Carbon storage and sequestration
in the forests of Northern Ireland
M. G. R. CANNELL1, M. M. CRUICKSHANK2 AND D. C. MOBBS1
1
2
Institute of Terrestrial Ecology, Bush Estate, Midlothian, EH26 OQB, Scotland
School of Geosciences, The Queen's University Belfast, Belfast, BT7 INN, Northern Ireland
Summary
The rate of accumulation of carbon in forests and woodlands in Northern Ireland was estimated
using the record of forest planting since 1900 and a model that calculated the flow of carbon
from the atmosphere to trees, litter, soil, wood products and back to the atmosphere. It was
assumed that all conifer forests had the carbon accumulation characteristics of Picea sitchensis,
and upper and lower estimates of carbon storage were calculated assuming Yield Class 16 m3
ha"1 a"1 unthinned and Yield Class 14 m3 ha"1 a"1 thinned. Broadleaved woodlands were assumed
to have the carbon accumulation characteristics of Fagus sylvatica, Yield Class 6 m3 ha"1 a"1.
Northern Ireland currently has about 78 300 ha of forest, 83 per cent of which is coniferous, 77
per cent state-owned, mostly planted since 1945, with peak planting in 1960-1975. In 1990,
conifer forests contained 3—4 MtC (trees + litter) and broadleaved woodlands contained about
0.8 MtC (trees + litter + new forest soil). In 1990, conifer forests were sequestering 0.15-0.20
MtC a"1 and broadleaved woodlands about 0.025 MtC a"1. To maintain these sink sizes, new
conifer forests need to be planted at 1500-2000 ha a"1, and new broadleaved woodland at
100-150 ha a"1 in addition to full restocking. Current carbon sequestration by Northern Ireland
forests represents around 6.5-8.2 per cent of the total for UK forests and is greater per hectare
than in Britain because the average forest age is younger in Northern Ireland.
Introduction
The UK is party to the Framework Convention
on Climate Change, which includes a commitment to publish inventories of national emissions of greenhouse gases and removals by
sinks, and to take measures to protect and
enhance sinks in the UK. Forest plantations, or
managed forests, may currently represent a substantial CO2-carbon sink in many nations,
because of recent planting, under-harvesting or
recovery from fire (e.g. New Zealand, Maclaren
C Imitate of Chgirtcrcd Forejten, 1996
a n d W a k e l i n ) 1991. Fin]and;
Karjalainen et al.,
1995; Canada, Kurz et al., 1992). In Britain, the
current carbon sink represented by plantation
forests was estimated by Cannell and Dewar
(1995) to be about 2.5 MtC a"1, equivalent to
about 1.5 per cent of the total annual emission
of carbon in the UK (about 164 Mt Ca" 1 ). In
this paper, the results are reported of a parallel
study on the forests of Northern Ireland, using
the same carbon accounting model as before
(Dewar, 1991). Readers should refer to Cannell
Forestry, Vol. 69, No. 2, 19%
FORESTRY
156
2500V
•
•
•
•
Privately owned broadleaf
Privately owned conifer
State owned broadleaf
State owned conifer
Year
Figure 1. Approximate areas of forest that have been planted each year in Northern Ireland since 1900, divided
into privately and state owned broadleaved and coniferous forest. The data are largely based on forest areas
that exist at present of different ages and so slightly overestimate state-sector plantings since 1970 owing to
restocking (see Table 2 and text).
Table 1: Approximate cumulative areas of conifer and broadleaved forest in Northern Ireland in the period
1900 to 1993, divided between the state and private sectors (thousands of hectares). Only woods over 0.5 ha
are included. The areas attributed to the state sector before 1910 were inherited by the Forest Service when
it was founded in that year
State
Year
Conifer
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
1993
0.3
0.3
0.3
0.5
1.6
5.0
14.9
33.3
46.1
55.3
57.7
Totals
Private
Broadleaved
1.2
1.2
1.2
1.2
1,3
1.5
1.8
2.0
2.1
2.6
3.0
Conifer
0.3
0.4
0.5
0.5
0.7
1.0
1.9
3.5
4.4
6.4
7.4
Broadleaved
2.3
2.5
2.7
2.9
3.5
5.1
7.2
8.6
8.9
9.8
10.2
Conifer
0.6
0.7
0.8
1.0
2.3
6.0
16.8
36.8
50.5
61.7
65.1
Broadleaved
3.5
3.7
3.9
4.1
4.8
6.6
9.0
10.6
11.0
12.4
13.2
CARBON STORAGE AND SEQUESTRATION
157
Table 2: Areas of state owned forest (thousands of hectares) in Northern Ireland which were known to be newly planted (i.e. on previously
non-forested areas) in each decade, compared with the areas that still
exist in each decade age-class. Suggested reasons for the discrepancies
are given. Data on the newly-planted areas were supplied by the Northern Ireland Forest Service
Decade
Area of
new planting
Areas that existed Main reason for
of that age in 1993
difference
1940-1949
1950-1959
1960-1969
1970-1979
1980-1989
1990-1993
4.4
10.0
18.1
12.8
6.5
2.0
3.2
9.2
18.7
13.6
9.6
3.8
Totals
53.8
58.1
and Dewar (1995) or to Dewar and Cannell
(1992) for a full description of the model.
Scenario assumptions and their rationale
The model simulates the growth of even-aged
plantations that are clearfelled and then
replanted. Total carbon accumulation in the
trees is derived from stemwood volumes, and
the fate of that carbon is then tracked in litter,
soil organic matter, wood products and back to
the atmosphere. Four types of input data are
required for the model: (1) the areas of forest
that have been planted in the past, obtained
from historic records; (2) estimates of the average stemwood growth rates of the forests and
their average patterns of harvesting, obtained
from forest yield tables; (3) parameter values to
derive changes in foliage, branch and root carbon over time from stemwood volumes; and (4)
coefficients to calculate the rates of loss of carbon by decomposition of litter and soil organic
matter and decay of wood products. The major
assumptions made in the choice of these input
data for Northern Ireland are given below.
Not restocked
Not restocked
Not restocked
Restocking
Restocking
Restocking
Planted areas
Assumption: substantial new forest planting
began in 1930 and followed the pattern
shown in Figure 1.
The approximate increase in areas of conifer and
broadleaved forests in Northern Ireland were
constructed, for the state sector, from records
supplied by the Department of Agriculture for
Northern Ireland and, for the private sector,
from a Private Woodland Inventory (to 1975;
Graham, 1981) and from Annual Reports of the
Forest Service, which list the areas planted each
year as the areas grant-aided (from 1975) (Table
1). The following approximations were made.
First, the data in the Private Woodland Inventory included only forests and woodlands over
0.5 ha. Very small, privately-owned woods,
rows of trees, single trees and scrub woodland,
were excluded; they may contain substantial
amounts of carbon, but probably represent a
minor carbon sink. Second, state-owned areas
described as mixed-species were classed as
either conifer or broadleaved, according to
which species were dominant. Third, the Private
Woodland Inventory gave the total areas
planted in 1900-1929 and the totals planted in
5-year spans until 1975; it therefore had to be
158
FORESTRY
assumed that equal areas were planted each
year during each time-span. Fourth, it was
assumed that 29 per cent of forests planted privately in all years since 1975 were broadleaved,
this being the average percentage for grantaided private plantings in 1989—1993. And
lastly, in the calculation of harvested volumes,
allowance was made for the fact that 15 per
cent of the area may not be productive forest.
Figure 1 presents estimates of the areas of forest planted each year, based on the age structure
of the forests that are present now. These are
only estimates of new forest planting, because
double-counting occurs when areas are restocked
and underestimates occur when forests are not
restocked. We estimated the magnitude of those
errors for the state owned forests for the period
1940-1993 using annual new planting data
(conifers and broadleaves combined) from the
Forest Service (Jack, 1992). Table 2 shows that
the areas of forest that date from 1940 to 1959
are, in fact, less than the new areas planted in
those years, mainly due to clearfelling without
restocking, while the areas that date from 1960
to 1993 are greater than the new areas planted in
those years, mainly because of restocking. Overall, the difference between (1) the areas that exist
at present in different age classes (as in Figure 1)
and (2) the areas that were known to be new
plantings, was only 4.3 thousand hectares for
1940-1993 (Table 2, 58.1-53.8 thousand
hectares).
No attempt is made in this study to evaluate
the likely or possible future pattern of forest
planting in Northern Ireland. Instead, a range of
future planting scenarios were chosen in order
to estimate the consequences for future carbon
sequestration. The scenarios chosen were either
to stop all new planting or continue planting
50-200 ha a"1 of broadleaves and 500-2000 ha
a"1 of conifers. All scenarios assumed full
restocking following clearfelling.
Species and Yield Class
Assumption: all conifers may be regarded as
having the carbon storage characteristics of
Picea sitchensis (Bong.) Carr., and follow
growth and harvesting patterns that range
between Yield Class 14 m3 ha"1 a"1, subject to
intermediate thinning, and Yield Class 16 m3
ha"1 a"1, unthinned; and all broadleaves may
be regarded as having the carbon storage
characteristics of Fagus sylvatica L. Yield
Class 6 m3 ha"1 a"1 (Edwards and Christie,
1981).
About 80 per cent of the state and privatelyowned conifer forests in Northern Ireland consists of P. sitchensis. The other 20 per cent
consists of species such as Pinus sylvestris L. and
Pinus contorta Dougl., which have volume
growth curves that differ from those of P.
sitchensis in ways that are important silviculturally, but which Dewar and Cannell (1992)
showed have a minor impact on carbon storage.
The average Yield Class of conifers in the
1974/5 inventory in Northern Ireland was 14.8
m3 ha"1 a"1 (Purcell, 1977) and was quoted as
14-16 m3 ha"1 a"1 by Kean (1993). The values
for P. sitchensis range from 9—14 m3 ha"1 a"1 on
deep peats to 16-21 m3 ha"1 a"1 on gley soils,
and, according to Schaible (1992) there has been
an increase in Yield Class of plantations established over the last three decades of about 4 m3
ha"1 a"1 owing to improved nutrition. Older
stands were thinned, but because of the risk of
windthrow, about 70 per cent of the conifer
forests are now unthinned (McKenzie, 1976;
Phillips, 1980).
Clearly, different fractions of the areas
planted each year will have had different Yield
Classes and thinning histories, and both the
means and frequency distributions of Yield
Classes seem to have changed over time. In the
absence of comprehensive historic data on the
Yield Classes and silviculture of all state and
private forests, the approach taken in this study
was to calculate the range of possible carbon
storage values based on two simple scenarios.
These were to assume that all conifer forests
were P. sitchensis either at (1) Yield Class 14
subject to intermediate thinning—which was
the assumption made by Cannell and Dewar
CARBON STORAGE AND SEQUESTRATION
(1995) for conifer forests in Britain, but which
probably underestimated carbon storage in
Northern Ireland—or, (2) Yield Class 16 and
unthinned, which may have overestimated carbon
storage in older plantings, but have been closer to
the average for young and future plantations in
Northern Ireland. As in the previous study, further simplifying assumptions were made that
forests followed the growth curves modelled by
Edwards and Christie (1981), that all plantings
were clearfelled at the time of maximum mean
annual increment, and that there was no change
in Yield Class in successive rotations.
To be consistent with the previous study, we
assumed that all broadleaved forests had the
carbon storage charaaeristics of Fagus sylvatica
L. of Yield Class 6 m3 ha"1 a"1- clearfelled at age
92 years. Although beech is uncommon in
Northern Ireland, its carbon storage characteristics are similar to those of oak, ash and
sycamore (Dewar and Cannell, 1992). In fact,
the broadleaved forests in Northern Ireland
consist of a mixture of species, with an estimated mean Yield Class in pure stands in the
range 4.8—5.2 m3 ha"1 a"1 and a highly variable
rotation period.
159
It should be noted that one of the effects of
assuming that all conifer and all broadleaved
forests had the same Yield Class was that the
total forest area planted in any year followed
the same time course of carbon storage and was
harvested at the same time; this had the effect
of exaggerating annual fluctuations in the estimates of carbon storage.
Foliage, branch and root carbon
Assumption: the parameter values 4-8 given
in Table 3 apply to forests in Northern Ireland.
The green stemwood volumes derived from
yield tables were converted to total woody biomass carbon by multiplying by the stemwood
basic density, the mean fraction of carbon in
dry biomass (0.5) and mean fractions of the
total woody biomass occurring in branches and
woody roots. The parameter values used were
the same as those abstracted from the literature
by Dewar and Cannell (1992) for P. sitchensis
and F. sylvatica in Britain (Table 3) which probably apply equally in Ireland (Carey and Farrell,
1978; Carey and O'Brien, 1979). Foliage and
Table 3: Attributes of conifer and broadleaved forests assumed in the calculation of carbon storage in Northern Ireland. Two options were used for conifers, based on P. sitchensis Yield Class 16 unthinned and Yield
Class 14 thinned. (1 tonne = 1 Mg = 10*g)
Conifers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Rotation length (years)
Initial spacing (m)
Year of first thinning
Stemwood basic density (t m"3)
Maximum carbon in foliage (tC ha"1)
Maximum carbon in fine roots (tC ha"1)
Fraction of wood in branches
Fraction of wood in woody roots
Maximum foliage litterfall (tC ha"1 a"1)
Maximum fine root litter (tC ha"1 a"1)
Foliage decomposition (a"1)
Wood decomposition (a"1)
Fine root decomposition (a"1)
Soil organic matter decomposition (a"1)
Broadleaves
P. sitchensis
YC16
P. sitchensis
YC14
55
2.0
Unthinned
0.34
7.2
2.7
0.09
0.19
1.4
2.7
1.0
0.06
1.5
0.03
57
2.0
23
0.35
6.3
2.7
0.09
0.19
1.3
2.7
1.0
0.06
1.5
0.03
F. sylvatica
YC6
92
1.2
30
0.55
1.8
2.7
0.18
0.16
2.0
2.7
3.0
0.04
1.5
0.03
160
FORESTRY
fine root masses were assumed to increase exponentially to reach the maximum values given in
Table 3 at a quarter rotation age. Thinnings
were subtracted from the stand volumes and
convened to carbon mass as described by Cannell and Dewar (1995).
It should be stressed that greater uncertainty
and model sensitivity occurred in the choice of
parameter values 4-8 in Table 3 than in the
choice of Yield Class and thinning regimes
(Dewar and Cannell, 1992).
Litter, soil and wood product decomposition
Assumptions: (1) conifer forests increase the
amount of organic carbon in litter (including
the debris and stumps left after harvesting),
with the decomposition rates given in Table
3 (parameters 11-14), but do not increase the
amount of organic carbon in the soil, whereas
broadleaved forests increase the amount of
organic carbon in litter and soil, and (2) the
average lifetime of wood products is equal to
the rotation period.
Carbon was transferred to the litter pool from:
(1) foliage and fine roots, with annual input
rates increasing exponentially to reach parameter values 9 and 10 (Table 3) at the time of
canopy closure; (2) small inputs of woody litter
(Dewar and Cannell, 1992); (3) dead trees in
unthinned stands, and branches, foliage and
roots from thinnings in thinned stands; and (4)
branches, foliage, stumps and roots at the time
of clearfelling.
For each component of litter, it was assumed
that a constant fraction decomposed each year
(parameters 11-13, Table 3), half of which was
transferred to the soil organic matter pool,
which then decomposed at a slower rate (parameter 14). The decomposition rates in Northern
Ireland were assumed to be the same as in
Britain.
In the case of conifer forests, the carbon
transferred from the litter to the soil organic
matter pool was ignored. This was based on the
assumption that conifers have been planted primarily in organic soils and that disturbance and
drainage of these soils has increased the rate of
organic matter decomposition, offsetting the
addition of new organic matter to the soil as a
result of litter decomposition. In Northern Ireland, 64 per cent of the state forests planted up
to 1989 were on soils classed as peats (over 50
cm depth) or peaty (5-50 cm depth) (mainly in
Tyrone, Fermanagh and Antrim) and 33 per
cent of the forests in the Private Woodland
Inventory (to 1975) were classed as being on
peats or peaty soils. In the long term, the loss of
carbon following drainage of these peats and
peaty soils is likely to be at least as large as the
contribution of the forests to the soil, excluding
litter (Cannell et al., 1993); policy to meet the
Convention commitments will need to take this
into account. In recent years, economic and
environmental considerations have argued
against planting on deep peats (Jack, 1992).
Results
Planted areas
Table 1 shows that there was, in 1993, a total
of about 78 300 ha of forest in Northern Ireland, 83 per cent of which was coniferous (17
per cent broadleaved) and 77 per cent of which
was state-owned (23 per cent privately owned).
There were only 4100 ha of forests in 1900,
mostly broadleaved private woodlands. Very little planting occurred until after World War II,
when the state sector began its main programme
of conifer planting, with over 1500 ha a"1
planted in the period 1960-1975 (Figure 1). The
rate of conifer planting by the state sector has
averaged less than 1000 ha a"1 since 1975.
Because most planting has occurred in recent
decades, the forests are relatively young: 67 per
cent of the total forest present in 1993 had been
planted since 1960, 39 per cent since 1970 and
21 per cent since 1980.
Interestingly, most of the broadleaved planting has been done by the private sector (Table
1) and the private sector has planted relatively
large areas of both conifer and broadleaved
trees in recent years (1990-1993, Figure 1; see
CARBON STORAGE AND SEQUESTRATION
-23
161
00
2000
1500
15- -20
3 '
1000
-15
10-10
5-3
I ' ' ' ' I '
1900
1925
'
1950
1975
I '
2000
2025
2050
2075
2100
Year
Figure 2. Modelled estimates and extrapolations of the cumulative amounts of carbon stored in forests in
Northern Ireland, based on the historic planting record (Figure 1) and theoretical scenarios of future new
planting after 1995 (in hectares per year of new forest, assuming full restocking), (a) Conifer forests (assumed
to be P. sitchensis): the carbon stored in trees and tree litter. The outside axis (0-20 X 10* kgC) refers to estimates based on the assumption that all conifer forests arc Yield Class 14 and subject to intermediate thinning;
the inside axis (0-25 X 109 kgC) refers to estimates based on the assumption that all conifer forests are Yield
Class 16 and unthinned. (b) Broadleaved woodlands (assumed to have the carbon storage characteristics of F.
sylvatica): the carbon stored in trees, tree litter and soil derived from tree litter.
Discussion). However, it should be noted that
the apparent 5-yearly bursts of broadleaved tree
planting by the private sector between 1930 and
1975 are largely an artifact, because the planting data were in 5-yearly totals (see above).
Amounts of carbon stored in trees, litter and
soil
Figure 2 presents estimates of the amount of
carbon added to the land surface as a result of
planting forests in Northern Ireland. The figures
for conifer forests (Figure 3a) include tree litter,
but assume no net gain in soil organic carbon;
two values are given on the y-axis, the smaller
value gives the lower boundary estimate, assuming Yield Class 14 and thinning, the larger value
gives the upper boundary estimate, assuming
Yield Class 16 and no thinning. The data for
broadleaved woodlands (Figure 3b) include the
trees, tree litter and soil carbon derived from
tree litter.
It is estimated that, in 1990, 3—4 million
tonnes of carbon (MtC = 109 kgC) will be
162
FORESTRY
-0.1
1900
1923
2100
0.06-,
1900
1923
2073
2100
Figure 3. Modelled estimates and extrapolations of the rates of increase (or decrease) in the amount of carbon stored in the forests and their wood products in Northern Ireland, with different scenarios of new planting after 1995 as in Figure 2. Positive values indicate the amounts of carbon removed from the atmosphere
each year (the size of the carbon sink); negative values indicate a net emission of carbon to the atmosphere,
(a) Conifer forests (assumed to be P. sitchesis): the carbon pool includes trees + litter + products. The outside axis (0-0.3 X lO'kgC a"1) refers to estimates based on the assumption that all conifer forests are Yield
Class 14 and subject to intermediate thinning; the inside axis (0-0.4 X 10*kgC a"1) refers to estimates based
on the assumption that all conifer forests are Yield Class 16 and unthinned. (b) Broadleaved woodlands
(assumed to have the carbon storage characteristics of F. sylvatica): the carbon pool includes trees + litter +
soil derived from tree litter + products.
stored in conifer forests in Northern Ireland.
With no further new planting (but full restocking), this figure will rise to 6-7 MtC by about
2015 and with continued restocking the equilibrium storage will vary between about 5 and 7
MtC. Equivalent estimates for broadleaved
woodlands are 0.8 MtC in 1990, rising to an
equilibrium storage of about 1.9 MtC by 2040.
With modest rates of new conifer planting, of
1000 ha a"1, the carbon store in conifer forests
would increase to 12-15 MtC by 2100, while
continued planting of 100 ha a"1 of new
broadleaved woodland would increase the carbon stored in broadleaved woodlands to about
3 MtC by 2100.
Carbon removal from the atmosphere
Figure 3 gives the net annual flux of carbon
between the atmosphere and the forests plus
their products. Positive values are the amounts
of carbon removed annually from the atmosphere and stored in forests plus their products
(i.e. representing a carbon sink); negative values
CARBON STORAGE AND SEQUESTRATION
are amounts of carbon added to the atmosphere
(i.e. representing a carbon source). As in Figure
2, two boundary values are given for conifer
forests.
The amount of carbon removed annually
from the atmosphere by forests in Northern Ireland has increased steadily from a negligible
value before about 1940, to 0.15-0.20 MtC a"1
in conifer forests and about 0.025 MtC a"1 in
broadleaved woodland in 1990. If there were no
further new planting, only restocking, then
these carbon sinks would decrease to zero by
about 2020 for conifer forests and 2045 for
broadleaved woodlands (Figure 3). In order to
maintain the forest and woodland carbon sinks
at their present level it would be necessary to
increase the conifer forest area, by new planting, by 1500-2000 ha a"1 and the broadleaved
woodland area by 100-150 ha a"1. However,
even with these planting rates, the carbon sink
provided by Northern Ireland forests is likely to
decrease in the period 2000-2025 (and again in
2065-2085), reflecting the dip in conifer planting
following the peak in 1960-1975 (Figures 1 and
3).
Discussion
Planting history in Northern Ireland
The amount of carbon being sequestered by
forests in Northern Ireland is strongly determined by the magnitude and annual fluctuations
in past forest planting. Below, we make some
comparisons with Britain, drawing mainly on
the report of Jack (1992).
The fraction of land currently covered with
woodland and forest in Northern Ireland is only
about 5 per cent, compared with 12 per cent in
Britain. This is partly attributable to the small
area of forest that existed in Northern Ireland
at the turn of the century and partly to a modest rate of afforestation, which did not begin in
earnest until after World War II (Figure 1). In
1945, an Agricultural Inquiry Committee recommended planting a modest 60 000 ha of state
forest in Northern Ireland by the year 2000,
163
which will probably be exceeded (Table 1). In
1970, a government White Paper increased the
target to 90 000 ha, which now seems unattainable—requiring a four-fold increase in recent
rates of planting (from about 1000 to over 4000
ha a"1). The White Paper set the target for the
private sector by the year 2000 at 30 000 ha,
which compares with only 17 600 ha planted by
1993.
This modest rate of afforestation may be
attributed to difficulties that the Forest Service
in Northern Ireland has had in the acquisition
of land for forestry (Jack, 1992). Before 1987,
the land acquired had to be designated as not
essential for agricultural purposes (by the
Department of Agriculture of Northern Ireland
agricultural inspectorate), which restricted
forestry largely to scattered sites in the poorer
farming areas in the west and the uplands in the
east. After 1987, attempts to acquire better land
were restricted by the high price of farmland in
Northern Ireland—double that of equivalent
land acquired by the Forestry Commission in
Scotland, and in much smaller blocks (Jack,
1992). Additionally, Jack (1992) stated that:
'Before the Forest Service purchases land it is
required to carry out an investment appraisal
which discounts estimated future net revenue
back to present value at an annual rate of 5 per
cent. This represents a disadvantage compared
with the Forestry Commission, which uses a
discount rate of 3 per cent, because it lowers the
price the Forest Service is able to pay for land.'
The decadal variation in total forest planting
since 1945 has been much greater in Northern
Ireland than in Britain, as might be expected
when dealing with a much smaller scale operation that is dominated by one enterprise—the
state sector. In Northern Ireland, the annual
rate of new planting in the state sector showed
a ten-fold upward trend from 1945 to 1963
(from about 200 to 2000 ha a"1) which then
decreased to half by 1980 (about 2000 to 1000
ha a"1). By contrast, planting by the Forestry
Commission in Britain was maintained at
around 15 000 ha a"1 from 1947 to about 1976
(Cannell and Dewar, 1995). New planting by
164
FORESTRY
the Forestry Commission has decreased greatly
since 1976, but this decrease was compensated
in many years by planting in the private sector,
so that total new planting in Britain was kept in
excess of about 20 000 ha a"1 for four decades
(about 1949 to 1989, with a peak of over 35 000
ha a"1 in 1970-1975; Cannell and Dewar, 1995).
The large fluctuations in historic planting
rates in Northern Ireland inevitably mean that
there could be large fluctuations in future rates
of carbon sequestration. However, the fluctuations will be less than shown in Figure 3,
because of variation in Yield Class and rotation
period among stands, and because harvesting
may be staggered to even out timber supplies.
Another important consequence of the slower
build-up in planting rate in Northern Ireland
than in Britain is that the forests in Northern
Ireland are, on average, younger. Thus, in 1990,
over 60 per cent of Northern Ireland forests
were less than 20 years old, compared with
about 40 per cent in Britain. Again, this difference has consequences for future patterns of
carbon sequestration.
It is noteworthy that the private sector in
Northern Ireland is proportionately much
smaller in relation to the state sector than in
Britain. In 1993, the private sector owned only
22 per cent of the total forest area in Northern
Ireland (17 200 ha out of 78 300 ha, Table 1),
whereas in Britain the total private sector holding, including all woodlands, accounts for 60
per cent of the total (about 1.35 million ha out
of 2.25 million ha). In Britain, institutional and
other investors in the private sector were
responsible for substantial new areas being
planted since the 1950s, with annual new planting rates equalling those of the Forestry Commission in the early 1970s and exceeding them
since 1985. There were few such investors in
Northern Ireland and the private sector played
a minor role in new planting until 1990
(1989-1990). At that time, substantial afforestation was promoted, at least temporarily, following the 1988 'Lawson budget', which
removed tax incentives for investors in Britain,
but allowed grant rates to rise (Woodland and
Farm Woodland Grant Schemes) which
attracted landowners in Northern Ireland (Jack,
1992; Hunter Blair, personal communication).
Carbon sequestration by forests in Northern
Ireland
The current rate of carbon removal from the
atmosphere into forests and their products in
Northern Ireland was estimated to be in the
range 0.175-0.225 MtC a"1 in 1990 (0.15-0.20
MtC a"1 by conifers and 0.025 MtC a"1 by
broadleaves). This represents 7-9 per cent of the
2.5 MtC being sequestered by forests in Britain,
or 6.5-8.2 per cent of that being sequestered by
forests throughout the UK. Given the uncertainties in the model calculations (especially in
the conversion of merchantable stemwood volume to total above and belowground carbon) a
figure of 8.2 per cent actually falls within the
margin of error of the total UK estimate.
Nevertheless, this study has shown that
Northern Ireland makes a positive and
significant contribution to carbon sequestration
in the UK, and the higher Yield Classes and
more widespread no-thinning policy in Northern Ireland compared with Britain probably
means that the higher of the two estimates for
conifers is closer to reality, especially for future
projections. Also, because of their younger average age, the current average rate of carbon
sequestration per hectare by forests in Northern
Ireland is greater than that in Britain. Thus, in
Northern Ireland, 0.175-0.225 MtC is being
sequestered by 78 300 ha, averaging 2.2-2.9 tC
ha"1, whereas in Britain 2.5 MtC is being
sequestered by 1.3 million ha of plantation
forests, averaging 1.9 tC ha"1 (or 2.5 MtC by
2.25 million ha of total forest, averaging 1.1 tC
ha- 1 ).
This study showed that, as in Britain, the
annual rate of carbon sequestration by forests in
Northern Ireland has increased since the 1950s
and can only be maintained with continued new
planting (Figure 3). The future new planting
rate for conifers required to maintain the current carbon sink in Northern Ireland is
CARBON STORAGE AND SEQUESTRATION
1500-2000 ha a"1, approximately that achieved
or exceeded in 1960-1975, 1990, 1991 and 1993.
Broadleaved woodlands currently make a small
contribution to the total sequestration, but
would be much more dominant if it were economic to plant large areas of poplars on agricultural land (Dewar and Cannell, 1992).
Finally, we would note that by far the largest
carbon pool in Northern Ireland, as in Britain,
occurs in peatlands, and that any disturbance or
drainage that increases the rate of peat oxidation is likely, in the long term, to return more
carbon to the atmosphere than can ever be
stored in forests (Cannell et al., 1993). Bearing
in mind that nearly two-thirds of the state
forests and one-third of private forests have
been planted on peats or peaty soils, what happens to those peats may be just as important as
future forest policies.
165
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Edwards, P.N. and Christie, J.M. 1981 Yield models
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Acknowledgements
Kurz, W.A., Apps, M.J., Webb, T.M. and McNamee,
This study was funded by the Department of the
P.J. 1992 The carbon budget of the Canadian forEnvironment, under Contract No. EPG1/1/3. We are
est sector. Phase I. Forestry Canada, Information
grateful to Pat Hunter Blair (Forest Service, DepartReport NOR-X-326.
ment of Agriculture, Northern Ireland) for supplying McKenzie, R.F. 1976 Silviculture and management in
planting, yield and new planting data, for informarelation to risk of windthrow in Northern Ireland.
tion on forest soils and for helpful comments on a
lr. For. 33, 29-38.
draft of this paper.
Maclaren, J.P. and Wakelin, S.J. 1991 Forestry and
forest products as a carbon sink in New Zealand.
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Received 11 December 1994