Edgar Karofeld

Mud-bottom hollows: exceptional features in carbon-accumulating bogs?

Mud-bottom hollows are depressions on the bog surface where Sphagnum mosses have died and peat accumulation is retarded or even replaced with loss by oxidation. Results of measurements carried out at Männikjärve Bog, central Estonia, con” rmed that peat accumulation has stopped and that in 1999–2001 the uppermost 2–3 cm thick surface layer of mud-bottom hollows became thinner by 1.95 6 0.75 mm yr2 1 (n = 188). Different methods revealed the corresponding carbon loss from the mud-bottom hollows 25 mm thick surface layer as 50–60 g C m2 2 yr2 1. As a result, the surface of mud-bottom hollows becomes lower as compared to surroundings with peat accumulation c. 1.5 mm yr2 1, and they are likely to have an important role in the differentiation of bog microtopography. Owing to the combination of a cessation in peat accumu lation, carbon loss by oxidation and increased emission of decomposition gases, mud-bottom hollows could have an important in‘ uence on the carbon budget of bogs.

How Does Tree Density Affect Water Loss of Peatlands? A Mesocosm Experiment

Raised bogs have accumulated more atmospheric carbon than any other terrestrial ecosystem on Earth. Climate-induced expansion of trees and shrubs may turn these ecosystems from net carbon sinks into sources when associated with reduced water tables. Increasing water loss through tree evapotranspiration could potentially deepen water tables, thus stimulating peat decomposition and carbon release. Bridging the gap between modelling and field studies, we conducted a three-year mesocosm experiment subjecting natural bog vegetation to three birch tree densities, and studied the changes in subsurface temperature, water balance components, leaf area index and vegetation composition. We found the deepest water table in mesocosms with low tree density. Mesocosms with high tree density remained wettest (i.e. highest water tables) whereas the control treatment without trees had intermediate water tables. These differences are attributed mostly to differences in evapotranspiration. Although our mesocosm results cannot be directly scaled up to ecosystem level, the systematic effect of tree density suggests that as bogs become colonized by trees, the effect of trees on ecosystem water loss changes with time, with tree transpiration effects of drying becoming increasingly offset by shading effects during the later phases of tree encroachment. These density-dependent effects of trees on water loss have important implications for the structure and functioning of peatbogs.

Drastic Turnover of Bryophyte Vegetation on Bog Microforms Initiated by Air Pollution in Northeastern Estonia and Bordering Russia

Human influence on bogs, including air pollution, causes changes in vegetation leading to the degradation of an ombrotrophic bog ecosystem into a more uniform transitional mire-like system. We have hypothesized that intensive atmospheric alkaline pollution will cause an increase in water pH and convergence of bryophyte species composition among microforms. We also expected that bog-specific acidophilic species will be replaced by species indigenous to neutral pH habitats. Through GLM and DCA analyses, we found that although natural acidic bogs are more species poor than polluted bogs, the increase in pH can lead to a decrease in bog-specific vegetation. In polluted bogs, the species composition in different bog microforms will become similar; in particular bog-specific Sphagnum mosses will be increasingly replaced by more tolerant brown mosses, particularly in lawns.

Latitudinal limits to the predicted increase of the peatland carbon sink with warming

The carbon sink potential of peatlands depends on the balance of carbon uptake by plants and microbial decomposition. The rates of both these processes will increase with warming but it remains unclear which will dominate the global peatland response. Here we examine the global relationship between peatland carbon accumulation rates during the last millennium and planetary-scale climate space. A positive relationship is found between carbon accumulation and cumulative photosynthetically active radiation during the growing season for mid- to high-latitude peatlands in both hemispheres. However, this relationship reverses at lower latitudes, suggesting that carbon accumulation is lower under the warmest climate regimes. Projections under Representative Concentration Pathway (RCP)2.6 and RCP8.5 scenarios indicate that the present-day global sink will increase slightly until around ad 2100 but decline thereafter. Peatlands will remain a carbon sink in the future, but their response to warming switches from a negative to a positive climate feedback (decreased carbon sink with warming) at the end of the twenty-first century.Analysis of peatland carbon accumulation over the last millennium and its association with global-scale climate space indicates an ongoing carbon sink into the future, but with decreasing strength as conditions warm.