Introduction

Effect of Old Millpond Dike on Cottonwood Growth in Cowling Arboretum,Northfield, Minnesota

DeFRIES, Natalie, MIDGET, Rosie, and RATLIFF-CRAIN, EthanCarleton Summer Science Institute 2010

Figure 1. Map of cottonwood sample sites in Cowling Arboretum (modified from Case, 2010).

Figure 2. Graph of the ratio of adjacent ring widths (normalized width) from cottonwood core #6 overlaid with yearly runing average of monthly rainfall ploted relative to the average rainfall (rainfall anomaly) from 1982-2010. Average monthly rainfall (1982-2010) is 2.3 inches.

This study focuses on the growth and development of cottonwood trees (Populus deltoides) on either side of the Old Millpond dike in Cowling Arboretum, Carleton College, Northfield, Minnesota (see Fig. 1). The dike creates two distinct forested areas, one of which may experience reduced water availability, and thus reduced growth rate. After analyzing tree core samples, we have found that the presence of the dike has no significant effect on tree ring growth; both regional climate and topography play a larger role.

Results

DiscussionWe are not able to find clear differences between cottonwood growth on opposite sides of the dike. Tree growth correlates much more with precipitation levels and topography. This suggests that local factors are the driving forces behind the variations in ring size. Low precipitation caused decreases in growth for some years, though several trees had high growth during these years (Fig. 3a). During high precipitation years, this trend was reversed (Fig. 3b). These graphs show how many trees had positive and negative correlations to different levels of rainfall. Temperature only appeared as a major factor in years with low precipitation and above-average temperatures, which tended to cause smaller rings.

Another key environmental factor is the location of the tree. Trees in flood plains on both sides of the dike were clearly affected by variation in precipitation. If precipitation was low, less water would have reached trees that were higher in elevation or out of the flood plains, which would have caused a significant reduction in growth.

The only possible indication of dike impact on cottonwood growth is the absence of older trees on the east side of the dike. The parallel planting pattern and similar age of the older trees on the west side suggest that they were planted as a windbreak. Because there probably weren’t any cottonwoods planted by humans on the east side of the dike, seeds needed to travel by natural means over the dike to grow.

References Case, Nat, Comp. "Arb Map." Map. Carleton Col-lege. 5th Ed. Minneapolis: Hedberg Maps, MN.

Speer, James H. Fundamentals of Tree-Ring Research. The University of Arizona Press, Tucson, 2010.

MethodsWe used increment borers to collect 5 mm-diameter cores from 11 cottonwood trees. Typical recovery of the cores was from 20 to 80 cm. Cores were stored in straws and placed in denatured alcohol to dehydrate overnight.

After drying, cores were secured to core holders with wood glue and planed down to an even, flat surface so the rings were easier to see. They were then sanded and lacquered and, once dry, they were scanned into the computer using 2400 dpi and 48 bits. Photoshop was used to measure and keep track of the rings, which were then plotted in Excel using a mean sensitivity formula (Speer 2010) .

ConclusionsIn our study of the cottonwood trees in the Arboretum, we found that the Old Millpond Dike has little effect on growth. The topography and annual precipitation have a much more pronounced impact of ring width than the tree’s position on either side of the dike. However, the sample size of trees on the east side was limited due to the sheer lack of cottonwood trees. The effects of the dike could be measured more accurately with other trees, such as maples or willows included in the study

We cored 11 cottonwoods total, 3 east of the dike and 8 west of the dike. The three eastern trees were relatively young at 17, 28, and 32 years old; the others ranged from 25 to 82 years old, with several trees older than 70.

Growth of cottonwood trees varies in relation to regional climate changes, and the ring width of trees east and west of the dike follow similar trends (accounting for individual variation in growth rate). Precipitation and temperature both contribute to a tree’s annual growth, but their effects can be somewhat delayed. A trend of increased or decreased temperature for a year caused a reaction in growth for that year, while the effect of precipitation on growth appeared in the growth ring of the year following the significant event, or not at all. Tree ring width doesn’t vary exactly with rainfall; however, these data are correlated in trends over the course of several years. For example, Fig. 2 shows a U-shaped decrease and increase for both rainfall and tree ring width from 1993 to 1998.

Many tree cores reflected years of unusually high and low precipitation and temperature. In years with high temperature and low precipitation, 14 trees correlated with the low rainfall (experienced a decrease in growth rate) and 3 trees had an inverse relationship (experienced an increase in growth rate). Years with high or low temperatures and high precipitation had many instances of correlation with the precipitation (increase in growth). Low precipitation with high temperatures inhibited tree ring growth more than low precipitation and low temperatures.

0.2

Year

Nor

mal

ized

tree

ring

wid

th

Rain

fall

anom

aly

(in)

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

2002

2004

2006

2008

2010

0

0.4

0.6 0

0.5

1.0

-0.5

-1.0

-1.5

0.8

1.0Correlations for Low Precipitation Years

Correlations for High Precipitation Years

Figure 3.a,b. Graph of positive and negative ring growth correlations to anomalous precipitation years.from 1931-2008.

Fig. 3.a

Fig. 3.b

112

3

1

4

7 89

10

65

0

1

2

3

4

5

6

7

1931

19

34

1937

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40

1949

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50

1951

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56

1957

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58

1959

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63

1964

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68

1977

19

88

1989

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90

1996

19

97

2003

20

04

# of

Rin

g C

orre

latio

ns

Years With Below Average Precipitation

Positive Negative

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1

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6

7

1932

19

39

1942

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43

1945

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46

1952

19

60

1965

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69

1971

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73

1983

19

87

1991

19

92

1993

19

94

1998

19

99

2004

20

05

2008

# of

Rin

g C

orre

latio

ns

Years With Below Average Precipitation

Positive Negative