Vascular Plant Responses to Elevated CO2 in a Temperate Lowland Sphagnum Peatland

Vascular plant responses to experimental enrichment with atmospheric carbon dioxide (CO₂), using MINIFACE technology, were studied in a Dutch lowland peatland dominated by Sphagnum and Phragmites for 3 years. We hypothesized that vascular plant carbon would accumulate in this peatland in response to CO₂ enrichment owing to increased productivity of the predominant species and poorer quality (higher C/N ratios) and consequently lower decomposability of the leaf litter of these species. Carbon isotope signatures demonstrated that the extra 180 ppmv CO₂ in enriched plots had been incorporated into vegetation biomass accordingly. However, on the CO₂ sequestration side of the ecosystem carbon budget, there were neither any significant responses of total aboveground abundance of vascular plants, nor of any of the individual species. On the CO₂ release side of the carbon budget (decomposition pathway), litter quantity did not differ between ambient and CO₂ treatments, while the changes in litter quality (N and P concentration, C/N and C/P ratio) were marginal and inconsistent. It appeared therefore that the afterlife effects of significant CO₂-induced changes in green-leaf chemistry (lower N and P concentrations, higher C/N and C/P) were partly offset by greater resorption of mobile carbohydrates from green leaves during senescence in CO₂-enriched plants. The decomposability of leaf litters of three predominant species from ambient and CO₂-enriched plots, as measured in a laboratory litter respiration assay, showed no differences. The relatively short time period, environmental spatial heterogeneity and small plot sizes might explain part of the lack of CO₂ response. When our results are combined with those from other Sphagnum peatland studies, the common pattern emerges that the vascular vegetation in these ecosystems is genuinely resistant to CO₂-induced change. On decadal time-scales, water management and its effects on peatland hydrology, N deposition from anthropogenic sources and land management regimes that arrest the early successional phase (mowing, tree and shrub removal), may have a greater impact on the vascular plant species composition, carbon balance and functioning of lowland Sphagnum–Phragmites reedlands than increasing CO₂ concentrations in the atmosphere.