PARENT SESSION
Oral Session # 8: Plant Ecology I: Physiology and Function I.
Presiding: E Hamerlynck
Monday, August 4. 8:00 AM to 11:30 AM, SITCC Meeting Room 200.
Root branching order and responses of fine roots to altered carbon source and sink strength.
Guo, Dali1, Mitchell, Robert 1, Hendricks, Joseph2, 1 Joseph W. Jones Ecological Research Center, Newton, GA, USA2 State University of West Georgia, Carrollton, GA, USA
ABSTRACT- Root branching order, a measure of root structure, can strongly influence key root functional parameters such as root diameter, specific root length, carbon (C) and nitrogen (N) concentrations, and root longevity. Yet it is not clear how the flow of C and N is regulated by root branching order when fine roots are under concurrent alterations in C source strength and C sink strength. To answer this question, we manipulated C source strength via canopy scorching (80% needle biomass reduction), and C sink strength via N fertilization (50 kg N /ha/yr) in a longleaf pine ecosystem in the southeastern USA. Our results showed that root starch concentration increased, whereas N concentration decreased with root order almost linearly in the controlled plots. More importantly, our results demonstrated that root branching order regulates the flow of C and N in the fine roots in a manner contrasting with some of the previous predictions. Root starch concentration was significantly (p <0.05) reduced by canopy scorching as compared to the control, but only in root orders 3-5 (with the distal roots = the first-order roots, and the fifth-order roots = the highest-order roots we sampled). Root N concentration increased significantly (p <0.05) with N fertilization as compared to the control, but also only in root orders 3-5. These results suggest that stored carbohydrates (e.g., starch) were preferably maintained in the roots of the lowest-order (1-2), and that N concentration in these roots may not be elevated easily by N fertilization (rather, extra N was exported to the higher-order roots). Because these lowest-order roots are also the roots with greatest metabolic activity (i.e., highest N concentration and root respiration) and may potentially consume the majority of the carbon transported to roots, the manner by which C and N are allocated in these roots may be critical to understanding C and N cycles in forest ecosystems. The results of this study also offer new perspectives for modeling root longevity based on a cost-benefit approach.
Key words: longleaf pine, non-structural carbohydrates, defoliation, N fertilization