Differences in CO2 dynamics between successional mire plant communities during wet and dry summers

Questions: What impact do a wet and a dry growing season have on CO₂ dynamics of mire plant communities along a primary succession gradient from the initiation stage to the bog stage? Location: Mires on a land uplift coast, Finland. Methods: We measured CO₂ dynamics and vascular plant green area development on five mires that form a sequence of mire succession. TWINSPAN was used to define successional mire plant communities and regression analyses were used to explore the temporal variation in CO₂ dynamics of the communities. Results: CO₂ dynamics of successional plant communities reacted differently to a wet and a dry growing season. The net CO₂ uptake rate of the earlier successional communities decreased in the dry growing season due to a decrease in photosynthesizing leaf area. Concurrently, CO₂ uptake of the later successional communities moderately increased or did not change. Generally, the difference in net ecosystem exchange (NEE) between a dry and a wet year resulted from the altered rate of gross photosynthesis (P_G) rather than ecosystem respiration (R_E). Conclusions: Critical factors for the more stable carbon (C) gas dynamics in the later stages of mire succession were (1) higher autogenic control of the physical environment and (2) an increase in the number of factors regulating the P_G rate. These factors may buffer mire ecosystems (in terms of the C sink function) from extreme and unfavourable variations in environmental conditions.