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Holland et al. – Vegetation changes in temperate forest

Vegetation changes in temperate forested island communities

in Lake Winnipesaukee, New Hampshire, USA

Department of Biology, University of Mississippi, University, MS 38677


USDA-ARS, Appalachian Farming Systems Research Center, Beaver, WV 24957

ABSTRACT. We document changes in overstory species composition and foliar cover during a twenty-three year sampling period, comparing woody species on three small islands in Lake Winnipesaukee, New Hampshire, and determining changes in dominant plant species over time. Floristic surveys began on these islands in 1901, and provide valuable information about earlier vascular plant composition. The three islands, named Three Mile Island, Hawk’s Nest Island, and Blueberry Island, were first sampled quantitatively in 1978, again in 1991, and most recently in 2001. Data are compiled here for woody vascular plant species found in long-term monitoring plots on the islands owned and/or managed by the Appalachian Mountain Club. On Hawk’s Nest Pinus resinosa, Pinus strobus and Tsuga canadensis were the three most dominant species. On Blueberry Island Acer rubrum, Betula populifolia and Tsuga canadensis were the three most dominant woody species when sampling began. However, by 2001 Ilex verticillata, Myrica gale, and Vaccinium corymbosum were the three most dominant. On Three Mile Island Pinus strobus and Quercus rubra were the two most dominant species in 1978, but by 2001 Acer pensylvanicum and Hamamelis virginiana were the dominants. Overall species richness within the permanent plots increased, suggesting that natural succession has followed similar patterns on all three islands.
Key words: Acer pensylvanicum, Hamamelis virginiana, Quercus rubra, Pinus strobus, Tsuga canadensis, island plant diversity, vegetation composition, disturbance, bootstrapping, forest succession, freshwater island vegetation

An understanding of human disturbance is increasingly recognized as critical for the interpretation of modern landscapes, the evaluation of conservation objectives, and the development of management approaches for natural areas (Egan and Howell 2001; Foster 2000; Foster and Motzkin 1998; Hall et al. 2002; Marcucci 2000). Despite widespread alteration of the New England landscape since European settlement, there have been few major shifts in species distributions, but rather several notable changes in the relative abundance of tree taxa (Hall et al. 2002). Several comparisons of early settlement and modern data from across the northeastern United States have documented decreases in Fagus spp., Tsuga spp., Picea spp., Ulmus spp., Carya spp., Pinus spp., and Castanea dentata (Glitzenstein et al. 1990; Nowacki and Abrams 1992; Russell 1981; Siccama 1971). Taxa that have increased in relative abundance across the region include: Acer spp., Betula spp., Populus spp., Abies balsamea Mill., Quercus spp., Prunus spp., and Fraxinus spp. (McIntosh 1972; Siccama 1971; Whitney 1990). The most dramatic change has involved Acer spp. (primarily Acer rubrum), which increased from 7 to 27% in Massachusetts from colonial to modern times (Hall et al. 2002). An examination of pre-settlement tree surveys for all six New England states indicates that in the 47 towns surveyed in New Hampshire, Fagus grandifolia represented 25.5%, Tsuga canadensis represented 14.7%, and Pinus spp. represented 12.3% of tree species across the state (Cogbill et al. 2002).

In this study, we review the influence of environmental variation and land-use history on forest composition and distribution in islands of central New Hampshire. We focused on the vegetation of three of the 253 islands in Lake Winnipesaukee, New Hampshire: Blueberry, Hawk’s Nest, and Three Mile Islands. These three are either owned or managed by the Appalachian Mountain Club (AMC), which allowed the authors, both of whom are members of the Club, easy access for sampling. All three islands are located in the northwestern arm of the lake, and the last of these three received its name because it is located three miles from the town of Center Harbor, NH.

Plot-based ecological studies build on the floristic surveys summarized in Holland and Sorrie (1989). Based on earlier observations of the three islands (Maciejowski et al. 1981), we predicted species richness would be highest on the largest island (Three Mile), lowest on the smallest (Blueberry), and that species richness would increase over time (Briggs et al. 2008). Secondly, we did not expect to see a great change in the importance of Tsuga canadensis and Fagus grandifolia, which are currently threatened in other places by the non-native invasive species, the hemlock woolly adelgid beetle (Orwig and Foster 2002; USDA 2005) and beech bark disease (Gilman and Watson 1993; Houston 1997). Thirdly, we present results from plot based ecological studies which quantify the frequency, distribution, and dominance of woody species listed in Holland and Sorrie’s (1989) floristic survey of the vegetation of Three Mile Island.

In 1900 the Appalachian Mountain Club (AMC) established a permanent camp on Three Mile Island (Rogovin 1983). By the early 1970’s the Camp had grown, and managers realized that certain uses needed to be restricted, leading to the implementation of a land use plan (Clapham et al. 2010). Adapted from Odum (1969)’s four types of environments needed by humans, the Camp was divided into Protective, Productive, Urban, and Compromise zones (Holland and Sorrie 1989). Protective and Productive zones see relatively little human use, while Urban and Compromise zones support the majority of human activity. By the 1970s five islands were affiliated with the Camp: Blueberry, Hawk’s Nest, Nabby, Rock, and Three Mile. Since Three Mile is the largest island, experiencing the greatest human impact and supporting structures which require regular maintenance of electrical and water fixtures, vegetation sampling was begun there. In July 1978, a pilot environmental assessment study at Three Mile Island Camp commenced with three purposes: (1) to establish a qualitative and quantitative vegetative baseline to allow future measurement of the floristic change and rate of change; (2) to document the recent history of the ecosystem and the role of disturbance (logging, fire, windthrow, and hiking) in its maintenance and development; and (3) to determine whether or not the land use plan “was working” by providing managers with vegetation data to establish a foundation for critical natural resource decisions (Maciejowski et al. 1981). During the winter of 1978-1979, it was agreed to sample Hawk’s Nest and Blueberry Islands during summer 1979, however, Nabby and Rock Islands would not be sampled due to the small size of these islands.


The shore of Lake Winnipesaukee is 154 meters above sea level in the foothills of the White Mountains. The lake has a water surface area of 187 square kilometers, and the distance around the lake is 463 kilometers (New Hampshire GRANIT 2009). The lake is fed by a number of assorted springs, and has a forested shoreline and crystal clear water. The three islands studied are erosional remnants left by the last glacier in the final retreat of the ice sheet between 10,000 and 15,000 years ago (Likens et al. 1967). The islands range in size from Blueberry (0.27 ha) to Hawk’s Nest (0.41 ha) to Three Mile Island (17.4 ha).

The geology of the islands in Lake Winnipesaukee in the reach between Moultonboro Neck and Meredith Neck (Bear, Pine, Black Cat, Three, Five, and Six Mile Islands) is quite uniform. These islands are all comprised of outcrops of the Winnipesaukee Quartz Diorite, which is generally described as a medium grained, gray quartz diorite (New Hampshire Highway Department 1940). In the places in which it is found it is often seen in contact or in gradational sections with granite, granodiorite and diorite. The bedrock core of Three Mile Island is covered with a surface mantle of broken blocks of bedrock from the local area produced as a result of the intense frost-prying action which came to bear on this area immediately following the departure of the ice. The mantle also contains unevenly distributed pockets of glacial till, which in this case is comprised largely of silt, sand and small stones.

Three Mile Island. Records from the 1890’s indicate that Three Mile Island (TMI) was then largely covered with a second-growth forest of Betula spp. and Populus spp., with an abundant understory of evergreen conifers (Holland and Sorrie 1989; Rogovin 1983). At the turn of the 20th century the island was described as “picturesquely rocky,” supporting a heavy stand of mixed timber with a richly diversified undergrowth of herbs and shrubs (Atkins 1972; Holland and Sorrie 1989). In May 1901, as a member of the Three Mile Island advisory board, H. P. Kelsey introduced 481 plants representing 60 predominantly woody species on Three Mile Island, contributed from his plant nursery (Kelsey 1902). A list was prepared in August of 1903 of the more important species of plants found on Three Mile Island (Kelsey 1903). By 1911 when Pease published his list of the 265 plant species at Three Mile Island, 51 of Kelsey’s 60 introductions persisted (Pease 1911; Table 1). By 1978, the island’s forests were dominated by Fagus grandifolia, Quercus rubra, and Pinus strobus with Acer rubrum, Betula papyrifera, and Tsuga canadensis also prominent (Maciejowski et al. 1981; Table 3). In a floristic survey in the 1980s, Holland and Sorrie (1989) reported 243 plant species on the island (Table 1).

Hawks’s Nest Island. Hawk’s Nest Island (HNI) is 100 m east of TMI and currently has no structures on it. HNI was privately owned by a family of AMC members through the mid-1960s, but their house fell into disrepair, and when HNI management was assumed by the AMC in the 1960s, the old house was dismantled, and management was assigned to the TMI Camp. Currently HNI is managed as part of the Protective zone, so permission for campfires is limited by the TMI Camp management. No botanical sampling had been conducted at HNI prior to 1979.

Blueberry Island. Blueberry (BI; also known as Minister Island) is 3.2 km northeast of TMI and is privately owned by AMC members who live outside of New England, so BI is managed by TMI staff. Blueberry Island is primarily used for picnicking, although the owners sometimes spend time in a primitive cabin there during the summers. William I. Haven, along with two college friends, purchased Blueberry on 22 December 1887 for $25.00 (Albert Gray, co- owner, pers. comm. 1980). An etching of the BI cabin dated 1899 shows an open area surrounding the structure with a few small conifers close by. Today Haven’s descendants in the North, Anderson, and Gray families share ownership for BI. The northeast and southeast ends of BI are managed by TMI staff as part of the Protective zone, while the central area near the 1899 cabin is managed as part of the Compromise zone. No botanical sampling had been conducted at BI prior to 1979.

Plots were established and quantitative sampling of the canopy, shrub, and herb strata of the islands’ mixed deciduous-coniferous forest were initiated at TMI in the summer of 1978, and extended to the two smaller islands in 1979 (Holland et al. 1983; Maciejowski et al. 1981). A circle, often used as a sample unit in forestry studies (Mueller-Dombois and Ellenberg 1974) was chosen for two reasons: it is easy to mark and relocate using only two coordinates after a period of time (Maciejowski et al. 1981) and it minimizes edge effect (Chapman 1976). To ensure random distribution a grid was drawn over a map of the three islands (Phillips 1959), each intersection on the grid was numbered consecutively, and numbered slips of paper were randomly drawn from a hat until 25 plot-centers had been identified. All 25 plot-centers have been permanently marked with plastic stakes, and further studies during 1991 and 2001 utilized the same plots. Each circle used to sample the overstory has a diameter of 34 m; 19 plots were established on TMI (the largest island), and three each were located on BI and HNI (the two smaller islands) in order to sample 12.55% of the overall islands’ area. Nomenclature followed USDA, NRCS (2011). Taxonomic authorities are provided in Tables 3, 4, and 5.

Within each circle, woody species (both trees and shrubs) composition, numbers of individuals of each species, and dominance were recorded. Total frequency of each species was recorded as the number of plots in which a species was present out of the total number of plots on each island (19 on TMI, 3 on HNI and 3 on BI). Importance values were calculated using the sum of the relative frequency (frequency of species A / total frequency) × 100), relative dominance (dominance of species A / total dominance) × 100, where dominance = rating of species A / sum of ratings for all species), and relative density (density of species A / total density) × 100, where total density = sum of the number of individuals of species A from all plots) of each species. All of the trees in each circle were identified and rated as to whether they were Dominant, Codominant, Intermediate, or Overtopped in relation to each other (Maciejowski et al. 1981; Smith 1962, 1986). Dominant trees have crowns extending above the general level of crown cover and receiving full light from above and partly from the sides (thus, received the highest rank of #4). Co-dominant trees (ranked #3) had medium-sized crowns and received little light from the sides. Intermediate trees (ranked #2) were shorter than those in the two preceding classes; receiving a little direct light from above but none from the sides. Overtopped (formerly “Suppressed” in Smith 1962) trees [ranked # 1; e.g. short trees, saplings, and tall shrubs] had crowns entirely below the general level of the crown cover (Smith 1986). Species richness was recorded as the number of species per plot sampled on each island.

The number of overstory species was summarized for each of the four land use zones each year of measurement using bootstrap resampling (sampling with replacement) (Efron and Tibshirani 1998). The resampling procedure involved pooling the data for each year and zone, for example one data pool contained data from the protective plots assessed in 1978. Then N samples were randomly selected with replacement from a given data pool where N equals the number of circular plots within a given land use zone. The mean of the N samples was then calculated and recorded and the procedure repeated for each measurement year and land use zone combination through 10,000 iterations using @Risk software (Palisade, Inc., Ithaca, NY), within Excel (Microsoft Inc., Redmond, WA). The frequency distributions of the 10,000 generated means are presented as probability density plots and cumulative frequency curves. Changes in mean species number between sample years were assessed using a non-parametric two-sample bootstrap hypothesis testing procedure described by Efron and Tibshirani (1993). In this procedure the null hypothesis, H0, is that the two distributions of interest (i.e. the mean number of species for years 1978 and 1991 for the Urban Zone) are equal. Output of the test is the probability that H0 is true.


Distributions of overstory species were modeled by bootstrapping data for each sampling period. Figure 1 (a-d) displays frequency distributions and cumulative probabilities for the total numbers of overstory species per plot for the four land uses for the three sampling periods. The increase in total overstory species in the protective and compromise zones between 1978 and 1991 was significant (Table 2). No other significant differences in the total overstory species in any of the other land-use zones over the periods 1978-1991 and 1991-2001 were significant (Clapham et al. 2009).

The number of species of woody plants increased in the permanent plots between 1978 and 2001 (Tables 3, 4, and 5; Figure 1). For the twenty-three year sampling period, overstory species richness increased on TMI and BI (Tables 3 and 5). Due to the marked increase in species richness on Three Mile Island from 1978 to 1991, which was an increase of 18 species, we predicted that in the 2001 sampling year the species richness would have increased again. However, species richness on Three Mile Island decreased slightly between 1991 and 2001 (Table 3). In the late 1970’s, the forests on Three Mile Island were dominated by Fagus grandifolia, Quercus rubra, and Pinus strobus (Maciejowski et al. 1981). Other prominent species included Acer rubrum, and Tsuga canadensis (Table 3) and these five have continued to be important into the 21st century.

As of 2001, there was no evidence of any hemlock wooly adelgid outbreaks on the three islands sampled. Other species of interest (Hawboldt 1944; Houston 1994; Siccama 1971) included Fagus grandifolia, Betula alleghaniensis and Betula papyrifera (Tables 3, 4, and 5), and Vaccinium corymbosum (Tables 4 and 5). While evidence of beech bark disease was observed on Three Mile Island by 1981 (Holland photo collection), beech tree density had increased over the sampling period (Table 3). As the island’s name suggests, blueberries (Vaccinium corymbosum) were a prominent component of the Blueberry Island flora throughout the sampling period (Table 5). While Betula alleghaniensis has not been sampled in the permanent plots on Blueberry Island (Table 5), it disappeared on Hawk’s Nest (Table 4), and on Three Mile Islands (Table 3). Betula papyrifera thrived on all three islands in 2001 (Tables 3, 4, and 5).

On Three Mile Island, Betula papyrifera, Fagus grandifolia, Tsuga canadensis, Acer rubrum, Pinus strobus, and Quercus rubra all decreased in importance values from the sampling in 1978 to the sampling in 2001 (Table 3). In 1978 Pinus strobus and Quercus rubra exhibited highest relative frequency (Table 3). Based on 1991 data, we expected Pinus strobus and Acer rubrum to be the most frequent species in 2001, but A. rubrum, A. pensylvanicum, and Quercus rubra were each found in all 19 plots, so were the three most frequent species in 2001 (Table 3).

Species with the highest dominance values in 2001 on Hawk’s Nest Island (Table 4) were Pinus resinosa (3.33), Betula papyrifera (2.75), and Pinus strobus (2.72). On HNI, Tsuga canadensis maintained an importance value over 15 throughout the sampling period, while Pinus resinosa, Pinus strobus, and Quercus rubra all decreased in importance values from 1978 to 2001 (Table 4). In 2001 Amelanchier laevis, Pinus resinosa, Pinus strobus, Quercus rubra, and Tsuga canadensis shared the highest relative frequency on HNI, while in 1978 Pinus strobus and Quercus rubra exhibited highest relative frequency.

Throughout the sampling period on Blueberry, Hawk’s Nest, and Three Mile Islands, the overall density recorded changed for individual species (Tables 3, 4, and 5). On Blueberry Island, Vaccinium corymbosum, Ilex mucronata, and Ilex verticillata increased in density (Table 5). On Hawk’s Nest, Tsuga canadensis showed the largest increase in density followed by Alnus incana spp. rugosa, which showed the second largest increase in density (Table 4). On Three Mile Island, Hamamelis virginiana and Acer pensylvanicum had the largest change in density over the three sampling years (Table 3). On Blueberry Island, three dominant species in 1979 (Acer rubrum, Pinus strobus, and Tsuga canadensis) all decreased in importance values by 2001, when Ilex mucronata, Ilex verticillata, and Vaccinium corymbosum exhibited the highest importance values (Table 5).

The greatest change in species richness is seen on Three Mile Island, which is most likely due to movement into the plots of species previously established in other locations on the island. Based on sampling in the 1970s, we expected to see Acer pensylvanicum, Acer rubrum, Fagus grandifolia, Pinus strobus, Quercus rubra, and Tsuga canadensis to have high importance which they did, but Hamamelis virginiana had the highest 2001 importance value (Table 3). Hamamelis virginiana, Acer rubrum, and Acer pensylvanicum were expected to have the highest density in 2001, but Fagus grandifolia had more individuals than Acer rubrum (Table 3). Quercus rubra, Quercus alba, and Betula alleghaniensis were expected to have the highest dominance (mean rating) for Three Mile Island in 2001, but in 2001 the four most dominant species were Betula lenta (3.5), Populus grandidentata (3.42), Quercus alba (4.00) and Quercus rubra (3.23) according to the mean dominance ratings (Table 3). Hamamelis virginiana (972) and Acer pensylvanicum (776) had the highest densities in 2001 on Three Mile Island, while Fagus grandifolia (531) also had a high density (Table 3).


Overall, overstory species numbers per plot and their standard deviations increased from the earliest sampling to the more recent on all three islands during the three sampling periods. Frequency distributions (Figure 1) provide an excellent means to display information about populations, mean variability and ‘shape.’ Since the data were bootstrapped, most distributions show a well-developed normal distribution. The insets showing the cumulative probability show clearly the differences among the distributions - their position relative to the x-axis, and the degree of variability – the slope of the probability function can be observed directly. The increase in overstory species richness may be attributed to a combination of natural succession (including blowdowns and lightning strikes) and the implementation of a land-use management plan on all three islands. Use of a similar land-use management plan for other islands could be helpful in protecting rare species and habitats.

In a study of Timber Island, Bradley and Crow (2010) compared the flora of Three Mile Island to that of three other Lake Winnipesaukee islands (Bear, Rattlesnake, and Timber). They speculated that an overall higher than anticipated species richness on Three Mile Island (compared to other island flora surveyed) may be related to the fact that the floristic study of TMI involved multiple-year sampling. We believe that increased protection of the island through a land use management plan which restricted heavy use of TMI to the southern end of the island (Clapham et al. 2010) may have allowed species with varied requirements to survive in different habitats spread throughout the island. Perhaps multiple-year sampling and protection have both played a significant role.

Regardless of what factors may have influenced the high species diversity on Three Mile Island, we can compare relative dominance and relative frequency of woody plants between Timber and Three Mile Islands. At Timber Island, Acer rubrum was present in all plots found in the Acer rubrum - Dulichium arundinaceum cover type (Bradley and Crow 2010), while Acer rubrum was present in all 19 plots on Three Mile Island (Table 3). Tsuga canadensis was present in 18 plots (95%) on Three Mile Island (Table 3), while it was found in 100% of the Tsuga canadensis cover type plots on Timber Island (Bradley and Crow 2010). Pinus strobus also occurred in 18 plots (95%) on Three Mile Island, while it was found in 94% of the Pinus strobus cover type plots on Timber Island (Bradley and Crow 2010). Quercus rubra occurred in 19 plots (100 %) on Three Mile Island, while it was found in 90% of the Pinus resinosa cover type plots on Timber Island (Bradley and Crow 2010).

Application of the Simple Matching Index to the island floras of Three Mile Island and Timber Island produced a value of 65.59% (Bradley and Crow 2010). This high similarity value is striking given the difference in human use of the two islands. Timber Island (135 acres) is the largest undeveloped island in Lake Winnipesaukee, while Three Mile Island (43 acres) experiences regular human disturbance from June through September when an AMC Camp is in session. It is possible, however, that this heavy human use has protected Three Mile Island from deer grazing in recent years. In fact, one moose was sighted on TMI about a decade ago, but only at the beginning of the Camp’s season. Deer have not been reported at TMI since the start of the vegetation sampling in 1978, so impacts from large grazing herbivores have not been noted.

Five out of the 25 plots sampled on BI, HNI, and TMI are shoreline plots, bordered by the Lake. It is in these plots that woody shrubs have proliferated, and the preponderance of Gaylussacia baccata, Ilex mucronata, Ilex verticillata, and Vaccinium corymbosum have been documented. Undoubtedly the easy access to sunlight has allowed these shrubs to dominate the shorelines of the three islands. By the mid-1980s, the number of non-indigenous species introduced to TMI by Kelsey had dropped to 10 species (Holland and Sorrie 1989), and most of these are growing in the island’s center in “Rhododendron Swamp,” part of the Protective Zone.

Cogbill et al. (2002) reviewed pre-settlement tree surveys to determine the natural patterns of vegetation throughout historical New England. The pre-settlement northern hardwood forests dominated by Fagus grandifolia were distinct from the Quercus spp. forests in southern New England (Cogbill et al. 2002). Since the earliest records for BI, HNI, and TMI date from the 1890s, we assume that the trees noted in pre-settlement New Hampshire may have provided the seed source for these islands. Seeds of Acer spp., Betula spp., and conifers are wind dispersed, thus with the mainland less than 0.5 miles away from the islands, the mainland could have provided the seed source. Specific changes on the three Lake Winnipesauke Islands sampled can be attributed to natural phenomena such as lighting strikes and other storm events, along with human uses associated with summer recreation (primarily in the Compromise and Urban zones). . Whereas records from the 1890’s (Rogovin 1983) indicate that Three Mile Island was covered by Betula spp. and Populus spp., by the late 1900’s the most frequently encountered species were Acer rubrum, Fagus grandifolia, Pinus strobus, Quercus rubra, and Tsuga canadensis.

The sampling of the three islands on Lake Winnipesaukee is part of ongoing floristic and ecological studies (Clapham et al. 2010; Hartmann 1941; Holland and Sorrie 1989; Kelsey 1902, 1903; Maciejowski et al. 1981; Pease 1911). As of this writing, healthy populations of Tsuga Canadensis were found on all three islands. In spite of the infestation of beech bark disease on Three Mile Island, density of Fagus grandifolia has increased over the study period; beech has not been found on Hawk’s Nest, but the one lone beech on Blueberry Island continues to survive. Numbers of “overtopped” woody specimens (shrubs and short trees) grew throughout the plots on all three islands. On Blueberry Island the density of Nemopanthus mucronatus, Vaccinium corymbosum, and Ilex verticillata increased each sampling year. On Hawk’s Nest Island, the species richness remained relatively constant, but density of Alnus incana ssp. rugosa, Lyonia ligustrina, and Viburnum cassinoides increased over the twenty-three year sampling period. On Three Mile Island, the density of Acer pensylvanicum and Hamamelis virginiana increased consistently from one sampling period to the next. We predict when the plots are next sampled the shorter woody species (both trees and shrubs) currently exhibiting high importance values on the islands’ edges will continue their importance there.

ACKNOWLEDGMENTS. The authors thank Marin Andrus, Raymond W. Prach Sr., Hannah Holland, and Raymond W. Prach Jr. for assistance with 2001 field work; James Fedders for help with statistical analyses; and C. John Burk, Elizabeth Farnsworth, Louis Palmer, Jr., Jay Maciejowski, Bruce Sorrie, Jason Hoeksema and two anonymous reviewers for reviews of various drafts of the manuscript. Logistical and moral support is acknowledged from Three Mile Island Camp managers, committee members, crew, and campers. Financial support from the Three Mile Island Camp Committee, the College of New Rochelle, and the University of Mississippi is gratefully acknowledged. Help from Forrest Briggs, Dustin Culver, Tara Davis, Shannon Moore, and Cherrelle Williams with manuscript preparation is greatly appreciated.

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Siccama, T.G. 1971. Presettlement and present forest vegetation in Northern Vermont with special reference to Chittenden County. American Midland Naturalist, 85, 153–172.

Smith, D. 1962. The Practice of Silviculture. John Wiley and Sons, Inc., Chapter 2.

______. 1986. The Practice of silviculture. Eighth edition. John Wiley and Sons, Inc. Ch. 2.

USDA Forest Service. 2005. An Exotic Pest Threat to Eastern Hemlock: An Initiative for Management of Hemlock Woolly Adelgid. Accessed 6 October 2011

United States Department of Agriculture Forest Service. August 2005. Hemlock Woolly Adelgid. ( Accessed 13 Nov. 2011.

USDA, NRCS. 2011. The PLANTS Database. National Plant Data Center, Baton Rouge, LA, Website ( Accessed September 2011.

Whitney, G.G. 1990. The history and status of the hemlock hardwood forests of the Allegheny Plateau. Journal of Ecology, 78, 443–458.
Table 1. Total reported, new taxa, and persistence from previous collections of plant species at Three Mile Island, New Hampshire, USA (from Holland and Sorrie (1989). * = Nine species planted by Kelsey were indigenous to the islands; ** = Ten species first observed on the islands by Hartmann persisted in 1989.

Number of Species


Total Species Reported

New to TMI

Persistence of Non- Indigenous Species Introduced by Kelsey in 1901

Persistence of Indigenous Species Reported by Pease (1911)

Kelsey (1902)*





Pease (1911)





Hartmann (1941)**





MMH/BAS (1989 study)





Table 2. Significance of differences of the mean number of overstory species per circular plot among the years indicated (from Clapham, Holland and Fedders 2009).

Land use zone

1978 to 1991

1991 to 2001













Figure 1: Frequency and cumulative frequency distributions for the number of overstory species per circular plot in the four ecological zones for the years 1978, 1991 and 2001.

Figure 1.

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