1. INTRODUCTION
Northern peatlands have acted as persistent sinks of atmospheric
CO2 throughout the Holocene (Loisel et al., 2014) and
today represent a globally important soil carbon (C) reserve
(~220–550 Pg C) (Turunen, Tomppo, Tolonen, &
Reinikainen, 2002; Yu, 2011). This long-term carbon sequestration is
largely the result of Sphagnum moss productivity exceeding moss
and peat decomposition and combustion rates (Rydin & Jeglum, 2013).
There is a concern, however, that Sphagnum mosses and their
associated peatland carbon stocks may be vulnerable to future climate
change (Ise, Dunn, Wofsy, & Moorcroft, 2008; Dorrepaal et al., 2009),
where evaporation rates are predicted to increase substantially within
the next century (IPCC, 2013). While drier conditions may inhibitSphagnum growth due to a greater frequency and severity of water
stress (Moore & Waddington, 2015), these peatland mosses are generally
considered resilient to drought owing to their water regulating traits
and a number of negative ecohydrological feedbacks that act to maintain
a wet near-surface (Waddington et al., 2015). Many of these key
autogenic negative feedbacks are stronger where peat depths are greater
(e.g. water table depth-peat deformation feedback) (Waddington et
al., 2010). Indeed, most peatland and moss water availability research
is biased to deep and large peatlands (e.g. Lindholm & Markkula,
1984; Price, 1997; Moore, Morris, & Waddington, 2015) where peatland
ecohydrological resilience is higher (Morris & Waddington, 2011) and
where the water table (WT) never falls below the peat layer. In
contrast, the likelihood of drought-driven water table drawdown well
below the moss layer is greater in shallow peat systems such as recently
restored peatlands or natural locations undergoing primary peat
formation (i.e. peat formed directly on mineral soil or rock).
Dixon et al. (2017) used modelled water balance simulations to
determine that peat deposits less than 0.5 m thick were least able to
buffer prolonged periods of evaporation due to limited labile water
storage and quickly experienced moss stress.
Mosses are non-vascular and must rely
on water supplied to the growing surface via capillary rise from deeper
in the profile and water storage in the capitula (apical bud) during
rain-free periods (Thompson & Waddington, 2008). Both water storage and
capillary transport in moss and the underlying peat is a nonlinear
function of soil water tension (𝜓) (e.g. McCarter & Price, 2014;
Moore et al., 2015). As 𝜓 increases with a falling WT, capillary films
become thinner and less connected, thus reducing the hydraulic
conductivity of the peat matrix (Price & Whittington, 2010; Rezanezhad
et al., 2010). Consequently, under dry conditions, steep hydraulic
gradients in the near-surface are unable to counteract sharp declines in
hydraulic conductivity, resulting in capillary transport being less than
evaporative losses, and thus leading to near-surface desiccation
(Schouwenaars & Gosen, 2007; McCarter & Price, 2014; Kettridge &
Waddington, 2014). Under these conditions, Sphagnumchlorophyllous cells use water stored in large, adjacent, dead hyaline
cells to maintain metabolic processes. However, hyaline cells are
expected to drain when 𝜓 exceeds 100-600 mb (or hPa) (Hayward & Clymo,
1982; Lewis, 1988), leading to rapid desiccation of the chlorophyllous
cells with continued evaporative losses. The nature of the relationship
between water storage, hydraulic conductivity, and 𝜓 is dependent on the
botanical origin and degree of decomposition of the peat (Rezanezhad et
al., 2010; McCarter & Price, 2014; Taylor & Price, 2015). For example,
in the near-surface and living moss layer, Sphagnum species with
higher stem and branch packing densities (e.g. Sphagnum sectionsAcutifolia and Sphagnum ) have generally been shown to
possess both greater volumetric water content (VWC) and hydraulic
conductivity for a given 𝜓 (Titus & Wagner, 1984; Strack & Price,
2009; Hájek & Vicherová, 2014; McCarter & Price, 2014). Moreover, the
greater ability of these Sphagnum species to retain and conduct
water gives them the ability to maintain a hydrological connection with
the WT at greater WT depths than species with weaker moisture retention
and hydraulic conductivity (e.g. Sphagnum sectionCuspidata ), and thus they possess a greater ability to avoid
desiccation in a given environment (Hayward & Clymo, 1982; Rydin,
1985). Water stress and the vulnerability to future drying, therefore,
will likely be lower for peatlands where water table depth (WTD) and
variability are minimized (Holden, Wallage, Lane, & McDonald, 2011;
Moore & Waddington, 2015), and for moss species which possess stronger
capillarity (Rydin, 1993). As such, to extend the modelling work of
Dixon et al. (2017) to field conditions, we hypothesized that due to the
link between WT depth, soil water tension, and moss water stress, mosses
on shallower peat will experience greater frequency and severity of
water stress due to greater WT variability and shorter hydroperiods
(i.e. period of WT presence) compared to the same species growing
on deeper peats. To test this ’survival of the deepest’ hypothesis we
measured near-surface tension, moisture content, and water table
position in both Sphagnum dominated shallow peat wetlands and
deeper peatlands on Canadian Shield rock barrens during a summer
drought. By examining the effect of peat depth on the frequency and
severity of water stress in moss during meteorological drought we aim to
better identify factors which may make peatlands more vulnerable to
longer term climate change mediated drought as a result of decreased
moss productivity.
2.
METHODS
2.1 Study area
This study was conducted at peatlands north of Parry Sound, Ontario,
Canada within the Georgian Bay Biosphere Mnidoo Gamii, a UNESCO
biosphere, situated within the Robinson-Huron Treaty of 1850 and the
Williams Treaty of 1923, and located on Anishinabek territory. This
eastern Georgian Bay region is on the Canadian Shield and is
characterized by multiple west-east oriented granite bedrock ridges and
valleys with numerous bedrock depressions of various depths and
landscape positions along the ridges. The low bedrock permeability in
these depressions support wetlands and peatlands with a perched water
table. The wetland and peatland soil is mostly organic soil (peat)
situated on a thin layer (0–5 cm) of mineral soil. The surface cover of
the ridges tends to consist of either small thin patches of mineral
soil, moss cushions, lichen mats, or exposed bedrock, while the
intervening valleys more commonly consist of deeper mineral soil, ponds
or deep and expansive peatlands.
To examine the effect of peat depth on moss water availability, we
categorized peat-filled wetlands and peatlands perched on the bedrock
(hereafter referred to as sites) into two different classes based on
average peat depth: i) sites with depths < 40 cm hereafter
referred to as shallow depth (S), and ii) sites > 40 cm in
depth hereafter referred to as deep (D). In Canada, peatlands are
defined as wetlands with peat depths greater than 40 cm (National
Wetlands Working Group, 1997). For each depth class, the catchment area
of the site was categorized as being either large (L, >
4,000 m2) or small (S, < 4,000
m2) providing a total of four depth/catchment
categories. Three representative sites for each of these four categories
(12 in total), were monitored (hereafter referred as the main sites)
throughout the 2015 growing season and identified by a site depth and
catchment area as well as three-digit site number. For example, site
DL-234 describes site number 234, which is a deep site in a large
catchment area. An additional five sites from each category (for a total
of 32 sites), were selected for an intensive field survey conducted in
mid-summer during a meteorological drought period (hereafter referred as
IFS sites).The sites are dominated by Sphagnum moss, and vascular
vegetation including leatherleaf (Chamaedaphne calyculata ),
sedges (Carex spp. ), tamarack (Larix laricin a), and jack
pine (Pinus banksiana ). Site and watershed characteristics of the
main sites are summarized in Table 1.
2.2 Hydrological
measurements
All hydrological measurements at the main sites were made over the 2015
growing season from day of year (DOY) 152 to DOY 325. Rainfall was
recorded at 30-minute intervals at two tipping bucket rain gauges at
sites DS-808 and DL-415. Measurements from the tipping bucket rain
gauges were validated with at least three manual rain gauges placed near
each of the tipping buckets and at all sites. Water table position was
measured at each of the main sites at 15-minute intervals using a water
level sensor (Solinst Level Logger, Georgetown, ON) in a PVC groundwater
well (5 cm inner diameter) installed to bedrock at the deepest position
at the site.
Measurements of near-surface soil water tension and volumetric moisture
content were made two times per week from June to September 2015 for the
two most common Sphagnum species (S. fallax and S.
palustre ) at each of the main sites. Soil water tension was measured at
5 cm depth (𝜓5 cm ) using tensiometers (2 cm outer
diameter, Soil Measurement Systems, Tucson, Arizona, USA) and an UMS
Infield tensicorder (Munich, Germany) accurate to ±2 mb (or hPa).
Integrated near-surface volumetric moisture content was measured
manually over the 0–3 cm (VWC0–3 cm) depth ranges
using a ThetaProbe Soil Moisture Sensor ML3 (Delta-T Devices, Burwell,
Cambridge, UK).
Near-surface tension and volumetric moisture content were also measured
during a mid-summer meteorological drought for all 32 IFS sites on DOY
221 for a plot of S. fallax and S. palustre following the
same methods for soil water tension and moisture content methods as
described above. IFS tensiometers were installed on DOY 212 (July 31).
2.3 Modelling moss moisture
stress
We modelled moss moisture stress by combining field data (see above) and
the relationship between VWC and chlorophyll fluorescence for S.
fallax and S. palustre derived from a controlled lab experiment.
Four replicate cylindrical samples of each species were taken from the
field using a PVC collar (10 cm diameter, 5 cm depth). Samples were
under-cut, carefully removed, and cheesecloth affixed to the PVC to
support the sample from underneath. Vascular vegetation was clipped from
the moss surface and the samples were saturated with water. Immediately
prior to the start of the drying experiment, the samples were allowed to
free drain for 24 h and then placed in a growth chamber to dry. The
temperature and light levels of the growth chamber were kept constant at
25oC and 300 µ mol m−2s−1 throughout the 14 days of the drying experiment.
VWC was measured by weighing the samples daily, and subtracting the
sample dry weight which was determined by oven drying at 60°C at the end
of the drying experiment. Chlorophyll fluorescence was measured using a
modulated chlorophyll fluorometer (Opti-Sciences, Inc. model OS30p+) for
three capitula using clipped sub-samples that were dark adapted for 20
minutes. The optimal quantum yield of photosystem II as represented by
Fv/Fm was measured, and reported
Fv/Fm values represent the average of
the three replicate capitula. Due to the destructive nature of
chlorophyll fluorescence measurements, measurements were initially done
every 2–3 days, and with greater frequency as the moss samples began to
appear stressed.
An empirical relation between lab-measured VWC and
Fv/Fm was used to define moss moisture
stress. A rectangular hyperbola was used to represent the relation:
\(F_{v}/F_{m}=-\frac{1}{\alpha}\left(\beta\cdot VWC+\gamma-\sqrt{\left(\beta\cdot VWC+\gamma\right)^{2}-4\cdot\alpha\cdot\beta\cdot\gamma\cdot VWC}\right)\)(1)
where 𝛼 is the curvature parameter, 𝛽 is the initial slope, and 𝛾 is the
maximum Fv/Fm. Because of the
uncertainty of the onset of moss stress in relation to fluorescence
parameters, we adopted a high and low VWC threshold corresponding to
75% and 25% of the fitted 𝛾 parameter (i.e. maximum
Fv/Fm), respectively.
In order to estimate the proportion of the snow-free season (April to
October 2015, inclusive) where moss was likely to be stressed, we used
an empirical relation between VWC and WT to determine when modelled VWC
was below the threshold defined by the lab-measured chlorophyll
fluorescence. The relation between VWC and WT was modelled using a
modified van Genuchten equation for S. fallax and S.
palustre separately (Figure S1), where WT depth was used in place of
tension. Based on lab samples (i.e. 5 cm deep surface samples)
porosity was taken to be 98% and residual water content was assumed to
be 0% (c.f. McCarter and Price, 2014). The fit function
in Matlab (MATLAB R2020a - The Mathworks Inc.) was used to estimate
curve parameters, and predint used to generate prediction
intervals for new observations. A simple Monte Carlo approach using 1000
iterations was used to generate modelled VWC using predint where
new modelled observations were generated using normally distributed
errors. For cases where the WT was lost, VWC was modelled based on the
measured distribution of VWC in the field. A logarithmic distribution
was used to represent measured VWC data.
2.4 Statistical
analyses
All statistical analyses were done using Matlab. Unless otherwise
stated, value pairs in parentheses represent the mean ± std. A general
linear model (glmfit ) was used to test for the significance of
site depth (Depth – shallow and deep), catchment area
(Catchment – small and large), and species cover (Species- S. fallax and S. palustre ) and their one-way
interactions on 𝜓5 cm , where the following model
was evaluated: Tension ~ 1 +Depth ×Catchment + Depth ×Species +Catchement ×Species (Wilkinson notation). Tension
measurements consisted of random paired plots at all 32 IFS sites. A
log-link function was used due to the right skew of the
𝜓5 cm data. Pairwise differences in the marginal
means were assessed using Tukey’s post-hoc HSD.
3.
RESULTS
3.1 Precipitation and water table
depth
During the study period the region received a total of 350 mm rainfall
with higher rainfall in early June, October, and early November than
during the middle of the summer (Figure 1). Specifically, only about
one-third of the study period rainfall (98 mm) occurred from DOY 174
(June 23) and DOY 271 (September 28) which was less than half of the
long-term mean for this period (Environment Canada, 2015). We refer to
this as the period of meteorological drought.
The WTD at all sites reached a springtime minimum in mid-June (DOY
164–166) (Figure 1). Following this period, the WT at deep sites
experienced an overall decline until a maximum WTD occurred on DOY 272
(September 29) (Figure 1). The magnitude of deep site WT decline was
unrelated to catchment size, with a range in WT decline of 48–65 cm.
Similar to the deep sites, each shallow site experienced an overall
increase in WTD following the minimum in mid-June (Figure 1). All
shallow sites lost their WT (i.e. the groundwater well was dry)
for considerable time during the drought period. The first shallow site
WT loss occurred between DOY 182 and DOY 196 and there was no WT at any
shallow sites between DOY 202 and DOY 232 (July 21 and August 20), and
with the exception of SS-407, no WT was present in any shallow site from
DOY 202 to DOY 289 (July 21 to October 16) (Figure 1).
3.2 Moss water availability
Between DOY 176 (June 25) and DOY 211 (July 30) all shallow sites lost
their WT providing an opportunity to evaluate the responsiveness of
near-surface VWC and 𝜓 to this WT loss. Both S. fallax andS. palustre experienced slight declines in VWC0–3
cm over the drought period, decreasing 6% and 12% on average for both
deep and shallow sites, respectively (Figures 2 and 3). When WT was
present, a general linear model (one-way interactions with WTD, species
and depth category) showed that trends in VWC0–3 cmwere significantly related to WTD (slope = -0.35, F=-5.18, p=7.4E-07),
where shallow sites tended to have slightly lower VWC0–3
cm (difference = 3.4%, F=-1.54, p=0.126), and the WTD response being
more subdued (WTD×shallow slope = -0.19, F=1.68, p=0.096) (Figure
4a–b).
Near-surface tension, however, tended to be more responsive during the
drought period. In particular, 𝜓5 cm increased
dramatically at shallow sites following WT loss (Figure 2) increasing by
219 and 122 mb on average for S. fallax and S. palustre ,
respectively. By comparison, the maximum change in 𝜓5 cmat deep sites was on average 29 and 25 mb, respectively (Figure 3).
Moreover, with the exception of one (potentially erroneous) measurement,
𝜓5 cm at deep sites did not exceed 40 mb (Figure 4c).
While average tension during the drought period was much higher at the
shallow sites, 𝜓5 cm generally followed a 1:1 relation
with WTD when WT was present for both deep and shallow sites (Figure
4c–d).
3.3 Mid-summer drought intensive
field
survey
The mid-summer drought IFS allowed for a comparison of how near-surface
VWC and 𝜓 are both affected by WTD over a larger range of sites. On the
day of the IFS (DOY 221) all 16 shallow sites did not have a WT present
while, in contrast, all 16 deep sites had a WT present between depths of
34 and 70 cm.
VWC0–3 cm was low at shallow sites regardless of
species or catchment size, having a median VWC0–3 cm of
1.1% at the time of the IFS. S. fallaxVWC0–3 cm at the deep sites was similarly low to
shallow sites, regardless of catchment area class. In contrast, theS. palustre VWC0–3 cm at the deep sites was
comparatively higher during the IFS (5.8%).
𝜓5 cm during the IFS varied significantly by site
depth classification (F=53.4, p<<0.001), catchment
size (F=5.32, p=0.025), and depth × size (Figure 5a; F=3.43, p=0.070),
but not by species (F=0.12, p=0.73). In particular, shallow sites with a
large catchment had the highest median 𝜓 (153 mb). Median 𝜓 measured at
the shallow sites during the IFS was 105 and 130 mb, for S.fallax and S. palustre , respectively (Figure
5b). Using the 100 mb 𝜓 threshold to infer water stress (Price
and Whitehead, 2001), 60% of S. fallax plots and 67% ofS. palustre plots in shallow sites exceeded this threshold
during the IFS. The median 𝜓5 cm measured at deep sites
during the IFS was much lower (41 and 35 mb) for both S.palustre and S. fallax , respectively. Unlike the shallow
sites, 𝜓 typically did not exceed the 100 mb threshold at the deep
sites. At deep sites during the IFS, the 100 mb 𝜓 threshold was only
exceeded in S. palustre plots at a single site, and in S.
fallax plots at 3 sites (~20% of sites).
3.4 Modelled moisture
stress
Lab-measured chlorophyll fluorescence
(Fv/Fm) for S. fallax andS. palustre capitula was positively related to sample VWC and
well described by a hyperbolic fit
(r2adj of 0.95 and 0.90,
respectively). There was a relatively sharp transition from high to low
Fv/Fm between VWC of 1–10% (Figure 6),
which represents the onset of moisture stress in the capitula samples.
Due to the rapid transition (relative to the measurement interval) in
Fv/Fm, results are lacking over the
~3–6% VWC range and so we adopted a conservative
approach of defining a high and low VWC threshold for the onset of
moisture stress based on the VWC where
Fv/Fm was ¼ and ¾ of the saturation
value (see Eq. 1). Correspondingly, the high and low VWC thresholds were
5.4% and 1.8%, and 3.8% and 1.3% for S. fallax and S.
palustre , respectively.
Although there were only slight differences in the estimated VWC
threshold for the onset of moss stress, there was a greater
differentiation in moisture retention by species where S.
palustre tended to retain more water than S. fallax for WTD
greater than ~10 cm (Figure S1). Using species dependent
VWC-Fv/Fm thresholds and VWC-WTD
relations, Monte Carlo simulations suggest that moss has nearly twice
the likelihood of being stressed at shallow sites (0.38 ± 0.24) compared
to deep sites (0.22 ± 0.18). The effect size of moss species on the
likelihood of being stressed was greater than site depth category
(Figure 7). Our model estimate of the likelihood of moss stress was
three times greater for S. fallax (0.46 ± 0.17) compared toS. palustre (0.14 ± 0.13). Unsurprisingly, a high VWC-stress
threshold resulted in a greater likelihood of moss stress (0.40 ± 0.23)
compared to a low VWC-stress threshold (0.19 ± 0.16), having a similar
effect size to site depth category.
4.
DISCUSSION
4.1 Influence of site depth and
species on moss water
availability
Our findings are broadly similar to other studies which have shown that
both WTD and species were critical factors affecting near-surface water
availability for Sphagnum moss (Clymo, 1973; Luken, 1985; Rydin,
1985; Li, Glime, & Liao, 1992; Strack & Price, 2009). Our results show
that during a meteorological drought period, 𝜓5 cm was
greater in shallow sites versus deep sites, where species did not appear
to have a significant effect (Figure 5). The Monte Carlo modelling
exercise similarly predicted a greater likelihood of stress (magnitude
cannot be directly inferred) for shallow sites, but where species was
shown to be a significant factor.
The consistency in results with respect to site depth suggests that the
loss of the WT at shallow sites has a strong influence on near-surface
water availability, particularly with respect to 𝜓 (e.g. Figure 4).
While the WT was present, 𝜓5 cm was near a hydrostatic
equilibrium tension relative to WTD (Figure 4c and d). It was only after
a loss of the WT in the shallow sites that the 𝜓5
cm–WTD relation substantially differed between shallow and deep sites
(e.g. Figures 2 and 3). A similar finding was shown by Price and
Whitehead (2001) where a change in the relationship between 𝜓 and WTD
occurred when WTD was greater than ~70 cm. The shift in
the relation between 𝜓 and WTD, and a rapid increase in near surface 𝜓
suggests that capillary transport is not sufficient to meet evaporative
losses (see McCarter & Price, 2014; Waddington et al., 2015).
The seemingly inconsistent result with respect to species may have a
number of contributing factors. First, the “likelihood of stress”
metric is binary and so is insensitive to the magnitude of difference
between species once the threshold is crossed (e.g. high versus extreme
tensions are similarly classified as ‘likely to be stressed’). Second,
we used a VWC relation rather than a gravimetric water content (GWC)
relation for ease of comparison to our field data. GWC is a more
physiologically relevant measure of moss water availability (e.g.
Schipperges & Rydin, 1998; Nungesser, 2003; Van Gaalen, Flanagan, &
Peddle, 2007); Hájek & Beckett, 2008) as it integrates differences in
moss shoot (i.e. branch, stem, and capitula) and colony structure.
Nevertheless, GWC was quantified for the lab experiment from which our
VWC–Fv/Fm threshold was estimated. A
comparison with GWC showed broadly similar results (Figure S2) where
there was no major difference in the relationship between species, nor
was GWC better correlated with Fv/Fm.
Third, the Monte Carlo simulation incorporates all available WTD for the
growing season while the 𝜓–WTD data spans the roughly one month
meteorological drought period. Fourth, and finally, based on field data
there was a fundamental difference between species in the VWC-𝜓
relationship where the two were more strongly correlated for S.
fallax compared to S. palustre (Figure S3).
During drought periods near-surface moisture is predominantly accessed
through upward capillary transport (as opposed to direct wetting from
rainfall) (McCarter & Price, 2014) and we found that S. palustrewas able to maintain higher near-surface moisture at considerably deeper
WT depths than S. fallax (Figure S1). The ability of S.
palustre to maintain a higher water content for a given WTD is likely a
combination of the species dependent structural characteristics of
individual shoots which can enhance capillarity (Hayward & Clymo, 1982;
Rice, Aclander, & Hanson, 2008) and colony structure which affects
evaporative losses (e.g. Rice, Collins, & Anderson, 2001; Elumeeva,
Soudzilovskaia, During, & Cornelissen, 2011). Moreover, moss capitula
water supply is mechanistically linked to the water transport ability
(unsaturated hydraulic conductivity; Kunsat ) of
the underlying peat matrix. While we are unaware of any studies that
have derived Kunsat relationships for S.
fallax and S. palustre to date, species with a lower
near-surface Kunsat have been shown to desiccate
at shallower WT depths than species with higherKunsat (Titus & Wagner, 1984; McCarter & Price,
2014).
However, it is important to note that if a moss species ordinarily
desiccates at WT depths shallower than the depth of peat on which it
grows, then the frequency with which it experiences water stress will
not necessarily be intensified by the loss of the WT. Indeed we found
that the loss of a WT had a larger overall influence on the likelihood
of moss stress for the species with relatively high (S. palustre )
versus low (S. fallax ) moisture retention (Figures S1 and S3).
Therefore, despite similar VWC-Fv/Fmstress thresholds (Figure 6), S. fallax was predicted to become
stressed at shallower WT depths (Figure S1). Site depth may not only
affect moss stress by influencing total saturated storage capacity, but
may also influence near-surface moisture availability through
differences in rates of WT decline as a result of basin area volume
relationships (e.g. Brooks & Hayashi, 2002) or peat properties such as
specific yield (e.g. Granath, Moore, Lukenbach, & Waddington, 2016).
Wilkinson, Tekatch, Markle, Moore, & Waddington (2020) showed that
shallow peat-filled depressions in the region tended to have a more
rapid WT decline compared to deeper peatlands, which is supported by the
lower average specific yield for shallow sites (Didemus, 2016) and more
rapid WT decline during the meteorological drought period of this study
(Figure 1).
4.2 Influence of catchment size
on moss water
availability
In addition to species-dependent moisture retention characteristics and
site storage dynamics as mediated via peat properties, the likelihood of
stress for moss species may also be affected by hydrological
connectivity to the surrounding upland where groundwater-surface water
interactions have been shown to influence peatland water storage
dynamics (Devito, Hill, & Roulet, 1996; Glaser, Siegel, Romanowicz, &
Shen, 1997). Given the study sites chosen, there was a moderately strong
positive correlation between site area and peat depth (Figure S4).
Meanwhile, a linear relationship between site and catchment area was not
evident from our data. Since site area and depth are well correlated for
the chosen study sites, it is likely that the storage capacity for water
is proportional to site area. If storage capacity increases without a
proportional increase in catchment area, one would not necessarily
expect catchment size to have a strong control on site storage dynamics,
all else being equal (particularly with respect to increases in storage
during/following rainfall). Nevertheless, since storage dynamics were
shown to be empirically linked to near-surface 𝜓 (e.g. correlation
between WTD and 𝜓 as shown in Figure 4c–d) we might expect to see an
influence of catchment size on 𝜓 if catchment size affects site water
balance.
During the meteorological drought period, we found that catchment area
had a significant effect on moss water availability for shallow sites,
but not deep sites (Figure 5). Paradoxically, shallow sites in large
catchments experienced higher median 𝜓 compared to shallow sites in
small catchments. Sphagnum species have a certain degree of
phenotypic plasticity associated with environmental gradients. For
example, the total hyaline cell volume can increase under drought
conditions for certain Sphagnum species (Li, Glime, & Liao,
1992) including S. palustre (Bu , Zheng, Rydin, Moore, & Ma,
2013). Although conjectural, Sphagnum moss at shallow sites with
small catchments could be adapted to drier conditions, and thus
experience lower 𝜓 during meteorological drought conditions. Unlike
other peatlands studied on the Canadian Shield of southern Ontario (e.g.
Devito, Hill, & Roulet, 1996), the catchments in this study do not have
extensive mineral soils (shallow or otherwise) in the surrounding upland
which could provide surface- or ground-water connections between
rainfall events. Rather, hydrological connectivity with the surrounding
landscape appears to be a function of fill and spill processes (e.g.
Spence & Woo, 2006) where upland storage within the various study site
catchments is largely in other similar small moss dominated wetlands.
Consequently, catchment storage elements will become isolated during
drought as their WT drops below the sill thus limiting hydrological
connectivity.
4.3 Survival of the
deepest
Given that site depth is linked to moss water availability during
meteorological drought and the cumulative likelihood of stress
throughout the growing season, our results suggest that shallow sites
might have inherently lower moss productivity compared to deeper sites.
Lab results show that the moss enters dormancy at low water content
(i.e. near-zero Fv/Fm – Figures 6 and
S2), where others have shown that recovery from desiccation is
hysteretic and that the negative impact of repeated and/or prolonged
drought can have a multiplicative type effect (Schipperges & Rydin,
1998; McNeil & Waddington, 2003). The greater WT drawdown rate and the
regular absence of a WT in shallow sites during the growing season
likely contributed to their overall higher 𝜓5 cm and
lower VWC0-3 cm as compared to deep sites. The larger
fraction of time that a given moss species experiences water stress in
shallower sites should lead to increased periods of dormancy and lower
annual growth. As Sphagnum is considered a keystone species for
peat formation (Van Breeman, 1995), there may exist a critical depth
threshold that must be exceeded for many locations on the landscape for
the production of Sphagnum peat to continue, despite seasonal
drought. With drought expected to increase in length and severity under
climate change, this could potentially lead to an ecological shift in
peatlands from a Sphagnum dominated system to one dominated by
forests (Dang & Leiffers, 1989; Kettridge et al., 2015) or more drought
tolerant mosses, such as Polytrichum species (Laine, Vasander, &
Sallantaus, 1995; Benscoter & Vitt, 2008). This then leads to two
important ecohydrological questions: 1) what allows a shallow site to
become deep?; and 2) what prevents a shallow site from becoming deep?
An answer to these questions could simply be time, where deep sites
initiated earlier than shallow sites. Disturbance such as fire is one
mechanism by which barrens landscapes in an otherwise forested biome are
maintained (e.g. Asselin, Belleau, & Bergeron, 2006). Wildfire could
maintain heterogeneity in site peat depth (and age if burned to bedrock)
due to differences in smouldering vulnerability between deep and shallow
peat-filled depressions (e.g. Wilkinson et al., 2020). Moreover,
wildfire disturbance may readily remove surface vegetation cover thus
promoting higher/rapid erosion of thin low density soils following fire
(e.g. Markle, Wilkinson, & Waddington, 2020).
An alternative hypothesis, assuming basal dating shows similar or more
random initiation periods with respect to site depth, would be that
local bedrock morphometry imposes a strong control on peat accumulation
in a rock barrens landscape, similar to the control of lake basin size
on total sediment accumulation (e.g. Ferland, del Giorgio, Teodoru, &
Prairie, 2012). For example, shallow sites may be located in small
basins. Once a small bedrock depression is filled with peat, additional
long term peat accumulation must be supported by a groundwater mound
(Clymo, 1984). The process of groundwater mounding is scale dependent,
where a greater absolute mound height (i.e. at the center of the
accumulating peat deposit) and therefore greater potential peat depth is
partly controlled by the lateral extent of the peat deposit (Ingram,
1982; Belyea & Baird, 2006). Thus for the infilling of a bedrock
depression, depressions that are larger in area ought to be able to
support deeper peat accumulation in the long-term (assuming otherwise
favourable conditions and under steady state). However, greater
long-term peat accumulation is possible if the peatlands are able to
paludify the surrounding landscape, as has been shown for peatlands
developing on mineral soils (e.g. Le Stum-Boivin, Magnan, Garneau,
Fenton, Grondin, & Bergeron, 2019). To our knowledge, paludification ofSphagnum mosses onto bare rock has not been documented in the
academic literature. In general, there is no surrounding mineral soil to
paludify in the study catchments, therefore lateral expansion onto rock
must be supported wholly by climatic conditions and water supply from
the peatland itself. With no underlying soil water storage at the edge,
expanding moss is likely to experience extreme/frequent desiccation. In
fact, the small isolated patches of mineral soil outside of the
peat-filled depressions tend to be dominated by more drought tolerant
species such as Polytrichum moss and lichens (Markle, Sandler, Freeman,
& Waddington, 2020).
Given that both the magnitude and frequency of drought are expected to
increase due to climate change (IPCC, 2013) the differential response of
shallow and deep peatlands are potentially far reaching. For example,
while deep pristine peatlands will likely be resistant and resilient to
drought, shallow peatlands such as younger and/or slow-accumulating
peatlands (Vardy, Warner, Turunen, & Aravena, 2000), recently restored
peatlands (Granath et al., 2016), and organic soils under moss and
lichen mats on the upland rock barrens (Moore, Smolarz, Markle, &
Waddington, 2019; Hudson, Markle, Harris, Moore, & Waddington, 2020)
will be more vulnerable and conservation and potential adaptive
management efforts may be necessary to maintain the carbon storage
function of these sites. Given that peatland restoration has been
emphasized as an important nature-based solution to mitigate climate
change (Humpenöder et al., 2020), our research also highlights the
vulnerability of peatland restoration efforts in situations where the
peat and moss layer are shallow (Grand-Clement et al., 2015). We argue
that shallow peat-filled depressions remain understudied within the
peatland literature and that further research examining the role of
hydrology and hydrogeological setting in controlling peat development,
carbon cycling and disturbance history are critical to better understand
how these systems might respond to future climate change.
ACKNOWLEDGMENTS
This research was funded by a Seed Research Grant from the McMaster
Centre for Climate Change. We thank Cam McCann and Alanna Smolarz for
assistance in the field and to Madeleine Hayes and Justin Kruse for
assistance with the analysis. We thank Dr. Sophie Wilkinson for comments
on an earlier draft of this manuscript.
DATA AVAILABILITY
STATEMENT
The data that support the findings of this study are available from the
corresponding author upon reasonable request.
ORCID
Paul Moore https://orcid.org/0000-0003-1924-1528
Alexander Furukawa https://orcid.org/0000-0001-6437-3314
J. Michael Waddington https://orcid.org/0000-0002-0317-7894
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Table 1: Average peat depth, area and catchment area characteristics of
the main sites.