Biomass allocation and canopy development in spruce model ecosystems under elevated CO₂ and increased N deposition

Area of Science:

• Forest Ecology
• Plant Physiology
• Environmental Science

Background:

• Ecosystem-level experiments are crucial for understanding forest responses to rising atmospheric CO2 and nitrogen (N) deposition.
• Spruce (Picea abies) model ecosystems were used to investigate these effects on nutrient-poor montane forest soil.

Purpose of the Study:

• To assess the impact of elevated CO2 and N deposition on forest ecosystem biomass, allocation, and leaf area.
• To determine if CO2 enrichment leads to increased ecosystem biomass production under competitive conditions.

Main Methods:

• Nine model ecosystems, each with spruce trees and a herbaceous understory, were subjected to varying CO2 concentrations (280, 420, 560 μl l-1) and N deposition rates (0, 30, 90 kg ha-1 year-1) for 3 years.
• Measurements included total ecosystem biomass, biomass allocation (roots vs. aboveground), leaf mass ratios (LMR), leaf area ratios (LAR), and leaf area index (LAI).

Main Results:

• Total ecosystem biomass increased with N deposition but was unaffected by CO2 concentration.
• Elevated CO2 led to increased root biomass and decreased leaf mass ratios (LMRs) and leaf area ratios (LARs).
• Leaf area index (LAI) decreased significantly with elevated CO2, while N deposition did not alter it. Aboveground N content declined with CO2 enrichment but increased with N deposition.

Conclusions:

• Under competitive conditions on natural soil, CO2 enrichment may not enhance ecosystem biomass production, whereas N deposition is likely to increase it.
• Reduced LAI under elevated CO2 suggests down-regulation of photosynthetic carbon uptake at the canopy level.
• Declining tree nitrogen mass with elevated CO2 may indicate reduced soil N availability.