The effects of weevil seed damage on germination, seedling vigor, and population growth of pure and hybrid American chestnut

Background/Question/Methods

Seed predation reduces reproductive output, potentially limiting regeneration and influencing both plant population dynamics and the evolution of seed traits. Weevils, Curculio spp, represent an important pre-dispersal seed predator in eastern US forests. We examine the individual and population effects of weevil infestation on American chestnut, historically a dominant tree species of eastern US forests. Although chestnut blight effectively eradicated sexually reproducing populations within the native range, the development of a blight-resistant hybrid makes reintroduction of this species possible within a decade. We addressed two questions: 1) What are the effects of weevil infestation on pure American and hybrid chestnut seed germination and seedling growth? 2) What are the potential effects of weevil infestation on chestnut seedling recruitment and population growth? We x-rayed pure and hybrid chestnuts to non-destructively assess weevil damage and grew the seeds in a growth chamber to determine the effect of the amount of weevil damage on germination and growth. Finally, we used the experimental data to within a matrix population model of chestnut to incorporate both pre- and post-dispersal seed damage and determine their influence on both short-term and long-term population growth.

Results/Conclusions

Weevil infestation rates in pure American chestnut were very low in both years: 0.003% in 2009 (3/580 chestnuts) and 0% in 2010 (0/281 chestnuts). Weevil infestation rates in hybrid chestnut were higher: 25% in 2009 (252/995 hybrid chestnuts) and 3% in 2010 (13/431 hybrid chestnuts). Germination probability declined with damage with the smallest damage level reducing germination from 94% to ~30%. No seeds with damage to greater than 50% of the nut germinated. Weevil damage reduced growth: Seeds with the smallest damage level were 50% shorter on average than undamaged seeds. While the sensitivity of long-term population growth to post-dispersal predation (-0.135) had a higher magnitude than weevil infestation (-0.013), long-term population growth was the most sensitive to survival and growth of mature trees. Model results indicate that although individual effects of weevils are severe, the long-term growth of established populations is unlikely to be affected. However, preliminary analysis of transient dynamics indicates that both pre-and post-dispersal seed predators may represent an important ecological barrier to successful restoration.