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Experimental studies of the multitrophic effects of anti-herbivore defense in three pine barrens shrub species

Coffey, Elizabeth R.

Titre : Experimental studies of the multitrophic effects of anti-herbivore defense in three pine barrens shrub species

Auteur : Coffey, Elizabeth R.


Grade : Master of Science (2012)

Plants serve as the foundation of two ecosystem trophic levels. Above ground, they are the primary food source for animals, while their roots and deposited leaves interact with the soildwelling community below ground. Plants have evolved the ability cope with herbivory and other environmental stressors by producing a number of chemical defense compounds that are manufactured through complex chemical reactions within the leaves and result in an array of direct and indirect defense responses, much like our own immune system. Induced chemical defenses potentially influence soil dynamics by causing changes in the nutrient ratio of the leaves, which are then seasonally shed and contribute to the organic layer of the soil, or by affecting the detritus community through leaching of chemicals in the root zone. Manual clipping was utilized in this study to induce the production of chemical compounds with the goal of examining the secondary effects of induced plant defenses. Soil invertebrate communities were assessed subsequent to the removal of 25% of the plant mass of three plant species in the Albany Pine Bush to detect downstream consequences of plant defense mechanisms. I also compared the leaf chemistry, whole plant growth and ensuing herbivory rates of damaged and undamaged Salix humilis, Quercus prinoides, and Q. iliciofolia shrub species. Thirty plants of each species were examined, half of which were treated by clipping annually for two consecutive years. Subsequent to final treatment, Nitrogen and Carbon contents of fallen leaf samples were analyzed periodically. A Berlese funnel extraction method was used to identify invertebrates from soil samples taken within 120 mm of the main plant stem for twenty plants from each species, ten of each treatment group. Soil pH was also determined for all soil samples taken for invertebrate analysis. Results indicate that soil invertebrate abundance and richness was significantly lower in the treatment groups of two of the three plant species. Soil samples from untreated plants revealed 2.62 times more soil invertebrate abundance and included 25% more groups of taxa. Soil invertebrate populations were not correlated with soil pH or closest plant species. These findings support the claim that soil communities are influenced by the changes caused by induced plant defense mechanisms. Variation in soil invertebrate communities between treatment groups did not correspond to differences in plant chemistry, as C:N did not vary between damaged versus undamaged leaves. Possible explanations for this include the possibility that changes were not reflected in leaf litter samples due to the reabsorption of many nutrients prior to leaf shedding, a concentration of defense chemicals towards more sensitive parts such as seeds and roots, or that soil invertebrates are more influenced by induced chemical changes that occur in the root region. It was found that C:N varied significantly among plant species and sampling periods, and that the ratio of browsed branches one year after treatment was different among treatment groups and plant species. Treated dune willow shrubs had a reduced number of browsed branches, indicating a possible whole-plant response. This study displays the importance of preserving these unique simplistic system composed of an arid environment with poor, relatively homogenous soils and redundant vegetation, such as the Albany Pine Bush, which allow for an exceptional opportunity to investigate plant-soil interactions.

Subject : Biology ; Ecology ; Plant sciences

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