Oak seedling growth and ectomycorrhizal colonization are less in eastern hemlock stands infested with hemlock woolly adelgid
than in adjacent oak stands
James D. Lewis (1, 2), Jeff Licitra (1, 3), Amy R. Tuininga (1), Abby Sirulnik (1), Gregory D. Turner (1, 4) and Jacqui Johnson (1)
1. Louis Calder Center, Biological Field Station and Department of Biological Sciences, Fordham University, Armonk, NY 10504,
USA / 2. Corresponding author () / 3. Department of Mathematics, Stony Brook University, Stony Brook, NY 11794, USA / 4. Department of Biology, West Chester University of Pennsylvania, West Chester, PA 19383, USA / Received April 17, 2007; accepted June 13, 2007; published online February 1, 2008
Summary
Invasive, non-indigenous, phytophagous insects have caused widespread declines in several dominant tree species. The decline
in dominant tree species may lead to cascading effects on other tree and microbial species and their interactions, affecting
forest recovery following the decline. In the eastern USA, eastern hemlock (Tsuga canadensis (L.) Carr) is declining because of infestation by the hemlock woolly adelgid (HWA; Adelges tsugae Annand). Northern red oak (Quercus rubra L.) is a common replacement species in declining hemlock stands, but reduced mycorrhizal inoculum potential in infested hemlock
stands may cause oak to grow more slowly compared with oak in oak stands. We grew red oak seedlings for one growing season
in declining hemlock-dominated stands infested with HWA and in adjacent oak-dominated stands. Ectomycorrhizal root tip density
and morphotype richness in soil cores were 63 and 27% less, respectively, in declining hemlock stands than in oak stands.
Similarly, ectomycorrhizal percent colonization and morphotype richness on oak seedlings were 33 and 30% less, respectively,
in declining hemlock stands than in oak stands. In addition, oak seedlings in declining hemlock stands had 29% less dry mass
than oak seedlings in oak stands. Analysis of covariance indicated that morphotype richness could account for differences
in oak seedling dry mass between declining hemlock stands and oak stands. Additionally, oak seedling dry mass in declining
hemlock stands significantly decreased with decreasing ectomycorrhizal percent colonization and morphotype richness. These
results suggest that oak seedling growth in declining hemlock stands is affected by reduced ectomycorrhizal inoculum potential.
Further, the rate of forest recovery following hemlock decline associated with HWA infestation may be slowed by indirect effects
of HWA on the growth of replacement species, through effects on ectomycorrhizal colonization and morphotype richness.
