Fire frequency exerts a fundamental control on productivity and nutrient cycling in savanna ecosystems. Individual fires often increase short-Term nitrogen (N) availability to plants, but repeated burning causes ecosystem N losses and can ultimately decrease soil organic matter and N availability. However, these effects remain poorly understood due to limited long-Term biogeochemical data. Here, we evaluate how fire frequency and changing vegetation composition influenced wood stable N isotopes (15N) across space and time at one of the longest running prescribed burn experiments in the world (established in 1964). We developed multiple 15N records across a burn frequency gradient from precisely dated Quercus macrocarpa tree rings in an oak savanna at Cedar Creek Ecosystem Science Reserve, Minnesota, USA. Sixteen trees were sampled across four treatment stands that varied with respect to the temporal onset of burning and burn frequency but were consistent in overstory species representation, soil characteristics, and topography. Burn frequency ranged from an unburned control stand to a high-fire-frequency stand that had burned in 4 of every 5 years during the past 55 years. Because N stocks and net N mineralization rates are currently lowest in frequently burned stands, we hypothesized that wood 15N trajectories would decline through time in all burned stands, but at a rate proportional to the fire frequency. We found that wood 15N records within each stand were remarkably coherent in their mean state and trend through time. A gradual decline in wood 15N occurred in the mid-20th century in the no-, low-, and medium-fire stands, whereas there was no trend in the highfire stand. The decline in the three stands did not systematically coincide with the onset of prescribed burning. Thus, we found limited evidence for variation in wood 15N that could be attributed directly to long-Term fire frequency in this prescribed burn experiment in temperate oak savanna. Our wood 15N results may instead reflect decadal-scale changes in vegetation composition and abundance due to early-to mid-20th-century fire suppression.
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Acknowledgements. We thank the editor, Edzo Veldkamp, and the reviewers, Peter Hietz and Rodica Pena, whose critiques and suggestions improved the paper. This work was supported by grants from the US National Science Foundation Long-Term Ecological Research Program (LTER), including grant nos. DEB-0620652 and DEB-1234162. This research was also supported by grant no. NSF-DEB-1655148 to Kendra K. McLauchlan, grant no. NSF-DEB-1655144 to Daniel Griffin, and the NSF REU program and the University of Minnesota Undergraduate Research Opportunities Program to Matthew L. Trumper. Fire frequency treatments have been maintained with support from the Cedar Creek LTER program. Further support was provided by the Cedar Creek Ecosystem Science Reserve and the University of Minnesota. Daniel Acker-man, Kate Carlson, Daniel Crawford, Mara McPartland, and Madison Sherwood assisted with collecting tree core samples. Ryan Mat-tke and the UMN Borchert Map Library provided access to historical aerial imagery. Jack Dougherty assisted with tree core specimen preparation, and Robin Paulman conducted the isotopic analysis at CASIF. We thank Kurt Kipfmueller for helpful discussions.
Financial support. This research has been supported by the Na-
tional Science Foundation, Division of Environmental Biology (grant nos. 0620652, 1234162, 1655148, and 1655144).