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All organisms, including plants, experience variability in the environment which puts pressure on organisms to evolve flexible responses. The study of these responses by organisms falls into the discipline of behavioural ecology. In this thesis, I am interested in the foraging behaviour of plant roots and I have two goals. First, I will use foraging theory from the animal literature to determine whether plants forage in ways that are similar to animals. Second, I will show how the adoption of foraging theory for plants can lead to a better theoretical understanding of coexistence of plants. I begin with a discussion of the major differences between plants and animals in their foraging behaviour and how this can be incorporated in to a more general predictive framework of plant foraging behaviour. I follow this discussion with two empirical tests of classic foraging models. First, I test a patch use model from the animal literature to determine if it can predict plant foraging behaviour. My results show that plants foraged for patches using the same strategies used by animals. Second, I test a resource choice model from the animal literature. These data indicated that plants select different types of nitrogen using the same resource choice strategies as foraging animals. These two studies reveal some basic foraging abilities of plants, however the experiments were performed in the absence of resource competition, a condition seldom experienced by plants in nature. To overcome difficulties in studying plant roots grown with neighbours I developed a molecular method for the identification of visually indistinguishable plant roots from competition experiments. Finally, I apply the molecular method to examine whether resource patchiness in soil can increase the intensity of competition experienced by foraging plants, and that the presence of neighbours influences the foraging strategies of plants. Together the results presented in this thesis show that plants use the same basic foraging strategies as animals, and that foraging behaviour can be linked to competition and coexistence of plant species. / Ecology

Soil nutrients are often heterogeneously distributed in space and time at scales relevant to individual plants, and plants can respond by selectively proliferating their roots within nutrient-rich patches. However, many environmental factors may increase or decrease the degree of root proliferation by plants. I explored how soil fertility, nitrogen (N) or phosphorus (P) limitation, and soil oxygen availability affected root system response to nutrient heterogeneity in forested wetland ecosystems of southeastern United States. Fine root biomass was not correlated with soil nutrient availability within wetland ecosystems, but was related to ecosystem-scale fertility. Root systems generally did not respond to P-rich patches in both floodplain (nutrient-rich) and depressional swamps (nutrient-poor) swamps, but results were inconclusive because the growth medium (sand) potentially hindered root growth. In floodplain forests, roots proliferated into N-rich patches but not P-rich patches, even though litterfall N:P ratios were > 15, which suggested that these ecosystems were P-limited. The combination of nutrient and oxygen heterogeneity affected root proliferation and biomass growth of three common floodplain forest species (Liquidambar styraciflua, Fraxinus pennsylvanica, and Nyssa aquatica) in a potted study, which was related to species' flood tolerance. My results suggest that the environmental context of plants can affect roots system response to nutrient heterogeneity in forested wetland ecosystems and highlights the need for field studies that investigate this phenomenon. Learning how environmental conditions affect plant response to nutrient heterogeneity at a fine-scale will provide better predictions of nutrient cycling, plant competition and succession, and forest productivity, which are important factors that determine carbon sequestration and timber production. / Ph. D.

Resources in the soil are heterogeneously distributed. We know that plant species differ in their root responses to nutrient patches and that these differences in foraging can influence plant competition. However, most studies of root-resource interactions overlook the potential top-down influence of root herbivores. While root herbivores can influence plant community structure, the extent to which they influence ecosystem-scale factors such as net primary production is unclear. In addition, little is known regarding root herbivore foraging behaviors and, more importantly, whether these foraging behaviors can actually influence species interactions. In this dissertation, I present a conceptual model of soil-root-herbivore interactions in which soil resource heterogeneity structures both root dynamics and the abundance and influence of root herbivores. I conducted two field and one greenhouse experiment examining this proposed model. The dissertation includes an introductory chapter (Chapter 1), a field study examining root responses to manipulations of soil fertility and root herbivory (Chapter 2), a greenhouse study that used plant species responses to heterogeneity to develop predictions about the role of root herbivores in mixed-species neighborhoods (Chapter 3), and a field study of planted communities examining soil fertility and fauna effects on above- and belowground structure and function (Chapter 4). In all cases, there were significant effects of root herbivores on community structure and components of net primary production. Resource distribution had a strong effect in studies conducted in sandy, nutrient-poor soils (Chapter 2 and 3), but had a reduced effect in the study conducted at Kentland Farm in loamy soils (Chapter 4). Interactions between resource availability and root herbivory were common. These results support the theory that the potential benefit of resource-rich patches may be constrained by root herbivores. This research complements recent findings that demonstrate other potential costs of species foraging behaviors (such as exposure to soil anoxia and increased drought stress), as well as potential effects of root herbivores and other soil fauna on plant diversity. / Ph. D.

Relationships between nutrient heterogeneity, root foraging behavior and short-term competitive interactions were investigated for six species native to southeastern USA. Monoculture, two- and six-species garden plots were established and fertilized to create spatially homogeneous or heterogeneous nutrient conditions. After 3.5 months, root proliferation in rich patches (precision) and aboveground biomass response to heterogeneity were assessed in monocultures, and competitive outcomes (aboveground biomass) were determined from mixed-species plots. In monoculture plots, two species were relatively precise foragers, but no species showed significant aboveground biomass response to nutrient treatment. Correlations between precision and aboveground biomass were weak (-0.40 < r < 0.17). In two-species plots, interspecific competition was influenced by soil heterogeneity in two of six cases tested (P < 0.05), and precision was the behavior most correlated with competitive success. In six-species plots, spatial pattern of nutrients had no influence on aboveground growth or competition. Results suggest that heterogeneity influences competition, but the influence is context-specific and generally small. Precision may be the foraging behavior that most influences interspecific interactions. / Ph. D.

La nutrition minérale des plantes dépend à la fois du développement et du fonctionnement de leur appareil racinaire, incluant l'absorption mais aussi la capacité des plantes à influencer les cycles des nutriments, notamment par l'exsudation. Le but de cette thèse est de lier les rétroactions plantes-sol impliquant les cycles des nutriments aux stratégies d'exploration racinaire. Dans la revue bibliographique, je recense des mécanismes d'interaction plantes-sol et leurs échelles spatiales et temporelles. En considérant, à l'échelle de la rhizosphère, les interactions directes entre racines et sol, je propose que la combinaison entre exsudation et absorption des nutriments mène à des synergies entre racines d'une même plante. Ma seconde hypothèse est celle de l'existence d'un compromis entre l'exploration du sol et son occupation (défini comme la capacité des plantes à influencer efficacement le cycle des nutriments). Dans un premier chapitre, je développe un modèle général de recyclage des nutriments afin de déterminer sous quelles conditions les plantes auraient intérêt à limiter leur exploration du sol. Je montre qu'une exploration limitée est une stratégie de nutrition efficace sous certaines conditions, dont l'existence de synergies entre racines et le fait d'être dans un sol pauvre en nutriment. Dans un deuxième chapitre, je mesure le patron d'exploration racinaire et évalue le recyclage de l'azote à l'aide des outils isotopiques, chez trois espèces de Poacées pérennes de la savane de Hwange (Zimbabwe). Cette étude de terrain montre un gradient d'hétérogénéité racinaire entre ces trois espèces. Les Poacées exprimant le patron d'exploration le plus hétérogène ont un cycle de l'azote plus lent, mais potentiellement plus efficace. Dans un dernier chapitre, je développe un modèle mécaniste à l'échelle de la rhizosphère, pour une plante absorbant le phosphore et contrôlant sa disponibilité par l'exsudation de citrate. Je montre que, selon l'échelle d'influence des racines en terme d'exsudation et d'abaissement de la concentration en phosphore, la combinaison de l'exsudation et de l'absorption mène soit à une compétition, soit à une facilitation entre les racines d'une même plante. En me plaçant à l'échelle du système racinaire, je montre que les pertes en phosphore sont limitées par une exploration limitée du sol. Ce dernier résultat va dans le sens du compromis exploration/occupation. Au cours de cette thèse, j'ai donc développé des approches complémentaires, mettant en jeu différents mécanismes et échelles d'interactions plantes-sol. Le fait que les racines ne se limitent pas à un rôle d'absorption, mais agissent activement sur les cycles de nutriments a mené à deux résultats originaux : la facilitation inter-racinaire et intra-plante, et le fait qu'une exploration limitée puisse être considérée comme une stratégie efficace de nutrition. Enfin, ce travail souligne l'importance d'intégrer les divers mécanismes d'interaction plantes-sol pour comprendre les stratégies de nutrition des plantes et mieux prédire leur impact sur les cycles de nutriments à l'échelle des écosystèmes. / Plant nutrition depends on complementary mechanisms : the development of root systems, root uptake and plant ability to control nutrient cycling, e.g. through exudation. The aim of this thesis is to link plant-soil feedbacks involving the cycling of nutrients and root foraging strategies. I first review the different mechanisms of plant influence on nutrient cycling within the soil and assess their respective scales. Considering the direct effect of roots on the soil at the scale of the rhizosphere, I hypothesize that the combination of absorption and exudation may lead to synergies between the roots of a plant. At the scale of the whole root system, I propose a second, heuristic hypothesis: the existence of a trade-off between soil exploration and soil occupation (defined as the ability of plants to influence efficiently nutrient cycling). In a first chapter, I develop a general model of nutrient cycling, to determine under which condition plants should limit the exploration of soil by their roots. I show that limited exploration is an efficient strategy under specific conditions, especially nutrient-poor soils and the existence of synergies between roots. In a second chapter, I characterize soil occupation and nitrogen cycling, by the use of isotopes ratios, in the plant-soil system of three perennial grasses of the savanna of Hwange (Zimbabwe). This field study shows a gradient of root heterogeneity among these grass species. Those showing the more heterogeneous root pattern have a slower but potentially more efficient nitrogen cycling. In a last chapter, I develop a numerical mechanistic model at the rhizosphere scale for a plant taking up phosphorus and increasing its availability through exudation of citrate. I show that, depending on the extent of root influence on soil by exudation and nutrient depletion, competition between roots as well as facilitation arise from the combination of root uptake and exudation. By upscaling rhizosphere processes to the root system, I show that phosphorus losses are minimized by a restricted soil exploration, which backs the hypothesis of a trade-off between soil exploration and occupation. Overall, I developed complementary approaches that took into account several mechanisms and scales of plant-soil interactions. Considering that root functions are not limited to nutrient uptake, but also involve their influence on nutrient cycling, lead to two novel results: the potential existence of intra-plant and inter-root facilitation, and limited soil exploration as an efficient foraging strategy. This work underlines the importance of accurately integrating the mechanisms of plant-soil interaction to assess their nutrient strategies and to predict their impact on nutrient cycling within ecosystems.

Nutrients are usually patchily distributed in natural soils. Plants are often able to respond to nutrient heterogeneity in artificial conditions by active plastic changes of root system morphology. The occurrence or magnitude of a foraging response can be altered by the presence of competition. However, it is unclear to what extent root foraging takes place in the field. I conducted a field experiment in order to determine the effect of an artificial nutrient patch on fine belowground biomass of (a) an established community and (b) model plants. The study array consisted of a grid of 30×30 cm plots with model plants located in the centre. Half of the plots contained the artificial patch located 5.5 cm from the model plant. Fertilizer patch treatment did not increase mean plot fine underground biomass. Instead, fine underground biomass was higher in places of greater soil moisture estimated from mean plot EIVs. Neither total model plant root biomass nor proportion of roots in the enriched quarter increased in the fertilizer treatment. Competition was probably higher in fertilized than in control plots judging by a 2-fold increase in death rate of model plants. However, greater proportion of model plants flowered in the treatment plots. Possible causes include a plastic response to the patch as well...