Michael Eisenring: Within-crown heterogeneity can affect herbivore performance in tree canopies

Dr. Michael Eisenring presents his work at University of Wisconsin titled “Spatial, genetic and biotic factors shape within‐crown leaf trait variation and herbivore performance in a foundation tree species”. He discusses the importance of sub-individual trait variation and how overcoming his fear to heights was worth it. 

Michel at the WisAsp common garden
Michael Eisenring at the WisAsp common garden

About the paper 

To date, research on plant trait variation has focused mainly on trait differences among individual plants. Much less is known about within-plant (i.e., sub-individual) trait variation – its magnitude, driving factors, and the ecological consequences thereof. Our paper documents how chemical and morphological leaf traits vary among different vertical crown positions in mature trembling aspen trees and how this sub-individual variation affects gypsy moth caterpillar performance.   

The initial interest for conducting a study on sub-individual leaf trait variation in aspen was prompted by a review paper co-authored by Sybille Unsicker (who is also a co-author of this study). That review pointed out that the extent and ecological consequences of within-crown leaf trait variation are surprisingly poorly understood in trees. I was in the fortunate position to address this knowledge gap by having access to the WisAsp common garden established by the Lindroth research group. WisAsp encompasses several hundred trembling aspen genotypes that were collected throughout Wisconsin. We included multiple replicates of different tree genotypes and also used a controlled herbivory treatment in our study. This design allowed us to study not only how leaf traits change depending on canopy position within trees, but also how genotypic variability and herbivory affected sub-individual leaf trait variation. 

Crown position, tree genotype, and to a lesser degree, herbivory, create a chemically and morphologically diverse foliar mosaic within tree crowns. We demonstrated that morphological leaf traits were driven mainly by canopy positional effects, whereas defensive chemistry was governed mostly by tree genotypic effects. Interestingly, for many defensive compounds total within-crown variance itself was heritable. Furthermore, feeding studies with caterpillars revealed that the effects of crown position on caterpillar performance varied among tree genotypes and caterpillar sexes. As such, sub-individual variation in tree crowns can affect higher trophic levels. 

Lymantria dispar L. caterpillar 

About the research 

Manoeuvring 10m tall ladders through the dense undercover of our common garden and collecting leaf samples from dozens of mature aspen canopies was a challenge. Also, the fact that I am slightly afraid of heights certainly did not help during fieldwork. Overcoming my fear was totally worth it, though, since our study revealed some very interesting results. One of the more surprising findings was that for many defensive compounds total within-crown variance is heritable. This suggests that sub-individual variability might be beneficial and possibly under positive selection.  Also, I was surprised to see that within-crown heterogeneity can affect herbivore performance. An interesting and important follow-up study that I would be interested in doing would be to quantify how overall phytochemical diversity is linked to overall herbivore diversity in tree crowns. 

Michael at work collecting leaf samples

About the author 

Already in Kindergarten, I used to collect snails and beetles. I kept the fascination for ecology and the many diverse forms of life throughout my teenage years. Once I realized that one could make a living by studying insects and plants and how they interact with their environment I was thrilled! Today I am convinced that being an ecologist, or a scientist in general, is an extreme privilege. However, as exciting as ecological research can be, conducting ecological experiments often require a high degree of flexibility and the ability to cope with unforeseen situations. So my piece of advice for someone in my field would be to not get discouraged by unforeseen situations. The sooner one learns to live (and maybe even to embrace) the unforeseen, the better and more interesting life as an ecologist gets. 

Read the article in full here. 

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