sion efficiency of light energy in photosynthesis which is 40 higher than C
3
plants Monteith, 1978. For C
4
plants the optimal conditions are most frequently either sub-tropical or tropical
but, interestingly a clone of Miscanthus has been recently shown, even in the temperate climate of
southern UK to achieve efficiencies 37 above those of native C
3
plants Beale and Long, 1995. This is probably due to the fact that Miscanthus
naturally occurs in, and is adapted to, cooler climates than most other species which exhibit C
4
photosynthesis Numata, 1979. Furthermore, it has also been shown that the environmental im-
pact of cultivation of Miscanthus is less than annual crops because a large proportion of fer-
tiliser inputs are effectively recycled from one year into another via the perennial rhizomatous system
Beale and Long, 1997. Finally, Miscanthus has a very good combustion quality due to low Cl, N, S
and ash contents Lewandowski and Kicherer, 1997.
To develop an energy industry which uses biomass as a raw material, high yield potential is
essential and it is also necessary to show how this yield potential varies with climatic conditions.
Crop growth models are now used widely to predict yields based upon prevailing climatic con-
ditions Schapendonk et al., 1998. Many of these are based on principles established by Monteith,
1977. Here the dry matter at final harvest W
h
is the product of the integral of incident solar radiation S
t
, the fraction of radiation which is intercepted by the canopy e
i
and the efficiency with which intercepted radiation is converted into
biomass e
c
, so that W
h
= S
t
· e
i
· e
c
1 Assessment of the yield from a promising clone,
Miscanthus × giganteus Greef and Deuter, 1993, has been attempted in field trials established at 16
sites across Europe in the European Miscanthus Network Project Walsh, 1997. In this paper data
collected in 1994 and 1995 from field trials estab- lished in southern central Ireland in 1990 were
used to parameterise the model described by Eq. 1. By using data from 23 meteorological stations
in Ireland and incorporation of the model results into a geographic information system GIS the
model has been scaled up to produce countrywide values of potential primary production of above
ground dry matter. It was shown that the poten- tial above-ground productivity of M. × giganteus
in Ireland could vary from 16 to 26 t DM ha
− 1
year
− 1
. It is important to note that M. × gigan- teus is just one of several clones of the Miscanthus
genus used in biomass trials and the model devel- oped here, although generic in nature, has been
parameterised specifically for this clone.
2. Materials and methods
2
.
1
. Field trial
2
.
1
.
1
. Establishment and management A field trial, using the clone M. × giganteus was
planted in June 1990 at Cashel, Co. Tipperary, Ireland Grid reference: 07°50W 52°39N; 80 m
above sea level on a gley soil. The effective planting density was two plants m
− 2
. The total area of the plantation was 800 m
2
divided into eight 10 × 10 m plots. In the fifth and sixth grow-
ing seasons 1994 and 1995 intensive measure- ments of plant growth were made. At the start of
each growing season fertiliser was applied to all eight plots at 120 kg N ha
− 1
, 36 kg P ha
− 1
and 72 kg K ha
− 1
. Weed-control using herbicides was carried out before the Miscanthus plants emerged
in spring each year. The crop was rain fed, and no irrigation system was installed because drought
occurs rarely.
2
.
1
.
2
. Field measurements
1994
and
1995 2
.
1
.
2
.
1
. Leaf area index LAI
. On two occasions during 1994 the relationship between leaf area
and the product of leaf length and width was established. This was done by using a leaf area
planimeter Delta-T Devices, Cambridge, UK, and the product of leaf length and width mea-
sured with a ruler Fig. 1. This relationship was used in the 1994 and 1995 growing seasons to
estimate the green leaf area per shoot. Five shoots were measured from each plot and therefore a
total of 40 shoots were used from the eight plots for each leaf area estimation. LAI was calculated
from the product of green leaf area per shoot and shoot density per m
− 2
measured at approximately 2 week intervals.
2
.
1
.
2
.
2
. Canopy radiation interception. Measure- ments of incident and transmitted PAR 400 – 700
nm above and at the base of the canopy were preferably made 9 1 h of midday Local Time
with a Decagon Sunfleck Ceptometer Delta-T Devices, Cambridge, UK. At approximately 2
week intervals, the mean of six radiation measure- ments at the base of the canopy and two measure-
ments above the canopy were used to calculate the proportion of radiation intercepted e
i
by the canopy for each plot. To avoid errors due to
fluctuating incident radiation, all measurements for a plot were made within a period of 2 min on
each occasion interception values were calculated.
2
.
1
.
2
.
3
. Seasonal standing biomass. Forty shoots were randomly sampled five shoots from eight
plots every 2 weeks by cutting at ground level from emergence in spring until the first frost in
autumn. Two extra harvests were made in 1994 after the growing season to assess pre-harvest
losses. Shoots were dried to constant weight at 80°C to determine the dry matter. An estimate of
the above ground dry matter standing biomass was calculated from the shoot density per m
− 2
.
2
.
1
.
2
.
4
. Climate at the field site. Daily maximum and minimum air temperatures were obtained
from a climate station 1 km from the site. Data was recorded using a datalogger Type CR10,
Campbell, Leicestershire, UK.
Daily incident radiation values were calculated for the site from the mean daily radiation received
at two meteorological stations located 62 km north Birr, 53°05N 7°47W and 43 km east
Kilkenny, 52°40N 7°16W of the site Met E ´ ire-
ann, Glasnevin, Dublin. These stations measure global radiation, which was converted to PAR by
multiplication by the factor 0.5 Jones, 1992. In the 10-year period 1984 – 1993 annual total of
incident radiation at Kilkenny and Birr differed by 7. This indicates that the incident radiation
environment at Cashel is probably reasonably well estimated from either of these meteorological
stations.
Soil moisture deficit SMD was assumed to be at 0 mm in January 1994 i.e. field capacity and
was thereafter estimated from the difference be- tween precipitation and potential evaporation cal-
culated by the Penman formula Penman, 1948 at Met E
´ ireann from records at Kilkenny meteoro- logical station.
2
.
2
. Growth model structure The model consists of four components. Firstly,
a thermal leaf area coefficient t
l
was obtained by regression of LAI on accumulated degree days
above a base temperature DD
TBX
calculated ac- cording to McVicker, 1946 using daily minimum
and maximum air temperatures. Secondly, the radiation extinction coefficient k of the Monsi –
Saeki equation Monsi and Saeki, 1953 was derived from the relationship between e
i
and the LAI according to Eq. 2.
k = expe
i
− 1LAI
2
Fig. 1. The relationship between the product of leaf length and width and leaf area cm
2
for Miscanthus leaves collected on two occasions in 1994. The regression equation was used to
convert measurements of leaf length and width into area in all the field determinations of leaf area.
Table 1 The 23 meteorological stations from which data was used
a
Station Latitude
County Longitude
Altitude m Yield t DM ha
− 1
Mean S.E.M.
54°39N 6°13W
Aldergrove 68
Antrim 19.6
0.7 54°04N
7°47W Ballinamore
80 Leitrim
16.8 0.9
54°30N 8°10W
36 Donegal
18.6 Ballyshannon
0.5 54°53N
6°58W 216
18.0 0.8
Banagher Derry
54°14N 10°00W
9 Mayo
19.5 Belmullet
0.7 Offaly
Birr 53°05N
7°53W 70
17.4 0.9
Monaghan Bryansford
54°13N 5°57W
85 20.1
0.9 53°43N
8°59W 69
Mayo 16.5
Claremorris 0.9
Monaghan Clones
54°11N 7°14W
87 18.3
0.7 51°51N
8°29W 151
Cork 25.5
Cork Airport 0.8
53°26N 6°14W
68 Dublin Airport
22.5 Dublin
0.7 52°05N
7°40W 14
Waterford 22.4
Dungarvan 0.9
Cork Fermoy
52°10N 8°16W
52 18.4
1.1 53°17N
9°4W 11
Galway 22.1
Galway UCG 0.5
52°40N 7°16W
63 Kilkenny
18.9 Kilkenny
1.1 55°22N
7°20W 20
Donegal 18.2
Malin Head 0.5
53°31N 7°21W
108 Mullingar
18.7 Westmeath
0.8 53°55N
9°34W 11
Mayo 20.7
Newport 0.7
52°52N 6°55W
58 Oak Park Carlow
19.9 Carlow
0.9 52°15N
6°20W 23
Wexford 24.6
Rosslare 0.8
52°41N 8°55W
3 23.1
Shannon Airport 0.5
Clare 51°29N
9°26W 16
Cork 25.6
Sherkin Island 0.9
Kerry Valentia Observ.
51°56N 10°15W
9 22.5
0.6
a
The table shows the county location, latitude and longitude, altitude m above sea level and the mean yield and S.E.M. in the 10 year period 1984–1993 predicted by model.
Thirdly, an estimate of the radiation use effi- ciency e
c
was obtained from the regression of the standing aerial dry matter on intercepted radia-
tion. Fourthly, the length of the growing season was determined by the number of days between
the last spring air frost and the first autumn air frost since the leaves of M. × giganteus are frost
sensitive air frost threshold 0°C.
The model assumes that water and nutrient supply are non-limiting for crop growth.
2
.
3
. Scaling of the model countrywide
2
.
3
.
1
. Climate data Daily sunshine hours, and maximumminimum
air temperatures 1.2 m above the ground for the period 1984 – 1993 were obtained from Met E
´ ire- ann, the Irish Meteorological Service, for 20 sta-
tions in the Republic of Ireland, and from the Northern Ireland Meteorological Office for three
stations in Northern Ireland. The stations were chosen to give good data cover for both inland
areas and coastal regions Table 1. The minimum and maximum temperature values were adjusted
to sea level mean sea level is taken to be 2.505 m above Irish Ordnance Datum in accordance with
standard methods. The values for 29 February in leap years were excluded from the calculations.
Missing values, where they occurred, were inter- polated using values from the years before and
after.
As solar radiation values were not available for most of the meteorological stations, it was neces-
sary to convert sunshine hours to global solar radiation MJ m
− 2
. The A , ngstro¨m equation
A , ngstro¨m, 1924,
QQa = a + b · nN 3
has been widely used to estimate solar radiation from records of sunshine duration McEntee,
1980, where Q = global solar radiation MJ m
− 2
day
− 1
; Qa = the extraterrestrial radiation MJ m
− 2
day
− 1
, the global solar radiation in the absence of an atmosphere received on a horizon-
tal surface at the station; n = duration of bright sunshine in hours; N = day length in hours; and a
and b are constants. In this study Qa and N were calculated according to McMurtrie 1993. This
regression equation was applied to the mean daily data of the stations at Valentia, Malin Head,
Dublin and Kilkenny, where both sunshine hours and global radiation were recorded for the period
1984 – 1993. The overall values for parameters a and b for the four stations were, a = 0.22 and
b = 0.62. These compare favourably with those produced by McEntee 1980 where a = 0.21 and
b = 0.67.
2
.
3
.
2
. GIS and surface interpolation Degree days and incident radiation were calcu-
lated for each climate station. The model was run with parameters derived from the field measure-
ments for each climate station. The point data of mean degree days, incident radiation and pre-
dicted yields from the 23 meteorological stations across Ireland was incorporated into a geographic
information system GIS, IDRISI v4.1, Clarke University, Massachusetts, USA to produce a
digital elevation model DEM. Surface interpola- tion of the scattered point data on a regular grid
was done using a inverse distance weighting inter- polation routine in the GIS which calculates a
complete surface from point data according to distance weighted averages. Degree day, radiation
and yield values are displayed at a grid size of 1 × 1 km and referenced in Irish National Grid
co-ordinates.
3. Results
