Nitrogen accumulation accompanies ectomycorrhiza formation in pine germinants the first growing season after wildfire or clearcutting.

Abstract

Early stages of the ectomycorrhizal symbiosis have rarely been studied on seedlings germinating in the field. By collecting lodgepole and ponderosa pine seedlings during their first growing season in recent clearcuts and burned areas, we were able to identify when colonization of pine roots first began, the rate at which ectomycorrhizal fungi colonized new germinants, and how this related to nitrogen nutrition and growth. Pine seedlings were first colonized in July, a month after germination was first observed. As the first seedlings became mycorrhizal, ectomycorrhizal lodgepole pine seedlings contained approximately 40% more nitrogen and > 60% greater biomass compared to uncolonized seedlings collected at the same time. Nitrogen content was 47% higher in mycorrhizal than nonmycorrhizal naturally-regenerating ponderosa pine seedlings. Ascomycetes, with a Pustularia sp. and Wilcoxina spp. most abundant, formed 80% of the ectomycorrhizae. Because all collected seedlings had ectomycorrhizae present on their roots by the end of the season, we concluded that inoculum of ectomycorrhizal fungi, especially of ruderal ascomycetes, was not limiting colonization of seedlings on these severely burned or recently clearcut sites. Our results are consistent with a role for ectomycorrhizal fungi in nitrogen acquisition, even within the first weeks after mycorrhiza formation; however, it is also possible that larger, more nitrogen-replete seedlings became colonized earlier than smaller seedlings. We saw no evidence of nitrogen loss by mycorrhizal pine seedlings as observed in previous studies.