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Post on 21-Feb-2016
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Distribution of a plant parasite on its host: gall distribution on the flowering dogwood, Cornus florida
Purpose• To determine the distribution of
midge galls on dogwood trees and to investigate some environmental factors that might influence distribution.
Questions1) How are the galls distributed spatially?
• Random vs. clumped vs. uniform
2) Number or distribution of galls influenced by:• Size/age of the plant? Nearest neighbor
distance? Sunny versus shady microhabitat?
Midge fly, Resseliella clavula
Random Distribution• Parasites are randomly distributed
within the environment. If midges flew until they “hit” a dogwood haphazardly …
• Predict:
4020
20
30
30
20
2020
10
2030
4030
20
20
30
30
30 20
20
40
40
0 10 20 30 4002468
10121416
# Parasites
# Tr
ees
Even Distribution• “Regular”: Organisms are evenly
spaced in the environment. Then every tree would have the same number of galls. This would suggest that the parasites are competing for the trees.
• Predict:
0 10 20 30 4002468
10121416
# Parasites
# Tr
ees
10
10
10
101010
1010
10
10
10
1010
10
10
Clumped Distribution• Most trees have few parasites, and some a
lot.• Typically associated with:
– disease outbreaks, tree densities (“contagious”), age
• Predict:
0 10 20 30 400
2
4
6
8
10
12
14
16
# Parasites
# Tr
ees
030
30
3030
0
0
00
0
0
0
0
0
00
0
0 030
• Tree size• Older and infirm trees might be more
likely to be attacked by the midge parasite.
• Distance to nearest neighbor• If trees are close together, they might be
more likely to attract galls.• Collect gall data in two habitats• Some microhabitats might be better for
galls than others.
Environmental Factors We are Measuring
Dogwood (Cornus florida) Identification
Dogwood Identification
Flowers: white, four petals (spring)
Fruit: bright red, in clusters (late summer, fall)
Leaves: opposite, veins curved
Dogwood Identification (Winter)
Flower buds(reproductive)
Leaf bud(somatic)
Both
Bark: dark brown with squarish, scaly blocks.
Young trees will be smooth and not as distinctive.
Terminal Buds
Examples of different numbers of bud scales
Dogwood with two bud scales. Twigs are slender, green or purple.
Leaf Scars
Dogwood leaf scars are opposite, small, and encircle twig.
Example of alternate leaf scars
Opposite leaf scars
Terminal bud
Methods Groups of 4 Go to 2 sites and survey 20 trees
each
1) Record how many galls on 100 branch nodes per tree
2) Measure the DBH (cm) of each tree trunk• DBH = “Diameter at breast height”
3) Measure the distance (m) of the three closest dogwood neighbors (DNN = nearest neighbor dist.)
1
2
3
4
5
67
8
10
9
1112
13
Dogwood Branch
Galls
…so this branch is good for 13 out of the 100 counts you need for each tree, and the number of galls is 2
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