Decoding Tree Rings: Can They Unlock Secrets of Nitrogen Pollution?
"New research explores how long-term nitrogen deposition impacts tree growth, offering insights into forest health and productivity in a changing world."
The rise in nitrogen (N) deposition, caused by burning fossil fuels and fertilizer use, is a major environmental change. While it's known to boost tree growth, the long-term effects on tree health remain unclear. Factors like climate change and rising CO2 levels further complicate the picture.
Just as analyzing leaf traits helps us understand plant function, studying wood traits is gaining importance. Nitrogen levels affect photosynthesis and growth rates in trees, but a lack of comprehensive models linking nitrogen and wood characteristics has limited long-term research.
Recent research published in Tree Physiology aims to bridge this gap. Borghetti et al. (2016) investigated how environmental factors, including nitrogen deposition, influence wood traits like tree-ring width and xylem anatomy (conduit area and density).
What Tree-Ring Nitrogen Isotopes Reveal
Anthropogenic nitrogen deposition has more than doubled reactive nitrogen inputs across large areas of the tropics, according to the 2015 study. Elevated δ15N in some tropical trees’ growth rings has been proposed as a sign of increased leaching of 15N-depleted nitrate after nitrogen deposition. However, the study found that the temporal δ15N trend in Bolivian trees using the radial method was primarily caused by tree ontogeny, making a change in nitrogen cycling an unlikely explanation there. A separate 70-year study reported total nitrogen deposition of 472 kg/ha at its southernmost site and 123 kg/ha at its northernmost, while tree-ring δ15N ranged from −5.8 to +1.3 and was higher at northern sites.
Tree Rings as Pollution Indicators
A 2026 study title describes using nitrogen stable isotopes (δ15N) in tree rings to reconstruct air pollution in urban areas, alongside PCBs, PAHs, nitro-PAHs, and measurements of CO, NO2, PM10, PM2.5, and SO2. The listed material identifies the proposed application but does not provide results or establish how reliably the method performs. A research proposal by Sheppard describes comparing ring width and soil nitrogen availability at tree scale in subalpine forests, including sites downwind of Los Angeles with heavy nitrogen deposition and a site in Baja California presumed to have ambient deposition. A 2025 source is explicitly about strengths and limitations of indicators for assessing forest nitrogen status, but its supplied snippet gives no specific findings.
The Promise and Challenges of Tree-Ring Isotopes
A 2022 chapter, “Nitrogen Isotopes in Tree Rings—Challenges and Prospects,” describes tree-ring δ15N series as archival systems used to investigate environmental change. It notes that questions remain about the isotopic integrity of the records and whether observed trends are reproducible. A 2022 study titled “Changes in forest nitrogen cycling across deposition gradient revealed by δ15N in tree rings” aimed to use tree-ring isotopes to examine changes in forest nitrogen cycling across a deposition gradient. The supplied source material does not report that study’s results.
Nitrogen's Tricky Impact: More Isn't Always Better
The study by Borghetti et al. (2016) used a meta-analysis to explore the relationship between nitrogen deposition and xylem traits across various species and environmental conditions. Surprisingly, they found no consistent long-term trends in tree-ring width that would suggest a growth boost from increased CO2.
- The study suggests that nitrogen deposition may improve hydraulic efficiency in trees.
- It highlights the complexity of the relationship between nitrogen and tree growth.
- It emphasizes the need for long-term data and quantitative analysis of wood anatomy.
Nitrogen Addition and Forest Responses
A review on nitrogen and phosphorus addition in forest ecosystems discusses how nutrient availability can constrain net primary productivity and influence terrestrial carbon storage. Its supplied excerpt says that modelled productivity gains are lower when nitrogen and phosphorus availability limits are considered than when those nutrients are assumed unlimited. A study in a subtropical forest in South China examined leaf and xylem growth traits in Castanopsis chinensis and Schima superba under canopy nitrogen addition of 0 or 50 kg N ha−1 year−1. A study of Mediterranean tree species notes that evidence on functional-trait responses to nitrogen deposition in Mediterranean forests remains scarce.
Nitrogen Cycling Across Changing Conditions
A 2022 review considers how climate extremes, including droughts and floods, may alter carbon and nitrogen cycling in forest ecosystems. It also identifies increased atmospheric CO2 and warming-induced water limitation as factors that may affect those cycles. The review focuses on stable nitrogen isotope composition (δ15N) in tree rings as a means of examining variation and trends in forest nitrogen cycling. The supplied material provides no direct comparative results between forests or deposition levels.
Unlocking the Past: What Tree Rings Can Tell Us
Analyzing tree rings and xylem anatomical traits has been crucial in understanding the impact of temperature and drought on tree growth. By studying carbon isotopes in tree rings, scientists can also infer long-term changes in iWUE.
Tracking Long-Term Nitrogen-Cycle Change
A 2026 review describes nitrogen as an essential nutrient and a key limiting factor in ecosystem productivity, affecting ecosystem structure and function. It reports that anthropogenic nitrogen deposition has substantially altered terrestrial nitrogen cycling over the past fifty years. The review lists associated effects including soil acidification, nutrient imbalances, and changes in carbon sequestration. These impacts frame the need to understand nitrogen cycling in forest ecosystems over the long term.
Rising CO2 and Boreal Forest Nitrogen
A 2026 study reports that nitrogen-isotope tree-ring chronologies indicate rising atmospheric CO2 has reduced nitrogen availability in boreal forests in Sweden. The source presents this as evidence suggesting elevated CO2 is affecting forest nitrogen availability. The supplied material does not quantify the reduction or describe its consequences for people or forest management.
However, determining the effects of increased nitrogen deposition on forest productivity presents unique challenges. Nitrogen concentrations in woody tissues are typically low and vary significantly between species and tissues. Retrospective analysis of stable nitrogen isotopes (15N/14N, δ15N) in tree-ring series offers a promising avenue for studying the long-term effects of nitrogen availability.
Borghetti et al. (2016) have pioneered the use of wood-anatomical traits to understand the effects of long-term nitrogen deposition on wood function, tree growth, and forest productivity. Future studies should address existing caveats (tree size and ontogeny, plasticity, measuring and laboratory protocols) and incorporate other proxies of tree functioning, such as δ15N in tree rings. By distilling information from wood, we can gain a better understanding of tree growth and functioning, enabling us to manage forests more effectively in the face of environmental change.