Deidra Jacobsen Advisor: Dr. Svata Louda Committee member: Dr. Sabrina Russo Undergraduate thesis defense 17 April 2009 Impacts of plant size, density, herbivory, and disease on native Platte thistle (Cirsium canescens)

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

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: Damage and plant size Larger plants had a higher proportion of leaves herbivore damaged GLM p = 0.02 Proportion leaves damaged / plant

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 (0.25m 2 ) 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 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, F 2,93 = 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 m 2 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= g beige= 0.011g Tan with brown striations= g Tan with red striations= g significant linear regression: seed weight = *seed type (R 2 =0.99, F 1,2 = , 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, F 3,37 = 0.79, P = ).

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