Effects of abiotic and biotic stress on plant fitness and herbivore defense in Boechera stricta

Abstract

Abiotic and biotic factors influence plant fitness by altering the physical environment and resource availability. According to the stress gradient hypothesis, increasing physical stress in an environment increases positive plant-plant interactions and decreases negative interspecies interactions. I investigated how variance in abiotic stress modifies interactions between the wildflower Boechera stricta and its heterospecific plant neighbors. I also tested how heterospecific neighbor density may affect insect herbivory due to associational defenses. I conducted a common garden experiment where the density of neighbor vegetation naturally varied across microsites and rainout shelters were used to manipulate soil moisture. I collected data on B. stricta survival, fecundity, and insect herbivory under these varying combinations of drought and neighbor density. I also performed a potted plant experiment with B. stricta to isolate the relationship between neighbor vegetation density, herbivore defense, and plant fitness. From early season data in the common garden, I found a significant positive relationship between neighboring vegetation density and survival in B. stricta. I also found a significant positive relationship between neighboring vegetation and total fitness in B. stricta planted in 2015, suggesting facilitation. Consistent with the stress gradient hypothesis, plants in the rainout plots showed greater facilitation than those in the control plots. However, I found no significant interaction between neighboring vegetation and abiotic conditions (permissive vs. drought- stressed environments). From the potted plant experiment, I found a significant negative relationship between herbivory and neighboring vegetation, indicative of associational defenses. I also found an indirect negative relationship between neighboring vegetation and reproductive output, providing evidence for competition. Finally, I found genetic variation in these plant-plant interactions; of the two genotypes I tested, one experienced greater competitive effects of neighbors (evidenced by a stronger decrease in reproduction with increasing neighbor density), while the other benefited more from neighbors through associational defenses. Genotypic differences in the strength and direction of ecological interactions with neighbors indicates that the stress-gradient hypothesis may not be sufficient in understanding plant responses to stress.

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