Purpose

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