Nitrogen Uptake by Trees and Mycorrhizal Fungi in a Successional Northern Temperate Forest: Insights from Multiple Isotopic Methods
Forest succession may cause changes in nitrogen (N) availability, vegetation and fungal community composition that affect N uptake by trees and their mycorrhizal symbionts. Understanding how these changes affect the functioning of the mycorrhizal symbiosis is of interest to ecosystem ecology because of the fundamental roles mycorrhizae play in providing nutrition to trees and structuring forest ecosystems. We investigated changes in tree and mycorrhizal fungal community composition, the availability and uptake of N by trees and mycorrhizal fungi in a forest undergoing a successional transition (age-related loss of early successional tree taxa). In this system, 82–96% of mycorrhizal hyphae were ectomycorrhizal (EM). As biomass production of arbuscular mycorrhizal (AM) trees increased, AM hyphae comprised a significantly greater proportion of total fungal hyphae, and the EM contribution to the N requirement of EM-associated tree taxa declined from greater than 75% to less than 60%. Increasing N availability was associated with lower EM hyphal foraging and 15N tracer uptake, yet the EM-associated later-successional species Quercus rubra was nonetheless a stronger competitor for 15N than AM-associated Acer rubrum, likely due to the more extensive nature of the persistent EM hyphal network. These results indicate that successional increases in N availability and co-dominance by AM-associated trees have increased the importance of AM fungi in the mycorrhizal community, while down-regulating EM N acquisition and transfer processes. This work advances understanding of linkages between tree and fungal community composition, and indicates that successional changes in N availability may affect competition between tree taxa with divergent resource acquisition strategies.
Key wordsmycorrhiza hyphae canopy 15N tree competition
This research is supported by awards from the National Science Foundation (DEB-0911461) and U.S. Department of Energy (DE-FC02-06ER64158). The authors thank R. M. Miller for assistance with hyphal ingrowth methodology, and the following individuals for assistance with fieldwork, sample preparation, and analysis: A. Bajcz, A. Baldick, C. Bogdan, A. Brenske, A. DeGabriele, A. Do, Z. Fortier, A. Gold, M. Grant, J. Halick, S. Liao, K. McClure, J. Pollack, K. Sparks, S. Sheperd, F. Soper, C. Vogel, S. Webster, B. Weiler, S. Yassine, and Z. Zeneberg. Lastly, the authors are grateful to two anonymous reviewers who provided helpful feedback that improved this paper from its manuscript form.
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