In this Insight, Olivia Cope talks about how she got into ecology, long-term research and her recent paper Chemical defense over decadal scales: Ontogenetic allocation trajectories and consequences for fitness in a foundation tree species

About the paper

Many plants invest tremendous amounts of resources into chemical compounds that protect against herbivore attack. This paper is about long-term dynamics in those chemical defenses over tree lifetimes. Back in the 1990s, the Lindroth research group had been doing studies with potted aspen saplings that showed strong genetic and environmental variation in defense allocation. They wondered whether the trends they were seeing for young trees in a controlled environment would hold in more natural environments and timescales. Thus, the common garden used in this paper was born.

The key result from our paper is that not only does chemical defense change across tree ontogenetic stages (i.e. juvenile vs. mature), but that both defense allocation at any given stage and the rates of change in defense between stages are genetically variable in the field. We also found that defense levels and rates of change in defense allocation throughout tree ontogeny have consequences for the ultimate long-term size and survivorship of different tree genotypes. This correlation between ontogenetic defense trajectories and genotype-level fitness indicates that such trajectories should be considered as adaptive traits.

About the research

Such a long-term study requires a massive amount of effort and personnel. Over the years, 30-40 people were involved with sample collection and processing! And that’s not to mention the considerable amount of effort that went into planting the trees and keeping weeds and rodents at bay in the early years. I took the reins on this project when I entered graduate school, just as the tree crowns were growing difficult to access with telescoping pole pruners; ultimately, inaccessibility was what decided the end point of the study in 2015.

In the end, we were surprised to find just how much the genetic variation and growth-defense tradeoffs in this study matched what had been seen in potted saplings decades ago. A next step to further improve field relevance will be scaling up to population-level studies. Plant populations, especially of trees, are often mosaics of different-aged individuals; scaling up studies of defense ontogeny will help us understand plant-herbivore interactions across space and time. Variation in leaf defense chemistry also has implications for soil processes via litterfall, and the common garden established for this paper is currently being used to study such plant-soil feedbacks. 

What was learned through the experience of setting up and maintaining this long-term common garden was directly applied to the establishment of two new aspen common gardens in 2010. One contains a huge number of genotypes (500+) and is being used to detect the genomic underpinnings of ecological interactions; the other is testing the strength of growth-defense tradeoffs in different environments and looking at implications for population genetic trajectories.

About the author

I got involved in ecology research as an undergraduate, when I was stopping by a professor’s office and identified a tiger beetle that had been partially dissolved in pitcher plant fluid. I was offered a research assistant position and, as they say, the rest is history. I’m currently finishing up my PhD at UW-Madison and have spent the past four years studying the context-dependence of plant defense in aspen trees, and, more recently, in an invasive mustard. My favorite things about being an ecologist are that no two days are alike, and that we get time to think deeply about the fascinating interactions happening all around us.

Read the paper in full here or the plain language summary here.