The Causes and Fitness Benefits of Germinating Later in the Presence of Neighbors
Theoretical and empirical studies have consistently shown that the optimal timing of seed germination reduces exposure to physical stress and minimizes competitive interactions with neighbors. However, this research has not accounted for facilitative (positive) interactions among plants, which become more pronounced as environmental stress increases. Facilitation is more likely to occur early in a plant's life when it is more susceptible to stress. In seasonal environments, the stress a given individual experiences can change throughout the year, and some years are more stressful than others. These sources of temporal variation in stress will dictate the facilitation-competition balance that individuals experience. However, it remains unclear how this balance affects the optimal timing of germination. My dissertation research asks how the timing of germination responds to neighbors, how those responses affect the facilitation-competition balance individuals experience, and how that balance in turn affects fitness and demography. More generally, it asks how the timing of germination and other types of emergence affect the facilitation and competition that individuals experience throughout their lives.
I used laboratory, greenhouse, and field experiments to examine how the timing of germination in the winter annual Arabidopsis thaliana (Brassicaceae) responds to cues of neighbors and how those responses affect interactions with neighbors. I then developed a mathematical model of population growth in an annual plant to examine how intraspecific facilitation and competition over ontogeny affect the optimal degree of investment in dormancy (i.e., delayed germination) in variable environments.
My experiments revealed that seeds of A. thaliana typically delay germination in response to neighbors and that these responses can promote facilitative interactions and reduce competitive ones with neighbors. Selection against delayed germination, which occurs because of stress later in the season, can be mitigated by facilitation. Further, delaying germination can be beneficial by increasing the difference in sizes between seedlings and their neighbors, which may promote resource partitioning. In the theoretical study, I found that increasing the degree of investment in the fraction of dormant seeds (i.e., delaying germination) can promote the persistence of populations that experience both facilitation and competition in variable environments. This occurs because increased dormancy prevents high juvenile densities that promote facilitation and consequently limit reproduction in large populations. The findings of this research indicate that plant-plant interactions depend strongly on temporal context, and they reveal that the facilitation-competition balance determined by temporal variation in stress plays a key role in how germination and dormancy traits will evolve in variable environments.
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