An assessment of carbon sequestration in Irish forests
using the C-Flow model
Kenneth A. Byrne1 and Ronald Milne2
1
Centre for Hydrology, Micrometeorology and Climate Change,
Department of Civil and Environmental Engineering,
University College Cork, Cork, Ireland.
k.byrne@ucc.ie
2
Centre for Ecology and Hydrology (Edinburgh),
Bush Estate, Penicuik, EH26 OQB, United Kingdom.
r.milne@ceh.ac.uk
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Abstract
It is generally acknowledged by the scientific community that increasing atmospheric levels
of greenhouse gases are causing climate change (IPCC 2001). This is expected to continue
with consequences for the water and nutrient cycles, ecosystems, food production,
infrastructure and property. The United Nations Framework Convention on Climate Change
(UNFCCC) and its Kyoto Protocol are the first international attempt to deal with climate
change. As a signatory of the UNFCCC, Ireland is required to develop an inventory of its
greenhouse gas emissions by sources and removals by sinks. Forestry is one category for
which an inventory is required. Furthermore, under the terms of the KP Ireland is committed
to limiting its greenhouse gas emissions to 13% above 1990 levels by 2008-2012. The Kyoto
Protocol allows carbon sequestration in soils and vegetation to be used to achieve
greenhouse gas reduction targets. Article 3.3 refers to net changes in carbon stocks that
occur during 2008-2012 as a result of afforestation, reforestation and deforestation which
has taken place since 1990. Article 3.4 refers to carbon sequestration due to forest
management. This covers all managed forests apart from those afforested since 1990 and
Ireland’s credit limit is set at 50,000 t C yr-1 during 2008-2012.
Ireland has a rapidly expanding forest estate which has grown from approximately 1% of the
land area at the beginning of the 20th century to a current level in excess of 10%. These
forests are mostly monocultural with Sitka spruce (Picea sitchensis (Bong.) Carr.) being the
dominant species. Overall conifers represent 78% of the forest estate with broadleaves and
mixed woodlands making up the balance. Until the mid-1980s the state was the dominant
investor in afforestation, however the introduction of government incentive schemes has
made private sector afforestation more attractive and currently 31% of forests are in private
ownership.
The main objective of this work was to use a dynamic carbon accounting model (C-Flow)
(Dewar and Cannell 1992, Cannell and Dewar 1995, Milne et al. 1998) to estimate the rate of
carbon accumulation in Irish forests during 1906-2002. The model was extrapolated forward
using the average rate of afforestation during 1989-2002. The main inputs to the model were
1) afforestation data 2) forest productivity data. These were as follows: Sitka spruce planted
in the period 1906 – 1989 was assumed to be Yield Class (YC) 16 m3 ha-1 yr-1. After 1990
this was increased to YC 20 m3 ha-1 yr-1 in order to reflect the move to more productive soils
during the 1990’s. All other conifers were assumed to have the growth characteristics of YC
8 m3 ha-1 yr-1 lodgepole pine (Pinus contorta Dougl.). Broadleaves were assumed to have the
growth characteristics of YC 6 m3 ha-1 yr-1 beech (Fagus sylvatica L.).
C-Flow estimated that in 1990 the total forest carbon stock was 22.9 Mt C. Trees accounted
for 50% of this with 37, 9 and 4% stored in soil, wood products and litter respectively.
Gallagher et al. (2004) have estimated that the total carbon stock in forest biomass was 18.1
Mt C in 1990. There are a number of methodological differences which may contribute to
this. For example, Gallagher et al. (2004) fells crops at 43 years. This is earlier than C-Flow,
which fells crops at the age of maximum mean annual increment. Hence C-Flow will have
higher equilibrium C stocks that the model of Gallagher et al. (2004). C-Flow also estimated
that in 2002 the total forest carbon stock had increased to 37.7 Mt C and that the overall C
stock increase in that year was 1.4 Mt C.
Extrapolating forward to 2008-2012 using the average annual rate of afforestation during
1990-2002 found that afforestation since 1990 would create an average annual carbon sink
of 2.93 Mt CO2 equivalent (0.8 Mt C yr-1) during 2008-2012 (Byrne and Milne, submitted).
Given that Ireland’s greenhouse gas reduction commitment under the Kyoto Protocol is
estimated to be 13.05 Mt CO2 equivalent yr-1 (Department of the Environment and Local
Government, 2000) it is clear that carbon sequestration under article 3.3 can make a large
contribution to this reduction. Variation in the rate of afforestation during 2003-2012 was
found to have little effect on the net carbon sink during 2008-2012. This is because forests
planted after 2002 would still be immature by 2008-2012 and would have relatively small
carbon inputs. Such forests would be important carbon sinks in post 2012 greenhouse gas
reduction commitments.
The potential of forest management activities to sequester carbon was estimated by running
C-Flow from 1906 to 2012 with afforestation discontinued after 1990. While the rate of
carbon sequestration remained steady during 1990-2000 at 0.98 Mt C yr-1 it declines
thereafter to 0.85 Mt C in 2008 and 0.71 Mt C in 2012. The average carbon sequestration
rate during 2008-2012 is 0.78 Mt C yr-1. Discounting this by 85% to allow for factors such as
age class distribution (the majority of the forests are young and actively growing) gives an
estimated carbon sink due to forest management of 0.12 Mt C yr-1. While this is greater than
the allowable limit of 0.05 Mt C yr-1 the rate of decline in the overall sink suggests that this
may not continue.
This study demonstrates that plantation forests in Ireland have considerable potential to
store carbon. This is due to the relatively young age of the forests (>50% of are less than 20
years old) and the generally high rate of productivity. Continued afforestation will be required
if plantation forests in Ireland are to continue to sequester more carbon. The ability of C-Flow
to accurately estimate carbon stocks and stock changes in Irish forests would be greatly
increased by adapting it to better reflect conditions in Irish forests. Examples of such
changes would be rotation length, biomass expansion factors and soil carbon stock changes,
particularly in the years before canopy closure.
Key words: carbon stocks, plantation forests, Kyoto Protocol
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Reference
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