Provenance, not leaf physiological traits, explains enemy damage to deciduous understory shrubs

Background/Question/Methods

Exotic species vary widely in success and invasiveness in their introduced ranges. This variation has been attributed to multiple mechanisms including release from enemies. Successful invaders may possess attributes that enable them to benefit more from enemy release than their non-invasive relatives. Traits associated with rapid growth such as high photosynthetic capacity , short leaf longevity, and low leaf mass per area (LMA) may be advantageous, particularly when enemy pressure is low and tissue defense is less important. In a common garden experiment (Syracuse NY), we quantified foliar damage from insect herbivores and foliar fungal pathogens on 186 individuals of 68 understory shrub species common throughout eastern US forests. We included species from 7 genera that each included at least one native and one exotic species, plus 15 common native species that lacked exotic congeners. We analyzed traits including LMA, photosynthetic capacity, leaf longevity, and chlorophyll content in order to assess the resource acquisition capacity of individuals. We predicted that foliar damage would be greater on natives than exotics. We also expected non-invasive exotics to experience greater damage than invasive exotics. Additionally, we predicted species possessing traits associated with rapid growth to receive greater foliar damage.  

Results/Conclusions

On average, exotics experienced approximately 30% lower damage than natives (4.65% and 6.1%, respectively, of total leaf area; p=0.003). Furthermore, there was a significant interactive effect of provenance and leaf lifespan on foliar damage (p>0.0001). For exotics, foliar damage increased with leaf lifespan, whereas damage decreased with increasing leaf lifespan among natives. This suggests that different mechanisms underlie damage in natives and exotics. One possibility is that among natives, longer lived leaves are more resistant to damage, and among exotics, longer lived leaves are more tolerant of damage. We found no consistent relationship between foliar damage and any other leaf physiological traits suspected to influence enemy damage (LMA, photosynthetic capacity, chlorophyll content). Finally, we found no difference in damage when comparing non-invasive exotics to invasive exotics (p=0.65). These results suggest that while release from natural enemies does not explain current variation among exotic species in invasiveness, it may nonetheless contribute to ecologically important differences between native and exotic species.