Plant apparency drives associational resistance to insect herbivory

Castagneyrol and company have a paper in the Journal in Early View titled “Plant apparency, an overlooked driver of associational resistance to insect herbivory”. Read the paper here.

The authors have provided a short synopsis of the paper and a few photos.

Figure 1: Overview of the ORPHEE experiment showing two examples of plots corresponding to two different tree species assemblages.

Figure 1: Overview of the ORPHEE experiment showing two examples of plots corresponding to two different tree species assemblages.

Why be so concerned about biodiversity loss in forest ecosystems? Let’s leave aside ethical and cultural considerations and focus on a pragmatic answer: because forest diversity supports ecological processes that provide humans with ecosystem services such as wood production, carbon sequestration and soil protection.

And what about insects? They account for more than 50% of forest biodiversity. But at the same time, insect pest damage reduces forest productivity by ca. 20% every year, and this impact is likely to increase in the near future because a lot of forest insects will benefit from global warming and drying. But the good news is that a growing body of evidence suggests that tree diversity could also provide resistance to pest insects.

This is what we wanted to confirm by setting up in south western France “ORPHEE” a large scale tree diversity experiment (Figure 1), which is part of the worldwide Tree Diversity Network (http://www.treedivnet.ugent.be/). In this experiment, 31 different tree species assemblages from monocultures to five tree species mixtures were planted, for a total of 256 experimental plots and 25,600 saplings. The five species are native from SW France: maritime pine, silver birch, pedunculate oak, Pyrenean oak and green oak.

We quantified damage and abundance of insect herbivores occurring on pedunculate oak saplings (Quercus robur) in pure and mixed plots with oaks (Figure 2). We observed a lower abundance of insects specialized on oaks (mainly leaf-miners) in oak mixtures than in oak monocultures but no effect on insect generalists, those that are able to feed on different tree species. However we went further in the analysis and had a closer look at the identity and the size of the immediate neighbours of focal oaks.

Figure 2: Examples of herbivores and damage found on Quercus robur.

Figure 2: Examples of herbivores and damage found on Quercus robur.

Oaks grow slowly. At the time of the experiment pedunculate oak saplings were 75cm high on average. The other oaks (the Pyrenean oak and the Green oak) were even smaller. But birch and pine saplings grow much faster and reached more than 2m in height. We therefore made the hypothesis that high crown and long branches of birches and pines could at least partially hide small oak neighbours and then protect them from insects attacks.

We thus defined “oak apparency” as the mean difference in height size between oaks and their neighbours, a greater apparency leading to easier access to herbivores (Figure 3). We found that the abundance of leaf miners on oak sapling was better explained by oak apparency than by the diversity of tree species surrounding oaks. However the higher the diversity of trees, the higher the probability to incorporate fast growing species like pine and birch and then the greater the likelihood to get trees taller than oaks.

Figure 3: Tree diversity can create huge contrasts in tree size. Smaller trees get protection against their specialist herbivores from taller neighbours.

Figure 3: Tree diversity can create huge contrasts in tree size. Smaller trees get protection against their specialist herbivores from taller neighbours.

We could then conclude that tree diversity might reduce insect herbivory through reduced host tree apparency. This is an important outcome as it sheds new light on an overlooked mechanism explaining why mixed forests are less prone to insect damage. It confirms again that the composition of tree species assemblage is more important than tree species per se to explain the positive effect of biodiversity on ecosystem functioning.

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