How light availability and plant age affect responses to herbivory?

Philip Hahn and John Orrock have an interesting paper accepted in Journal of Ecology titled “Ontogenetic responses of four plant species to additive and interactive effects of land-use history, canopy structure, and herbivory“. The authors have provided a lay summary and a picture of their research below.

Herbivory can have destructive effects on a plant’s ability to survive, grow, and reproduce. Many plants are able to counteract the negative effects of herbivory by regrowing new leaves following attacks by herbivores. Typically, it is assumed that plants growing in rich habitats with plenty of light and nutrients are better able to tolerate herbivory (Maschinski & Whitham 1989). There is also some evidence for the opposite, that plants growing in resource-poor habitats are better able to tolerate herbivory (Hilbert et al. 1981).

We tested these possible outcomes using a landscape experiment that harvested trees in large patches, creating habitats that differed dramatically in environmental conditions (i.e., sunny vs. shady habitats). Within these experimental patches, we planted thousands of seeds and seedlings of four perennial herbaceous plant species and protected half of the plants with cages to exclude grasshopper herbivores. Over two years we measured growth, survival, reproduction and herbivore damage on each plant.

Pic Ontogenic

Longleaf pine ecosystem (South Carolina, USA). Top: Satellite image of one experimental plot showing a two hectare thinned patch surrounded by dense longleaf pine forest (Google Earth). Bottom: Thinned patch (left) and unthinned patch (right). (photo credit: PG Hahn)

Our landscape experiment is unique in that we found support for both models of how plants might respond to herbivory in different resource environments, but this depended on the age of the plants. Interestingly, young plants were more affected by herbivory in shady habitats (i.e., low-resource),whereas older plants were more affected by herbivory in sunny habitats (i.e., high-resource). These findings contribute to understanding when herbivores will negatively affect plants and help to resolve some of the previously conflicting results.

Philip G Hahn
Department of Zoology, University of Wisconsin-Madison, USA.

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