Theoretical and empirical approaches to examining the interplay between vascular constraints and plant responses to environmental heterogeneity

Environmental heterogeneity is ubiquitous in natural ecosystems, and plants, like other organisms, are constrained by their ability to make use of patchily distributed resources and to respond to localized stressors. Plants have developed numerous plastic responses to deal with environmental heterogeneity, but many of these responses are constrained by vascular architecture---the anatomical arrangement of the xylem and phloem. At the same time, vascular constraints---called sectoriality---are expected to have different effects on allocation of resources depending on the level of environmental heterogeneity. I combine modeling and empirical approaches to understanding ways that sectoriality interacts with plant responses to heterogeneity. The first two data chapters (Chapters 2 and 3) measure the movement of locally supplied isotopically labeled nitrate in basil, under different patchy scenarios. Chapter 2 shows that partial defoliation can cause increased crossover of nitrate between sectors. Chapter 3 shows that changes to local water uptake in the presence of a nitrate patch can affect inter-sector allocation. The third data chapter (Chapter 4) simulates water flow into, and transpiration and photosynthesis by high light leaves in trees whose crowns are otherwise shaded. The results of Chapter 4 suggest that sectoriality can constrain photosynthesis, but only when water is limited. The fourth data chapter (Chapter 5) examines crown morphology in understory saplings, to assess whether species with different wood types (and therefore different levels of sectoriality) use different strategies for branch biomass allocation in response to light gradients. The results of Chapter 5 did not support the hypothesized effect sectoriality on allocation patterns, suggesting that explanations are needed for the overall similarity between deciduous trees of different wood types. The final data chapter (Chapter 6) presents a model for the effects of sectoriality on root proliferation in a nitrogen patch. The model output suggests interaction roles for xylem and phloem sectoriality in shaping root allocation. The model from Chapter 6 may be particularly useful for designing and interpretting experiments on nutrient foraging by plant roots. In the concluding chapter (Chapter 7), the results of the various data chapters are tied together and future research directions are suggested.