Delaware Water Gap National Recreation Area (“the park”) covers 28,000 ha along the Delaware River in Pennsylvania and New Jersey. Eastern hemlock forests cover 2,200 ha (8%) of the park, and are associated with trout streams and scenic waterfalls. Activities like trout fishing, bird watching, and hiking are concentrated in hemlock forests, and many of these areas are officially designated as “Outstanding Natural Features” having “high intrinsic or unique values." In 1989 hemlock woolly adelgid (HWA) was detected in the park, and since 1993 the park has conducted a program to address the threats that hemlock woolly adelgid and hemlock decline poses to park resources. This program includes annual monitoring of HWA populations and hemlock tree health in permanent plots, studies of hemlock ecosystems and associated biodiversity, and efforts to suppress HWA infestations and maintain hemlock ecosystems in the park. Questions addressed include: What are the distinctive characteristics of hemlock ecosystems in the park? What effects do HWA infestations have on hemlock health? What is the rate and extent of hemlock decline and mortality? How effective are insecticides and biological controls at suppressing HWA infestations and sustaining hemlock health? What are the ecological effects of hemlock decline?
Results/Conclusions
Sustained monitoring and research in the park during the past 17 years have provided answers to these questions and informed management. Streams draining hemlock forests are 1oC–2oC cooler in summer, hydrologically more stable, three times more likely to support trout, and typically support 30% more species of aquatic insects than streams draining hardwood forests. Birds such as blackburnian warblers, black-throated green warblers, and blue-headed vireos are very strongly associated with hemlock forests in the park. Permanent plot trees first became infested with HWA in 1998, and by 2008 no healthy hemlocks remained, and 30% had died. A dendrochronology study in the park concluded that every 1% increase in average plot HWA infestation level resulted in an 8% increase in the likelihood of a tree being in decline. Mathematical models indicate that hemlock mortality in the park will reach 50% by 2015, and 80% between 2020 and 2030. Although we have released over 90,000 biocontrol beetles in the past ten years, they have not controlled HWA populations. Insecticides effectively suppress HWA populations and sustain tree health, and we have treated about 10,000 individual trees to date. Continued hemlock decline will have major effects on park ecosystems and biodiversity.