Fig. 5. a Predicted fraction of radiation intercepted solid line and measured triangles at Essex, E. UK. b Predicted
values of above ground dry matter yields solid and dotted lines when run with e
c
values indicated and a measured point near the end of the season in Essex, E. UK.
when an e
c
of 3.3 g MJ
− 1
derived at Essex is used, is close to that measured.
3
.
3
.
2
. Annual 6ariation Mean variation in annual radiation over all
sites ranged from 1583 to 1817 MJ m
− 2
12 between 1984 and 1994. In the same 10 years
variation in the frost free period was 89 days 33 variation in the length of the model’s ‘growing
season’. Mean degree days DD
TB10
calculated over the whole year for all stations ranged from
535 to 805°C day
− 1
33 over the 10 year period. Within the growing season, which is de-
pendent on the frost free period, degree days varied from 494 to 781°C day
− 1
36. The influence of annual variation in temperature
shown in terms of length of the frost free period Fig. 6a, degree days Fig. 6b and incident radi-
ation Fig. 6c conditions on the yield predictions are shown in Fig. 6d at Kilkenny meteorological
station. Predicted yields vary from 13 to 24 t DM ha
− 1
.
3
.
3
.
3
. Interpolation across Ireland Maps of annual incident radiation and degree
days show higher values in the south and south western areas of Ireland Fig. 7a,b, respectively.
Surface interpolation of results of the model run at each of the 23 meteorological stations showed
that potential peak productivity ranges from 16 to 26 t DM ha
− 1
in the MidlandsNorth Midlands region and South West region respectively Fig.
7c. The ‘bulls-eyeing’ of the data around stations is a result of the interpolation procedure which
accumulates distance weighted averages.
4. Discussion
4
.
1
. Model Parameterisation from field data
4
.
1
.
1
. Frost and length of season The length of the growing season is determined
in this model by the occurrence of air tempera- tures below 0°C. The validity of this at the begin-
ning of the growing season is supported by the observation that air temperatures below 0°C in
spring kill the newly expanded leaves. Measure- the predicted above ground yield is 8 t lower than
that measured. The yield prediction of the model
Fig. 4. a Relationship between leaf area index and degree days above 10°C DD
TB10
for M. × giganteus in 1994 and 1995 at Cashel. Error bar = 9 1 S.E.M. n = 40. Inset graph
shows the influence of base temperature X on the correlation coefficient r
2
of the relationship. b Relationship between radiation interception coefficient e
i
and green leaf area index LAI for M. × giganteus in 1994 and 1995 at Cashel. The Eq.
2 is fitted to the combined data for 1994 and 1995 to estimate the radiation extinction coefficient, k. Vertical and horizontal
error bars = 9 1 S.E.M. of interception n = 8 and LAI n = 40, respectively. c Relationship between above ground
dry matter DM of M. × giganteus and intercepted PAR at Cashel, Co. Tipperary. The slope of the regression is the
average radiation use efficiency e
c
for the crop for both the 1994 and 1995 growing seasons S.E.M. of slope = 0.075,
n = 16. Error bar = 9 1 S.E.M. n = 40. Data for 1995 was only used when the soil moisture deficit was less than 150 mm.
Fig. 6. Outputs from the model run with daily climate data from Kilkenny meteorological station from 1984 to 1993.
Annual variability in length of the growing season a, the seasonal totals of degree days b above a threshold of 10°C
and incident radiation c, and predictions of ‘peak’ yield at the end of the growing season t DM ha
− 1
. Averages for each parameter for the 10 years are indicated by column labelled
‘all’.
ments of temperatures during a frost event near the ground can be from 6 to 8°C lower than those
measured in a screen at 1.2 m above the ground and more recent data shows that leaves of M. ×
giganteus were destroyed by temperatures below −
6°C in an artificial freeze test Clifton-Brown, unpublished results. The evidence supporting the
choice of the first frost in autumn for the end of the growing season is weaker. It has been ob-
served in some trials that the crop stops produc- ing above ground biomass towards the end of
August although the exact environmental trigger for the reduction in e
c
is not known Vleeshouw- ers, 1998. Frost temperatures of − 4°C in Octo-
ber in UK have been reported to have effectively destroyed the photosynthetic capacity of the
canopy Beale and Long, 1995.
4
.
1
.
2
. Degree days and leaf expansion The linear relationship between canopy devel-
opment and thermal time Fig. 6a calculated from air temperatures has been demonstrated for
a wide range of crops including wheat Jamieson et al., 1995, barley Gallagher and Biscoe, 1979
and fibre hemp Van der Werf et al., 1995.
It has recently been shown that leaf expansion in M. × giganteus continues at temperatures down
to 6°C Clifton-Brown and Jones, 1997. How- ever, regression analysis of the relationship be-
tween LAI and degree days in the field revealed the highest correlation when the base was 10°C
Fig. 4a, inset. One possible reason for this dis- crepancy is that the thermal response of leaf
expansion rate is curvilinear Clifton-Brown and Jones, 1997. The linear portion occurs at temper-
atures above 10°C, and under field conditions only a very small amount of growth occurred
below this temperature in this genotype at the trial site because hourly temperatures within the
growing season where above 10°C, for 85 of the time.
4
.
1
.
3
. Radiation interception by the canopy The Monsi – Saeki model Eq. 2 provided an
adequate description of the relationship between leaf area index and the proportion of radiation
intercepted Fig. 4b. Here k, the extinction coeffi- cient, for M. × giganteus at Cashel was estimated
Fig. 7. a Total annual mean PAR MJ m
− 2
and b degree days above 10°C throughout Ireland and c mean simulated yield at the end of the growing season for M. × giganteus calculated using 10 years of daily radiation and air temperatures at 23 stations
in Ireland 1984 – 1993, climate data from Met E ´ ireann. Positions of the stations are indicated with a ‘+’ and the position of the
field trial is indicated with an ‘X’ in c.
to be 0.68 which is similar to the value of 0.7 obtained for maize Azam-Ali et al., 1994. This
relationship shows that LAIs above 3.5 confer no additional radiation interception efficiency and
therefore this represents the optimum LAI.
4
.
1
.
4
. Radiation use efficiency The average radiation use efficiency e
c
calcu- lated over 2 years from the Cashel field trial was
2.4 g MJ
− 1
PAR. This is comparable to the value of 2.6 g MJ
− 1
PAR determined for M. × gigan- teus growing in the Netherlands Van der Werf et
al., 1993, but it is significantly lower than those calculated for field trials in the UK 3.3 g MJ
− 1
PAR Beale and Long, 1995 and in Northern France 4.2 g MJ
− 1
PAR Tayot et al., 1995. A more recent study in the Netherlands in 1997
determined an e
c
of 3.3 g MJ
− 1
PAR Vleeshouw- ers, 1998. The proposed consistency of the empir-
ical coefficient e
c
has received some strong criticism Demetriades-Shah et al., 1992 because
if Monteith’s proposal is correct Monteith, 1977, then e
c
should be similar from different sites when the crop is grown under ‘optimal’ conditions of
water and nutrient supply. Evidently, other fac- tors might significantly influence e
c
. Photoinhibi- tion in response to chilling is one such candidate
Baker et al., 1988, although for M. × giganteus this was not found to be significant in E. UK
Beale et al., 1996. However, less specific effects of local conditions may be responsible for the
lower e
c
observed here. Mean temperature differ- ences between our site and Essex in the years of
measurement 13.2 and 13.6°C, respectively are unlikely to explain the difference. Further work is
necessary to establish why these differences in e
c
are observed.
4
.
2
. Model output
4
.
2
.
1
. Validation of output Data from other M. × giganteus field trials in
Ireland where a measurement of the peak above ground dry matter at the end of the growing
season has been made are scarce. This model predicts a 30 lower yield than measured at an
irrigated trial in south eastern UK when a e
c
value of 2.4 g MJ
− 1
PAR is used Fig. 5. However, if the e
c
which was calculated at the Essex site is used then the model predicts a peak yield to
within 5 of that measured. This shows the high dependency of model predictions on e
c
in this type of model Demetriades-Shah et al., 1992; Reddy,
1995. Variability in e
c
suggests that more mecha- nistic approaches to describing radiation conver-
sion efficiency could be developed but there is a problem with scaling more complex models.
4
.
2
.
2
. Predicted potential 6ersus actual har6estable yield
The model predicts yields of 19.8 and 18.6 t ha
− 1
in 1994 and 1995, respectively, which is the standing biomass yield in early autumn when the
first frost occurred. However, there are post grow- ing season and pre-harvest losses by both death
and detachment of leaves and the translocation of assimilates to the rhizomes. Yield determinations
in 1994 after the end of the growing season show a 30 decline in above ground dry matter Fig.
3d by mid December.
The model does not account for the effects of drought on yield reduction below potential yield.
Such drought conditions as those in 1995 in Ire- land are rare and mean cumulative soil moisture
deficits in the region where the trial was con- ducted are generally below 40 mm Collins and
Cummins, 1996.
4
.
2
.
3
. Potential annual 6ariation in yield The model has been parameterised using a well
established crop 4th and 5th season following planting. The Miscanthus stand normally reaches
full productivity in the third growing season fol- lowing planting Walsh, 1997. It is important to
point out that yields from a young stand cannot predicted by this model. Annual variation in the
yield estimates from the model depend more on temperature dependent length of the growing sea-
son than on radiation because of large variation in the frost free period and degree days Fig. 6.
4
.
2
.
4
. Scaling of the model countrywide GIS provided a means to interpolate the results
of the point data of the modelled yields predicted from the climate data of 23 meteorological sta-
tions over 10 years Fig. 7c. This made it possible to identify the areas in Ireland with the most
suitable temperature and radiation conditions for growing M. × giganteus. Simulated yields in the
frost free coastal regions are often higher than inland but wind shelter could also be critical to
realising these higher yields. Water supply to the crop is normally adequate because growing season
soil moisture deficits are on average lower than 50 mm over about 80 of the land surface of Ireland
Collins and Cummins, 1996. Since 97.4 of land surface of Ireland has a soil moisture storage
capacity of plant available water above 60 mm, water deficits are unlikely to limit yields signifi-
cantly FAO, 1995.
5. Conclusions
