The potential for plant-animal interactions to limit reintroduction of American chestnut (Castanea dentata)

Background/Question/Methods

Before the accidental introduction of chestnut blight (Cryphonectria parasitica (Murr.) Barr [=Endothia parasitica (Murr.) Anderson & Anderson]) over a century ago, American chestnut (Castanea dentata) was a dominant tree species throughout much of its 800,000 km² range in eastern North America, comprising 25-50% of the forest tree community. Breeding programs for blight resistance have progressed to the point that putatively blight resistant progeny are being field-tested and reintroduction plans are being discussed. Much ecological research on chestnut has focused on understanding abiotic conditions necessary to promote growth. We know less about how herbivores and seed predators may interact with reintroduced chestnut, and when or whether they may limit the establishment and spread of chestnut populations. We conducted a field experiment to test for the effects of light and herbivory on BC₃F₃ (C. dentata x mollissima) chestnut seedling survival and growth. We then combined these results with previous work on pre- and post-dispersal seed predators to parameterize a matrix population model of chestnut that incorporates both seed predation and deer herbivory to determine their influences on population growth.

Results/Conclusions

Light conditions had a greater effect on seedling survival than did the presence of herbivores, with first-year survival in canopy gaps at 0.58 ( ±0.12 sd) and only 0.05 (±0.006 sd) under the canopy. Browsing by herbivores in unfenced gap plots indirectly benefited seedlings by suppressing competition, and this effect was more pronounced in the older of 2 gap sites where the competing vegetation was more established. In the matrix model, long-term population growth was more sensitive to post-dispersal predation (-0.135) than pre-dispersal predation (-0.013), but was most sensitive to survival and growth of mature trees. Preliminary analysis of transient dynamics indicates that elasticities of both population size and growth to seedling survival were the largest in the early years. Over time, the elasticity of population size and growth to post-dispersal seed predation increased and the elasticities to seedling survival decreased. These results indicate that deer may present an important ecological barrier to successful population initiation, but that seed predators may be more important in limiting the growth of developing populations.