Eastern Redback Salamander Abundance at the Arnold Arboretum 2004-2005

Terrestrial salamanders are significant contributors to the overall vertebrate biomass in a forest (Burton and Likens 1975a). Due to this abundance, they are ecologically important as both predators of soil fauna, and as prey for larger animals such as birds, reptiles, and small mammals (Welsh and Droege 2001). In 1997, the invasive insect pest, hemlock woolly adelgid (Adelges tsugae) or HWA, was discovered on Hemlock Hill at Harvard University’s Arnold Arboretum in Jamaica Plain, Massachusetts. Unfortunately, this infestation has led to the death, decline, and removal of a larger percentage of the eastern hemlock (Tsuga canadensis) trees in this section of the Arboretum. In April 2004, researchers from the Harvard Forest and the Arnold Arboretum began studying the environmental impacts of this disturbance event (HF061). To complement this research twenty-four artificial cover objects (ACOs) were installed in three plots on Hemlock Hill in the summer of 2004 to monitor the relative abundance of eastern red-backed salamanders (Plethodon cinereus) and any other herpetofauna encountered. The timing of the installation permitted an analysis of the effects of a logging treatment conducted in the winter of 2004-2005 on the relative abundance of terrestrial salamanders.
Logging had an immediate impact on the relative abundance of eastern red-backed salamanders in both logged plots as seen by an 83 percent decline in the relative abundance of red-backs in plot 1 and a 63 percent decline in plot 2 in the spring following logging versus the fall prior to logging. In the unlogged control plot (plot 3) the relative abundance declined 9 percent in the spring versus the fall. The lower relative abundances in the logged plots were likely due to higher temperatures on the surface of the soil (5.7 deg C) and five centimeters below the soil surface (1.2 deg C), and lower relative humidity on the surface of the soil (3.4%) than in the unlogged plot. By Fall 2005, the relative abundance of red-backs had nearly recovered in plot 2 with only a 7% decline from Fall 2004, compared to a 15% increase in the control plot. However, in plot 1 the relative abundance of red-backs declined further in Fall 2005 with 94% fewer salamanders than in Fall 2004. These differences are likely because plot 1 is on a steeper slope leading to less retention of moisture and higher levels of sunlight reaching the forest floor than plot 2. This physical difference led to plot 1 having higher temperatures on the surface of the soil, higher soil temperatures, and lower relative humidity on the surface of the soil than plot 2.
In addition to the 147 Eastern Red-back Salamander observations, twelve American Toad (Bufo americanus), three Northern Dusky Salamander (Desmognathus fuscus), and one Garter Snake (Thamnophis sirtalis) observation were made under ACOs. It is important to point out that many repeat observations were likely made, and that 147 observations does not suggest 147 unique red-backs observed. Of the Eastern Red-backed Salamanders observed 40% were leadback morphs and 60% were red-back morphs. This is a higher percentage of leadback morphs than in any of the 50 Eastern Red-backed Salamander populations in New England studied by Lotter and Scott (1977), and more similar to the percentages seen in populations in Pennsylvania, Maryland, and Ohio. Interestingly, Lotter and Scott found that the frequency of leadback morphs is positively correlated with warmer climates. All three Northern Dusky Salamander observations were made under ACO 6 in plot 1 in 2004, and this species was not observed again in 2005. The Garter Snake observation was made in 2005 under ACO 10 in plot 2.
The relative abundance of red-backs in the control plot was higher than in any of the thirty second-growth eastern hemlock-dominated and mixed deciduous forests studied by the author during the same period in 2005. However, this may be more a reflection of a lower abundance of coarse woody debris on the forest floor of Hemlock Hill than in the unmanaged forests studied in north central Massachusetts than an actual higher relative abundance of salamanders. That is, fewer pieces of coarse woody debris would mean fewer natural cover objects for salamanders, thereby making ACOs more attractive. Evidence that natural cover objects were less abundant at the Arboretum than in north central Massachusetts was the much higher frequency of multiple red-backs observed under the same ACO. Two red-backs were observed under an ACO in 5% of observations at the Arboretum versus 0.6% of observations in north central Massachusetts, three observations were made under the same ACO in 2% of observations versus 0.08%, and four or more observations were made in 1% of observations versus 0%.
The Arnold Arboretum has a long history of breeding bird studies spanning from 1895 to 2005 which have allowed researchers to track changes in species composition over more than a century (Mayer 2005). Similarly, the baseline data provided here will hopefully be used by future studies to assess potential changes in the relative abundance of terrestrial herpetofauna on Hemlock Hill at the Arnold Arboretum as it undergoes significant vegetative changes due to HWA infestation. Another interesting question that future research will be able to assess is whether the percentage of leadback morphs in the population changes over time, given the potential changes in climate. In the future hopefully other areas of the Arboretum will be studied as well including the woods to the west of the Bradley Collection of Rosaceous Plants where spotted salamanders (Ambystoma maculatum) could be breeding in the adjacent ponds, and Bussey Brook and the stream running through the Meadow which may provide habitat for northern two-lined salamanders (Eurycea bislineata).