Charlotte Poeydebat: Effects of tree diversity on forest resistance to insect herbivores

Charlotte Poeydebat, postdoc at University of Bordeaux, presents her work “Climate affects neighbour‐induced changes in leaf chemical defences and tree diversity–herbivory relationships”, discusses the importance of research networks to address general questions in ecology and share her passion for ecosystems research. 

Charlotte Poeydebat taking notes in the field (c) Soline MARTIN-BLANGY
Charlotte Poeydebat taking notes in the field (c) Soline MARTIN-BLANGY

About the paper 

In an unprecedented context of human-induced biodiversity loss, research on the relationships between biodiversity and ecosystem functioning has accelerated during the last decades. Whether and how plant diversity mitigates pest abundance and damages (i.e. associational resistance) has been actively investigated, but conclusions are inconsistent. Recently, several literature reviews and latitudinal studies have pointed out that the diversity-herbivory relationship depends on the climatic context.  

Numerous mechanisms have been proposed, and demonstrated in some cases, to explain associational resistance, but very little is known about trait-mediated mechanisms, i.e. effects of diversity on pests mediated by changes in host traits. On this subject, recent research is bringing new insights: tree diversity effects on insect herbivory could be indirectly mediated by interspecific neighbour-induced changes in host tree chemical defences. Given the dependence of the physiology of both trees and insects on climate, this mechanism is likely to occur depending on the climatic conditions, which remains to be clarified.  

Taking advantage of the unique opportunity provided by the international network of tree diversity experiments TreeDivNet (https://treedivnet.ugent.be/), this paper is one of the first to investigate defence-mediated associational effects on insect herbivory in relation with the climatic context. Interestingly, we accounted for functional dissimilarity between the focal species (Betula pendula Roth) and its heterospecific neighbours to explain trait-mediated associational effects.  

With this study, we showed that the effect of tree species diversity on insect herbivory on silver birch leaves was climate-dependent and, specifically, varied with temperature. Our findings also showed that tree species diversity modified chemical defence levels in birch leaves, but further suggested that such changes in leaf chemistry induced by heterospecific neighbours are complex, as they are influenced in opposite ways by the number of species and the functional dissimilarity between species. Altogether, our results showed that neighbour-induced changes in birch chemical defences were climate-dependent and that they partly explained associational resistance to herbivory. 

Our work was correlative in essence and certain questions need to be further elucidated. Future research should investigate which traits of tree species drive associational effects on herbivory and address simultaneously multiple underlying mechanisms. For instance, it would be particularly interesting to explore the role of forest structure and tree spatial arrangement in associational effects, as it may be implied in both neighbour-induced changes in chemical defences through effects on individual crown illumination, as well as in focal plant apparency. 

Given the uniqueness of our dataset and our integrative approach of plant-insect interactions, we are confident that our study could be of interest to a broad readership of ecologists. 

A moth on a birch leaf (c) Inge VAN HALDER
A moth on a birch leaf (c) Inge VAN HALDER

About the research 

This work builds toward a more general predictive framework of the effects of tree diversity on resistance of forests to insect herbivores. It confirms the importance of climatic context and functional diversity in understanding biotic interactions.  

This work would not have been possible without the TreeDivNet platform. I believe that such networks of experiments are a promising tool to advance research in ecology because they create connections between researchers, facilitate collaborations and allow generating large datasets that cover wide ecological gradients. Their implementation should be encouraged and financially supported. 

On a more practical level, this kind of study could help forest managers to mitigate the risk of pest invasions that is likely to increase with the ongoing environmental changes.  

The ORPHEE experimental site in Southwestern France in 2018 (c) Bastien CASTAGNEYROL
The ORPHEE experimental site in Southwestern France in 2018 (c) Bastien CASTAGNEYROL

About the author 

I discovered ecology at high school, and I was instantly passionate about this science of interactions that was connecting everything and explained me how the world works. Since then, it was clear to me that I had found something to get involved in. I did so by pursuing an academic education fully dedicated to ecology. For my first experiences as an ecologist, I had the chance to study the behaviour of dolphins in La Réunion Island and of king vultures in the Yucatán Peninsula. I discovered the marvellous world of the tropics and the joy of fieldwork. Soon after, I became aware of the major role of our agricultural system in the loss of biodiversity and the alteration of ecosystem functioning. I then became interested in biodiversity-ecosystem functioning relationships applied to cultivated ecosystems, with the idea of trying to reconcile agriculture and biodiversity conservation.  

Currently, I am working on ecological multifunctionality in mixed vs pure Scots pine forests across boreal and temperate biomes of Europe. This work involves about 50 scientific partners that all provided data from “their” forests. My task is to evaluate the effect of mixing tree species on 4 tree-related ecological functions: tree production, the richness of microhabitats provided to forest fauna, wood quality and resistance to defoliation. I explore whether the effect size of mixing tree species on the functions vary according to the associated species (beech, oak or spruce) or Scots pine proportion in the mixture, as well as whether effect size is contingent upon the climatic context. I also aim at evaluating how species composition affect forest multifunctionality. 

To end, I would say that the best thing about being an ecologist is to understand the nature that surrounds us and to be even more amazed.  

Read the paper in full here. 

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