I study the temporal context of plant-plant interactions, specifically how phenology and ontogeny affect the balance between facilitation and competition over the life cycle. What I most enjoy about science is sharing its marvels with others, be they mentees, students, or the public. In my spare time I read a lot of fiction and spend time with my cat Wilson. Visit my personal page here: http://lindsayleverett.weebly.com/
Durham, North Carolina
Skills and Expertise
Aug 2010 - May 2012
East Carolina University · Department of Biology
Greenville, United States
Aug 2008 - May 2010
Troy University · Department of Biological and Environmental Sciences
Troy, United States
A single plant can interact both positively and negatively with its neighbors through the processes of facilitation and competition, respectively. Much of the variation in the balance of facilitation and competition that individuals experience can be explained by the degree of physical stress and the sizes or ages of plants during the interaction. Germination phenology partly controls both of these factors, but its role in defining the facilitation-competition balance has not been explicitly considered. I performed an experiment in a population of the winter annual Arabidopsis thaliana (Brassicaceae) to test whether germinating during physically stressful periods leads to facilitation while germinating during periods that promote growth and reproduction leads to competition. I manipulated germination and neighbor presence across two years in order to quantify the effects of the local plant community on survival, fecundity, and total fitness as a function of germination phenology. Neighbors increased survival when germination occurred under conditions that were unsuitable for survival, but they reduced fecundity in germinants that were otherwise the most fecund. Later germination was associated with facilitation in the first year but competition in the second year. These episodes of facilitation and competition opposed each other, leading to no net effect of neighbors when averaged over all cohorts. These results indicate that variation in germination timing can explain some of the variation in the facilitation-competition balance in plant communities. This article is protected by copyright. All rights reserved.
Summary' I. 'Introduction' II. 'When are responses to parental vs progeny environmental cues adaptive?' III. 'Constraints, costs and conflicts' IV. 'Conclusions' References There is renewed interest in how transgenerational environmental effects, including epigenetic inheritance, contribute to adaptive evolution. The contribution of across-generation plasticity to adaptation, however, needs to be evaluated within the context of within-generation plasticity, which is often proposed to contribute more efficiently to adaptation because of the potentially greater accuracy of progeny than parental cues to predict progeny selective environments. We highlight recent empirical studies of transgenerational plasticity, and find that they do not consistently support predictions based on the higher predictive ability of progeny environmental cues. We discuss these findings within the context of the relative predictive ability of maternal and progeny cues, costs and constraints of plasticity in parental and progeny generations, and the dynamic nature of the adaptive value of within- and across-generation plasticity that varies with the process of adaptation itself. Such contingent and dynamically variable selection could account for the diversity of patterns of within- and across-generation plasticity observed in nature, and can influence the adaptive value of the persistence of environmental effects across generations.
Background: Seeds adjust their germination based on conditions experienced before and after dispersal. Post-dispersal cues are expected to be more accurate predictors of offspring environments, and thus offspring success, than pre-dispersal cues. Therefore, germination responses to conditions experienced during seed maturation may be expected to be superseded by responses to conditions experienced during seed imbibition. In taxa of disturbed habitats, neighbors frequently reduce the performance of germinants. This leads to the hypotheses that a vegetative canopy will reduce germination in such taxa, and that a vegetative canopy experienced during seed imbibition will override germination responses to a canopy experienced during seed maturation, since it is a more proximal cue of immediate competition. We tested these hypotheses in Arabidopsis thaliana. Methods: Seeds were matured under a simulated canopy (green filter) or white light. Fresh (dormant) seeds were imbibed in dark, white light, or canopy at two temperatures (10°C or 22°C), and germination proportions were recorded. Germination was also recorded in after-ripened (less dormant) seeds that were induced into secondary dormancy and imbibed in the dark at each temperature, either with or without brief exposure to red and far-red light. Key results: Unexpectedly, a maturation canopy expanded the conditions that elicited germination, even as seeds lost and regained dormancy. In contrast, an imbibition canopy impeded or had no effect on germination. Maturation under a canopy did not modify germination responses to R and FR light. Seed maturation under a canopy masked genetic variation in germination. Conclusions: Our results challenge the hypothesis that offspring will respond more strongly to their own environment than to that of their parents. The observed relaxation of germination requirements caused by a maturation canopy could be maladaptive for offspring by disrupting germination responses to light cues after dispersal. Alternatively, reduced germination requirements could be adaptive by allowing seeds to germinate faster and reduce competition in later stages even though competition is not yet present in the seedling environment. The masking of genetic variation by maturation under a canopy, moreover, could impede evolutionary responses to selection on germination.
Seed dormancy can prevent germination under unfavourable conditions that reduce the chances of seedling survival. Freshly harvested seeds often have strong primary dormancy that depends on the temperature experienced by the maternal plant and which is gradually released through afterripening. However, seeds can be induced into secondary dormancy if they experience conditions or cues of future unfavourable conditions. Whether this secondary dormancy induction is influenced by seed-maturation conditions and primary dormancy has not been explored in depth. In this study, we examined secondary dormancy induction in seeds of Arabidopsis thaliana matured under different temperatures and with different levels of afterripening. We found that low water potential and a range of temperatures, from 8°C to 35°C, induced secondary dormancy. Secondary dormancy induction was affected by the state of primary dormancy of the seeds. Specifically, afterripening had a non-monotonic effect on the ability to be induced into secondary dormancy by stratification; first increasing in sensitivity as afterripening proceeded, then declining in sensitivity after 5 months of afterripening, finally increasing again by 18 months of afterripening. Seed-maturation temperature sometimes had effects that were independent of expressed primary dormancy, such that seeds that had matured at low temperature, but which had comparable germination proportions as seeds matured at warmer temperatures, were more easily induced into secondary dormancy. Because seed-maturation temperature is a cue of when seeds were matured and dispersed, these results suggest that the interaction of seed-maturation temperature, afterripening and post-dispersal conditions all combine to regulate the time of year of seed germination.
Seed mass variation and heteromorphism may afford plant species differential germination behavior and ultimately seedling success, particularly in disturbed habitats. We asked whether such variation occurs in Packera tomentosa (Michx.) C. Jeffrey (Asteraceae), a clonal species of the southeastern USA. Seed mass was compared within and among genetic individuals differentiated using amplified fragment length polymorphisms. We compared central and peripheral seeds produced by disc and ray florets, respectively, for their morphology, mass, and germination behavior, including response to water availability, aging, and cold stratification. Seed mass was highly variable both within and among individuals and influenced germination behavior. We found cryptic seed heteromorphism in P. tomentosa. Central and peripheral seeds had similar morphologies but dissimilar mass and biomass allocation. We used failure time analysis to detect different germination behavior. Central seeds were heavier, contained larger embryos, and germinated faster and at a higher proportion in most germination studies. Highly variable mass and heteromorphism of seeds may allow persistence of P. tomentosa in its disturbed habitats. Based on our results, some future studies of Asteraceae species with disc and ray florets may need to account for possible differences between seed types, even when morphological differences are not apparent. Evaluation of individual seed mass and maternal differences in germination studies may assist in the detection of cryptic seed heteromorphism, a phenomenon thought to be common, yet rarely documented.
The primary objectives of this project were to determine which species of Crotalaria (Fabaceae) occur in Alabama and the county distribution of each species. Crotalaria, known commonly as rattlebox, is recognized as consisting of seven species in Alabama. The most common species are Crotalaria sagittalis, C. rotundifolia, and C. spectabilis. The less common species are C. purshii, C. lanceolata, C. pallida, and C. ochroleuca. In Alabama, introduced species of Crotalaria (C. lanceolata, C. ochroleuca, C. pallida, and C. spectabilis) generally have showier inflorescences and reach greater maximum heights than native species (C. purshii, C. rotundifolia, and C. sagittalis). The dichotomous key and descriptions we present are modifications from earlier authors; however, all measurements are based on morphological features of the vegetative and reproductive structures of the more than 460 specimens studied during this project. Data for the county-level distribution maps were compiled entirely from herbarium vouchers.