- The document analyzes growth patterns of black spruce trees in a central Maine peat bog to understand relationships between tree growth, climate, and distance from the bog edge. - Tree cores were collected from black spruce trees along three transects at increasing distances from the bog edge. Tree and stand characteristics, growth rates, and influence of climate factors like temperature and precipitation were analyzed. - Preliminary results suggest that tree growth rates increased with distance from the bog edge likely due to higher nutrient availability further from the bog. Climate accounted for only small variations in growth, with temperature having a greater influence than precipitation.

1. 1
Black Spruce Tree Characteristics and Growth
Patterns in a Central Maine Peat Bog
Daniel Murphy – School of Forest Resources, University of Maine, Orono
Abstract
Bogs are an important component of
Maine’s ecosystem. Bogs are large carbon
sinks that mitigate greenhouse gas
emissions. Analyzing growth patterns can
help understand past, current, and future
climates. Trends between growth and
climate can help predict future functions of
bog ecosystems and their role in global
climate control. This study looked at the
influence the bog had on growth and
climate relationships moving along the
gradient from the bog edge into the forest.
Stand characteristics became denser with
larger stems compared to short, stunted
growth characteristic of the bog edge. Tree
growth rates increased further from the bog
edge likely due to higher nutrient
availability. Climate only accounted for
small variations in growth rates, with
temperature being more influential than
precipitation. This is likely due to high
water availability and increased soil
conditions with warmer, drier climate.
Climate appeared to have less influence on
tree growth than expected and it is likely
water table depth that is most influential on
growth.
Introduction
Peatlands are an environment in which
saturated conditions lead to an anoxic
environment, creating difficult conditions
for vegetation establishment due to
sphagnum moss. Peat bogs in boreal forests
can store up 30% of the world’s terrestrial
soil carbon, despite only covering about 3%
of the earth’s land surface (Turunen,
Tomppo, Tolonen, & Reinikainen, 2002).
Peat bogs are primarily in the northern
latitudes with 80% found in Russia, Canada,
and the United States. Bog ecosystems can
have up to 1-10m of peat on their surfaces,
thus they are considered the world’s
greatest organic material sinks among
wetland ecosystems (Foster & Wright Jr,
1990). However, peat bogs are fragile
structures and as the world’s climate
continues to change, projecting their future
conditions is important for their protection
and the amount of greenhouse gas
emissions.
Being such large carbon sinks, peat
bogs mitigate potentially harmful
greenhouse gas fluxes, most notably carbon
dioxide (CO2) and methane (CH4) that
significantly influence global climate. In
order to create a more productive forest,
some forestry practices have tried draining
bogs to unleash some of their potential to
grow that is hindered by poor growing
conditions. This improves substrate
aeration and speeds decomposition of
organic matter; however, the change in
growth rates can be delayed for 13-19
years. In addition, there is a 3-6 year span
where minimal to no changes are seen in
vegetation likely due to a delay in
photosynthetic material and fine root
development (Dang & Lieffers, 1989). This
delay is significant when looking at the big
picture of ecosystem degradation as it is
important to note that not all effects are
observed in short term studies. These
drainage implications could lead to a change
in species composition as soil conditions
change, favoring other species, thus further
affecting the rates of carbon sequestration

2. 2
and releasing more harmful greenhouse
gases to the atmosphere at an increasing
rate. A changing environment could lead to
degradation of bog habitats causing
increased sequestration of carbon as soil
characteristics change to better suit tree
growth. By examining tree ring increments,
potential insight on past trends can give a
depiction of the local climate, past and
present, and any associated climactic
relationships.
Previous research done at lower
latitudes has show that there is a weak
correlation between climactic influences on
yearly growth of pine species in bogs
(Dauškane & Elferts, 2011). Alternative
research has shown that trees on natural
peatlands are most greatly influenced by the
depth of peat moss and water table
fluctuations (Boggie, 1972). However,
temperature and precipitation are the main
drivers of water table depth (Linderholm,
2001), thus giving some significance to
these climactic influences when looking at
peat bogs. On drier upland sites,
temperature and precipitation are more
significant as there is less water availability.
This project was designed as a senior
thesis in an effort to fill the knowledge gap
regarding bog dynamics in Maine. As peat
bogs are a prevalent part of the ecosystem,
it is important to understand the
characteristics and functions of these
communities before understanding the
whole functioning of the forest ecosystem
and the complex interactions between
communities. This was a one time study
that will hopefully open the door to future
scientists to acknowledge that there is a
need for further understanding of these
critical ecosystems. This study was done in
order to understand how black spruce
(Picea mariana) characteristics and growth
patterns change in relation to the distance
from the bog edge beginning where black
spruce reaches breast height to the change
in composition along the 300-350 ft
gradient. The purpose of the study was to
understand black spruce tree and stand
characteristics in regards to the distance
from the bog edge and climactic variables.,
and The specific objectives of this study
were to analyze: tree and stand
characteristics with respect to the bog edge;
tree growth rates in relation to the distance
from the bog edge; and changes in tree
growth patterns with respect to climactic
factors, i.e. temperature and precipitation. I
hypothesize that the distance from the bog
edge serves as a proxy for water table
height and will significantly influence
growth patterns . As you move along the
gradient of the transect, I would predict
more competition, faster tree growth, and
less influence from the bog.
Site Description
The Orono Bog, located off Stillwater
Avenue (44°52’06”N 68°43’26”W), is a joint
venture by the city of Bangor, the University
of Maine, and the Orono Land Trust. The
bog is 616 acres and provides habitat to
numerous plants that evolved to this highly
specialized environment as well as some of
Maine’s rarest bird species (Bangor City
Forest website). The bog developed with
the recession of the glaciers roughly 10,000
years ago. Like most bogs, the Orono bog is
a low elevation site (147 ft) that was once a
lake. When the lake dried enough it allowed
the colonization of sphagnum moss and an
accumulation of peat developed. Black
spruce saplings are established scarcely on
the edge of the tree line transforming into a
more mature spruce fir forest. The stand
has a minor tamarack (Larix laricina)
component mixed throughout. At about 300
ft, the stand rapidly transitions into a mixed
hardwood stand dominated by red maple
(Acer rubrum).

3. 3
Methodology
Data Collection
Using aerial imagery from Google Earth and
shape files from the University Forest
database, a homogeneous stand of black
spruce was outlined as the designated
sample area located directly adjacent to the
Orono bog boardwalk (Appendix 1). The
rectangular stand was estimated to be 300
ft x 750 ft. Using 3 transect lines spaced 200
ft apart, 10 sample points per transect were
created. To avoid any recreational users, the
first transect line was set 100 ft off the
boardwalk and a random number generator
used produced a number between 1 and
100 (18) to further offset from the
boardwalk to establish the distance away
from the boardwalk for the first transect
line. The bog edge was determined to be the
presence of black spruce above breast
height (4.5 ft). From the edge, an azimuth
perpendicular to the bog edge (210°) was
taken to create the three transect lines. All
three transects have the same azimuth of
210° from magnetic north; magnetic
declination was not adjusted for.
At 30 ft intervals within each
transect, one tree core was collected that
was cored directly through the tree (bark to
bark) to gather two samples from one core
form the most dominant black spruce. The
twenty-seven tree cores collected were
stored in straws until further processing.
Other parameters collected include basal
area (BA) using a 10 BAF prism, height and
live crown ratios (LCR) using a Hagloff,
diameter at breast height (DBH) using a
diameter tape, and the distance and DBH of
the nearest competitor at each sample
point. When there were no black spruces to
be found, no core was taken but basal area
was still measured. Transects occurred at
118, 318, and 518 ft from the boardwalk
and all were started at the bog edge. Each
transect was done at 210°. All tree cores
were labeled and put into straws for storage
and further analysis of tree ring growth.
Climate data was downloaded from
the PRISM Climate Group website. Daily
precipitation and temperature was then
compiled into tables and analyzed on a
monthly and annual basis. Climate variables
include monthly precipitation and
temperature, annual precipitation and
temperature, and lag months and years.
Sums, means, and standard deviations were
calculated and analyzed for all variables
including growing season. For this study,
only one series of lag years was created.
Post Processing
Tree cores were removed from the straws
and mounted onto tree core mounts. Once
glued on, samples were sanded with a belt
sander and then further with fine grain
sandpaper for a clear visual on the tree
rings.
Table 1: Orono bog stand characteristics. Relative mean annual increment(MAI) was calculated by taking the
average of the BAI of a given year over its DBH of the same year (BAIyear/DBHyear) for each series. The coefficient of
variation was significantly lowered when the data was normalized by accounting for tree size in relation to BAI.

4. 4
Data Analysis
Under an electron microscope, annual ring
width measurements were taken using the
software WinDENDRO. Series data were
compiled into RStudio. All data analysis was
done using RStudio. Generalized Boosted
Regression Modeling calculated the relative
influence of growth characteristics in
relation to relative basal area increment
(rel.BAI). The highest importance factors
were further analyzed individually with
linear regression fits.
Results
The black spruce sampled in this stand had
an average DBH of 5.1 in with a maximum of
10.0 in and a minimum of 2.3 in. Basal area
was an average of 80.0 ft2. Basal area
measurements include alternative species
such as tamarack and hardwoods. The
tallest sampled tree was 57.0 ft and the
shortest at 18.1 ft. Stand characteristics are
represented in table 1.
Objective 1
Trees growing further from the bog edge
were generally of larger diameter than
those nearer the bog edge. A regression that
fits with an R2 value of 0.34 and a p-value of
0.003** relating diameter to the distance
from the bog edge can be seen in figure 1.
There is a very strong correlation of
height and distance from bog edge (R2=0.42;
p-value <0.001***). The strongest
relationship in the dataset was HT/DBH
ratio (R2=0.84; p-value <0.001***) but no
significant relationship when comparing
HT/DBH ratio in relation to distance from
bog edge. Live crown ratio proved to have
no statistical relationship, however, there
was a slight negative skew in the data.
Although there is no statistical basis to
make a valid correlation, this could be an
artifact of the process of branch shedding.
There is a slight relationship
between basal area and distance from the
bog edge (R2=0.23; p-value=0.025*) as seen
in figure 3. BA increases by 15.4 ft2/100ft
traveled from the edge into the forest. This
is the best regression fit for the data. A more
realistic representation would be a
logarithmic fit as basal area will not
continue increasing indefinitely. The equation
BA = 0.50 + 38.97 * log(DIST_EDGE) would
be a more accurate fit for this reason. When
logarithmic models were run, a lesser
correlation was seen. Because soils carrying
capacities differ with soil conditions, a
linear fit for this data is representative of
bog conditions.
Figure 2: Mean basal area increment vs. distance
from the bog edge. R2=0.32; p-value=0.005**;
MAI=DIST_EDGE*0.8659+45.2285
Figure 1: Diameter at breast height (DBH) vs.
distance from the bog edge. R2=0.34;
p-value=0.0035**; DBH=DIST_EDGE*0.012+3.227

5. 5
Objective 2
Using a linear fit, a relationship can be
described between the distance from the
bog edge (ft) and the mean annual basal
area increment (in^2) (MAI) as: MAI=
DIST_EDGE * 0.87 + 45.23. An R2 of 0.32 and
p-value of 0.005** describes its statistical
significance. There is considerably higher
variation associated with the sampled trees
as you move further from the bog edge.
Mean annual increment was assessed by
comparing radial growth in ring widths and
applying it to diameter to convert length to
an area. Accuracy may be limited by sample
size and dating accuracy as tree rings were
not cross-dated with pre-assessed historical
ring width data. When relative mean annual
increment (MAI) was plotted vs distance to
edge, a p-value of 0.93 suggests there is no
correlation.
Objective 3
A third and final objective was to evaluate
the influence of climate on growth rates. Out
of the 166 climactic variables, 28 were
found to be influential with respect to
relative mean annual increment (rel. MAI)
and 24 with respect to BAI. Relative
influence of precipitation and temperature
variables were evenly distributed in table 2
and skewed towards temperature in table 3,
suggestive of a strong co-dependency
between climactic variables with
temperature being slightly more influential.
Table 2 depicts the 28 climactic
variables in relation to relative mean basal
area increment (rel. MAI) and shows their
relative influences. The sum of July
temperatures is the most influential
variable followed by the sum of October
temperatures with relative influences of
31.34 and 13.85 respectively. Other notable
influences were the variations in December
temperatures, variation in June
precipitation, the sum of January
precipitation, the annual sum of
temperatures, and January temperature
sums to name a few. It is interesting to note
Table 2: Relative influences of climactic variables
influencing relative basal area increment (BAIyear/DBHyear).

6. 6
that the sum of October temperatures and
precipitation values have high relative
influence despite not being in the growing
season.
Relative influences of precipitation in
the winter months, December to February,
are small but have significant implications.
Variation of precipitation in winter months
is often a greater influence than that of the
sum of monthly precipitation.
Discussion
Bog dynamics have a strong relationship
with temperature and precipitation. Much
of this dependency relates to water storage
and fluctuations in water table depth
limiting availability of resources, i.e.
oxygenated soil. This study was designed to
create transects that would simulate water
table depth by distance from bog edge
serving as a proxy value. No water table
measurements were collected. For a more
accurate and detailed study, pressure heads
would be created at each sample location to
find actual water depths. Previous studies
have been done that show a dependency of
peatland pine growth on previous seasons
climate (Linderholm, Moberg, & Grudd,
2002). This was not seen in this study as no
lag variables showed relative influence on
basal area increment. Future analyses of the
data would focus on collecting more
samples from the stand, cross dating the
samples, and developing a further sequence
of lag years to determine multiple previous
season influences.
Objective 1
As demonstrated in figure 1, trees at a
higher distance from the bog edge are
generally of larger diameter and taller than
those closer to the edge. Scheffer et. al.
describes summer droughts influencing
seedling survival rates (Scheffer, Van Nes,
Holmgren, & Hughes, 2008), affected in part
by mosses drying out and lowering their
photosynthetic ability (Robroek, Schouten,
Limpens, Berendse, & Poorter, 2009).
Assuming distance from bog edge serves as
a proxy for water table depth, it can be
concluded that trees further from the bog
edge are more likely to have established
prior to the trees nearer the bog edge which
have a lower probability of germination
(Ohlson & Okland, 2001). This coupled with
drier soil conditions could propagate both
radial and apical growth due to higher
nutrient availability (Fenner & Freeman,
2011) and less competition from mosses.
Three locations along the transects
returned no black spruce trees in the
sample areas. In transect 1, a failure to
obtain a sample tree occurred at 312.3 ft
from the bog edge. Transect 2 had two
locations where samples could not be taken
at 227.6 and 257.6 ft from the bog edge. At
these locations, no black spruce was
available to sample. These locations were
mostly made up of tamarack as the primary
species. On transect 2, black spruce advance
regeneration approximately 7-10 ft tall but
too small to core was developing at the
location 227.6 ft from the bog edge. Further
on the transect, 256.6 ft from the bog edge,
the only tree in the sample area was a 9.2 in
red maple. The difficulty posing to find
black spruce along the far ends of the
transects is indicative of the species
composition change as the forest abruptly
changes from a black spruce dominated
stand to a mixed hardwood stand favoring
red maple. Species that are less tolerable of
moist soil conditions but still favor wet
sites, such as red maple, begin to out
compete the black spruce. It is at this
transition that I would suspect the water
table from the bog becomes non-influential
to the stand and individual tree growth
patterns.

7. 7
Basal area, an indication of stand
density and relative competition, increases
along the gradient of the transects. This is
indicative of better soil conditions able to
support more growth. On transect 2, where
there was black spruce regeneration
developing, a recent tree fall is suspected to
have induced growing conditions for
advance regeneration. Because there is less
influence from the bog, there is likely to be
more decomposition happening allowing
the turn over of nutrients from decaying
course woody debris and allocating those
resources to sapling growth.
Objective 2
When figures 1, 2, and 3were interpreted
together, the co-relationships between DBH,
BA, and MAI can be seen as all three are
increasing along the transect gradient and
influential on each other. As the trees with
larger DBH further from the bog edge
accrue new wood, it takes less radial growth
to produce the same amount of BAI than
that of smaller diameter trees (figure 5).
Surprisingly, when relative ring width
growths were compared to DBH, there were
only slight variances between the data, as
seen in figure 4.
The ability for trees to grow faster
the further they are from the bog edge is
likely a result of better growing conditions.
When the soil is supersaturated, such as it is
near the bog edge, water fills macropores
and inhibits growth. In drier conditions,
these macropores are channels where fine
roots can disperse and obtain more
nutrients (Lieffers & Rothwell, 1987). In the
harsh bog environments, decomposition
rates are incredibly slow due to lack of
aeration and inhospitable conditions for
decomposing species. As trees become more
established further from the bog edge, their
root systems can grow larger as they out
compete the sphagnum moss, allowing
uptake of more nitrogen (N), the limiting
nutrient for most plant ecosystems.
Previous studies have hypothesized that an
increase in N would ultimately lead to
higher productivity. In sites with high
atmospheric nitrogen supplies bogs are
faced with phosphorus (P) as a secondary
limiting factor (Aerts, Wallan, & Malmer,
1992; Small, 1972). A decrease in sphagnum
moss would ultimately allow more N to be
available for the trees. However, due to the
secondary limiting factor of P, production
rates would stay relatively equal while
decomposition rates would increase; thus
Figure 3: Basal area vs. distance to bog edge.
BA=57.71+0.15*DIST_EDGE
R2=0.23; p-value=0.025*
Figure 4: Relative ring width growth in relation to
tree size (DBH). There is very little relation to the
size of black spruce in the bog and their growth
rates. This suggests that growth is consistent and
independent of diameter. A lowess trendline is fit to
the data.

8. 8
resulting into more C-emitting systems
instead of C-accumulating systems, leading
to serious implications for the global carbon
budget (Aerts et al., 1992; Gorham, 1991). A
detailed soil survey for this site would give
further understanding to the mechanisms
driving tree growth. Based on the data
collected from this study, it is apparent the
bog has a negative influence on tree growth
but specific factors cannot be determined
without alternative data collection.
Objective 3
Previous research determined that 90% of
the maximum observed photosynthetic rate
for black spruce is maintained between 10
to 25 °C and that it peaks at 15 °C (Bonan &
Sirois, 1992). Bonan and Sirois (1992) also
state that in sites not limited by soil
moisture, temperature is the most
influential factor for black spruce growth by
analyzing growing degree-days (GGD). This
data lacks GGD and future analysis should
including GGD as a climactic variable.
However, it is important to note that the
second highest relative influence of
climactic variables in relation to BAI (not
relative BAI) was the standard deviation of
May temperatures. Based on the climate
data downloaded from the PRISM Climate
Group website, the average temperature in
the month of May between 1981 to present
is 53.5 °F. This compares to the optimum
growing temperature of 59 °F (15 °C)
previously determined by Bonan and Sirois
(1992). Complimenting their theory, the
highest relative influences for BAI were the
standard deviation of June precipitation
amounts and variation in May temperatures
being second. This further implies that
precipitation is a valid determinant of
annual growth in water dominated
ecosystems due to the strong correlation
between precipitation and water table
depth.
Both temperature and precipitation
sums, primarily during the growing seasons,
were highly correlated to annual growth
rates of black spruce. Factors such as
precipitation sums in February and March
as well as their relative temperature sums
were also of relative influence. Because
water table depths are so influential to the
growth of black spruce in bogs, it is likely
that these variables are correlated with
snowmelt and the commencement of the
growing season. Unexpected results
suggested October temperature and
precipitation sums were of relative
influence. This could be correlated with
elongated climactic conditions associated
with the previous months’ growing
conditions. Furthermore, frozen zones in
the peat that are dependent on water table
depth can be found as late as mid summer
(Lieffers & Rothwell, 1987). High insulation
prevents deep water tables from thawing
until the peak summer months. High
insulation could create a lag in peat
temperatures explaining the importance of
October climate variables. When most trees
have stopped their growing seasons, these
insulated root systems could see a delay in
both the start and completion of their
growing seasons leading to fine root
development in late autumn.
Figure 5: A horizontal fit shows the relation
between ring width growth and DBH to be
consistent (left) while basal area increases
exponentially (right). This relationship allows for
more total growth even with consistent radial
growth.

9. 9
The relative ring width growth
relative influences seen in table 3 have
more relative influence variables associated
with temperature than precipitation. This
implies that temperature is more important
to ring growth than precipitation.
Temperature being more influential to
growth is likely due to the saturated soils
characteristic of bogs. The sum of July
temperature still reigns as the most
important variable. I would expect warm
temperatures associated with July result in
a full thawing of frozen peat and a decline in
the water table due to higher evaporation
rates allowed more root development.
Future studies for the bog could
address the implications of temperature
and precipitation on the water table. Change
in storage over time in the bog from a series
of storm events and evapotranspiration
rates could have significant implications on
the water table, thus growing conditions in
the bog. A comparison of above ground
temperatures and below ground
temperatures may draw further conclusions
regarding root development. This study was
developed with the assumption that
distance from the bog edge is correlated
with water table depth. In order to fully
address this assumption, a series of
pressure heads should be set up to
determine that accuracy of this assumption.
Conclusion
Data collected from the Orono Bog located
in central Maine indicate black spruce is
highly tolerable of wet soil conditions. In
bog environments, water table depth
appears to be the limiting factor regarding
growth. A study directed at water table
depth is needed to draw direct conclusions
on the influence of water table and related
growth rates. As you travel further from the
bog edge, increases in tree characteristics
such as DBH and height increase for this
reason. Stand characteristics such as BA
also follow this trend. Growth rates of black
spruce, similarly, see an increase in mean
annual increment with relation to the
distance from the bog edge.
Climactic factors related to
precipitation and temperature showed
strong correlations in regards to tree
growth. However, it is estimated that these
climactic variables only account for 6.5% of
Table 3: Relative influences of climactic variables
influencing relative ring width growth.
(RingWidth/DBHyear).

10. 10
the variation associated with tree growth.
Because temperature and precipitation are
the most influential variables in
determining water table depth (Paavilainen
& Päivänen 1995), I would conclude that
these variables have a lower relative
influence than water table depths as that
determines nutrient availability. Future
studies related to bog environments in
Maine should be conducted, as bogs are
such a prevalent part of the forested
ecosystem. Implications drawn can help
further understand long term implications
regarding the world’s carbon budget due to
bogs currently being such large carbon
sinks. Changes in atmospheric nitrogen
levels risk altering their function into
carbon emitters (Aerts et al., 1992).
Conclusions drawn from this study
are limited by a small sample size, the
assumption that distance from bog edge is
uniformly correlated with water table
depths, and precision in dating of tree cores.
Because black spruce in bogs grow on
unstable ground, variations in compression
wood produce extreme difficulties in cross
dating sample cores from bog environments
(Linderholm et al., 2002).
Acknowledgments
I would like to thank the University of
Maine and School of Forest Resources for
their encouragement and support in this
study. Professor Dr. Aaron Weiskittel’s
advising in the development,
implementation, and analysis of this study
was fundamental to its success. Shawn
Fraver’s expertise in tree ring analysis was
essential to all dendrochronological
analyses. I would like to give a special
thanks to Professor Dr. John Daigle and
other members of the Boardwalk
Management Committee for their
permission to access and study the Orono
Bog located directly adjacent the Bangor
City Forest. Without the help of the
Boardwalk Committee Members, guidance
from the School of Forest Resources
professors, and the permission to access the
University of Maine’s forest this study could
never have been accomplished. Help from
fellow undergraduates in data collection
and graduate student, David Carter, played
a pivotal role in the collection and analysis
of data. I hope future studies are conducted
by University of Maine researchers to
further understand the importance of bog
functions on Maine ecosystems and their
global importance.
Citations
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Growth-limiting nutrients in
Sphagnum-dominated bogs subject to
low and high atmospheric nitrogen
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11. 11
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12. 12
Appendix 1

13. 13
Appendix 1 is a map of the study area. The Orono boardwalk can be seen on the west side
of the delineated black spruce stand. Just outside of the black spruce boundary there is a
composition shift to a mixed hardwood stand.