Late Holocene Records of Changing Moisture Regime
from Wetlands in Southwestern Nova Scotia, Canada:
Implications for Wetland Conservation and Restoration
Ian Spooner, Sarah Principato, Nicholas Hill, Hilary White, Dewey Dunnington, Tom Neily, and Susann Stolze
Northeastern Naturalist, Volume 24, Issue 3 (2017): 331–348
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2017 NORTHEASTERN NATURALIST 24(3):331–348
Late Holocene Records of Changing Moisture Regime
from Wetlands in Southwestern Nova Scotia, Canada:
Implications for Wetland Conservation and Restoration
Ian Spooner1,*, Sarah Principato2, Nicholas Hill3, Hilary White4,
Dewey Dunnington1, Tom Neily3, and Susann Stolze5,6
Abstract - An understanding of the morphological stability and succession of open water
and wetland ecosystems in Nova Scotia is a priority for informing the conservation management
of critical habitats for a complex of nationally listed, rare, disjunct wetland species.
Baltzer Bog and Big Meadow Bog in southwestern Nova Scotia contain stratigraphic
records of late Holocene moisture variability. Baltzer Bog is a shrub bog that formed in
an elevated, enclosed kettle basin. Excavated sections exposed by peat mining revealed 2
distinct wood-rich horizons that are located above a well-developed soil and wood horizon
that yielded a radiocarbon-dated age of 3260 cal. BP from an upright stump. The overlying
wood-rich horizons were dated at 1640 and 1045 cal. BP and were overlain by Sphagnum
species transitions indicative of increasing wetness. At Big Meadow Bog, a thin wood mat
in Sphagnum at 90 cm depth was dated at 1760 cal. BP. These records are broadly correlative
with pollen and stratigraphic data from Pleasant River Fen in central Nova Scotia
that indicate periods of high and low productivity and a fluctuating water table from 1950
cal. BP until present. Though other high-resolution paleoclimate records from the region
indicate that the late Holocene was a time of increasing precipitation and cooler air temperatures,
these wetland records demonstrate that in Nova Scotia this time period was
characterized by rapid variations in effective moisture and that significant and sustained dry
periods likely occurred. This record of late Holocene moisture variability and its influence
on habitat structure serves to better establish the potential for long-term residency of threatened
and endangered species at wetland sites.
Introduction
Nova Scotia, Canada has one of the densest archives of regional paleoenvironmental
data in Eastern North America because of the excellent preservation of lake
sediments of Late Wisconsinan and Holocene age (Stea and Mott 1998). Although
climate-change records interpreted from lake sediments provide a detailed chronology
for the Last Glacial Maximum, the Younger Dryas, and the Early Holocene,
there are gaps in the record during the last 5000 years. Wetlands have received far
1Department of Earth and Environmental Science, Acadia University, Wolfville, NS, B4P
2R6, Canada. 2Environmental Studies, Gettysburg College, 300 North Washington Street,
Gettysburg, PA 17325. 3Fern Hill Institute for Plant Conservation, 424 Bentley Road, Berwick,
NS, B0P 1E0, Canada. 4Department of Geography and Environmental Studies, Wilfrid
Laurier University, 75 University Avenue West, Waterloo, ON N2L 3C5, Canada. 5Department
of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois
Street, Golden, CO 80401. 6Institute of Arctic and Alpine Research, University of Colorado
Boulder, Boulder, CO 80303. *Corresponding author - ian.spooner@acadiau.ca.
Manuscript Editor: Gail Chmura
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less attention than lake sites, though they have the potential to preserve long (>9000
years) and continuous records of hydrological variability, a proxy for moisture regime
(Robichaud and Bégin 2009, Spooner et al. 2014, Suarez-Gonzalez et al. 2016). Abiotic
proxy records indicate that the middle Holocene thermal maximum was followed
by a transition to cooler and moister climate, though the timing of this transition is
poorly constrained (Lennox et al. 2010, Spooner et al. 2014). Similarly, most pollen
records from lake and wetland archives in Nova Scotia indicate a muted response to
this transition (Davis 1983, Green 1987, Ogden 1987). At Pleasant River Fen, southcentral
Nova Scotia, sediment stratigraphy, pollen data, and various proxies suggest a
changing late Holocene moisture regime, in particular an increase in precipitation and
water-table fluctuation from 5250 cal. BP onwards (Spooner et al. 2014).
An understanding of moisture regime in Nova Scotia and its impact on both open
water and wetland environments is a priority because these environments provide
habitat for disjunct and genetically distinct populations of a number of threatened
and endangered species such as Emydoidea blandingii (Holbrook) Blanding’s
Turtle; COSEWIC 2017, Mockford et al. 2005) and a suite of rare Atlantic Coastal
Plain, arctic–alpine, and boreal plants including Geum peckii Pursh (Eastern
Mountain Avens; Hill and Keddy 1992, Munro et al. 2014). The survival of these
species may depend, in part, on the stability of wetland habitat when subjected to
environmental change. Recent studies on the suitability of wetlands in Nova Scotia
for restoration and species re-introduction have focused on reversing the impact of
anthropogenic alteration of local hydrology (e.g., Eastern Mountain Avens at Big
Meadow Bog) and determining optimum forest and wetland structure (e.g., Blandings’s
Turtle at Pleasant River Fen) (Newton and Herman 2009, Spooner et al.
2014). An understanding of how these wetlands responded to past environmental
change is required to provide context for habitat management and restoration efforts
given predicted future changes.
In this study, we investigated biostratigraphic records from 2 wetlands in southwestern
Nova Scotia. At Baltzer Bog, in the Annapolis Valley, a peat section 2.3 m
thick and containing wood-rich layers and upright stumps is exposed in a kettlelake
bog (elevated, closed basin wetland) that overlies Wisconsinan glacial outwash
sand. At Big Meadow Bog on Brier Island, NS, trenching and vibracoring the central
zone of a raised bog formed on the base of marine sand revealed a wood-rich
layer at the base of the peat and a succession of Sphagnum types.
As Smith (1957) speculated, the former population ranges of many species are
based almost exclusively on modern ranges. The Baltzer Bog and Big Meadow
Bog paleoenvironmental records provide a 3500-year perspective on moisture variability
that may offer insight into species resilience to environmental change and
habitat suitability for wetland restoration and/or species re-introduction (Spooner
et al. 2014).
Study Sites
Baltzer Bog is a 28-ha shrub bog located in Coldbrook, NS, Canada (45°04'N,
64°35'W; Fig. 1). Abies balsamea (L.) Mill (Balsam Fir), Rubus allegheniensis PorNortheastern
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ter (Common Blackberry), Juniperus communis L. (Common Juniper), Pinus strobus
L. (White Pine), Pinus resinosa Aiton (Red Pine), Betula papyrifera Marshall
(Paper Birch), and Acer rubrum L. (Red Maple) are all common along the edges of
Figure 1. Location of the Baltzer Bog study site and sections (indicated by stars). The site
is located within a closed basin on top of an extensive kame deposit and is presently being
mined for both peat and aggregate. Site stratigraphy was obtained from drainage ditch exposures
at sections S1 and S2 and logs from hydrogeological wells (after Hennigar 2006).
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the bog. Plants found in the bog include: Picea mariana (L.) Mill. (Black Spruce),
Gaylussacia dumosa (Andrews) Torr. & A. Gray (Dwarf Huckleberry), Larix laricina
(Du Roi) K. Koch (Eastern Larch), Rhododendron groenlandicum (Oeder) Kron
& Judd (Labrador Tea), Sphagnum spp. (peat moss), Rhododendron canadense (L.)
Torr. (Rhodora), and Carex scoparia Schkuhr ex Willd. (Broom Sedge) (Munro et
al. 2014). Standing water is often present where the bog has been cut at Wood Lake
(Fig. 1), which is the lowermost point in the bog. Baltzer Bog has been mined for
peat and aggregate and has been farmed sporadically over the past 50 years. The
site is located within an elevated, closed basin on top of an extensive kame deposit.
The sand and gravel that underlies the peat deposits is relatively coarse and contains
a low percentage of silt- and clay-sized material. As a consequence, this sand is
very well drained and has high conductivity and low soil-moisture retention capacity
(Langille et al. 1993). The well-drained sand is underlain by a silty sand that
is moderately to poorly drained and results in the development of a local perched
water table (Fig. 1).
Baltzer Bog likely initiated as a kettle-lake bog when reeds, sedges, and mosses
slowly expanded across a very shallow pond (now Wood Lake; Fig. 1) in response
to a rising water table. A paludification bog likely formed around the periphery
of the study site as previously dry land was covered by bog vegetation. These
conditions can occur due to an elevation in the water table brought about by climatic
change, local hydrological change caused by beaver dams or logging, or the
natural advancement of a peatland. In both kettle-lake and paludification bogs, the
accumulation and compression of lower layers of peat can detach the underlying
mineral-rich soil from the water table creating acidic conditions that may kill existing
trees and allow bog vegetation to dominate (Cwikiel 2003).
Excavation and trenching corresponding with past commercial peat extraction
has exposed 2–3-m sections of peat and wood mats (Fig. 2). A recent hydrological
study of Wood Lake in Baltzer Bog (Jacques Whitford 2005) indicated that water
is lost by evaporation and seepage through the bottom of the lake at approximately
equal rates. Natural groundwater fluctuations average about 3 m in range.
Big Meadow Bog is an oval-shaped peatland 2 km in length with a central raised
bog that formed between the East Ferry and Brier Island member basalt ridges of
the North Mountain at Brier Island, the westernmost land of the Digby Peninsula
(44°15'N, 66°21'W; Fig. 2). The climate is boreal with average summer temperatures
below the July isotherm delimiting the southern edge of the Boreal forest.
Groundwater flow along the east and west edges of the peatland from minerotrophic
swamps on the basalt ridges creates a marginal fen bounded and influenced by both
the central raised bog and the surrounding swamps. This is a classic “lagg” ecotone
(Howie and Tromp-van Meerveld 2011), and the Big Meadow Bog lagg originally
consisted of a system of pools and streams that set up a dynamic habitat for a complex
of rare boreal organisms (e.g., Carex livida (Wahlenb.) Willd. (Livid Sedge),
Betula michauxii Sarg. (Michaux’s Birch), the fishing spider Dolomedes striatus
Giebel, and the endangered (COSEWIC 2010) Eastern Mountain Avens. Ditching
of the lagg zones and the central raised bog in 1958 accelerated outflow and lowered
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the water table (Environment Canada 2010). The lowered water table made the bog
attractive for nesting gulls, and the accumulated guano from nearly 40 years of
gull nesting (1978 to present) has transformed an ombrotrophic, Sphagnum-based
bog containing Sphagnum Section Acutifolia, S. magellanicum Brid. (Magellan’s
Sphagnum), and S. affine Ren. & Card. with ericad shrubs and Rubus chamaemorus
L. (Cloudberry), into a weedy mosaic of introduced grasses, canes (Rubus spp.) and
non-ericad shrubs (Environment Canada 2010).
Brier Island at the mouth of the Bay of Fundy is believed to have been the earliest
land on Nova Scotia to deglaciate during the Chignecto Phase (15.9–14.7 cal.
BP; Stea et al. 2011). The relatively early deglaciation of the Digby Peninsula may
have afforded boreal and arctic–alpine plants a refuge as competitive processes at
the margin of the retreating continental glacier led to arctic–alpine communities
being replaced by boreal species and these in turn by the temperate flora as climate
Figure 2. Location of the Big Meadow Bog study site on Briar Island, southwestern Nova
Scotia. Big Meadow Bog is an oval-shaped peatland 2 km in length that has drainage
ditches excavated in the late 1950s. A minerotrophic swamp on basalt drains into the lagg
on each side of the bog. The star indicates the section location.
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rapidly warmed. Upon deglaciation, the Eastern Mountain Avens would have had
to follow its arctic–alpine community type north and eastward to keep from being
outcompeted by the more competitive vegetation that would have replaced the low
biomass communities of stress-tolerant plants at the glacier edge. Brier Island may
have deglaciated 2000–2500 years before alpine New Hampshire (cf. values in Stea
et al. 2011 and Thompson et al. 1999). Stea et al. (2011) suggested that a corridor
for animals and plants likely was available along the edges of the Bay of Fundy in
the Chignecto phase of the Appalachian Glacier Complex (R.R. Stea, Nova Scotia
Depratment of Natural Resources, Halifax, NS, Canada, 2015 pers. comm.).
The thin maritime band of boreal climate along Nova Scotia’s shoreline suggests
a potential pathway for the continued migration of the boreal organisms on Brier
Island; however, a geological interval of almost 500 km of incompatible rock type
(Meguma Formation slate and metaquatzite) exists between Big Meadow Bog and
calcium-rich rock of Richmond County, Cape Breton, limiting eastern migration.
Climate
Due to the high precipitation rates associated with maritime air masses and the
wide annual temperature range associated with its continental location, the climate
of Nova Scotia is classified as a moist continental climate (Shaw et al. 1996). The
ocean modifies the continental climate of Nova Scotia. The North Atlantic Drift,
an extension of the Gulf Stream, brings warm water (~16 °C) several hundreds of
kilometers south of Nova Scotia, warms summer air temperatures, and extends the
fall season by several weeks (Jetté and Mott 1995). The cool Nova Scotia Current,
an extension of the Labrador Current, has the opposite effect: this cold water
current (~8–12 °C) decreases sea surface temperatures along the Scotian Shelf,
moderates winter temperatures, and delays the onset of spring by a couple of weeks
(Jetté and Mott 1995). The Digby peninsula (Big Meadow Bog) has a total annual
precipitation of 140 cm, an average low temperature of 4 °C , and an average high
temperature is 11 °C., whereas the eastern Annapolis Valley (Baltzer Bog) is somewhat
drier (average precipitation = 100 cm) and warmer in the summer months
and cooler in the winter (average high = 12 °C, average low = 2 °C) (Environment
Canada 2016).
Methods
We conducted stratigraphic description and macro- and microfossil collections
at Baltzer Bog and Big Meadow Bog at peat sections centrally located within the
bogs (Figs. 1, 2) and exposed by past commercial activities. At the Baltzer Bog
site, we used a small percussion corer to obtain samples from below the waterline.
Peat samples were refrigerated until analysis at Acadia University. At Big Meadow
Bog, we dug a pit 90 cm deep to intersect a wood mat revealed in a nearby drainage
ditch. At both sites, we sampled the sections at 10-cm intervals to determine
section stratigraphy and to interpret vegetation assemblages. At the Big Meadow
Bog site, the peat stratigraphy exposed in the dug pit was extended by vibracorNortheastern
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ing. We collected macro- and microfossils as potential indicators of changing local
environmental conditions and for radiocarbon dating. We measured water content
for peat samples at Baltzer Bog and Big Meadow Bog using a drying oven at 50 °C
for 24 hours, and recorded the Munsell color of dry samples. We determined the
degree of decomposition (humification index-bog surface wetness index) of peat in
the sections at both Big Meadow Bog and Baltzer Bog using the methods of Blackford
and Chambers (1993). A 0.2-g ground sample was simmered in 100 mL of 8%
NaOH for one hour. Humic acids were extracted from this solution following a
series of filtrations. The percent light transmission and absorbance of the extracted
humic acid were measured using a spectrophotometer set at 540 nm. We present
humification data for the excavated section as raw data and normalized values using
the calculation of Blundell and Barber (2005:fig. 2). Normalized data are referred
to as the bog surface wetness (BSW) index.
We developed a stratigraphic profile of the Sphagnum moss community for the
Big Meadow Bog section to explore whether shifts in the relative abundances of
Sphagnum species might reflect changes in peatland conditions. The Sphagnum
study was initiated in conjunction with and supported by Environment and Climate
Change Action Plan for the Eastern Mountain Avens in Canada.
Sphagnum species are known indicators of both the hydrological and nutrient
status of peatlands (Galka and Lamentowicz 2014, McQueen 1990) and we developed
a successional history for each peatland from the relative abundance of
Sphagnum leafs in each 10-cm depth interval. Five forceps samples (less than 1 cm3) were
taken from refrigerated peat collections from each 10-cm interval of the section and
vibracore from Big Meadow Bog. For each sample, we made a peat slurry in 25 mL
of water and counted the number of different Sphagnum leaf morphotypes using a
dissecting microscope. Section Sphagnum was recognized at low magnification by
its large, ovate, cucullate branch leaves and confirmed at high power, by the resorption
furrow evident in cross section on the leaf margin. Sphagnum magellanicum
of Section Sphagnum was readily recognizable with large, spoon-leaves (~2.5 mm
long) and chlorophyllose cells of branch leaves that were completely enclosed
(McQueen and Andrus 2007). The other Section Sphagnum species was identifiable
as either S. affine or S. austinii Sull. in Aust. (Austin's Sphagnum); green cells
were not enclosed and were shaped like equilateral triangles, and hyaline cells had
ridge-like comb fimbrils (Andrus and McQueen 2007). There were no attached
stem leaves observed in this study, a common impediment to specific determinations
in Sphagnum (Barber et al. 1998). At another fen on Brier Island, stem leaves
for S. affine/S.austinii were found but only in peat just below surface (ca. 300 BP
applying the depth-to-age relation from Big Meadow) in 3 of 5 subsamples. The
lack of comb-fimbrils in the stem leaves in these subsamples identified the material
as S. affine (Allen 2006). The 2 taxa (S. affine and S. austinii) occur in distinct regions
and microhabitat of peatlands; whereas S. affine occurs in minerotrophic fens,
S. austinii is usually found in bog hummocks (Gunnarsson et al. 2002). Sphagnum
affine is more common in the contemporary fen landscape on Brier Island (T.
Neily, unpubl. data). Smaller acute and narrow leaves in the slurries belonged to
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Sphagnum Section Acutifolia, which we recognized at higher magnification by triangular
to trapezoidal green cells with broader exposure over the inner leaf surface
(Andrus and McQueen 2007). Leaves of Section Acutifolia could not be identified
at the species level but were most common overall in the peat profile and in the contemporary
peatland flora around Big Meadow Bog which contained 4 species of the
Section in abundance (S. girgensohnii Russ. [Girgensohn’s Sphagnum] in swamps,
S. flavicomans (Card.) Warnst. and S. capillifolium (Erh.) Hedw. in open fen and
bog, and S. fuscum (Schimp.) Kinggr. on hummocks of raised bog). These species
cover large ecological gradients, meaning that there is little habitat information in
the Section Acutifolia data.
We established the chronology of the Baltzer Bog and Big Meadow Bog sections
using conventional radiocarbon dates of upright stumps located at the top
of each wood mat exposed in excavated sections and from wood recovered from
vibracoring (Table 1). The age assigned to each was calculated from the best-fit
intercept of the radiocarbon age to the calibration curve using INCAL13 (Reimer
et al. 2013).
Results
Baltzer Bog
A gleyed soil horizon is located at the contact between the kame sand and the
overlying organic sediment at Baltzer Bog (Fig. 3). The parent material is siliceous
outwash sand and the gravel/cobble content is generally low in surface horizons.
Drainage is moderate to poor due to its location in a depression and restricted vertical
drainage due to the proximity of the water table (Fig. 1). The soil contains
terrestrial vegetative matter including coarse woody material as well as seeds and
other non-woody detritus. Mineral matter constitutes about 20% of the sediment by
volume. A coarse wood mat ~50 cm thick overlies the soil horizon at 1.55 m depth
(Figs. 3, 4). A date of 3260 cal. BP was obtained from a stump in life position.
Stumps with boles are common in this lowermost wood mat and consisted primarily
of Black Spruce and Chamaecyparis thyoides (L.) Britton, Sterns, & Poggenb
(Atlantic White Cedar). Logs are also common in the lowermost layer and can
have diameters in excess of 35 cm. The lowermost wood mat is overlain by wellpreserved,
red-hued Sphagnum peat as well as Sphagnum leaves and stems. This
sediment contains very little woody material or minerogenic sediment.
Table 1. Radiocarbon data from wood collected from Baltzer Bog and Big Meadow Bog, NS, Canada.
Radiocarbon analyses were performed at Beta Analytic, Miami, FL. Calibrated ages are given as mean
calendar years (Stuiver and Reimer 1998).
Depth Age Age Max–min age
Site Lab # (cm) (14C yr BP) (2б-cal. BP) (2б-cal. BP) Material δ13C (‰)
Baltzer Bog 204970 82 3070 ± 50 3260 3380–3150 Wood -21.7
Baltzer Bog 287386 61 1730 ± 40 1640 1720–1540 Wood -25.0
Baltzer Bog 299069 8 1110 ± 50 1045 1160–930 Wood -24.7
Big Meadow Bog 360367 92 1820 ± 30 1760 1820–1700 Wood -24.3
Big Meadow Bog 409374 193 8370 ± 30 9345 9385–9305 Wood -26.0
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There were 3 discrete interruptions by wood layers above the lowermost wood
mat at Baltzer Bog. Wood mats were encountered at 1.0 m and 0.6 m depths, and
stumps were dated at 1640 cal. BP and 1045 cal. BP, respectively (Fig. 4, Table 1).
Each of these mats contain upright stumps that have bole diameters of less than
10 cm. Small branch debris was very common in these wood mats, and Sphagnum
was absent or occurred in trace amounts in the largely organic matrix surrounding
the wood debris. Sphagnum samples above and below these 2 wood mats
exhibited significant variability in percent light transmission values and Munsell
color, which is indicative of changing degrees of humification. High percent light
transmission values correspond with light Munsel colors (e.g., 51% and 7.5YR 4/6,
strong brown) and are associated with a low degree of humification. Low percent
light transmission corresponds to dark Munsel colors (e.g., 14% and 7.5YR 5/2,
very dark brown) and a high degree of humification. The bog surface-wetness data
(BSW; Fig. 4) record 3 increases indicative of relatively high decomposition that
occur above wood mats at 1.25 m (ca. 3200 cal. BP), 0.77 m (ca.1550 cal. BP) ,and
0.53 m (ca. 1000 cal. BP). Strata in which little wood was found tended to exhibit
Figure 3. Sediment section at Baltzer Bog (S1 in Fig. 1) showing (A) the contact between
the kame sand and the overlying organic sediment, (B) coarse wood above the soil horizon,
and a (C) wood layer overlain and underlain by Sphagnum peat.
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higher BSW values, whereas BSW values for the wood-rich sections was relatively
low. The upper most wood mat occured at 10 cm depth, continued to the bog surface,
and contained no stumps and few logs.
Big Meadow Bog
The single wood layer at 90 cm in the Big Meadow Bog occurred in a peat
matrix dominated by Sphagnum magellanicum with Sphagnum Section Acutifolia
where there were upright trunks of Black Spruce with root and trunk bases but
little remnant preserved trunk material. A peat moss layer dominated by S. affine
occurred directly above the S. magellanicum-dominated wood layer. The greatest
percentages (≥ 50%) of S. affine among Sphagnum leaf types in the profile occurred
from 60 to 90 cm; the last 700 years of peatland (i.e., 40 cm) was entirely
Sphagnum Section Acutifolia.
Figure 4. Section stratigraphy at Big Meadow Bog and Baltzer Bog showing age of wood
mats, Sphagnum species composition (Big Meadow Bog), and BSW index.
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Similar to Baltzer Bog, Sphagnum samples exhibited variability in percent
light transmission values and Munsell color. The bog surface wetness data (BSW)
for Big Meadow Bog record an increase above the wood mat at 90 cm, indicating
relatively high decomposition (Fig. 4). Conventional radiocarbon dates were from
an upright stump (Fig. 5A), and a lower wood mat yielded an age 1760 cal. BP. A
radiocarbon sample from the underlying marine mud yielded a date of 9345 cal. BP.
Discussion
High-resolution paleoclimate records from southwestern Nova Scotia generally
indicate that the late Holocene was a time of increasing precipitation and cooler air
temperatures (Lennox et al. 2010, Spooner et al. 2014). The Baltzer Bog and Big
Meadow Bog records, however, demonstrate that during this time the water table
Figure 5. Sediment
section at
Big Meadow
Bog showing
(A) an upright
s p r u c e log
with an intact
root ball removed
from a
nearby drainage
ditch at
approximately
the same depth
from the bog
surface as (B)
the wood mats
exposed in the
test pit .
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fluctuated and that significant and sustained dry periods likely occurred. The stratigraphic
record at Baltzer Bog indicates that at least 3 woodland–wetland transitions
have taken place. The wetland is located in a closed basin on an elevated kame and
initially developed in response to rising water table at less than 3200 cal. BP. The presence
of alternating wood mats and Sphagnum indicates that the water table fluctuated
substantially after a rise in the water table inundated the site. The lack of appreciable
Sphagnum and the development of a soil underneath the lower-most wood
mat indicates that the Baltzer Bog site was not wet from the time of deglaciation
(ca. 12000 cal. BP) until at least 3260 cal. BP.
The preservation of the wood above the soil and the abrupt transition to Sphagnum
likely indicates a rapid increase in water-table elevation. The preservation of
wood depends upon its susceptibility to both aerobic and anaerobic decomposition
by microbes, where it is located in the peat profile, how long it remains in the oxygenrich
upper layer of the bog, and the density of the overlying Sphagnum which might
reduce the accessibility to the microbial population (Moore 1989). While the earliest
wood mat occurred on mineral substrate, its preservation was made possible by the
Sphagnum matrix. The elevated water table resulting from the peat moss’s capillary
action restricts oxygen diffusing into the surface peat layer. As a result, tree root
metabolism and lignin oxidase of decomposers are inhibited causing trees to die and
the lignocellulose of the dead trees to be preserved. No community-level insight into
the Sphagnum Section Acutifolia community at Baltzer Bog was possible, but the 3
transitions to Sphagnum as well the pattern of tree decomposition (mostly stump and
roots alone preserved) represent rapid increases in the water table.
At Big Meadow Bog, a Black Spruce swamp with Sphagnum magellanicum had
a 400-year occupancy, and the disappearance of the wood mat from the core coincided
with the first appearance of Sphagnum affine, a fen species. The transition
from swamp to fen runs counter to most cases of autogenic peatland succession
models, although cases of forest paludification have been documented in Alaska
(Noble et al. 1984). The ability to separate cases of Sphagnum successions driven
by autogenic versus allogenic factors is informed by understandings of paleohistoric
climatic changes particular to a regional setting (Payette 1988). Single
landscapes may have wetlands that are out of phase due to autogenic (or cyclic
autogenic) processes (Payette 1988). When different landscapes within a single
region have wetland processes that are chronologically in-phase, paleo-reconstruction
data serve to reinforce our understanding of allogenic, cl imatic influence.
Our reconstruction of the Big Meadow Bog wetland history based on wood and
the 3 Sphagnum types is a simplified analog of a 6000-year peatland history for a
Baltic bog which featured the same Sphagnum type transitions (Gałka and Lamentowicz
2014). In that study, S. magellanicum was a pioneer species correlated with
treed phases of the bogs. At Big Meadow Bog, S. magellanicum (Section Sphagnum)
with S. girgensohnii (Section Acutifolia) are the most common mosses in the
swamp surrounding the open peatland. Sphagnum magellanicum is morphologically
adapted to the low UV levels in the understory of trees or shrubs (Searles et
al. 2002). Sphagnum affine is a known fen species (Crum 1992, Malmer et al. 2003)
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that occurs in the fen lagg of Big Meadow Bog in the contemporary landscape.
Whereas S. magellanicum may be indicative of either shade or relative dryness,
the appearance of S. affine at Big Meadow Bog may indicate periods of higher
water table, and its placement in the succession after the demise of the trees (Black
Spruce) strongly suggests that the disappearance of the trees is related to their inability
to tolerate root-zone anaerobiosis. Despite the distribution of Black Spruce
over a variety of treed wetland types, its root tips are highly sensitive to flooding
(Levan and Riha 1985).
A wetland stratigraphic record from Pleasant River Fen about 100 km southwest
of Baltzer Bog demonstrated an increased influx of coarse minerogenic matter at
5250 cal. BP, which coincides broadly with other records of the development of
cooler and wetter conditions during the late Holocene (Spooner et al. 2014). The apparent
disparity in timing between these 2 records of increased precipitation might
be attributed to the location of the 2 sections at Baltzer Bog at the side rather than the
base of the kame depression leading to a lag between initial increase in precipitation
and flooding at the location of the sections. Though the Baltzer Bog record does not
constrain the timing of initiation of the late Holocene transition to moister climate, it
provides a minimum date. It is likely that the elevation of the base of the Baltzer Bog
section is sufficiently above the local (perhaps perched) water table that a significant
time lag existed between the initial increases in moisture and the drowning of the
forest that records the rise in the local water table. The basal date of 3260 cal. BP indicates
increased moisture, a rising local water table, and drowning and subsequent
preservation in a bog environment of a forest that existed at the site.
The 2 later woodland–wetland transitions at the Baltzer Bog site, sometime after
1640 cal. BP and 1045 cal. BP, respectively, indicate 2 previously unrecognized
transitions in the Nova Scotian late Holocene moisture record (Spooner et al. 2014).
We dated tree samples at the top of the wood mat and as close to the woodland–wetland
transition as possible to better constrain the timing of increased moisture. In
the Pleasant River Fen record, rhythmicity is apparent from 1950 cal. BP onwards
and is distinguished by slight changes in sediment color, likely indicating periods
of high and low productivity, a fluctuating water table, and/or episodes of flooding,
though this sequence is poorly constrained.
The Baltzer Bog and Big Meadow Bog records represent 2 independent wetlands
that, along with the Pleasant River Fen record, suggest the occurrence of a series of
Holocene wetting periods as far back as 5250 cal. BP and as recent as 1045 cal. BP.
In addition, the data indicate an apparent overlap between the timing of the loss of
wood at Baltzer Bog (1640 cal. BP) and at Big Meadow Bog (1760 cal. BP). The 2
most recent of the apparent wetting events (i.e., 1045 cal. BP and 1640–1760 cal.
BP) may be region specific given that these dates were interpreted by Payette (1988)
as dry Holocene periods related to a period of expansion of Sphagnum nemoreum
and drought stunting of Black Spruce. Paleoecological records from northeastern
Canada indicate significant climate variability during the Little Ice age rather than
sustained cooling, a condition also likely in Nova Scotia (Lemus-Lauzon et al.
2016, Roy et al. 2011).
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Both Baltzer and Big Meadow bogs are now highly disturbed wetlands. The
former was ditched and harvested for peat, the latter ditched for agriculture and
then abandoned. In both cases, there are conservation reasons for wetland restoration:
peat harvest has been halted at Baltzer Bog, and a detailed restoration effort
is beginning at Big Meadow Bog that is driven by the Recovery Plan for the endangered
Eastern Mountain Avens. This globally rare, arctic–alpine plant thrives
only in the mountains of New Hampshire and wetlands on Brier Island (Munro et
al. 2014). The lagg fen of Big Meadow Bog provides a paleorecord of community
change from sea lagoon to spruce swamp to fen then raised bog, direct evidence
of the need for dynamic conservation land planning. Prior to the transition from
Sphagnum affine to a community composed of Sphagnum Section Acutifolia, the
entire “bog” expanse was likely a south- and north-flowing fen originating in
the middle of the peatland. Though possible, it is unlikely that Eastern Mountain
Avens would have successfully occupied a fen-only peatland because it is absent
from such landscapes on Brier Island and over a 20-km stretch of intermittent fen
north of Brier Island that supports the rare Lophiola aurea Ker Gawl. (Coastal Plain
Goldcrest). However, in planning for critical habitat for the endangered Eastern
Mountain Avens, the paleorecord suggests that in half a millennium, parts of this
linear expanse of fen, between the same basalt ridges that formed Big Meadow
Bog, will become raised bog with the attendant essential lagg habitats on either
side. Rare plants are most often specialists (Griggs, 1940, Mobaied et al. 2015) and
indicators of particular landscape features (Crain et al. 2015, Edwards and Weakley
2001, Miller 1986). This is clear for rare wetland Atlantic Coastal Plain herbs, the
majority of whose Canadian habitat cases were associated with the low-biomass
shorelines of lakes with large catchment-areas (Hill and Keddy 1992). A similarly
strong landscape attachment may also hold for the arctic–alpine Eastern Mountain
Avens and its associated complex of rare boreal organisms (Carex livida, Betula
michauxii, Dolomedes striatus, and various Sphagna [S. teres, S. contortum, and a
putative S. carolinianum]) that are most common in the portions of lagg fen where
near-neutral swamp water forms pools at the edge of the bog rand that are least
affected by ditching and gull guano enrichment. It should be emphasized that the
lagg is a dynamic landscape both in terms of its contemporary natural disturbance
regime that fosters diversity and also as a somewhat ephemeral feature at the millennial
time scale. These 2 time scales inform the conservation planning for the
Eastern Mountain Avens (Environment and Climate Change Canada 2016).
Though the water table rise is a local phenomenon, the apparent persistent
increase in moisture (and perhaps decrease in temperature leading to decreased
evapotranspiration) recorded in all 3 sections at the 2 wetland sites likely indicates a
significant climate regime shift. Other paleoenvironmental records indicate similar
but much subtler late Holocene trends. Lennox et al. (2010), in a paleolimnological
study at Canoran Lake, located about 80 km to the southeast of Big Meadow Bog,
noted both fluctuating and higher than normal lake levels in the late Holocene.
Historical records show that over the past 100 + years, Baltzer Bog and Big
Meadow Bog have been in transition from wetland to woodland, which may be an
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indication of a declining water table but also may be influenced by drainage modification
and peat harvesting on site, and the inability of Sphagnum to regenerate
(Environment Canada 2010). The woodland transition at Pleasant River Fen from
200 cal. BP onwards was likely influenced by human activity in the region (fire,
water-level management), which may have accelerated the trend in declining moisture
availability (Lemus-Lauzon et al. 2016, Spooner et al. 2014).
Conclusions
Baltzer Bog and Big Meadow Bog contain stratigraphic records of a late Holocene
transition to moister climate shortly after 3000 cal. BP. The 2 later woodland–
wetland transitions at the Baltzer Bog site, sometime after 1640 cal. BP and 1045
cal. BP, indicate 2 previously unrecognized transitions in the Nova Scotian late Holocene
moisture record. However, significant periods of late Holocene drying took
place and are recorded stratigraphically by the presence of a conifer forest at both
sites indicating a significantly lowered local water table. The Baltzer Bog record is
in general agreement with other moisture-sensitive proxy records from the region,
though the dry period that occurred ca. 1640 cal. BP was previously unrecognized.
In light of recent studies that suggest long-term residency of disjunct species such
as Eastern Mountain Avens and Blanding’s Turtle, the variability in water table and
the coincident change in wetland habitat structure observed in wetlands in southwestern
Nova Scotia can be used to better gauge species resilience in light of both
anthropogenic and natural environmental change.
Acknowledgments
The authors acknowledge the assistance of Bryan Martin (Acadia University) in initial
site investigation. We gratefully acknowledge the Natural Science and Engineering
Research Council (NSERC) and Acadia University for financial support of this research
project. We thank the editor and 2 anonymous reviewers for their valuable comments on
this manuscript.
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