Impacts of plant size, density,
herbivory, and disease on native
Platte thistle (Cirsium canescens)
Deidra Jacobsen
Advisor: Dr. Svata Louda
Committee member: Dr. Sabrina Russo
Undergraduate thesis defense
17 April 2009
Platte thistle (Cirsium canescens):
Background
 Native to Nebraska Sandhills
 Previous focus on floral herbivory and
predispersal seed predation
 We concentrated on the seedling and
especially the juvenile stages
S. M. Louda
S. M. Louda
Key parts of study:
1. Effects of plant size and density on
herbivory and disease in Platte thistle
juveniles
2. Seed addition experiment to examine the
effect of seed density on germination
3. Seed quality experiment to determine how
seed morphology influences germination
success
1. Juvenile experiment:
methods
 5 sites in Arapaho
 Patches of different
natural densities
 223 total juveniles
monitored
 Half sprayed with
fungicide
S. M. Louda
Insect herbivores and damage
S. M. Louda
Juvenile experiment:
fungal infection
 Low fungal infection
 Only on 5 experimental plants (~2%)
 Survey of plants in other areas of the prairie did not
reveal any other evidence of S. cirsii infection
 However, this allowed us to examine the impact of
fungicide on plant growth and herbivory in the
absence of external fungal infection
Juvenile experiment:
treatment on growth
 Fungicide treated
plants had:
 Faster increase in leaf
number than control
plants
 Higher final leaf
number
Juvenile experiment:
treatment effects on herbivory
 Fungicide treated plants
had fewer leaves
damaged than control
plants
GLM
p = 0.008
Juvenile experiment:
 Larger plants had
a higher
proportion of
leaves herbivore
damaged
Proportion leaves damaged / plant
Damage and plant size
GLM
p = 0.02
Juvenile experiment:
spatial variation and density
 High variation in
damage and plant
size between
patches
 Density did not
affect the proportion
of leaves damaged
ANOVA
p = 0.749
Juvenile experiment:
conclusions
 High variation in damage between patches
 Not due to density differences
 Large plants had more damage
 Fungicide-treated plants had higher growth and less
herbivory
 Insect damage slows plant growth
 Damaged plants likely to flower later
2. Seed Addition Experiment:
2006 plantings
 1 block= 2 rows of 4 (0.25m2) plots
 4 plots of 5 seeds
 2 plots of 20 seeds
 2 plots of 100 seeds
 3 block replicates at 4 sites= 12 total
blocks
Seed addition experiment:
Monitoring
 2007= seedlings emerged
and counted
 Half sprayed with fungicide
 Growth and damage
measurements every 10-14
days
 Number of seedlings
surviving at the end of the
season
Seed addition experiment:
recruitment
 Very low recruitment
overall
 Fewer seeds= larger
proportion germinating
(ANOVA, F2,93 = 10.36, P < 0.001)
 Mean=0.23, SE=0.03 for 5-
seed added treatment
 Mean=0.03, SE= 0.03 for
100-seed added treatment
Seed addition experiment:
conclusions
 More seeds=more seedlings, but
 Negative density-dependence in germination success
 May be due to granivory by ants or small mammals
 Did not affect survival of seedlings
3. Seed Quality Experiment:
Seed types
 4 color groups:
 Dark brown
 Beige
 Tan & brown
striations
 Tan & reddish
striations
 Seed weights
Seed quality experiment:
2006 plantings
 0.5 m2 plot
 20 seeds/quadrant
 1 of 4 seed types in
each quadrant
 Emerged in 2007
 Same growth, damage,
and survival measures
as other experiments
Seed quality experiment:
seed weight variation among types
 Seed weights confirmed differences other than color in
seed types
 Dark brown= 0.010 g
 beige= 0.011g
 Tan with brown striations= 0.013 g
 Tan with red striations= 0.015 g
 significant linear regression: seed weight = 0.008 +
0.002*seed type (R2=0.99, F1,2 = 166.22, P = 0.006)
Seed quality experiment:
color/weight and germination
 Again, low germination
 No difference in
proportion of seeds
germinating based on
seed color/weight
(ANOVA, F3,37 = 0.79, P = 0.5047).
Seed quality experiment:
conclusions
 No effect of seed color/weight on proportion of seeds
germinating= unexpected
 Flowerhead damage that results in discolored, lighter
weight seeds may not decrease germination success if
seeds are intact
Synthesis of 3 experiments
 Many factors influence flowering success in plants
 Juvenile stage is understudied
 herbivore damage could delay flowering
 Seed germination success depended more on seed
abundance than seed quality
 More information needed on seed weight independent
of seed morphology
 Chlorothalonil appears to have insecticidal properties
Thank you!!!
 Dr. Svata Louda and lab members
 Dr. Sabrina Russo
 Those who provided field and statistical
support
 Personnel of Cedar Point Biological Station
 UCARE program
S. M. Louda