann also coauthored the paper. This work was supported by National Key Research and Development Program and NSF-China.  

The dependence of Triadica sebifera (a) seeding number and (b) percent chewing damage on seedling distance to a focal adult T. sebifera tree in the field sites in the native and introduced range. Letters denote means that did not differ in post hoc tests. Means + 1 SE (Image by YANG Qiang)