In our new post Emily Holden, PhD candidate at University of Alberta (Canada), presents her work ‘Plant trait dissimilarity increases competitive interactions among co-occurring plants’. She talks about confronting contrasting hypotheses, shares how she overcame COVID-19 to do her experiment, and celebrates her grandmother’s gardening skills.
About the paper
A foundational theory in ecology is that similar species will compete strongly as they share niche requirements. The prediction follows that when a plant attempts to grow in a new place, it should experience less competition if it is less similar to the plants already growing there. However, certain species characteristics may be necessary to survive in an environment (e.g., taller plants live in shady habitats), making species more functionally similar over environmental gradients. Luckily, we can use plant functional traits, or the plant attributes shaped by the environment (e.g., height or size of leaves) to examine how similarity among neighbours impacts competition. The issue of whether trait states reflect trait hierarchy or niche differentiation remains an active area of investigation in ecology, so in 2020, I sought to examine how a plant’s trait dissimilarity with its neighbours impacted performance. To determine if trait dissimilarity-performance relationships are context-dependent, I fertilized half of the pots. To test this, I added seedlings of five species to pots of grassland turf from 20 different plant communities and followed their growth and survival over the summer.
At the start of my degree program, my supervisor and I talked about influential theories in ecology that lack direct experimental tests. The idea of limiting similarity and competition quickly came to mind. With input from my supervisor and lab mates, I developed and refined the research questions that became this paper. A novel aspect of our paper is that we examined how dissimilarity in root traits affects competition, which has been unaddressed in previous experimental tests. I hope that other ecologists who are interested in how plant traits function in competition read this paper.
The mesocosm setup. (Credit: Emily Holden)
About the research
When I created the experiment, I was truly unsure what theory (limiting similarity vs. trait hierarchy) the results would support. After processing samples, cleaning the data, and making the first figures, I was surprised to see that neighbour effects were more negative with increasing dissimilarity!
COVID-19 lockdowns took effect just before I was to set up my experiment. I initially proposed this experiment to occur in the field and to transplant seedlings in situ, then measure their traits and the traits of the community. However, because of COVID-related restrictions and potential issues affecting sustained access to my field site, I pivoted. Once I received approval to conduct a single field trip from my institution, I went to the field and cut out natural plant communities, then brought them back to campus and grew them in pots. I also germinated and grew the seedlings that I transplanted into the mesocosms in my parents’ greenhouse where I was living during the initial COVID lockdowns. There was a lot of uncertainty related to doing research during the early days of the COVID pandemic, but I consider myself very lucky for facing only minor delays and for finding ways to keep myself and my colleagues safe during the experiment.
Elly Smith applies fertilizer pellets to mesocosm pots in May 2020. (Credit: Emily Holden)
I hope other ecologists will work to answer what plant traits function in competitive response. Our study contributes to a growing understanding of what traits function in competitive effect, or the ability of one plant to suppress the growth of another. However, there is less conclusive evidence regarding what plant traits function in the ability of plants to resist growth suppression due to neighbours (aka competitive response). I also hope others will replicate our experiment to examine trait dissimilarity’s role in competition in other biomes, field contexts, and with other experimental approaches. Moving forward, I think it is essential that we establish traits function in certain contexts before inferring causality from trait patterns. For instance, in my study, I found that community-level traits are not inherently competitive. I hope that future functional trait work will ensure traits function in the purported context before moving to inferring mechanisms.
An example of a grassland sample in a mesocosm pot at the start of the experiment. (Credit: Emily Holden)
About The Author
I am currently a Ph.D. Candidate in the Cahill Lab of Experimental Ecology at the University of Alberta in Edmonton, Alberta, Canada. My interest in ecology began at a young age. I grew up on a rural acreage and spent much of my childhood playing outside. My mom is a teacher and created many nature-based learning activities for me and my siblings, encouraging my keen interest in the natural world and science. I spent my childhood summers with my grandmother in her garden: she was a masterful gardener, and I believe her influence “planted the seed” for my interest in plants.
After completing two years of an undergraduate degree I did not know in what direction I wanted to take my studies, so I took a gap year to explore other opportunities. During this gap year, I realized how much I enjoyed being outdoors and that I could make a career out of being outside and studying nature. Upon my subsequent return to university, I changed my major to ecology. In the last year of my undergraduate degree, I conducted an independent research project which I loved. I went on to start my graduate studies in that lab.
I am from the parkland ecoregion of Canada, and I enjoy spending time in the prairies learning about the plants and mechanisms underlying their diversity. I feel very connected to the land where I do my research and consider myself very fortunate to study this system. I also love clever experimental tests of ecological theory – I am an empiricist through and through!
Emily consults a key book to identify plants in the grassland turf samples. (Credit: Elly Smith)
I am very fortunate that Canada has an excellent primary and secondary education system and a robust funding mechanism for science. I’ve had great mentorship and support in applying for funding, which has been foundational to securing funding for my research. I recognize my privilege in having encountered few barriers in my scientific career, so I try to contribute to initiatives to remove barriers for other folks and groups (e.g., by mentoring undergraduate students, high school students, and an Indigenous summer intern in research).
My advice to other ecologists or new graduate students is to have a variety of interests and seek out professional development opportunities not directly related to your research. Developing leadership, project management, and science communication skills have made me a better scientist because I have a better understanding of how to collaborate with colleagues, plan, and execute projects through learning in other facets of my life.
The author, Emily Holden, takes community abundance measures in a different experiment. (Credit: Stella Gibbon)
Like the blog post? Read the research here.
You can find out more about the work that the Cahill lab is doing here.