H. Lux, D. Orwig, P. Del Tredici
Our research on Hemlock Hill at the Arnold Arboretum continued into its 4th and final year in 2007. High levels of nitrogen availability and dramatic growth of herbaceous and shrub vegetation as well as increasing importance of black birch saplings are among the interesting highlights of this year.
In 2003, we began to study the effects of post-adelgid logging on ecosystem function at the urban Arnold Arboretum. Hemlock woolly adelgid (HWA) was discovered at the Arboretum on blown down foliage in 1997, and had likely infested trees there for years before. Hemlock Hill is a heavily used portion of the Arboretum and many moribund hemlocks needed to be removed to address safety concerns. At this time collaboration between the Arboretum and Harvard Forest was initiated to study ecosystem responses to removal of adelgid-infested hemlocks in this park setting. During 2004, six 15 x 15 meter plots were fenced off, and baseline data on soil nutrient cycling, microclimate, and vegetation dynamics were collected. Hemlocks were removed from four plots in February of 2005, while two remained untreated, as control plots. Slash was chipped and left on site in two chipped treatment plots, or removed from two logged (only) treatment plots. Minimal impact logging was conducted using a crane. This series of treatments coincided in time with Harvard Forest’s Hemlock Removal Experiment at the Simes Tract, where we are collecting analogous data, allowing for an urban vs. rural comparison.
To date we have completed one year of pre-treatment and three years of post-treatment measurements. Nitrogen availability, estimated with the use of resin bags, is very high at the Arboretum in comparison with rural sites, but did not increase significantly with tree removal. Results from winters in 2005 and 2006 trend toward increases in the cut plots, and analyses from the growing season and winter of 2007 may help clarify the nitrogen availability story on the hill (Fig. 1). There continues to be a dramatic vegetation response to hemlock removal in the logged plots. Herbaceous cover in both the logged and logged + chip plots have increased over time to between 50 and 60% cover. The control plot also exhibited a recent increase in herb cover due to declining hemlock health and several treefalls adjacent to the plot (Fig. 2). Similarly, average shrub cover has increased in all plots, but particularly in the cut plots (Fig 2). Rubus sp. continues to be the dominant shrub, while Frangula alnus averages about 10% cover in the chipped plots. Total seedling densities were highest in the control plots, but many of these were small, 1 or 2 year old hemlocks that are establishing under thinning canopies. In contrast, large Betula lenta seedlings with higher cover are driving the changes in both the logged and chipped plots. Understory species richness continues to be largely unchanged.
The last overwinter nitrogen mineralization cores and resins will be pulled at Hemlock Hill in April and May of 2008. These overwinter data will complete our data set and provide 4 years of nutrient and vegetation dynamics for synthesis.