Reliability of Stable Nitrogen Isotope Dendrochronologies for Long-Term Ecological Reconstruction
Abstract
In recent decades, anthropogenic activities have doubled the amount of reactive nitrogen (N) available to ecosystems. However, the effects of increased N availability on ecosystem function are complex and not fully understood. In forests, stable N isotope analysis of tree ring chronologies offers a possible methodology for unraveling millennial-scale changes in forest ecosystem N cycling and plant responses to changing N dynamics. Currently, more than 50 studies have used tree-ring N chronologies to assess the impacts of ecosystem N perturbations from fertilization, pollution, exogenous marine sources, and disturbance. A key limitation in all of these studies is the potential for translocation of N across annual ring boundaries during the lifetime of the tree. Translocation has been documented in 15N-labeled tracer studies; however, the magnitude and extent of translocation remains uncertain. If translocation of N across annual ring boundaries is of sufficient magnitude to alter the interannual signature and/or decadal trajectory of annual ring δ15N, then stable N isotope chronologies of ring wood will not reliably reconstruct ecosystem N cycling. This study addresses the question of translocation by resampling previously-studied trees in order to quantify the magnitude of N movement across annual ring boundaries. During the 1985 growing season, three five-year-old ponderosa pine trees in Blodgett Forest, California were labeled with 15N by applying 15N-enriched urea to fine roots. In 1999, the then 19-year-old trees were cored and their δ15N ring chronologies were published by Hart and Classen (2003). 15N enrichment was documented in growth rings laid down prior to the fertilization event in 1985, providing the first documentation of N translocation in tree ring chronologies. In 2018, new cores were obtained from these trees (now 38 years old) and reanalyzed to determine the magnitude and direction of N translocation that has occurred during the last two decades. Preliminary results indicate significant movement of 15N-enriched compounds towards more recent annual growth rings corresponding to calendar years in the early 2010s. These results call into question the utility of tree ring δ15N for long-term ecosystem N cycling and forest responses to nutrient dynamics.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B44F..06G
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0439 Ecosystems;
- structure and dynamics;
- BIOGEOSCIENCES;
- 0466 Modeling;
- BIOGEOSCIENCES;
- 0476 Plant ecology;
- BIOGEOSCIENCES