The controls and constraints of fine-root lifespan

Open Access
Author:
Adams, Thomas
Graduate Program:
Ecology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 06, 2014
Committee Members:
  • David Eissenstat, Dissertation Advisor
  • David Eissenstat, Committee Chair
  • Roger Tai Koide, Committee Member
  • Kathleen Marie Brown, Committee Member
  • Consuelo M De Moraes, Committee Member
Keywords:
  • root lifespan
  • minirhizotron
  • phenolics
  • herbivory
  • 13C
Abstract:
Despite fine roots accounting for up to 50% of global terrestrial net primary productivity and 60% of soil respiration, surprisingly little is known about their ecology. Much of our ignorance involving fine-root ecology stems from the difficulty in observing roots in situ without disturbing the environment they inhabit. As a result, the ecological study of roots is still in its infancy. Through the use of minirhizotrons and isotopic techniques, we are beginning to gain a better understanding of how long roots live. However based on the different methodological approaches employed, the answer to this seemingly basic question can differ by as much as five fold. Beyond these methodological discrepancies, a basic understanding of the controlling factors that govern root lifespan remains elusive. Marshall and Waring put forward one of the early hypotheses regarding the controls of fine-root lifespan. They hypothesized that fine roots are initially constructed with a static carbohydrate reserve and the use of this finite reserve to fuel the metabolic demands of the root dictates the root’s longevity. In Chapter 2, we examine this hypothesis in greater depth by labeling Sassafras albidum trees with 99% 13CO2 and tracking the fate of the label in fine roots that were at least two weeks old at the time of labeling. If a root’s carbohydrate reserves truly are determined at initiation, than no 13C labeled photosynthate should appear in the carbohydrate pools of existing, non-elongating roots. We found that both root non-structural and structural carbon pools incorporate carbon from current photosynthate and as a result we found no support for the underlying assumptions of hypothesis put forward by Marshall and Waring. In Chapter 3, we investigate another hypothesis concerning the control of fine-root lifespan, namely that root lifespan is dictated by some metric of the costs of building and maintaining the root compared to the benefits the root supplies in terms of nutrient or water acquisition. Here we used a combination of minirhizotron tubes and in-growth cores fertilized with nitrogen to see if roots supplying greater levels of a limiting nutrient do indeed have extended lifespans. We found that for species with fine-root morphology, root lifespan was significantly extended by localized nitrogen fertilization, but this trend was not observed in species with coarse-root morphology. Finally, in Chapter 4 we investigated the role herbivory plays in fine-root lifespan. We know that herbivores and pathogens can significantly reduce root longevity, but how well roots are defended against such attacks remains unanswered. We therefore investigated the relationship between levels of fine-root soluble phenolics, a putative measure of chemical defenses against root herbivory, and specific factors that have been shown to be related to fine-root lifespan. Although we found significant correlations between fine-root phenolic concentrations and both root order and localized nitrogen availability, we were unable to find general utility in relating phenolic concentrations with factors that have been shown to extend fine-root lifespan. Combined, the research described in the following chapters represents a significant scientific contribution in furthering our understanding of the controls and constraints of fine-root lifespan.