Isotopic evidence indicates saprotrophy in post-fire Morchella in Oregon and Alaska

Abstract

We assessed the nutritional strategy of true morels (genus Morchella) collected in 2003 and 2004 in Oregon and Alaska, 1 or 2 y after forest fires. We hypothesized that the patterns of stable isotopes (δ¹³C and δ¹⁵N) in the sporocarps would match those of saprotrophic fungi and that radiocarbon (Δ¹⁴C) analyses would indicate that Morchella was assimilating old carbon not current-year photosynthate. We compared radiocarbon and stable isotopes in Morchella with values from concurrently collected foliage, the ectomycorrhizal Geopyxis carbonaria (Alb. & Schwein.) Sacc., the saprotrophic Plicaria endocarpoides (Berk.) Rifai, and with literature to determine isotopic values for ectomycorrhizal or saprotrophic fungi. Geopyxis, Plicaria and Morchella, respectively, were 3‰, 5‰ and 6‰ higher in 13C than foliage and 5‰, 7‰ and 7‰ higher in ¹⁵N. High ¹⁵N enrichment in Morchella indicated that recent litter was not the primary source for Morchella nitrogen, and similar ¹³C and ¹⁵N enrichments to Plicaria suggest that Morchella assimilates its carbon and nitrogen from the same source pool as this saprotrophic fungus. From radiocarbon analyses Morchella averaged 11 ± 6 y old (n = 19), Plicaria averaged 17 ± 5 y old (n = 3), foliage averaged 1 ± 2 y old (n = 8) and Geopyxis (n = 1) resembled foliage in Δ¹⁴C. We conclude that morels fruiting in post-fire environments in our study assimilated old carbon and were saprotrophic.

• carbon sources
• ectomycorrhizal
• life history
• post-fire
• saprotrophy

Acknowledgments

Financial support for this research was provided by the USDA Forest Service, Pacific Northwest Research Station, and by the U.S. National Science Foundation, grant DEB-1146328. Mention of trade or firm names does not constitute an endorsement by the US Department of Agriculture. We thank Knut Kielland for boat transportation to the Tanana River site in Alaska and thank Steve Macko for stable isotope analyses. Tricia L. Wurtz contributed collections and initial ideas for the study. Tom Horton, Ari Jumpponen, Amy Rossman and Steve Trudell provided comments on an earlier version of the manuscript, and Ernst Linder and Greg Brenner provided statistical advice.