The central objective of this study was to identify the mechanisms driving the formation of new plant species. Elucidating these processes is critical in the Neotropics, a biodiversity hotspot facing unprecedented species loss due to human-induced factors such as climate change and altered land use. We used spiral gingers (Costus species) as a model to explore how plants adapt to both living organisms, like pollinators and herbivores, and abiotic conditions, such as soil and climate. The study was organized around three aims. The first was to document the evolutionary history and ecological niches of spiral gingers, which will inform hypotheses about what factors are crucial in the formation of new species. The second aim involved hands-on field experiments to test how these plants adapt to their local environments, and how these adaptations could lead to the formation of new species. The third aim of the study focused on identifying the specific genetic factors that influence a plant's adaptability and potential for evolving into a new species. The project included an interdisciplinary group of researchers from several institutions. The grant made to Chapman University supported plant ecophysiology research which contributed to aims 1 and 2. Thus, this Project Outcomes report will focus on those results.

In support of aim 1, we examined the variation in plant traits (such as photosynthetic rate and root morphology) among closely-related Costus species across different environmental settings. We found that, rather than having functional trait values that reflect genetic differentiation, the species we studied simply exhibited phenotypic plasticity in response to their environment. Thus, we did not find strong evidence for the role of functional traits in promoting speciation among these recently diverged taxa. Root traits are not as well understood as leaf and stem traits; thus, we also explored how root, leaf and stem traits coordinate to form cohesive strategies that inform resource acquisition and use. We found coordination among leaf, stem and root traits, and that Costus species could be categorized based on their whole-plant functional strategies—some species were more geared toward resource acquisition, while others were more conservative in their resource use. Specifically, the degree to which plants regulate water loss from leaves was negatively related to both stem and rhizome tissue construction, and these relationships became stronger after accounting for evolutionary relatedness, indicating correlated evolution. Finally, we conducted a greenhouse experiment to determine how light and water availability influence ecophysiology of two closely related Costus pairs. Costus allenii is found in wet, shady environments relative to C. villosissimus, which can occur in open, sunnier habitats. Our preliminary results suggest that light availability is a strong driver of physiological function but that both species respond similarly to variation in light and water availability.

In support of aim 2, we measured physiological data for several pairs of species associated with a field transplant experiment, where a species from one environment (e.g., warm low elevation forest) is grown in a very different environment (e.g., cool high elevation forest). Our preliminary analysis suggests that low elevation populations have higher long-term photosynthetic function than high elevation populations (genetic variation), but photosynthetic function is higher at high elevation in the short-term (environmental variation). This aligns with the countergradient variation model, in which genetic factors affecting a particular trait, such as photosynthetic function, act in opposition to environmental factors.

This project had several broader impacts. First, a diverse group of undergraduate students, high school students, two graduate students, and one postdoctoral scholar contributed to the ecophysiology research. Many of these trainees were female and from underrepresented groups in STEM. The high school outreach program, in partnership with Davis Senior High School, aimed to bolster science education for female high school students by providing them hands-on research experience. Every year, 10 female students engaged in 20 hours of greenhouse and laboratory work, participating in activities like measuring plant growth and processing leaves for elemental analysis. We recruited and educated local community members to serve as field assistants in Costa Rica, simultaneously providing them with economic benefits and educational experiences while deepening the connection between the field stations and the local community. Finally, understanding the mechanisms driving plant speciation is crucial for addressing our current biodiversity crisis exacerbated by climate change, invasive species, and land use change. Gaining insights into how new plant species evolve can inform conservation strategies, enabling us to better preserve unique genetic lineages and enhance the resilience of ecosystems in the face of environmental challenges.

Last Modified: 10/19/2023
Modified by: Jennifer L Funk