Spruce-fir forest changes during a 30-year nitrogen saturation experiment.

Area of Science:

• Ecology
• Forestry
• Environmental Science

Background:

• High elevation red spruce-balsam fir forests face decline.
• The nitrogen (N) saturation hypothesis proposes excessive N causes forest damage.
• Understanding N impacts is crucial for forest management and conservation.

Purpose of the Study:

• To test the nitrogen (N) saturation hypothesis in a red spruce-balsam fir ecosystem.
• To investigate the mechanisms behind forest decline attributed to N deposition.
• To assess the long-term effects of N fertilization and subsequent recovery.

Main Methods:

• Established a field experiment with control, low, and high N addition plots (0, 15.7, 31.4 kg N ha⁻¹ yr⁻¹).
• Applied N treatments annually from 1988 to 2010, with ongoing monitoring.
• Measured forest floor properties (C:N, N mineralization, nitrification), foliar N and Ca content, and tree basal area.

Main Results:

• Increased N addition led to decreased forest floor C:N, N mineralization, and spruce basal area.
• Elevated N addition resulted in increased spruce foliar N% and forest floor nitrification.
• Control plots accumulated N, while N addition plots lost N from the forest floor.
• Post-treatment, tree components normalized, but forest floor processes recovered slowly.
• Forest mortality on N addition plots coincided with drought and freeze injury years.

Conclusions:

• Nitrogen addition significantly altered forest floor dynamics and tree health, supporting aspects of the N saturation hypothesis.
• Ecosystem recovery after N fertilization cessation was slow, with forest floor processes lagging.
• Climate change (warming, drought) and other stressors interact with N deposition, complicating ecosystem response.
• Spruce-fir ecosystem regeneration is unlikely in the next century due to cumulative stressors, despite recent growth increases.