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Effect of increased solar radiation on the south aspect of Rock Canyon, Provo, UT on the width
of tree rings of Douglas-fir trees (Pseudotsuga menziesii)
Leah Davis, 2208 Stover Hall Provo, UT 84604 Email: smilesrockon118@gmail.com. April 2011.
Permission to share granted to CRN, 15 April 2011.
Abstract
This study examines how slope-aspect (the direction a slope faces) affects the tree rings
of Douglas-fir trees (Pseudotsuga menziesii) relative to the increased amount of sunlight on the
southern aspect. Core samples were taken from trees on both the southern and northern aspects
of Rock Canyon in Provo, UT to determine whether the rings differed in width. Variables such as
number of trees in the sampled tree stand, elevation, and angle of the canyon slopes were
controlled in an attempt to isolate the variable of solar radiation. However, results showed that
on average, the samples from the southern aspect did not have larger tree rings than the northern
aspect. While researchers have indeed proven slope-aspect to cause differences in growth, other
factors present in this particular ecosystem, such as competition, temperature, or precipitation,
may have had a greater impact on the width of the tree rings.
Key words: Dendroclimatology, sunlight, conifers, growth responses, precipitation, slope angle
Introduction
Tree rings have a high sensitivity to changes in their surrounding environments. In
gymnosperms such as conifers, the growth of each annual ring reflects fluctuating climatic cycles
(Schweingruber 1988). Ring growth can also show the effects of environmental factors from
previous years, a process known as autocorrelation (Speer 2010). Abiotic factors such as light,
temperature, precipitation, water sources, nutrient supply, wind, fungi attacks, air pollution, and
damage to the crown, roots or stem all affect the width of the rings (Schweingruber 1996). For
example, when light conditions decrease, photosynthesis is reduced and trees form narrower
rings (Schweingruber 1988).
Due to the angle of the sun on different parts of the earth, in the Northern Hemisphere,
southern aspects receive more annual solar radiation per unit area than northern aspects (Haase
1970). For example, in a study by Haase (1970), the south aspect in the South Catalina
Mountains of Arizona had the greatest solar radiation for eight months out of the year 1967,
compared to other aspects. In another study on the effect of light on Douglas-fir trees, the
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greatest light intensity occurred on the south-facing slope at midday (Schweingruber 1996).
Accordingly, Douglas-fir trees are found mainly on southern aspects, and often grow at high
elevations in the Rocky Mountains (Hermann and Lavender, n.d.).
For this experiment, the sensitivity of tree rings to solar radiation was tested by
comparing trees on the sunnier southern aspect to those on the northern aspect. I hypothesized
that increased exposure to sunlight on Douglas-fir trees growing on the southern aspect of Rock
Canyon, in the Wasatch Mountain Range of the Rocky Mountains, would result in wider rings.
Methods and Materials
On Friday, March 11, 2011, I selected six Douglas-fir trees (Pseudotsuga menziesii) in
Rock Canyon, Provo, UT. As aspect was the variable in this experiment, I chose three trees on
the northern aspect and three on the southern aspect, and marked each with red tape. To regulate
the impact of elevation on the tree rings, I used a GPS unit to choose trees located at the same
elevation, which was within 30 meters of 1675 meters above sea level (Table 1). In order to
equalize the amount of shade coverage, I picked trees within similarly dense stands, with about 3
or 4 other trees within a 3-meter radius. I used a tape measure to find trees with circumferences
between 115 and 165 cm, which was intended to control for similar ages of the trees (Table 1).
On Saturday, March 12, 2011, I used an increment borer to extract core samples between
10 and 23 cm long from the six trees at breast height. I used beeswax to lubricate the blades of
the increment borer before boring into the trunks. Next, I took each sample at a parallel to the
slant of the hill on the side of the tree closest to the uphill: on the north side for the trees growing
on the southern aspect, and on the south side for those on the northern aspect. Finally, I placed
the samples in plastic straws and labeled them with red tape and a permanent marker. During the
process, I took pictures and notes on the field conditions.
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On Tuesday, March 15, 2011, I observed each of the six core samples with an Olympus
camera microscope and Microsuite Basic Edition camera software (Figures 1 and 2). Using
increments of millimeters, I measured fifteen ring widths, representing the past 15 years of
growth (Table 1). Next, I averaged the tree rings for each individual tree (Table 2). My final
calculation was to average the ring width for all three trees on the northern aspect and then for all
three trees on the southern aspect (Table 2).
Results
Although one of the individual samples from the southern aspect had larger rings, the
other two trees from that same slope showed overall narrower average ring-widths (Table 2). For
example, Tree 1 (Figure 1) had rings of an average ring width of 6.97 mm, whereas Trees 2 and
3 had rings of much smaller widths of 2.74 mm and 2.46 mm, respectively (Table 1).
A similar trend occurred on the northern aspect. For instance, Tree 4 had an average ring
width of 6.29 mm. Measurements of Tree 5 showed an average ring width of 2.07 mm. Tree 6
(Figure 2) had an average ring width of 4.20 mm, neither of which showed a relationship to each
other or to Tree 4 (Table 1).
In addition, the average ring width of all three trees on the southern aspect was nearly
equal to the average of the three trees on the northern aspect, with averages of 4.06 mm and 4.19
mm, respectively (Table 2).
Discussion
When I measured the tree rings, I could not identify a consistent pattern. On the southern
slope, the ring widths were not uniformly as large as I had expected from the increased sunlight
exposure (Table 1). Then on the northern aspect, where I had expected to find smaller ring
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widths on average, the results were similarly inconsistent (Table 2). Finally, the average of ring
widths of all samples from each aspect did not reveal clear results (Table 2). Since individual
tree ring widths were so varied, this may have altered the averages. The data could be more
pertinent if additional samples were taken to show general trends in tree ring growth of Douglasfir trees growing on each aspect.
As neither aspect seemed to favor the width of the tree rings, my experiment did not
support my hypothesis. However, according to Schweingruber (1995), there is a wide variability
within a single species and within individual trees, and it is often impossible to prove the effects
of a single climatic factor. Speer (2010) remarks that it may be difficult to determine a single
limiting factor, as trees may be limited by multiple factors at once. Furthermore, Haase (1970)
discusses that unless every possible environmental factor is addressed, the importance of slope
exposure on plant growth cannot be considered complete. I tried to eliminate variables in order to
isolate the effect of aspect, but I subsequently learned that there were countless other factors that
may impact the growth of tree rings.
The lack of a clear pattern in my results could be attributed to the particular geographical
features of Rock Canyon, if the narrowness of the canyon mouth caused only a slight increase in
the amount of time that the sun hit the southern aspect versus the northern aspect. Furthermore,
though the trees grew in similar conditions, many environmental factors specific to each
individual tree could have impacted the rings. Schweingruber (1988) took samples of spruce
trees in Finland growing on uniform sites, and found that individual tree ring features still varied
greatly. Fritts (1974) also discusses that samples obtained from the same site do not have the
exact responses to climatic factors.
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According to Speer (2010), younger trees sometimes experience a period of juvenile
growth during which they produce larger-than-average growth rings. Schweingruber (1988)
affirmed this notion, explaining that on occasion tree-ring width decreases rapidly as trees
become older. Perhaps in this experiment the age of the Douglas-fir trees was an independent
factor in the width of the rings. I tried to limit this factor by choosing trees with similar
circumferences that might reflect annual cambial tree growth, but perhaps that was not the best
indicator of their ages. Wind also may impact tree growth. Trees that lean due to extreme wind
storms form pressure, or compression wood, resulting in narrower rings (Schweingruber 1988).
In this experiment, if trees on either aspect experienced different wind conditions, their rings
may have responded to this factor independently of light.
Position as a dominant organism in a tree stand is sometimes critical when competing for
sunlight. Schweingruber (1996) studied the development of a Douglas fir stand in Switzerland, in
which trees grew optimally when they outcompeted neighboring ferns and thus had more access
to light. Devine and Harrington (2010) planted trees in close proximity to each other and
discovered that seedlings competing for sunlight developed modified crown morphology, which
greatly affects the growth of the whole tree. Other cases, however, show that competition for
light may be less important because fir trees are more shade-tolerant than other species such as
larch and pine (Schweingruber 1996). Thus, this could indicate that the position of Douglas-fir
trees on sunnier or shadier slopes is not as important a factor. Either theory—the effect of stand
competition or the trees’ indifference toward shade—could be considered in accordance with my
data. Perhaps in this experiment, samples with similar distances to other trees could have been
chosen to equalize the effect of competition for light.
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Holland and Steyn (1975) discuss the angle of the slope as a factor affecting plant
growth. Their field experience showed that slopes steeper than about 15 to 25 degrees have
limited water storage capacity in their soil, which would lessen the advantages of sun exposure.
As Branson and Shown (1989) discovered in their studies on slope angles on Green Mountain in
Denver, Colorado, vegetation growing on southern-facing slopes experiences higher soil
temperatures, greater evaporation, and decreased soil moisture, in addition to periods of freezing
and thawing that cause erosion and runoff. Their study of western-snowberry plants on the
southern aspect of Green Mountain showed that individuals thrived better in a less steep area that
could retain moisture in the soil more abundantly (Branson and Shown 1989). From this
observation they surmised that increased solar radiation, especially on steeper angles, causes
greater evaporative water loss and decreases the water available for plants. However, Hermann
and Lavender (n.d.) discuss that Douglas-fir trees growing on the south aspect do adapt to drier
conditions on the southern aspect by growing larger roots and setting buds earlier compared to
those on adjacent northern aspects, which may be an adaptation that compensates for drier soil
conditions. According to these sources, in my experiment, reduced soil moisture caused by the
angle of the slope could have had a substantial impact on tree ring growth.
Holland and Steyn (1975) discuss how the intensity and duration of direct beam solar
radiation influence temperatures and water movement. According to Fritts (1974), temperature
and precipitation are the most important elements of tree growth. His experiments on the ringwidth responses of coniferous trees during a 14-month period in western North America showed
that the aspect of slope did have a critical impact on growth. However, he accounts this to
temperature differences caused by more shade, rather than the impact of solar radiation on
photosynthesis, as I had hypothesized.
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High temperatures and low rainfall correlated with the formation of narrower rings in
conifers (Fritts 1974). Haase (1970) studied how rainy and arid seasons during the year 1967
affected different slopes, and found that the south aspect was warmest and driest compared to
other aspects. Drier and warmer climates considerably reduce net photosynthesis (Fritts 1974).
However, high precipitation sometimes has negative effects: poor aeration on some sites,
reduced root growth, and reduced water absorption when soils begin to dry (Fritts 1974). Indeed,
Hermann and Lavender (n.d.) explain that Douglas-fir trees do not grow well on soils that are
poorly drained. Overall, however, high precipitation tended to result in greater ring-width growth
in conifers on arid sites in a study by Fritts (1974). In my experiment, differences in each
individual tree’s access to water might have directly affected the width of the rings more than
access to sunlight.
Further research on the relationship between aspect and tree ring width could be
conducted by collecting samples from Douglas-fir or other tree species at different latitudes,
elevations, or stand sizes. Future studies could focus on the effects of precipitation on tree ring
growth, as this seemed to be a more important trend than sunlight as supported in previous
studies (Fritts 1974; Haase 1970).
Acknowledgements
Thanks to John Sproul and Charles Riley Nelson for helping me to develop my
hypothesis and for providing equipment and feedback. Thanks to the research assistant and the
employee at the Biology Reference desk at the BYU library for help with finding research
materials. Thanks to Scott Maughan for driving me up to Rock Canyon and helping with the data
collection. And thanks to Stuart Davis, Sam Clyde, Ashley Hurst, and the many others who
listened to my concerns about the project and were supportive.
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Works Cited
Branson, F. A. and Shown, L.M. 1989. Contrasts of Vegetation, Soils, Microclimates, and
Geomorphic Processes Between North- and South- Facing Slopes on Green Mountain
Near Denver, Colorado: U.S. Geological Survey. Water-Resources Investigations Report
89-4094. Denver, Colorado.
Devine, W.D. and Harrington, T.B. 2010. Aboveground growth interactions of paired conifer
seedlings in close proximity: New Forests, Vol. 41, No. 2., pp. 162-178.
Fritts, H. C. 1974. Relationships of Ring Widths in Arid-Site Conifers to Variations in Monthly
Temperature and Precipitation: Ecological Society of America. Ecological Mongraphs,
Vol. 44, No. 4 (Autumn 1974), pp. 411-440.
Haase, E.F. 1970. Environmental Fluctuations on South-facing Slopes in the Santa Catalina
Mountains of Arizona: Ecology, Volume 51, No. 6. pp. 963-971.
Hermann, R.K. and Lavender, D.P. n.d. Pseudotsuga menziesii (Mirb.) Franco Douglas-Fir:
United States Department of Agriculture Forest Service, Northeastern Area.
Holland, P.G. and Steyn, D.G. 1975. Vegetational responses to latitudinal variations in slope
angle and aspect: Journal of Biogeography Vol. 2, pp. 179-183.
Schweingruber, F. H. 1996 Tree Rings and Environment Dendroecology: Birmensdorf, Swiss
Federal Institute for Forest, Snow and Landscape Research. Berne, Stuttgart, Vienna,
Haupt. 609 pp. 26-29.
Schweingruber, F. H. 1988 Tree Rings Basics of Dendrochronology: D. Reidel Publishing
Company, Dordrecht, Holland. pp. 11, 123.
Speer, J. H., 2010, Fundamentals of Tree Ring Research: The University of Arizona Press,
Tucson, AZ. pp. 16, 17.
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Table 1. Location, slope aspect, circumference, and tree ring width of core samples of Douglas-fir trees in Rock
Canyon, Provo, UT.
WIDTH OF DOUGLAS-FIR TREE RINGS (mm)
N
1691
15 Years Ago
6
14 Years Ago
1688
13 Years Ago
N
12 Years Ago
5
11 Years Ago
1671
10 Years Ago
N
9 Years Ago
4
8 Years Ago
1709
7 Years Ago
S
6 Years Ago
3
5 Years Ago
1698
4 Years Ago
S
3 Years Ago
2
2 Years Ago
1660
1 Year Ago
S
Circumference
of Tree (cm)
1
Location of
Tree
GPS Reading
Core Samples
from Tree #
Direction
Slope Faces
Elevation of
Tree (m)
Note: This study assumes that conifer trees in temperate areas,
such as Douglas-fir trees, grow one ring per year (Speer 2010)
N 401555.2
W1113703.6
N 401555.8
W1113703.6
N 401556.3
W1113702.0
N 401551.9
W1113700.9
N 401552.4
W1113702.0
N 401550.7
W1113702.9
162.6
5.17
7.22
6.52
5.15
6.21
10.99
8.09
5.87
4.47
7.58
7.34
8.48
6.91
7.97
6.57
144.8
3.03
3.08
1.94
2.66
3.98
1.94
1.99
2.43
1.34
1.47
4.24
3.70
3.00
2.77
3.54
157.5
2.87
3.49
2.46
1.71
2.33
2.56
0.83
2.15
1.63
1.50
1.97
1.99
4.19
3.44
3.83
116.8
3.67
7.09
7.66
3.75
5.38
7.09
5.66
4.97
6.28
7.14
8.07
5.74
7.37
7.50
7.06
106.7
1.47
2.12
2.15
1.81
1.76
2.12
0.85
1.97
2.25
2.09
3.57
2.74
1.71
2.12
2.30
144.8
4.03
3.39
3.34
4.22
2.51
3.96
5.20
3.10
2.87
3.03
2.79
2.84
3.67
4.27
6.57
Table 2. Average tree ring width of individual and slope-specific core samples of Douglas-fir trees in Rock Canyon,
Provo, UT.
Core Samples from Tree #
Average Tree Ring Width
Over Past 15 Years (mm)
Average Tree Ring Width of
All Trees on Slope (mm)
1
6.97
SOUTH-FACING SLOPES
2
3
2.74
2.46
4.06
Figure 1. Tree 1, South-facing slope, Rings 1 to 5, representing
most recent 5 years of growth.
4
6.29
NORTH-FACING SLOPES
5
6
2.07
4.20
4.19
Figure 2. Tree 6, North-facing slope, Rings 1 to 9, representing
most recent 9 years of growth.