Environmental Reviews
BOREALIZATION OF THE NEW ENGLAND-ACADIAN FOREST: A REVIEW OF THE EVIDENCE
Journal: Environmental Reviews
Manuscript ID er-2019-0068.R1
Manuscript Type: Review
Date Submitted by the 06-Feb-2020 Author:
Complete List of Authors: Noseworthy, Joshua; University of New Brunswick, Faculty of Forestry and Enviromental Management Beckley, Thomas; University of New Brunswick, Faculty of Forestry and EnviromentalDraft Management Is this manuscript invited for consideration in a Special Not applicable (regular submission) Issue? :
Borealization, New England, Maritimes, Acadian Forest, Forest Keyword: Composition
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1 2 3 4 5 6 7 8 BOREALIZATION OF THE NEW ENGLAND-ACADIAN FOREST: A REVIEW OF THE 9 EVIDENCE 10 11 12 13 JOSHUADraft NOSEWORTHY 14 GLOBAL CONSERVATION SOLUTIONS 15 AND 16 THOMAS M. BECKLEY* 17 UNIVERSITY OF NEW BRUNSWICK 18 19 20 21 22 23 24 *CORRESPONDING AUTHOR: 25 FACULTY OF FORESTRY AND ENVIRONMENTAL MANAGEMENT 26 UNIVERSITY OF NEW BRUNSWICK 27 P.O. BOX 4400 28 FREDERICTON, NB CANADA E3B 5A3 29 EMAIL: [email protected] 30 506-453-4917
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31 TITLE: BOREALIZATION OF THE NEW ENGLAND-ACADIAN FOREST: A REVIEW OF THE
32 EVIDENCE
33
34 ABSTRACT
35 The New England-Acadian Forest (NEAF) is an ecoregion spanning 24 million hectares of the
36 northeastern U.S. and eastern Canada. The region is characterized as a transitional forest naturally
37 composed of both boreal and temperate species. The term “borealization” is sometimes used to
38 describe various processes driving the NEAF toward a more boreal character at the expense of its
39 temperate forest species and ecological communities. That the NEAF has undergone significant
40 landscape-scale change in the last four centuries since European settlement is well understood.
41 The purpose of this manuscript is to reviewDraft the literature on the forest composition and dynamics
42 of this region to investigate whether past, current, and/or predicted future processes of change are
43 indeed driving the forest toward a more boreal character. We examine studies on the historical
44 forest composition, impacts of past and current land-use practices, as well as indirect
45 anthropogenic changes that are predicted to influence future forest compositions of the NEAF. We
46 review over 100 peer-reviewed scientific journal articles and government reports related to this
47 issue. We find ample evidence to suggest that, at the landscape scale, there has been widespread
48 replacement of temperate tree species by boreal species since European settlement. Five primary
49 drivers have facilitated borealization across the NEAF: logging and high-grading, natural
50 reforestation of abandoned farmland, industrial clearcutting, anthropogenic fire, and boreal conifer
51 plantations. Furthermore, the borealization of the NEAF has continued to occur in direct contrast
52 to the predicted impacts of climate change. We encourage future scholarship to tackle these aspects
53 of borealization in the NEAF, including its social, economic, and ecological implications.
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54 INTRODUCTION
55 The New England-Acadian Forest (NEAF) is an ecoregion located in eastern North America.
56 Although geographic interpretations of the NEAF differ among sources, it can generally be
57 described as spanning the majority of New England (USA), the three Maritime Provinces of
58 Canada (with the exception of the highlands of New Brunswick and Nova Scotia), and
59 portions of southeastern Quebec (Figure 1). The NEAF is characterized as a transitional
60 forest composed of both northern boreal and southern temperate tree species. We are
61 interested in the long-term trend and trajectory of the forest composition of this region. The
62 term “borealization” is increasingly used to describe both current and historic land-use
63 practices that have driven the NEAF toward a more boreal character at the expense of
64 temperate tree species and forest communities.Draft Although landscape-scale changes since the
65 onset of European settlement are well documented across the NEAF, this paper addresses
66 whether these changes truly reflect a shift toward a more boreal tree species composition.
67 In order to proceed with this investigation, we first define borealization in the specific
68 context of the NEAF. Concurrently with the review of tree species compositional changes, we
69 summarize the key drivers of change that led to the current composition of the NEAF. In the
70 discussion section, we speculate about possible future trajectories for the NEAF given
71 climate change, as well as identify other distinct but related structural changes to the NEAF
72 that warrant further investigation.
73 DEFINING BOREALIZATION
74 Outside of North America, the term borealization has been used in several contexts, such as a
75 turnover in Arctic fish populations as a result of climate change (Fossheim, et al., 2015), to describe
76 the process of soil acidification due to the impacts of acid deposition and industrial conifer
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77 plantations (Emmer et al., 1998), and most frequently, to describe declines in temperate tree
78 species due to land-use practices that directly or indirectly favour boreal tree species (Jedrzejewska
79 et al., 1994; Fanta, 1997; Emmer et al., 2000; Lindbladh et al., 2014). Ideally, a wholistic and
80 comprehensive assessment of borealization would include examination of a range of biotic and
81 abiotic factors; However, historical forest scholarship in the region almost exclusively focuses on
82 trees. As such, we confine our definition of borealization to refer to forest tree species composition,
83 which is also the definition most often used when cited in the context of the NEAF (see Loo et al.,
84 2005; Diamond, 2008; Taylor et al., 2017; Lahey, 2018). With this in mind, it is important to
85 distinguish between native boreal and temperate tree species that occur within the NEAF for
86 comparison. The boreal forest is often characterized as coniferous and the temperate as deciduous,
87 but these generic associations do not reflectDraft the true nature of tree species distributions. Coniferous
88 and deciduous tree species occur in both boreal and temperate biomes, and as such, to determine
89 whether changes across the NEAF constitute borealization, a species-specific approach is required.
90 To establish a baseline for comparison, we conducted a spatial assessment to categorize the boreal
91 and temperate affinities of 30 tree species native to the NEAF, as presented in Burns & Honkala
92 (1990). Using a Geographic Information System (GIS), we calculated the proportion of each
93 species’ native range (spatially delineated by Little, 1971; 1976; 1977) that overlaps the North
94 American boreal zone as delineated by Brandt (2009). Tree species with >50% of their native
95 range overlapping the North American boreal zone were assumed to have a stronger boreal affinity
96 and were categorized as such. Incidentally, the results of this assessment align with the species
97 identified by Brandt (2009) as boreal, as well as other studies that identify boreal tree species in
98 North America (see Greene et al., 1999; Chen & Popadiouk, 2002; Taylor & Chen, 2011;
99 Nienstaedt & Zasada, 1990). Based on these results (Figure 2), if borealization is a true
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100 phenomenon, we would expect two outcomes from the literature review: (1) evidence of a general
101 increase in one or more of the following species: Jack Pine (Pinus banksiana), White Spruce (Picea
102 glauca), Black Spruce (Picea mariana), Balsam Poplar (Populus balsamifera), Tamarack (Larix
103 Laricina), White Birch (Betula papyrifera), Balsam Fir (Abies balsamea), Trembling Aspen
104 (Populus tremuloides), Pin Cherry (Prunus pensylvanica); and (2) evidence of a general decline
105 in one or more of the remaining species listed in Figure 2. In the following sections, we summarize
106 our findings from the literature review, which are structured across three time periods: pre-
107 European Settlement (pre-1600), European Settlement (1600-1890), and post-European
108 Settlement (1890 – 2019).
109 LITERATURE REVIEW
110 Pre-European Settlement Draft
111 In order to determine whether there has been a change in tree species composition in the NEAF,
112 we need to establish a baseline. Betts and Loo (2002) contrast two methods for setting a pre-
113 European settlement baseline for the NEAF - the Witness Tree and Potential Forest methods. Each
114 has its strengths and weaknesses, but taken together, such methods can provide a partial view of
115 tree species distributions and community types prior to 1600 AD. The processes that led to the
116 current landscape of the NEAF began approximately 12,000 years ago following the last glaciation
117 event. As the Laurentide Ice Sheet retreated, the exposed substrate was first colonized by tundra
118 vegetation, which was then replaced by boreal forest as the climate warmed over time (Anderson,
119 1980; Anderson et al., 1986). Tree species with temperate affinities began colonizing the region
120 approximately 9,600 years ago, and the NEAF as first encountered by Europeans came into being
121 approximately 3,000 years ago, coinciding with an increase in tolerant hardwoods (see Neily et
122 al., 2011) and Eastern Hemlock (Tsuga canadensis), followed by Red Spruce (Picea rubens), the
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123 so-called “signature” species of the NEAF (see Lorimer, 2001; Loo et al., 2010). The region has
124 been inhabited by humans for at least 10,000 years, albeit initially in relatively low densities, with
125 an estimated population between 90,000 – 120,000 at the time of European contact (Cronon, 1983;
126 American Friends Service Committee, 1989). Unlike elsewhere in eastern North America,
127 Aboriginal peoples of the NEAF generally did not rely on fire for hunting or clearing land, and
128 there was very little agriculture practiced north of the Kennebec River in Maine (Cronon, 1983).
129 In general, there is little evidence to suggest that Aboriginal peoples exerted landscape-scale
130 ecological change across the region, both owing to their low population density and hunter-
131 gatherer lifestyles (Anderson et al., 1986; Cronon, 1983; Foster et al., 1998; Loo et al., 2010).
132 Natural disturbances within the pre-settlement NEAF were primarily driven by gap- and patch-
133 scale dynamics (Fraver et al., 2009; Lorimer,Draft 2001; Seymour et al., 2002). Large-scale natural
134 disturbance, such as fire, catastrophic windstorms and insect outbreaks occurred infrequently
135 (Anderson et al., 1986; Seymour et al., 2002). As a result, at any time over the 3,000-year period
136 prior to European settlement, it is estimated that between 60-85% of the NEAF persisted in a state
137 of old-growth, which is defined here as forest >150 years old (Mosseler et al., 2003a; Lorimer &
138 White, 2003). That the pre-settlement forest was dominated by long-lived, shade tolerant tree
139 species is well-established through reviews of historical records (Lorimer, 1977; Lutz, 1996;
140 Cogbill, 2000; Blackadar, 2002; Crossland, 2006; Aubé, 2008; Dupuis et al., 2011; Thompson et
141 al., 2013; Ponomarenko et al., 2013), palynological studies (Mott, 1975; Anderson et al., 1986;
142 Green, 1987), extrapolative assessments using site classifications (Siccama, 1971; Zelazny et al.,
143 1997; Stewart et al., 2003; Sobey & Glen, 2004), and characterizations of remaining old-growth
144 (Greenidge, 1987; Chokkalingam & White, 2001; Mosseler et al., 2003a; Fraver & White; 2005;
145 D'Amato & Orwig, 2008; Fraver et al., 2009). In many historical records, trees are often described
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146 only to the genus level, such as birch (Betula spp.), pine (Pinus spp.) and spruce (Picea spp.),
147 which prevents distinguishing between predominantly boreal versus temperate species within the
148 same genus. Nonetheless, the combined approaches clearly illustrate a pre-settlement forest
149 dominated by temperate tree species, such as American Beech (Fagus grandifolia), Sugar Maple
150 (Acer saccharum), Yellow Birch (Betula alleghaniensis), Eastern Hemlock, Red Spruce, and
151 Eastern White Cedar (Thuja occidentalis), and to a lesser extent, Eastern White Pine (Pinus
152 strobus), White Ash (Fraxinus americana), and Red Oak (Quercus rubra). The studies listed
153 above also illustrate that Balsam Fir played an important role in the pre-settlement forest, but with
154 higher proportions generally occurring in the north of the NEAF, as expected given its boreal
155 affinity. Furthermore, boreal tree species are known to have dominated under specific conditions,
156 such as in exposed coastal areas, acidicDraft wetlands, and high elevation zones (Seymour & Hunter,
157 1992; Mosseler, 2003a; Neily et al., 2017). Outside these localized conditions, tree species with
158 boreal affinities appear to have had minor compositional importance in the pre-settlement forest.
159 Lorimer (1977) suggests that, in northeastern Maine, “Shade-intolerant species, such as white
160 birch (Betula papyrifera), aspen (Populus spp.), pin cherry (Prunus pensylvanica), larch (Larix
161 laricina), and pine were of minor importance” (p. 145). Dupuis et al. (2011) found that, in eastern
162 Quebec, “…dominance of cedar, fir and spruce was strong and uniform across the study area. In
163 contrast, the dominance of maple and birches was patchy, especially for paper birch [White Birch]
164 …” (p. 6). In eastern New Brunswick, Crossland (2006) found that, “Shade-intolerant species,
165 such as Populus, Pinus resinosa, L. laricina, Prunus, Q. rubra (and probably B. papyrifera), were
166 only minor components of historic forest composition” (p. 121). Furthermore, within an old-growth
167 Red Spruce forest in northern Maine, Fraver & White (2005) found, “…no living or dead
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168 intolerant tree species, such as Betula papyrifera and Populus spp. ... despite their abundance in
169 the harvested landscape surrounding the Reserve” (p. 606).
170 European Settlement: 1600 – 1890
171 The period 1600–1890 marks the onset of European colonization of the NEAF. This period
172 consisted of nearly three centuries of timber harvesting and land clearing for agriculture (Keeton,
173 2006; Loo et al., 2010, Bell 1989, Parenteau 2013). Land clearing began as early as 1604-1605
174 with the first European settlements of Saint Croix Island in Maine and Port Royal in Nova Scotia
175 (Thierry, 2012). Commercial lumbering was also an important driver of forest change during this
176 period, beginning with intensive high-grading of Eastern White Pine for ship masts and ton timber
177 (Acheson, 2008; Loo et al., 2010; Crossland, 2006). Parenteau (2013) describes successive waves
178 of forest exploitation; Which species wereDraft sought, for how long, and to what extent were matters
179 closely tied to the geopolitics of Europe, as well as a variety of factors related to timber access.
180 Logging and land clearing continued to expand across the NEAF as European settlements grew
181 and new timber markets were exploited. Studies comparing pre- and post-settlement impacts to
182 the forest indicate that changes to forest composition were both widespread and severe. At one
183 extreme, approximately 70% of Prince Edward Island was cleared for agriculture by 1900, and
184 what forest remained was largely subject to burning and high-grading (Glen, 1997). Cavallin &
185 Vasseur (2009) suggest the impacts of land clearing and high-grading in Prince Edward Island
186 have resulted in a forest with, “…higher frequencies of balsam fir, red maple, white spruce, white
187 birch and trembling aspen” (p. 170). Both Loo et al. (2010) and MacDougall et al. (1999) identified
188 several forest communities that have been “greatly reduced” throughout eastern Canada in part
189 due to agricultural clearing, including rich tolerant hardwood, wet calcareous mixedwood, and
190 tolerant softwood. Forests on calcareous soils and floodplains were particularly susceptible to land
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191 clearing for agriculture, resulting in declines in Bur Oak (Quercus macrocarpa; McPhee & Loo,
192 2009), Silver Maple (Acer saccharinum; MacDougall & Loo, 2002), and upwards of a 99% loss
193 of rich hardwood forest containing Basswood (Tilia americana), Ironwood (Ostrya virginiana),
194 and Butternut (Juglans cinerea) in western New Brunswick and northeastern Maine (Betts, 1999).
195 Although Maine, New Brunswick, Nova Scotia and the Quebec portion of the NEAF were
196 generally less affected by agricultural expansion compared to southern New England and Prince
197 Edward Island, waves of commercial logging, high-grading, and human-caused fire has left little
198 of the pre-settlement forest intact (Loo et al., 2010; Cronon, 1983). Thompson et al. (2013) state
199 that, across the northeastern U.S., “…logging and agricultural clearing were initiated that
200 removed more than half of the forest cover and cut over almost all of the rest” (p. 1). Mosseler et
201 al. (2000) suggest that Red Spruce hasDraft undergone a severe decline throughout its range due to,
202 “…a long history of selective removal, particularly during the 1800s…,” (p. 929). Seymour &
203 Hunter (1992) suggest that species such as White Pine, Red Spruce, and Yellow Birch have been,
204 “…greatly reduced in certain stand types through preferential high-grading and disease” (p. 9).
205 In Kings County, New Brunswick, Lutz (1997) concluded that temperate species such as Eastern
206 Hemlock, Eastern White Cedar, and Ash (Fraxinus spp.) have all declined since European
207 settlement, and that the current frequency of Balsam Fir has more than doubled. He goes on to
208 conclude that, “…white birch, poplar and red maple increased in areas where they were absent
209 two hundred years ago” (p. 66). In eastern Quebec, Dupuis et al. (2011) also found a significant
210 decline of Eastern White Cedar since European settlement, as well as an increase in Balsam Fir,
211 White Birch, maple, and poplar. In the eastern lowlands of New Brunswick, Crossland (2006)
212 found that White Elm, Ironwood, Eastern Hemlock, Eastern White Cedar, Eastern White Pine, and
213 American Beech have all declined. She goes on to state that, “[Balsam Fir] has doubled on many
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214 sites, and Populus has become the most abundant hardwood species. [Jack Pine], nearly absent
215 ca. 1800, has become the most dominant pine species” (p. iii). In northern Vermont, Siccama
216 (1971) found upwards of a 90% decline in the abundance of American Beech, and this is also
217 supported by Thompson et al. (2013) for the entire northeast U.S., who state that, “…while climate
218 and disease (particularly beech bark disease) may be contributing factors, the primary cause of
219 beech reduction locally and regionally is the disruption of the forest by deforestation, logging and
220 fire” (p. 12). In the Miramichi River watershed of New Brunswick, Aubé (2008) found declines in
221 Eastern Hemlock, White Elm, Eastern White Cedar, and “…presumably Yellow Birch…” (p.
222 1179), and concluded that, “…the Acadian forest bears more resemblance to the boreal forest than
223 it used to, at least in New Brunswick” (p. 1179). Similar results were found by Blackadar (2002)
224 in southwest Nova Scotia, with significantDraft declines in Sugar Maple, Eastern Hemlock, Eastern
225 White Pine, and Red Oak. In a historical review, Loo & Ives (2003) conclude that Sugar Maple,
226 Red Spruce, Eastern Hemlock, Yellow Birch, American Beech, and Eastern White Cedar have
227 declined throughout the Maritime Provinces, while Balsam Fir, Red Maple, White Spruce, White
228 Birch, and Trembling Aspen have increased.
229 As previously eluded to, anthropogenic fire also played a significant role in influencing the forest
230 composition of the NEAF (Lorimer, 1977; Crossland, 2006), such as the Miramichi fire of 1825,
231 which burned nearly 16,000 square km of forest in New Brunswick and Maine alone (MacEachern,
232 2014). While detailed compositional changes resulting from human-caused fires are scarce,
233 Ponomarenko et al. (2013) found that, in the eastern lowlands of New Brunswick, “...Pinus
234 banksiana [Jack Pine] – a fire-dependent species that is common and widespread in the modern
235 landscape, was absent at the time of land clearance” (p. 212). Furthermore, Loo et al. (2010)
236 suggest that mixedwood communities of fire adapted species such as of Jack Pine, Trembling
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237 Aspen, and Black Spruce have increased throughout eastern Canada since European settlement.
238 Fobes (1948) claims that, as forest was cleared for agriculture across Maine, “…fires increased in
239 number and extent to change markedly even the character of the original forest” (p. 269). He goes
240 on to suggest that much of the original spruce-fir and tolerant hardwood forest was replaced by,
241 “…light-seeded hardwoods such as aspen, grey and white birch and cherry” (p. 269).
242 While the bulk of evidence points towards a general decline in abundance of temperate tree
243 species, there is also evidence that several temperate species have benefitted from European
244 settlement. Abrams (1998) suggests that Red Maple was historically restricted to poorly drained
245 soils but has since become ubiquitous across the landscape due to land clearing, and this is
246 supported by several studies (Lees, 1978; Lutz, 1997; Loo & Ives, 2003; Cavallin & Vasseur,
247 2008). Although not stated explicitly Draft in the literature, numerous studies make reference to
248 increases in “poplar”, which would undoubtedly include Large-tooth Aspen. At the northern limit
249 of the NEAF, Dupuis et al. (2011) found that Sugar Maple increased in abundance since European
250 settlement, which is attributed to its ability to colonize cutover land and potentially due to climate
251 change. Nonetheless, with the exception of Red Maple, the available evidence demonstrates an
252 overall decline in temperate tree species throughout the NEAF due to widespread forest clearing
253 during the period of European colonization.
254 Post-European Settlement: 1890 – Present Day
255 The late 19th Century marked an increase in forest cover throughout New England and Maritime
256 Canada as a result of farmland abandonment (Census of Agriculture, 1971; Bell 1989), which was
257 followed by the rapid industrialization of forest harvesting in the mid-20th Century (Thompson et
258 al., 2013; Cronon, 1983). The previous centuries of high-grading and forest clearing not only
259 altered the abundance and diversity of tree species across the landscape, but as stated by Cronon
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260 (1983), created a new suite of growing conditions that were, “…sunnier, windier, hotter, colder,
261 and drier than they had been in their former state” (p. 123). As a result, pioneer and early
262 successional tree species became much more abundant (Thompson et al., 2013). Many boreal
263 species flourished under these new conditions, such as White Spruce, which Nienstaedt & Zasada
264 (1990) suggest is, “confined to abandoned fields in New England and the Maritime Provinces…”
265 (p. 207). In an early government report, Drinkwater (1957) suggested that White Spruce likely
266 occupied 500,000 acres (202,340 hectares) of abandoned farmland across Nova Scotia. Neily et
267 al. (2011) suggest that old-field forests in Nova Scotia today are, “typically dominated by White
268 Spruce, Tamarack, White Pine or Balsam Fir” (p. 109), and this is also supported by Simmons et
269 al. (1984). Old-field White Spruce is now one of the dominant forest communities across Prince
270 Edward Island due to farmland abandonmentDraft (Loo & Ives, 2003). The species is also known to
271 colonize old fields in Maine (Seymour, 1992), and is commonly associated with abandoned
272 farmland in New Brunswick alongside, “…poplar, white birch, grey birch and alder…” (Zelazny
273 et al., 2007; p. 45). Betts & Loo (2002) suggest that, “…White Spruce, poplar, Balsam Fir, and
274 White Birch are predominant species on old fields in the Maritimes” (p. 424). Although “poplar”
275 can refer to both Trembling Aspen and Largetooth Aspen (Populus grandidentata), both of which
276 are known to colonize old fields, the former generally appears to be more common (D’Orangeville
277 et al., 2008; Neily et al., 201l; D’Orangeville et al., 2011).
278 Although boreal tree species dominate abandoned farmland in the Maritime Provinces and
279 northern Maine, temperate species can also dominate abandoned farmland throughout the NEAF,
280 particularly in southern New England (Hibbs, 1983; Loo et al., 2010). This is particularly the case
281 for Eastern White Pine, which Abrams (2001) suggests may be more abundant in New England
282 today than prior to European settlement due to its ability to colonize abandoned farmland. In
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283 northern Vermont, Siccama (1971) found that, “The present success of pine (mostly white pine),
284 hardhack [Tamarack] and poplar is related to their role in secondary succession on abandoned
285 farmlands” (p.170). It is also worth noting that both Red Spruce and Eastern White Cedar are
286 known to colonize old fields, although these tend to occur under specific localized conditions and
287 are uncommon (Seymour, 1992; Cavallin & Vasseur, 2009; Curtis, 1946).
288 In addition to farmland abandonment and natural reforestation, forest harvesting – in a variety of
289 forms - has also influenced the present-day forest composition in the NEAF. While logging and
290 high-grading have been ongoing since European settlement, commercial- and industrial-scale
291 clearcutting in the NEAF began in the mid-1900s (Seymour et al., 2006; Nelson et al., 2012a). The
292 large forest openings created by clearcutting continue to perpetuate the growing conditions
293 previously listed by Cronon (1983), andDraft as stated by Salonius (2007), “…has led to increasing
294 representations of formerly rare, large-opening opportunist species […] that are more common in
295 boreal ecosystems” (p. 91). Silviculture studies have consistently shown that clearcutting favours
296 early successional species, namely intolerant hardwoods and Balsam Fir (Saunders & Wagner,
297 2008; Weaver et al., 2009; Olson & Wagner, 2010; Arseneault et al., 2011; Nelson et al., 2012a;
298 Salmon et al., 2016). Seymour (2005), states that, “...typical stand compositions have shifted from
299 the slower-growing, late-successional species to those that are favored by frequent harvest
300 disturbance, such as red maple, paper birch (Betula papyrifera Marsh.), aspen (Populus spp.),
301 and balsam fir” (p. 42). According to Butler (2017), Balsam Fir accounts for 36% of all the trees
302 in Maine, which is three times more than any other species. Clearcutting has also been attributed
303 to the reduction of several forest communities throughout eastern Canada, including wet
304 calcareous mixedwood forests composed of Eastern White Cedar, Black Ash, and Red Maple, and
305 tolerant softwood composed of Red Spruce, White Pine, and Eastern Hemlock (Loo et al., 2010;
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306 MacDougall et al., 1998). Both high-grading and clearcutting are attributed to the severe decline
307 of Red Spruce throughout the Maritime provinces (Major et al., 2003; Dumais & Prévost, 2007;
308 Mosseler et al., 2003a; 2003b; Loo et al., 2010), and Fraver & White (2005) conclude that, “Only
309 0.02% of the pre-settlement [Red Spruce]-dominated forests remains unharvested in the
310 northeastern USA” (p. 598).
311 While abandoned farmland and clearcutting have directly favoured boreal tree species at the
312 expense of temperate ones, as stated by Salonius (2007), “Plantation silviculture with boreal
313 conifers to enhance softwood fiber production has hastened such species transitions” (p. 91).
314 Although industrial conifer plantations are generally uncommon in the New England portion of
315 the NEAF (Seymour & Hunter, 1992), there has been considerable focus on plantation forestry in
316 the Maritimes, particularly in New BrunswickDraft (Ross-Davis & Frego, 2002; MacDougall et al.,
317 1998; Salmon et al., 2016). According to the National Forestry Database (2018), over 778,200
318 hectares (1.92M acres) of conifer plantations have been established in the Maritime Provinces
319 between 1990 and 2016, 67% of which were in New Brunswick.
320 Plantation forestry has traditionally relied on a handful of fast-growing tree species that are
321 exposure-tolerant, such as Tamarack, Jack Pine, White Pine, Red Pine and White Spruce
322 (McWilliams et al., 2005; Nelson et al., 2012b; Etheridge et al., 2005). Norway Spruce (Picea
323 abies) – a Eurasian boreal species, was also commonly planted in the 1970s and ‘80s and is
324 considered the most widely planted exotic tree in eastern North America (NS DLF, 1990). While
325 industrial pine plantations are generally found in southern New England (Fisher, 1928), Jack Pine
326 has been planted extensively in New Brunswick, which Erdle & Pollard (2002) state is, “…three
327 times more abundant in plantations than in the replaced natural forest” (p. 816). In contrast, Red
328 Spruce, “…comprises 1% of plantation volume in contrast to the 14% it made up in natural forest
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329 replaced by plantations” (p. 816). Throughout the Maritime Provinces, Freedman et al. (1994)
330 makes reference to “…the widespread establishment of white spruce in intensively managed
331 plantations up until the mid-1980s” (p. 43). In a compositional review of a 189,000-hectare
332 land base in northwestern New Brunswick, Etheridge et al. (2006) state that, “Although the
333 overall amount of softwood was similar in 1945 and 2002, nearly all of this area (88%) was
334 comprised of plantations in 2002. These included 56% black spruce, 30% white spruce, 9%
335 Norway spruce….” (p. 513). Black, White, and Norway Spruce were also cited by Boucher et
336 al. (2009) as covering 22% of a 117,000-hectare land base in eastern Quebec.
337 While it is difficult to ascertain to what degree plantations have replaced temperate forest 338 communities across the NEAF, activeDraft conversion of hardwood forest to conifer plantation was 339 common in the Maritimes prior to the early 1990s (Salmon et al., 2016). Loo et al. (2010) also
340 identified several forest communities that have been impacted by plantation establishment in the
341 region, including ridge-top hardwoods composed of Sugar Maple, Beech, Yellow Birch, White
342 Ash, and Ironwood, as well as upland mixedwoods composed of Sugar Maple, Yellow Birch, Red
343 Spruce, Beech, Balsam Fir, White Pine, Eastern Hemlock, and Eastern White Cedar. While it is
344 clear that plantations have at least partially contributed to the decline of temperate tree species and
345 communities in the NEAF, it should be noted that this is not always the case. In many instances,
346 forest communities replaced by plantations were already largely comprised of boreal species
347 as a result of past land use practices and anthropogenic fire (see Etheridge et al., 2005).
348 Additionally, temperate tree species such as Red Spruce, Eastern White Cedar, and White Pine
349 are increasingly being used in mixed conifer plantations (Erdle & Pollard, 2002; Dumais &
350 Prévost, 2007), although the ratio of temperate to boreal species planted across the NEAF is
351 currently unknown.
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352 DISCUSSION:
353 Throughout the world, the term borealization is increasingly being used to describe
354 anthropogenic ecosystem changes that directly or indirectly favour boreal species at the expense
355 of non-boreal ones. This study was undertaken to determine whether these claims hold true for the
356 NEAF. While we recognize that forests are composed of many biotic and abiotic elements, and
357 are thus considerably more complex than the composition of trees alone, trees play an outsized
358 role in both the structure and function of forest communities, and for this reason, we have limited
359 the scope of this work to focus on changes to tree species compositions. Within this scope, our
360 research suggests that borealization is an accurate descriptor of what has occurred across the region
361 since European settlement, as the two definition criteria have been met: (1) evidence of a
362 significant decline in temperate tree speciesDraft that once dominated the landscape, such as Red
363 Spruce, Sugar Maple, Yellow Birch, Eastern Hemlock, Beech and several other species considered
364 rare or uncommon today, and (2) evidence of a significant increase in boreal tree species, including
365 Balsam Fir, White Birch, Jack Pine, Black Spruce, Trembling Aspen and White Spruce. While the
366 primary drivers are varied and often layered, in its simplest form, borealization is the product of
367 opening up what was previously a closed-canopy forest, allowing exposure-adapted boreal species
368 to colonize. In the first several centuries of European settlement, this occurred as a result of high-
369 grading for commercial and subsistence purposes and clearing to convert forested land to
370 agriculture. The novel conditions that resulted from opening up the forest allowed both accidental
371 and intentional fires to burn massive swaths of the region, thus further promoting the colonization
372 of boreal tree species. Although farmland abandonment near the turn of the 20th century increased
373 the overall forest cover, this was also to the benefit of boreal tree species, particularly in the
374 northern half of the NEAF. Modern-day forestry practices, such as clearcutting and intensive
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375 conifer-based silviculture, have perpetuated the growing conditions favourable to boreal tree
376 species, and continue to do so today. While our study shows that borealization has occurred
377 throughout the NEAF in response to these drivers, it appears that borealization is “truer” as one
378 moves north, which stands to reason given the north-south climate gradient of boreal-temperate
379 biomes.
380 The current research, although anecdotal in places, clearly illustrates that the NEAF has indeed
381 undergone borealization over the past 400 years. However, given the many challenges and
382 limitations to this work, we are unable to state with any confidence the actual percentages or total
383 area of land that has been impacted. The NEAF straddles two nations and more than half a dozen
384 states and provinces, and very few studies include both Canadian and U.S. sources. Additionally,
385 the supporting evidence varies significantlyDraft in its methods and spatial and temporal scales, making
386 it nearly impossible to assess the extent of borealization in its entirety. The degree to which we
387 can rely on any single study to accurately characterize species-specific forest compositions within
388 a defined geography is directly related to how far back in time the study aims to reveal; pre-
389 settlement forest characterizations are generally less reliable than those following European
390 settlement, which in turn are less reliable than modern silvicultural studies and characterizations
391 of remaining old growth. While the combined evidence plainly demonstrates an increase in boreal
392 tree species at the expense of temperate ones, quantifying this change over a period of 400 years
393 is near impossible. Nonetheless, summarising the available evidence at an ecoregional scale is a
394 significant contribution to this body of work. Furthermore, doing so has led to several interesting
395 findings that warrant further investigation. Firstly, it is clear that much of the NEAF has undergone
396 two or more successive rounds of high-grading, clearing for agriculture, fire and/or harvesting
397 since European settlement. Even within what we characterize as the modern period (1890 –
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398 present), a significant proportion of the region has been harvested multiple times, albeit under
399 different silvicultural regimes. For example, softwood high-grading would have been common
400 practice prior to the collapse of the sawmill industry in the post -WWI era. If harvested again in
401 the post-WWII era, it was likely clearcut and allowed to regenerate naturally. If harvested again in
402 the last 50 years, clearcutting was likely followed by one or more silvicultural treatments that
403 favoured boreal conifers, such as planting, thinning or herbicide application. Each successive wave
404 of high-grading, clearing and/or harvesting had the compounding effect of both promoting the
405 regeneration of boreal species, while simultaneously decreasing the available seed source of
406 temperate tree species (with notable exceptions, such as Red Maple and Large-toothed Aspen),
407 and thus limiting their ability to recover.
408 In addition to borealization, our researchDraft also reveals two distinct but related forest processes that
409 have also taken place across the NEAF over the last 400 years. The first we term “forest
410 infantization”, which refers to both the perpetually young state of the forest due to the short harvest
411 rotations that became commonplace following the pulp and paper era of the 1930s, as well as the
412 hyperabundance of pioneer and early-successional tree species, regardless of their boreal or
413 temperate affinity. As shown previously, the drivers of borealization also tend to favour early
414 successional temperate species such as Red Maple and Large-tooth Aspen, providing further
415 evidence for the wide-scale replacement of long-lived, shade-tolerant (i.e. late-successional) tree
416 species. In a status review of northeastern forests, Anderson & Olivero-Sheldon (2011) suggest
417 that, “…our forests are overwhelmingly similar in age...” (p. 4-19), with an average cutting
418 rotation of 60 years or less. Furthermore, it is well established that old forest is rare throughout the
419 NEAF and is widely recognized as a conservation concern (Mosseler et al., 2003a; Davis, 1996).
420 The second process we term “forest bifurcation”, which describes a transition away from natural
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421 mixedwood communities toward pure hardwood or softwood. The former is a result of the
422 common past practice of high-grading softwoods such as White Pine, Red Spruce and Eastern
423 Hemlock from tolerant hardwood and mixedwood stands (Kelty & D'Amato, 2006), whereas the
424 latter is a product of conifer-based silvicultural practices involving planting, thinning, and
425 herbicide application that have directly favoured softwoods to the exclusion of hardwoods
426 throughout the NEAF (see Etheridge et al., 2005; 2006). Combined, these processes illustrate a
427 400-year trend towards a more simplified forest structure and composition at the landscape scale.
428 However, similar to borealization, actual percentages or total area of impacted land is unknown
429 for each of these processes. Regardless of this uncertainty, these processes warrant further
430 investigation, both individually and in combination with borealization, as they undoubtedly have
431 both economic and biodiversity implicationsDraft that are not currently understood.
432 While our objective was simply to review past scholarship to better understand changes in tree
433 species over time, we were also naturally led to speculate about the future of the NEAF,
434 particularly in the context of climate change. As asserted by Salonius (2007), “… short-lived,
435 exposure-tolerant, boreal tree species that regenerate in large forest openings are believed to be
436 less able, than the late-successional Acadian species they replace, to adapt to the climate warming
437 expected during the next forest rotation” (p. 91). Among predictive studies that covered the NEAF,
438 we found general consensus that boreal tree species will decline in the face of climate change, and
439 that temperate species will increase (Iverson & Prasad, 1998; Bourque & Hassan, 2008; Taylor et
440 al., 2017). Bourque and Hassan (2008) predict declines in Black Spruce and Balsam Fir due to
441 climate change, while species such as Yellow Birch, White Pine and Red Oak are predicted to
442 increase. Taylor et al. (2017) support these findings, and state that, “…under rapid 21st century
443 warming, Canada’s Acadian Forest Region will begin to lose its boreal character (i.e.,
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444 “deborealize”) as key tree species fail to regenerate and survive” (p. 1). While there is general
445 agreement among these studies that boreal tree species will likely decline as a result of climate
446 change, whether these predicted impacts apply to high-input plantations composed of boreal
447 conifers is to be seen. None of the predictive studies account for the effects of intensive
448 competition control, and it is conceivable that the combined effects of intensive management
449 coupled with a warmer and wetter climate may in fact increase the productivity of boreal conifer
450 plantations.
451 The results of this study, while somewhat generalized, have significant management
452 implications, not least of which relate to the “shifting baseline syndrome” that is pervasive in
453 natural resource management (Papworth et al., 2009; Vera, 2010; Soga & Gaston, 2018). This
454 phenomenon occurs as each successive Draftgeneration of scientists and managers tend to perceive the
455 environment at the beginning of their careers as the unaffected baseline condition against which
456 changes are compared. Soga & Gaston (2018) suggest that this gradual trend results in three
457 consequences: (1) an increased tolerance of ecological degradation over time, (2) an altered sense
458 of what is “desirable” or “good” from a management perspective, and (3) the setting of
459 inappropriate management targets based on reference conditions that have already been drastically
460 degraded. The NEAF has not been immune to these consequences, and we hope that this body of
461 work can assist in combatting them. Given that many temperate tree species in the NEAF are of
462 conservation concern due to past land use practices, and that the same species are generally
463 predicted to fare well in the face of climate change, we provide the following four
464 recommendations: (1) forest conservation efforts should focus on protecting remaining examples
465 of temperate forest communities in the NEAF, with priority going towards those that have
466 witnessed severe declines. Not only are they important from a biodiversity conservation
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467 perspective, but also from a climate-adaptation one, as they will likely serve as important seed
468 sources as the forest transitions to a more temperate nature in response to climate change; (2) forest
469 managers should consider transitioning to silvicultural practices that reverse borealization and
470 promote the regeneration of temperate tree species. This holds true for ecological restoration to
471 support the recovery of uncommon temperate forest communities and the wildlife that depend on
472 them, and may also hold true for commercial and industrial forestry operations. Given the risks
473 associated with climate change and the lengthy timeframes required to realize a return on
474 investment (whether ecological or economic), the opportunity costs of a climate-adapted
475 management strategy cannot be overlooked or ignored; (3) forest education programs, both formal
476 and informal, should focus on promoting firsthand experiences with remaining old-growth forests
477 across the NEAF. The shifting baselineDraft will continue to shift unless forest managers and
478 conservationists are able to recognise the differences between past and current forest conditions,
479 and this requires directly interacting with those conditions; (4) further research on the broader
480 social, ecological and economic impacts of borealization in the NEAF is warranted, including
481 research that expands the scope of borealization to non-tree biodiversity. We advocate for
482 silviculture trials aimed at reversing borealization and regenerating temperate species, as well as
483 trials designed to assess whether the predicted effects from climate change hold true for intensively
484 managed conifer plantations. Given the rate of climate change and biodiversity loss, we
485 recommend adopting an applied-science model for these studies, so that lessons can quickly be
486 translated into management.
487 CONCLUSION
488 There have been numerous published references and anecdotal claims that the New England –
489 Acadian Forest has undergone “borealization” as a result of past land use practices. While the
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490 amount and degree is impossible to discern with any precision, our review concludes
491 unequivocally that borealization has occurred throughout the New England Acadian Forest over
492 the last 400 years. The main drivers which have contributed` to borealization are high-grading,
493 clearing and subsequent abandonment of agricultural land, anthropogenic fire, clearcutting and
494 preferential conifer silviculture that directly favours boreal species. As a result of these practices,
495 many temperate forest communities are of conservation concern today. However, climate change
496 is generally predicted to favour temperate species over boreal ones, providing opportunity for
497 rebalancing historic tree species distributions and forest stand composition as well as improved
498 forest conservation, restoration and commercial management practices.
499
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747 processing and freezing-tolerance traits in red spruce and black spruce: species and seed-source
748 variation. Tree physiology, 23(10), 685-694.
749
750 McPhee, D. A., & Loo, J. A. (2009). Past and present distribution of New Brunswick bur oak
751 populations: a case for conservation. Northeastern Naturalist, 16(1), 85-100.
752
753 McWilliams WH, Butler BJ, Caldwell LE, Griffith DM, Hoppus ML, Laustsen KM, Lister AJ,
754 Lister TW, Metzler JW, Morin RS, Sader SA, Stewart LB, Steinman JR, Westfall JA, Williams
755 DA, Whitman A, Woodall CW (2005) The forests of Maine: 2003. Resour Bull NE-164. US
756 Department of Agriculture, Forest Service, Northeastern Research Station. Newtown Square
757
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758 Mosseler, A., Lynds, J. A., & Major, J. E. (2003a). Old-growth forests of the Acadian Forest
759 Region. Environmental Reviews, 11(S1), S47-S77.
760
761 Mosseler, A., Major, J. E., & Rajora, O. P. (2003b). Old-growth red spruce forests as reservoirs
762 of genetic diversity and reproductive fitness. Theoretical and Applied Genetics, 106(5), 931-
763 937.
764
765 Mosseler, A., Major, J. E., Simpson, J. D., Daigle, B., Lange, K., Park, Y. S., ... & Rajora, O. P.
766 (2000). Indicators of population viability in red spruce, Picea rubens. I. Reproductive traits and
767 fecundity. Canadian Journal of Botany, 78(7), 928-940.
768 Draft
769 Mott, R. J. (1975). Palynological studies of lake sediment profiles from southwestern New
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772 National Forestry Database. (2018). 6.2.2 Area planted by jurisdiction, tenure and species group.
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774 http://nfdp.ccfm.org/en/terms.php
775
776 Neily, P., Basquill, S., Quigley, E & Keys, K. (2017). Ecological Land Classification for Nova
777 Scotia. Nova Scotia Department of Natural Resources, Renewable Resources Branch. REPORT
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780 Neily, P., Basquill, S., Quigley, E., Stewart, B. & Keys, K. (2011). Forest Ecosystem Classification
781 for Nova Scotia. Part I: Vegetation Types (2010). Nova Scotia Department of Natural Resources,
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784 Nelson, A. S., Wagner, R. G., Saunders, M. R., & Weiskittel, A. R. (2012a). Influence of
785 management intensity on the productivity of early successional Acadian stands in eastern Maine.
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787
788 Nelson, A. S., Saunders, M. R., Wagner, R. G., & Weiskittel, A. R. (2012b). Early stand production
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800 Olson, M. G., & Wagner, R. G. (2010). Long-term compositional dynamics of Acadian
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802 40(10), 1993-2002.
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804 Olson, D.M., E. Dinerstein, E.D. Wikramanayake, N.D. Burgess, G.V.N. Powell, E.C.
805 Underwood, J.A. D'Amico, I. Itoua, H.E. Strand, J.C. Morrison, C.J. Loucks, T.F. Allnutt, T.H.
806 Ricketts, Y. Kura, J.F. Lamoreux, W.W. Wettengel, P. Hedao, and K.R. Kassem. (2001).
807 Terrestrial Ecoregions of the World: A New Map of Life on Earth. BioScience 51:933-938.
808
809 Papworth, S. K., Rist, J., Coad, L., & Milner-Gulland, E. J. (2009). Evidence for shifting
810 baseline syndrome in conservation. Conservation letters, 2(2), 93-100.
811
812 Parenteau, B. (2013). Looking Backward, Looking Ahead:: History and Future of the New
813 Brunswick Forest Industries. Acadiensis,Draft 42(2), 92-113.
814
815 Ponomarenko, E. V., Crossland, D., & Loo, J. (2013). Reconstructing tree species composition at
816 the time of land clearance: two approaches compared. In Charcoal and microcharcoal. Continental
817 records. F. Damblon, ed., Acts of the 4th International Meeting of Anthracology. Proceedings of
818 the 4th International Meeting of Anthracology. BAR International Series (Vol. 2486, pp. 203-214).
819
820 Ramovs, B. V., & Roberts, M. R. (2005). Response of plant functional groups within plantations
821 and naturally regenerated forests in southern New Brunswick, Canada. Canadian Journal of Forest
822 Research, 35(6), 1261-1276.
823
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824 Ross-Davis, A. L., & Frego, K. A. (2002). Comparison of plantations and naturally regenerated
825 clearcuts in the Acadian Forest: forest floor bryophyte community and habitat features. Canadian
826 Journal of Botany, 80(1), 21-33.
827
828 Salmon, L., Kershaw Jr, J. A., Taylor, A. R., Krasowski, M., & Lavigne, M. B. (2016). Exploring
829 factors influencing species natural regeneration response following harvesting in the Acadian
830 forests of New Brunswick. Open Journal of Forestry, 6(03), 199.
831
832 Salonius, P. (2007). Silvicultural discipline to maintain Acadian forest resilience. Northern Journal
833 of Applied Forestry, 24(2), 91-97.
834 Draft
835 Saunders, M. R., & Wagner, R. G. (2008). Long-term spatial and structural dynamics in Acadian
836 mixedwood stands managed under various silvicultural systems. Canadian Journal of Forest
837 Research, 38(3), 498-517.
838
839 Seymour, R.S. (1992). The red spruce-balsam fir forest of Maine: evolution of silvicultural practice
840 in response to stand development patterns and disturbances. In: Kelty, M. J., Larson, B. C., &
841 Oliver, C. D. (eds.) The ecology and silviculture of mixed-species forests. Springer, Dordrecht.
842
843 Seymour, R.S. (2005). Integrating natural disturbance parameters into conventional silvicultural
844 systems: experience from the Acadian forest of northeastern North America. United States
845 Department of Agriculture Forest Service General Technical Report Pnw, 635, 41.
846
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847 Seymour, R. S., Guldin, J., Marshall, D., & Palik, B. (2006). Large-scale, long-term silvicultural
848 experiments in the United States: historical overview and contemporary examples. Allgemeine
849 Forst und Jagdzeitung. 177: 104-112.
850
851 Seymour, R. S., & Hunter, M. L. (1992). New forestry in eastern spruce-fir forests: principles and
852 applications to Maine (Vol. 716). College of Forest Resources, University of Maine.
853
854 Seymour, R.S., White, A.S. & deMaynadier, P.G. (2002). Natural disturbance regimes in
855 northeastern North America—evaluating silvicultural systems using natural scales and
856 frequencies. Forest Ecology and Management, 155(1-3), 357-367.
857 Draft
858 Siccama, T. G. (1971). Presettlement and present forest vegetation in northern Vermont with
859 special reference to Chittenden County. American Midland Naturalist, 153-172.
860
861 Sobey, D. G., & Glen, W. M. (2004). A mapping of the present and past forest-types of Prince
862 Edward Island. The Canadian Field-Naturalist, 118(4), 504-520.
863
864 Soga, M., & Gaston, K. J. (2018). Shifting baseline syndrome: causes, consequences, and
865 implications. Frontiers in Ecology and the Environment, 16(4), 222-230.
866
867 Stewart, B. J., Neily, P. D., Quigley, E. J., & Benjamin, L. K. (2003). Selected Nova Scotia old-
868 growth forests: age, ecology, structure, scoring. The Forestry Chronicle, 79(3), 632-644.
869
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870 Taylor, A. R., Boulanger, Y., Price, D. T., Cyr, D., McGarrigle, E., Rammer, W., & Kershaw, J.
871 A. (2017). Rapid 21st century climate change projected to shift composition and growth of
872 Canada’s Acadian Forest Region. Forest Ecology and Management, 405, 284-294.
873
874 Taylor, A. R., & Chen, H. Y. (2011). Multiple successional pathways of boreal forest stands in
875 central Canada. Ecography, 34(2), 208-219.
876
877 Thierry, E. (2012). French Settlement, 1604-1613. In: Pendery, S. (ed.) Saint Croix Island, Maine:
878 History, Archaeology, and Interpretation. Occasional Publications in Maine Archaeology, No. 14.
879 Maine Historic Preservation Commission and the Maine Archaeological Society, Augusta. 310 pp.
880 Draft
881 Thompson, J. R., Carpenter, D. N., Cogbill, C. V., & Foster, D. R. (2013). Four centuries of change
882 in northeastern United States forests. PLoS One, 8(9), 40.
883
884 Vera, F. (2010). The shifting baseline syndrome in restoration ecology. In Restoration and
885 History (pp. 116-128). Routledge.
886
887 Weaver, J. K., Kenefic, L. S., Seymour, R. S., & Brissette, J. C. (2009). Decaying wood and tree
888 regeneration in the Acadian Forest of Maine, USA. Forest Ecology and Management, 257(7),
889 1623-1628.
890
891 Zelazny, V. F., Martin, G. L., Toner, M., Gorman, M., Colpitts, M., Veen, H., ... & Roberts, M.
892 (2007). Our landscape heritage: the story of ecological land classification in New Brunswick.
39 https://mc06.manuscriptcentral.com/er-pubs Environmental Reviews Page 40 of 42
893 New Brunswick Department of Natural Resources, Hugh John Fleming Forestry Centre,
894 Fredericton, NB E3C 2G6, Canada.
895
896 Zelazny, V., H. Veen & M. Colpitts. (1997). Potential Forests of the Fundy Model Forest.
897 Department of Natural Resources and Energy, Fredericton, N.B.
Draft
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Draft
899
900 Figure 1. The New England – Acadian Forest Region; Map created by Josh Noseworthy in ArcMap
901 10.3 (ESRI, 2015). Ecoregional boundary adapted from the Terrestrial Ecoregions of the World
902 (Olson et al., 2001).
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Draft
903 Figure 2. The proportion of 30 native tree species ranges’ that occur within the North American
904 boreal region. Proportions were calculated in a GIS using United States Forest Service range maps
905 for each species (Little, 1971; 1976; 1977) overlaid on the North American Boreal Zone as
906 delineated by the Canadian Forest Service (Brandt, 2009). Species were considered boreal if >50%
907 of their range fell within the North American Boreal Zone.
42 https://mc06.manuscriptcentral.com/er-pubs