Climate change affects carbon exchange processes in northern peatlands
Jukka Laine
Peatland Ecology Group
Department of Forest Ecology
University of Helsinki
The importance of peat accumulating wetlands to global climatic change is via the
large carbon store accumulated in them over the millennia. The estimate for the longterm carbon accumulation rate for boreal and subarctic bogs is given as ca. 21 and for
fens as 17 g m2 yr-1. However, the delicate relationship of carbon balance on climatic
variations indicates that the past, long-term accumulation rates cannot be taken for
granted even in the near future. Global climatic warming affects peatlands mainly
through spatial and temporal changes in water balance components. Increased
evapotranspiration, altered precipitation and increased frequency of extreme events
(droughts) result in deeper water table levels during the growing season. Impacts of
hydrological changes on the ecosystem are mediated through autotrophic (vegetation)
and heterotrophic (microbes) communities.
The climate change impacts on the peatland ecosystems can be assessed using
paleoecological data showing the historical links between ecosystem and past
climates, using simulations of peatland ecosystem models, or acquiring experimental
information of the ecosystem responses to proposed changes.
Recently, experiments on climate change impacts were started in Finland (University
of Helsinki, Department of Forest Ecology and Department of Biological and
Environmental Sciences; Finnish Forest Research Institute, Joensuu Research Centre),
in Canada ( Université Laval, Dept. of Phytologie; McMaster University, School of
Geography and Geology; University of Waterloo, Department of Geography, and in
Ireland (University College Cork, Civil&Environmental Engineering; University
College Dublin, Forest Ecology Research Group).
The preliminary results from the ongoing research show that the photosynthesis of
plant communities, adapted to wet conditions (fens) is sensitive to temporal variation
in water table level, and drawdown in water table causes a decrease in photosynthetic
rates. The ombrotrophic bog communities appear to be less sensitive.
The heterotrophic respiration increases markedly in dryer conditions in both fens and
bogs. Increased respiration, together with decreased photosynthesis lead to negative
carbon balance soon after water level draw down. It is, however, possible that the
plant community structures adapt to new conditions, and a positive C balance, typical
of pristine peatlands may be reached in the near future.
Experiences from peatlands that have been drained for production forestry in Finland,
show that carbon accumulation into peat soil may continue, and even at a faster rate
after water level drawdown. This is especially true in the case of more nutrient poor
peatlands where the height growth of the moss layer still continues after drainage.
The reasons behind these findings may be attributed to following factors as concluded
in a recent research report:

During their Holocene-long persistence, northern peatlands have developed an
inherent autogenic changeability, and react fast to changing hydrological
conditions. It is important to account for the changes in plant-mediated organic
matter and nutrient fluxes when evaluating ecosystem response to changing
environmental conditions.

The main adaptation mechanism to long-term alteration of water-level is
change in species composition and relative abundance. Long-term water-level
drawdown induces a decline in graminoids and proliferation of shrubs and/or
trees. Mosses retain their functional importance after adaptation at the
community level.

Consequently, the tissue type composition of litterfall changes. The litter
material of dried-out sites is largely slowly decomposable, which will
counteract the enhancing effect of water-level drawdown on decomposition
processes.

Changes in physico-chemical environment also partially counteract the effects
of increased aeration on decomposition of organic matter. These changes
include increased acidity and decreased soil temperatures
Perhaps the most serious threat to boreal peatlands will be the predicted increase in
fire frequencies, especially so in the continental parts of North America and Eurasia.
This is also true for tropical peatlands, connected to El Niño events. The removal of
carbon to the Atmosphere in fires may overshadow the other climate change impacts.
The present knowledge of the global change impacts on peatlands will be gathered in
the “Assessment on Peatlands, Biodiversity and Climate Change” as part of a UNEPGEF Funded Project “Integrated Management of Peatlands for Biodiversity and
Climate Change” coordinated by the Global Environment Centre in Kuala Lumpur
during the next few years.