Impact of Hemlock Woolly Adelgid on Forest Ecosystem Dynamics

1. Effects of Simulated & Actual Hemlock Woolly Adelgid Infestation on Forest Ecosystem Processes • USDA Forest Service, Coweeta Hydrologic Lab • Background • Eastern hemlock, a long-lived, shade-tolerant, evergreen tree dominating ~1 M ha is undergoing mortality caused by attack by an invasive exotic insect, the hemlock woolly adelgid (HWA). • Hemlock has no known resistance to HWA. Decline is progressive & can occur after 4 or more years of infestation. Decline of hemlock may lead to loss of uniquely associated fauna, changes in soil ecosystem processes & altered hydrological regimes. Characterizing hemlock crown loss from HWA • HWA decline & growth response by co-occurring plants Hemlock in the GDL plots declined faster than in HWA plots, indicated by mortality surveys, BAI & leaf litter inputs. Cumulative 2-yr litter inputs were similar for the treatments indicating rapid leaf area loss. Hemlock woolly adelgid Adelges tsugae Annand Co-occurring hardwoods responded within two years. However, understory evergreen shrubs had the greatest response, occurring fours years after infestation and girdling. Cover, richness & diversity increased over time, although still less than that in non-hemlock forest. Changes in light transmittance with season Experimental design • Plots (n = 8) had >50% hemlock basal area • Half of plots had hemlock girdled (GDL) in July ‘04 to simulate mortality from HWA • Infestation occurred in fall of ‘04 (HWA plots) • Additional four plots used as reference (no hemlock) • Trees & understory shrubs and herbs monitored for growth & abundance • Light transmittance & canopy litter monitored