- 1600s-early 1800s – Extensive removal and/or culling of trees leave the hill in a constant regenerative state.
- ~1810 – Benjamin Bussey aquires much of what is named “Hemlock Woods.”
- 1840s – Margaret Fuller and other members of the Trancendentalist circle frequently visit the landscape.
- 1842 – Benjamin Bussey wills his estate to Harvard College.
- 1872 – Harvard College accepts the James Arnold bequest, executes the indenture of the Arnold Arboretum, and agrees to locate the Arboretum on part of its Bussey estate, “Woodland Hill” in West Roxbury.
- 1883 – Arboretum land is taken into the Boston Park System and leased ($1.00/yr.) back to Harvard College for one thousand years.
- 1907 – The first accession of Chinese hemlock (Tsuga chinensis 6851, E.H. Wilson #952) is planted and survives until 1921.
- 1932 – Fire, fanned by a stiff wind, runs up the southwestern face of the hill, scarring the landscape and destroying a grove of Japanese yew (Taxus cuspidata).
- 1930s – “Indian camp relics” (e.g. projectile points) are found in the area.
- 1938 – The Great New England Hurricane topples a significant number of eastern hemlocks, leaving a pit and mound topography along its northern slope; a combined total of 400 trees (oak, hemlock, etc.) are lost.
- 1940 – Approximately 200 additional hemlocks, having been grown in Arboretum nurseries, are planted on Hemlock Hill in the fall.
- 1946 – An Asa Gray collection (ca. 1843, Tennessee) of Buckleya distichophylla (piratebush) is transplanted from the Harvard Botanic Garden to Hemlock Hill and becomes the the oldest cultivated plant in the Arboretum’s collections.
- 1997 – Hemlock Woolly Adelgid (Adelges tsugae; HWA), an exotic pest native to Asia, is found on numerous native eastern hemlocks on Hemlock Hill.
- 1998 – Approximately 99% (1,904 plants) of the eastern hemlock population are accessioned for long-term monitoring.
- 2004 – Collaboration begins with Harvard Forest to examine the long-term ecological impacts of eastern hemlock loss. Three of the six research plots (15x15m) established on Hemlock Hill are logged (136 hemlocks removed).
- 2008 – Betula lenta (black birch) are found to be heavily represented in the seed bank and thickly populate the Arboretum’s logged plots (circa 2004). Since the discovery of HWA in 1997, 30% of the eastern hemlock population have died.
- 2013 – Elongate Hemlock Scale (Fiorinia externa; EHS), an invasive scale from Japan and China, are observed for the first time on both Tsuga canadensis and T. chinensis.
Management and Research
In 1997, the hemlock woolly adelgid (HWA; Adelges tsugae) was discovered on the Arnold Arboretum’s Hemlock Hill. This tiny insect, a relative of the aphid, feeds with lethal effect on the hemlock species of eastern North America. Native to eastern Asia, HWA was first detected in Virginia in the early 1950s and has since spread throughout the mid-Atlantic and southern New England. Sadly, researchers studying the insect have observed very high mortality rates among infested forests.
Beginning in 1997, Arboretum staff monitored the condition and rates of decline of roughly 1,900 eastern hemlocks (Tsuga canadensis) on Hemlock Hill. Presently, it is believed that 100 percent of the population is infested with HWA. To date, over 500 trees have been lost. As has been observed on other sites, those trees located in more marginal habitat conditions—thin soils or south- and west-facing exposures—have often been the first to succumb.
Because of the absence of host resistance and limited cultural control options, chemical treatments are the only reliable means of protecting hemlocks. Clearly any chemical treatment brings concern for the larger environment. At the same time, Hemlock Hill is an important resource for a large urban population that for over 150 years has enjoyed the singular educational and aesthetic experiences of a majestic hemlock-dominated forest.
Finding balance among stewardship, education, and public service goals, we are presently protecting hemlocks that are of sufficient vigor to recover and that grow in conditions that are favorable for treatment and do not present risk of water contamination. We control HWA on selected trees with applications of horticultural oil and, more recently, soil-injections of Imidacloprid (®Merit), a treatment now provided to over 40,000 trees at Great Smoky Mountains National Park. While using this method, we pay close attention to ongoing research that monitors for non-target effects and persistence in the environment. Ultimately, it is hoped that these treatments will “buy time” until biocontrols or other non-chemical options can offer reliable protection.
Other management measures include:
- Removal of hazardous trees. We continue to remove hemlocks that are in severe decline and present a potential safety hazard.
- Native regeneration. We encourage the growth of native tree species by removing competing invasive plants. We have also planted native hardwood species on the Hill’s south-facing slope. In the future, we envision a mixed deciduous and evergreen forest in areas where hemlocks have been lost to HWA.
- Research Programs on Hemlock Hill. The severe consequences of HWA infestation pose compelling questions about the ecological changes associated with the loss of eastern hemlock. Beginning in 2004, the Arboretum collaborated with the Harvard Forest, a research institute, to examine changes on Hemlock Hill. Harvard Forest scientists established six 15 x 15 meter research plots in order to measure the changes that occur when hemlock is removed from the forest system. Measurements established baseline data for soil temperature, available nitrogen, organic soil mass and understory vegetation. Analysis compared nitrogen cycling, decomposition rates and regeneration across the six plots. Completed in summer 2008, the study is part of a longer-term Harvard Forest effort to assess ecosystem impacts of HWA in southern New England.
A second research project determined that Chinese hemlock (T. chinensis), a close relative of our native species, is fully resistant to HWA. Ongoing field observations assess the suitability of Chinese hemlock as a landscape replacement for the eastern hemlock.