Monday, August 4, 2008

PS 6-91: Growth and survivorship of American chestnut trees in six Michigan populations: Influence of a blight pathogen and hypoviruses

Joshua C. Springer, Michigan State University, Anita L. Davelos Baines, The University of Texas - Pan American, and Andrew M. Jarosz, Michigan State University.

Background/Question/Methods

Trees at six populations of American chestnuts were monitored annually from 1996 to 2007. Two populations were experiencing epidemics of chestnut blight caused by Cryphonectria parasitica. Trees at two other populations displayed signs of recovery from blight epidemics Recovery was presumed to be caused by the invasion of hypoviruses that infected C. parasitica at both sites. Trees at the final pair of populations were initially disease free, but blight entered one site in 1997 and the other in 1998. Survivorship, diameter at breast height (DBH), and infection status were noted at each census. DBH was converted to cross-sectional area for analyses. Growth and survivorship of large trees (>10cm DBH) were compared across populations to determine the extent to which hypoviruses promote recovery of American chestnut. Small trees with DBH < 10cm are often unaffected by blight. These trees were monitored to determine if the understory trees in epidemic populations experienced ecological release as the large stems of overstory trees died from infection.

Results/Conclusions

Ninety-six percent of the 251 large trees in recovering populations survived from 1996 to 2007. Survivorship of the 70 large trees was only 67% in populations that were initially disease-free. However, all but one death was associated with blight infections that entered the population after monitoring began. One large healthy tree died from a lightning strike. 27% of the 77 large trees in epidemic populations died over the eleven year period. Growth of large and small trees varied across years and among populations. Year effects were most likely due to annual variation in weather patterns, while local habitat differences presumably underlie differences among populations. Disease status of individual trees had a significant effect on growth of large trees. Growth rates of large trees at recovering sites were small but relatively stable over the period from 1996 to 2007. In populations that were initially disease-free, large trees that remained healthy displayed slightly larger growth rates compared to large trees in recovering populations. In contrast, infected trees displayed negative growth. Large stems of these infected trees often died and were replaced by smaller stump sprouts. A similar pattern of negative growth was found for large trees in the epidemic populations. Patterns of growth and survivorship of small trees were not associated with disease classification. Small trees in epidemic populations did not display any evidence of ecological release. Our data suggest that hypoviruses can promote ecological recovery of American chestnut trees.