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).

陆生蝾螈是森林脊椎动物生物量的重要组成部分（Burton与Likens，1975a）。鉴于其种群丰度较高，陆生蝾螈在生态系统中兼具双重重要性：既是土壤动物的捕食者，亦是鸟类、爬行类与小型哺乳类等更大体型动物的猎物（Welsh与Droege，2001）。

1997年，美国马萨诸塞州牙买加普兰市哈佛大学阿诺德植物园（Arnold Arboretum）的铁杉山（Hemlock Hill）首次发现外来入侵害虫——铁杉球蚜（hemlock woolly adelgid，Adelges tsugae，简称HWA）。遗憾的是，此次虫害导致该园区内大量加拿大铁杉（Tsuga canadensis）死亡、长势衰退甚至被移除。2004年4月，哈佛大学森林（Harvard Forest）与阿诺德植物园的研究人员启动了针对此次干扰事件的环境影响研究（HF061）。为补充该研究，2004年夏季研究人员在铁杉山的3个样地内布设了24个人工隐蔽物（artificial cover objects，简称ACO），用于监测东部红背蝾螈（Plethodon cinereus）及其他两栖爬行动物的相对丰度。此次布设时机恰好可用于分析2004-2005年冬季实施的伐木处理对陆生蝾螈相对丰度的影响。

伐木处理对两个伐木样地内东部红背蝾螈的相对丰度产生了即时影响：与伐木前秋季相比，伐后春季样地1内红背蝾螈相对丰度下降了83%，样地2下降了63%。未进行伐木的对照样地（样地3）的相对丰度仅下降了9%。伐木样地内相对丰度较低的原因，可能是相较于对照样地，伐木后土壤表层（5.7℃）与5cm深土层（1.2℃）温度更高，且土壤表层相对湿度更低（仅3.4%）。

截至2005年秋季，样地2内红背蝾螈相对丰度已基本恢复，较2004年秋季仅下降7%；而对照样地的相对丰度则上升了15%。但样地1内红背蝾螈相对丰度在2005年秋季进一步下降，较2004年秋季减少了94%。这一差异可能源于样地1的坡度更陡：相较于样地2，其土壤保水性更差，且林床接受的光照更多。这种地形差异使得样地1的土壤表层温度、土层温度均高于样地2，而土壤表层相对湿度则更低。

此次研究共记录到147次东部红背蝾螈观测、12次美洲蟾蜍（Bufo americanus）观测、3次北方暗爪蝾螈（Desmognathus fuscus）观测，以及1次束带蛇（Thamnophis sirtalis）观测，所有观测均在人工隐蔽物下方完成。需特别说明的是，本次观测中存在大量重复记录，因此147次观测并不代表共观测到147只独立的东部红背蝾螈。在观测到的东部红背蝾螈中，40%为灰背型（leadback morphs），60%为红背型。该灰背型占比高于Lotter与Scott（1977）研究的新英格兰地区50个东部红背蝾螈种群，且与宾夕法尼亚州、马里兰州及俄亥俄州的种群占比更为接近。值得注意的是，Lotter与Scott的研究发现，灰背型个体的出现频率与气候温暖程度呈正相关。

三次北方暗爪蝾螈观测均于2004年在样地1的ACO6下方完成，2005年未再观测到该物种。此次束带蛇观测记录于2005年在样地2的ACO10下方完成。

2005年同期，对照样地内红背蝾螈的相对丰度高于作者研究过的30个次生东部铁杉林与混交落叶林。但这一现象更可能源于铁杉山林床内的粗木质残体（coarse woody debris）丰度低于马萨诸塞州中北部的未管理森林，而非蝾螈实际相对丰度更高。换言之，粗木质残体越少，蝾螈可利用的天然隐蔽物就越少，因此人工隐蔽物的吸引力也就越强。可佐证阿诺德植物园天然隐蔽物丰度更低的证据是：同一只人工隐蔽物下观测到多只红背蝾螈的频率显著更高。在阿诺德植物园的观测中，5%的记录为同一只ACO下有2只红背蝾螈（马萨诸塞州中北部仅为0.6%）；2%的记录为同一只ACO下有3只（马萨诸塞州中北部为0.08%）；1%的记录为同一只ACO下有4只及以上个体（马萨诸塞州中北部为0%）。

阿诺德植物园拥有长达百年的繁殖鸟类研究历史（1895年至2005年），使研究人员得以追踪一个多世纪以来鸟类物种组成的变化（Mayer，2005）。同理，本研究提供的基线数据有望为未来研究提供支撑，以评估因HWA虫害发生显著植被变化的阿诺德植物园铁杉山地区，其陆生两栖爬行动物相对丰度的潜在变化。未来研究还可探讨另一项有趣的问题：在气候潜在变化的背景下，该种群内灰背型个体的占比是否会随时间发生改变。

未来研究还可拓展至植物园的其他区域，例如布拉德利蔷薇科植物收集区（Bradley Collection of Rosaceous Plants）西侧的林地——该区域周边水塘可能为斑点钝口螈（Ambystoma maculatum）提供繁殖场所，以及巴塞溪（Bussey Brook）与穿过草甸的溪流——这些水域可能为北方双线蝾螈（Eurycea bislineata）提供栖息生境。