What happens when the neighbour is eaten?

Submitted by editor on 21 October 2016.Get the paper!

An important current focus in ecology is to better understand how species interactions can change in response to biotic and abiotic contexts. Herbaceous plants in many forests of the world face increased impacts from growing ungulate herbivore populations. However not all forest herbs interact directly with ungulate herbivores. Some plant species are unpalatable to ungulates and their interactions with ungulate herbivores occur through indirect pathways. For example, unpalatable forest herb species are thought to benefit from a reduction in expected competitors or they may experience costs from collateral damage of large herbivore actions.

Undergraduate student Jake Winkler collecting non-flowering Arisaema for transplant to the common garden.

Our research focuses on the large generalist herbivore, white-tailed deer, and a common and unpalatable forest herb, Arisaema triphyllum as a model system for our study of indirect effects. Arisaema is one of several forest herbs demonstrated to be generally unpalatable by deer but this species is also unique in that it exhibits size-dependent sex switching.

As biomass increases from season to season non-flowering Arisaema (A) will eventually pass a threshold size for onset of flowing and produce an inflorescence (B). Small flowering Arisaema will produce male flowers (C), and as individuals yearly biomass gains they will reach a threshold size for female flowering and produce female flowers on the inflorescence (D). Individuals can transition back and forth between flowering and sex states from year to year as biomass increases or decreases.

In order to estimate impacts from deer on a plant that doesn’t directly interact with deer, we measure the amount of browse on a favorite food, Trillium spp., as a proxy for indirect effects. Using this model system our previous research has shown that deer browse on co-occurring Trillium may not benefit unpalatable forest herbs but surprisingly have cryptic, negative indirect effects on life history traits of Arisaema and other forest herbs in Pennsylvania forests.

An Arisaema inflorescence between two deer browsed Trillium stems. White-tailed deer avoid Arisaema when more palatable herbs, like Trillium, are available.

Our research further suggests that these negative indirect effects are mediated largely through non-trophic pathways; deer are thought to interfere with or halt belowground processes generating stressful growth conditions for all plants.

Christopher Heckel transplanting non-flowering Arisaema from 6 different populations into the common garden environment. Arisaema individuals came from populations that exhibited and gradient of deer browse on Trillium for forest sites in Pennsylvania, USA, with historically high deer densities.

Our present study builds upon this research and asks if the life history trait differences we observed in the field can be explained by local adaptation or phenotypic plasticity. We used a common garden experiment with Arisaema from populations that experience a gradient of deer impacts to test this question. Plants were allowed to grow in the common garden for five seasons.

The raised bed common garden in 2006 as Arisaema transplanting was nearly completed.

The size at onset of flowering, biomass allocation, and asexual reproduction of Arisaema showed no differences among populations, differing from our field results, suggesting phenotypic plasticity in those traits. We did see significant differences among populations in relative growth rate and size at onset of female flowering in the common garden suggesting local adaptation. This exciting result suggests that non-trophic indirect effects can generate selection on life history traits on Arisaema in Pennsylvania forests. The extent to which this occurs more generally in forest plant communities needs further exploration.

The common garden in the spring of 2009. After just two years in the garden conditions many Arisaema had put up inflorescences.

Christopher Heckel

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