Ecology of Root Lifespan

Eissenstat, D.M.

August 1, 2006 - July 31, 2009. Funded by NSF IOB-Environmental and Structural Systems Cluster

Despite its importance, variation in root lifespan among species and in response to changes in the environment is poorly understood. Relatively few species have been examined and rarely have multiple species been compared in a common environment. The work proposed will examine the root lifespan of 12 tree species that vary widely in root diameter, root tissue density and potential growth rate using state-of-the-art approaches. Trees were transplanted as 1-yr-old seedlings in replicated plots in a common garden approximately nine years ago. Variation in root lifespan will be related to plant potential growth rate, root structure (specific root length, diameter, tissue density) and root N concentration. Do roots and leaves share parallel suites of traits commonly associated with their lifespan? Three hypotheses are proposed that attempt to explain what controls and constrains root lifespan: the “Starch depletion hypothesis” (SDH) of Marshall and Waring, the “Resource optimization hypothesis” (ROH) and a new hypothesis proposed here, which is referred to as the “Metabolic activity hypothesis” (MAH). The Starch depletion hypothesis assumes that a finite amount of stored carbohydrates (starch) is deposited at root formation and that the rate the carbohydrates are depleted by root respiration determines the lifespan of the root. The Resource optimization hypothesis assumes that root lifespan is optimized to provide the greatest benefit in terms of water and nutrients for the least cost (usually measured in carbon) over the lifespan of the root or cluster of roots. The Metabolic activity hypothesis suggests that root lifespan is mainly governed by metabolic rate; roots with higher respiratory activity live shorter lives than those with lower respiratory activity. Three experiments are proposed to help distinguish which of these three hypotheses best explains patterns of root lifespan. The first experiment proposes creating fertile patches which do not become depleted. These patches should increase the efficiency of nitrogen acquisition and also increase metabolic rate. If root lifespan is increased in the patch, then ROH is supported. If root lifespan is decreased then either SDH or MAH is supported, depending on how quickly the roots die in relation to their starch reserves. Respiration and nonstructural carbohydrates (including starch) of the roots of the different species will be examined as a function of root age in a second experiment. The third experiment examines the importance of current photosynthate on root lifespan of 1st-order roots by pulse-labeling carbohydrates with 13C. Collectively these experiments should greatly increase our understanding of the ecology of root lifespan.

This study will have several broader impacts. A better understanding of root lifespan will be valuable to those attempting to model ecosystem carbon cycles because of the important link root turnover has in this process. Many of the trees proposed to be examined in this study are forest dominants in much of eastern hardwood forests. Better understanding of their root lifespan will be useful to forest managers as well as investigators of climate change. This study will provide strong support for the training of graduate and undergraduate students in research.