Coming to Common Ground: The Challenges of Applying Ecological Theory Developed Aboveground to Rhizosphere Interactions

Accumulating evidence supports the importance of belowground interactions for plant performance, ecosystem functioning, and conservation biology. However, studying species interactions belowground has unique challenges relative to the aboveground realm. The structure of the media and spatial scale are among the key aspects that seem to strongly influence belowground interactions. As a consequence, our understanding of species interactions belowground is limited, at least compared to what is known about interactions aboveground. Here we address the general question: Do the ecological concepts that have been developed largely in aboveground systems apply to understanding species interactions in the rhizosphere? We first explore to what extent ecological concepts related to species interactions are considered in rhizosphere studies across various subdisciplines. Next, we explore differences and similarities above- and belowground for fundamental concepts in ecology, choosing topics that are underrepresented in rhizosphere studies but represent a swath of concepts: species diversity, island biogeography, self-organization and ecosystem engineering, trophic cascades, and chemical communication. Finally, we highlight to overcome major challenges of current methodologies to study rhizosphere interactions in order to advance the understanding of belowground interactions in an ecological context. By synthesizing literature related to rhizosphere interactions, we reveal similarities, as well as key differences, in how fundamental ecological concepts are used and tested in above- and belowground studies. Closing the knowledge gaps identified in our synthesis will promote a deeper understanding of the differences above- and belowground and ultimately lead to integration of these concepts

FRIEND OR FOE? THE CONDITIONALITY OF MYCORRHIZAE-CONFERRED RESISTANCE TO INSECT HERBIVORES

136 pagesPlants face a constant struggle to acquire nutrients and defend themselves against herbivores. Association with soil microbes like mycorrhizal fungi can increase plant growth, and alter resistance to herbivores. Mycorrhizae are traditionally seen as mutualists that increase plant growth, and as such are used in agriculture. However, the effects of mycorrhizae on resistance to herbivores are variable. The conditions that drive either mycorrhizae-conferred resistance or mycorrhizae-conferred susceptibility to herbivores are not well understood. To determine the conditions under which mycorrhizae confer resistance, I conducted a series of greenhouse experiments testing the effects of different abiotic and biotic conditions on mycorrhizae-conferred resistance, specifically manipulating intraspecific plant competition, fertilization, plant domestication, and plant species identity. For each of these experiments, I measured resistance traits within the plant to identify potential mechanisms by which mycorrhizae might change resistance. In my first experiment, mycorrhizae increased susceptibility to herbivores when the plants were not in competition, but had no effect in competition. I also showed that mycorrhizae induced jasmonic acid-mediated decreases in foliar nitrogen, a novel mechanism by which mycorrhizae affect resistance to herbivores. In my second experiment, I investigated mycorrhizae-conferred resistance along a gradient of fertilization treatments. I found that mycorrhizae only conferred resistance to herbivores at medium levels of fertilization. Increased resistance was again correlated with changes in the plant’s foliar nitrogen content. In my third experiment looking at the effects of domestication on mycorrhizae-conferred resistance to three different herbivores, I found that mycorrhizae changed the growth and resistance of undomesticated plants to a larger degree than domesticated plants. The change in mycorrhizae-conferred resistance in undomesticated, mycorrhizal plants corresponded with an increase in protease inhibitors, a class of chemical defenses. By changing the defensive chemistry and nutrient content of their host plants, mycorrhizae can shift plant resistance to herbivores. While mycorrhizae are traditionally seen as mutualists, under many conditions, and when viewed in a tri-trophic context, they can act parasitically. My research demonstrates the limitation of mycorrhizae as an agricultural tool and provides insights into ways that mycorrhizae can manipulate aboveground herbivore community composition. FRIEND OR FOE? THE CONDITIONALITY OF MYCORRHIZAE-CONFERED RESITANCE TO INSECT HERBIVORE

Data from Experiments 1 and 2

Data from both experiments. Column headings in ReadMe fil

Data from: Evolution of increased Medicaco polymorpha size during invasion does not result in increased competitive ability

Species invading new habitats experience novel selection pressures that can lead to rapid evolution, which may contribute to invasion success and/or increased impact on native community members. Many studies have hypothesized that plants in the introduced range will be larger than those in the native range, leading to increases in competitive ability. There is mixed support for evolution of larger sizes in the introduced range, but few studies have explicitly tested whether evolutionary changes result in decreased competitive responses or increased competitive effects on other species in the community. Here we show that introduced Medicago polymorpha genotypes produced 14% more aboveground and 41% more belowground biomass than genotypes from the native range, suggesting evolutionary changes in size occurred after introduction. However, these size differences were only observed in the absence of competition. The competitive effects of introduced and native range genotypes on three species that commonly co-occur with Medicago in invaded regions were remarkably similar. These results suggest that evolutionary increases in size during biological invasions do not necessarily alter the competitive effects of the invader on other community members but may increase invasion success in disturbed or low competition environments

Plant size, latitude, and phylogeny explain within-population variability in herbivory

Interactions between plants and herbivores are central in most ecosystems, but their strength is highly variable. The amount of variability within a system is thought to influence most aspects of plant-herbivore biology, from ecological stability to plant defense evolution. Our understanding of what influences variability, however, is limited by sparse data. We collected standardized surveys of herbivory for 503 plant species at 790 sites across 116° of latitude. With these data, we show that within-population variability in herbivory increases with latitude, decreases with plant size, and is phylogenetically structured. Differences in the magnitude of variability are thus central to how plant-herbivore biology varies across macroscale gradients. We argue that increased focus on interaction variability will advance understanding of patterns of life on Earth