Defences

History of deer colonization - Response of vegetation to deer - Response of animals to deer and squirrel - Deer biology

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Effect of browsing history on the vegetation - Effects of chemical defences on deer browsing - Effects of reduced plant abundance on pollination - Effect of hunting on tree regeneration - Effect of prolonged deer population reduction on the vegetation

Effects of chemical defences on deer browsing

Aim & scope - Where & when - How - Main results - Conclusion - Scientific Publications

Aim and scope

Deer browsing delays or prevents tree regeneration in the forests on Haida Gwaii (History of deer colonisation and Different browsing histories).

Anecdotal evidence suggested that delays were more severe on Haida Gwaii than in mainland sites with high deer populations. We hypothesised that this could be related to differences in plant chemical defences.

What are chemical defences?

Chemical defences can vary among species, populations and individuals. They can be independent of herbivory (constitutive) or stimulated by herbivory (induced).

Chemical defences can also vary with the age of a plant.

The level of chemical defences produced by an individual at a given place and at a given time depends on a balance between:

benefits: prevention from being eaten
costs: allocating energy to the production of chemical compounds

What could be special on Haida Gwaii?

Plants on Haida Gwaii were little or not exposed to mammalian herbivores before deer were introduced at the beginning of the 20^th century.

We hypothesized that in such a situation plants might have lost or reduced their chemical defences over time, thus the severity of deer impact on Haida Gwaii.

We tested this hypothesis on Western redcedar (Thuja plicata) and posed two questions:

Are trees on Haida Gwaii less defended against herbivores than trees on the mainland?

What determines the level of chemical defences in redcedars from Haida Gwaii?

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Where and when

Gwenaël Vourc’h and co-workers studied trees from Haida Gwaii and from the adjacent mainland between 1999 and 2001.

Mainland redcedar evolved in presence of deer, while Haida Gwaii redcedar evolved in the absence of deer for at least 10 000 years.

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How - Main results

Are Haida Gwaii redcedar less defended?

What determines the level of defences of redcedars on Haida Gwaii?

Are Haida Gwaii redcedar less defended?

Do deer prefer trees from Haida Gwaii to those from the mainland?

Feeding choice experiments offered foliage from Haida Gwaii and from mainland trees to wild deer on Haida Gwaii.

To avoid biases related to defence induction we used foliage samples that had never been exposed to deer.

Foliage from Haida Gwaii was more browsed than foliage from the mainland. Deer preferred trees from Haida Gwaii.

Is there a difference in the production of chemical compounds between trees from the mainland and trees from Haida Gwaii?

Chemical analyses by gas chromatography showed that Haida Gwaii redcedar had, on average, lower concentrations of monoterpenes.

Are Redcedar monoterpenes acting as deer repellent?

Using additional feeding choice experiments where apples stained or unstained with monoterpenes were offered to deer, we demonstrated that, at the concentration encountered in redcedar leaves, redcedar monoterpenes had a marked repellent effect in the three species of deer tested.

Thujone was the compound that had the strongest effect,

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What determines the level of defences in redcedar on Haida Gwaii?

On Haida Gwaii heavily and lightly browsed young trees of the same age occur side by side in localities where redcedar achieves some level of regeneration.

Where does the difference in browsing intensity come from?

This difference in the intensity of deer browsing could be explained either by the level of chemical defences produced by the two categories of trees, or by different nutritive values.

Chemical analyses were conducted to determine the levels of monoterpene and of nutritive compounds.

Heavily browsed redcedars had lower monoterpene concentrations and lower nutritive values.
Monoterpene concentration seems therefore to explain browsing intensity rather than nutritive value.

Are differences in defence intensity induced?

Herbivory can trigger the production of induced chemical defences unless excessive defoliation decreases carbon availability to a point that the plant becomes unable to produce the necessary compounds.

Defoliation experiments were conducted on young trees grown in nursery. We tested 4 intensities of defoliation: 0%, 25%, 50% and 75%.

Monoterpene concentrations were measured in the days following the defoliation.

We found no evidence of defence induction

Are the differences in the level of defences constitutive?

To confirm the constitutive nature of these defences, we protected heavily and lightly browsed trees from browsing.

We found no change in terpene production as a result of protection in either tree category.

To confirm the genetic basis of the plant's chemical make up, we exposed 3000 four year old redcedars to browsing. These trees had been protected from deer until then. The 3000 individuals belonged to 62 known genetic families.

Browsing intensity differed from family to family.

Trees belonging to lightly browsed families produced significantly more monoterpenes than the trees belonging to heavily browsed families.

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Conclusion

Redcedars from Haida Gwaii produce less chemical defences than redcedars from the mainland, which partly explains the high intensity of deer browsing on the archipelago.

However, some trees from Haida Gwaii are genetically able to produce more chemical defences than others, and thus are less severely browsed.

Following deer introduction, the individuals who have the genetic potential to produce more chemical defences are favoured. Less browsed they are able to escape deer in the sites where redcedar regeneration is possible.

This selective advantage may cause a change in the chemical profile of the redcedar population on Haida Gwaii.

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Scientific Publications

Gwenaël Vourc’h, Bruno Vila, Dominique Gillon, José Escarré, Frédéric Guibal, Hervé Fritz, Thomas P. Clausen and Jean-Louis Martin, 2002, Disentangling the causes of damage variation by deer browsing on young Thuja plicata, OIKOS 98: 271–283, 2002 – PDF

Bruno Vila, Gwenaël Vourc’h, Dominique Gillon, Jean-Louis Martin, Frédéric Guibal, 2002, Is escaping deer browse just a matter of time in Picea sitchensis? A chemical and dendroecological approach, Trees (2002) 16:488–496 – PDF

Gwenaël Vourc’h, Jean-Louis Martin, Patrick Duncan, José Escarré, Thomas P. Clausen, 2001, Defensive adaptations of Thuja plicata to ungulate browsing: a comparative study between mainland and island populations, Oecologia (2001) 126:84–93 – PDF

Gwenaël Vourc’h, Michel de Garine-Wichatitsky, Alice Labbé, Dimitri Rosolowski, Jean-Louis Martin, Hervé Fritz, 2002, Monoterpene effect on feeding choice by deer, Journal of Chemical Ecology, Vol. 28, No. 12, December 2002 (°C 2002) – PDF

G. Vourc’h, J. Russell, and J.-L. Martin, 2002, Linking Deer Browsing and Terpene Production Among Genetic Identities in Chamaecyparis nootkatensis and Thuja plicata (Cupressaceae), The Journal of Heredity 2002:93(5) 370:376 - PDF

G. Vourc’h, J. Russell, J.-L. Martin, Dominique Gillon, 2003, Short term effect of defoliation on terpene content in Thuja plicata, Ecocsciences, Vol. 10 (2) 2003) 161:167 – PDF

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