Tree-ring Nitrogen Isotopes Show No Direct Response to Fire Frequency from a 55-year Prescribed Burn Experiment in Oak Savanna
Abstract
Fire frequency exerts a fundamental control on productivity and nutrient cycling in savanna ecosystems. Fire often enhances short-term nitrogen (N) availability to individual plants, but repeated burning can decrease soil organic matter and N availability. However, these effects remain poorly understood in savannas due to limited long-term biogeochemical data. Here, we leveraged one of the longest running prescribed burn experiments (established in 1964) to evaluate how fire frequency influences N availability across space and time. We developed multiple stable nitrogen isotope ( δ 15 N) records across a burn frequency gradient from precisely dated Quercus macrocarpa tree-rings in an oak savanna at Cedar Creek Ecosystem Science Reserve, Minnesota, U.S.A. Sixteen trees were sampled across four treatment units that varied in temporal onset of burning and burn frequency, ranging from an unburned control to a unit burned four out of five years during the past 55 years. Given that wood δ 15 N integrates several N cycling processes, we hypothesized that measured declines in N stocks and net N mineralization rates in frequently burned units would result in corresponding wood δ 15 N declines in these experimental units. Overall, wood δ 15 N has declined at this site for the last 115 years, although specific trajectories varied across sampled units. In contrast, wood δ 15 N records within each unit were remarkably coherent in their mean state and trend through time. A large, temporally synchronous decline in wood δ 15 N occurred in the mid 20 th century in three out of four units. However, this decline did not systematically coincide with the beginning of the burn experiment and we found limited evidence for variation in δ 15 N of tree-rings due to long-term fire frequency after synthesizing data across experimental burn units.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP23F1720T
- Keywords:
-
- 0424 Biosignatures and proxies;
- BIOGEOSCIENCES;
- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCES;
- 1620 Climate dynamics;
- GLOBAL CHANGE;
- 4950 Paleoecology;
- PALEOCEANOGRAPHY