Reconstructing Nitrogen Availability in US Forests Using Tree-Ring Chronologies
Broadly, the Novus RCN addresses the impacts of disturbance on ecosystems. A disturbance event, as the name implies, is a temporary change in environmental conditions which disturbs the functioning of the ecosystem (such as landscape function, plant composition, stream chemistry, or elemental cycling). Novus focuses on five types of disturbance events: wind, fire, insects, drought, and mass movements. Climate change predictions anticipate changes in the type, magnitude, and frequency of disturbance events, making it important for us to understand the impacts disturbance can cause at the ecosystem scale. The Novus RCN combines research on current (neo-ecological) and past (paleo-ecological) ecosystems, utilizing sediment chronologies, tree ring chronologies, chronosequences, and long-term instrumental records in order to assess the importance of changing severity and frequency of disturbance events, as well as to deepen our knowledge of cascading impacts of single disturbance events.
My role in the Novus RCN relates primarily to nitrogen isotope analysis. Dr. McLauchlan and I recently published a review of the existing literature on dendroecological work with nitrogen isotopes. There are currently about 50 publications of wood nitrogen isotope records (termed δ15N) published in over 30 journals. These publications are disparate in methodology, study design, and interpretation of data. Consequently, our review focused on recommendations for unifying methodologies and identified major existing gaps in the available data.
Dr. McLauchlan and I hope to address some of these gaps by expanding wood δ15N records – both spatially (through a continental scale sampling of trees throughout the contiguous United States) and temporally (by accessing several repositories of wood from the mid-Holocene and the Pleistocene). We also hope to address lingering questions relating to the temporal fidelity of wood N records (due to nitrogen translocation across rings) and the replicability of wood N measurements across varying sample preparation and analytical equipment protocols.
At this point, we are focusing on the continental scale project (nicknamed ‘Continental N’). Our data for this project comes from a variety of sources:
1) Published data: we first mined the literature for data to include in this project. Unfortunately, much of the wood N work in the United States has focused on nitrogen fertilization (through experimental or anthropogenic N inputs), and even natural abundance records often were too short of a timeline to be useful for us (we require at least 60-year chronologies). Consequently, only two published records were included in our study, and both are Kendras – from Indiana and New Hampshire.
2) ‘Orphaned’ data: Kendra and several other researchers have wood δ15N datasets that have not been published, which we call ‘orphaned’ datasets. In some cases, these datasets may yet be published on their own in the future. In other cases, they were too small or not compelling enough to stand alone in a publication. Orphaned datasets were obtained for Kansas, Minnesota, Oklahoma, California, Virginia, Maryland, Colorado, and West Virginia.
3) Samples provided or collected by colleagues: Kendra and I tapped our scientific and personal networks to get assistance collecting samples from some of the more distant sites. In some cases, our colleagues had previously-collected samples already archived in their labs, which were loaned to us. In others, our colleagues, friends, and family hiked out to remote forests throughout the United States and cored trees for us. We are greatly appreciative of all the hard work that was ‘donated’ to this project, and I have a lot of science karma to pay back for all the help! Our network provided samples from Arizona, Arkansas, Florida, Idaho, Illinois, Kentucky, Maine, Massachusetts, Nevada, New Jersey, New Mexico, New York, Ohio, Oregon, Pennsylvania, South Carolina, Tennessee, and Utah.
4) Samples obtained by Paleoenvironmental Lab members: We didn’t pawn off *all* the work on our friends though! Paleo Lab undergrad Emily Sutton and I completed an 8-day, 14-state, 130 sample collecting trip over spring break in 2013. I also drove through the desert southwest at the end of May, and with the assistance of my in-laws, sampled an additional 3 states in the desert southwest. You can review the details of these trips in my field blog. Other sites were sampled by Kendra and by Paleo Lab PhD students Kyleen Kelly and Julie Commerford. Paleo Lab collecting provided samples from Alabama, Arizona, California, Georgia, Iowa, Louisiana, Mississippi, Missouri, Nevada, North Carolina, North Dakota, South Dakota, Texas, New Mexico, Montana, Wisconsin, and Wyoming.
Many of the sampling locations are part of the Organization of Biological Field Stations (OBFS), The National Atmospheric Deposition Program (NADP), and/or the Long-Term Ecological Research network (LTER). Each of these networks maintains long-term data on environmental conditions (temperature and precipitation) and/or atmospheric nitrogen deposition. By sampling near these stations, we can link our tree-ring nitrogen trends to environmental parameters that may be driving ecosystem nitrogen dynamics.
We are currently writing up the data for publication and plan to submit it soon!
Last updated September 2016