Directory UMM :Data Elmu:jurnal:I:Industrial Crops and Products:Vol12.Issue2.Aug2000:

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The modelled productivity of

Miscanthus

×

giganteus

(GREEF et DEU) in Ireland

J.C. Clifton-Brown

a,b,

_{*, B. Neilson}

_{, I. Lewandowski}

_{, M.B. Jones}

b a_Uni₆_{ersita¨t Hohenheim,}_{Institut fu¨r Pflanzenbau und Gru¨nland}₍₃₄₀₎_,_{Fruwirthstrasse} ₂₃_,_D-₇₀₅₉₉ _Stuttgart, _Germany

b_{Botany Department,}_{Trinity College,}_Uni₆_{ersity of Dublin,}_Dublin ₂_, _Ireland Received 27 July 1999; received in revised form 28 January 2000; accepted 14 February 2000

Abstract

The contribution of Miscanthus biomass to an energy or fibre industry in Ireland can only be estimated if the potential productivity is predicted on a regional basis. In order to parameterise a model to predict dry matter production, growth and climatic measurements were carried out in 1994 and 1995 on aMiscanthusfield trial, planted in 1990 in southern central Ireland. These were used to derive relationships between: (i) leaf canopy light interception and thermal time calculated from air temperatures; and (ii) radiation intercepted and above ground biomass. These relationships were used to parameterise an empirical productivity model in which water and nutrient supplies are assumed non-limiting. The output from this model was incorporated into a geographical information system (GIS) to map the predicted potential production ofM.×giganteusthroughout Ireland, using 10 year daily air temperatures and incident radiation from 23 climatic stations. Across the island, potential biomass yields at the end of the growing season, ranged between 16 and 26 t DM ha−1_{. The model approach and its predictions are discussed. © 2000 Elsevier}

Keywords:Miscanthus; Geographic information system (GIS); Growth model; Energy crops; Ireland

www.elsevier.com/locate/indcrop

1. Introduction

Herbaceous, rhizomatous crops can provide a renewable, and largely carbon-neutral, energy source. Miscanthus, a genus of rhizomatous, perennial C4 grasses with origins in E. Asia, is a

strong candidate as a biomass species on account of several advantageous physiological characteris-tics. Plants using C4 photosynthesis have the

po-tential to out-yield those with the more common C3photosynthesis because when grown under

op-timum conditions they have a maximum

conver-Abbre6iations: DDTBX, degree days accumulated above a base temperature of X°C; ec, radiation use efficiency (g DM MJ−1_{PAR intercepted);}_e

i, efficiency with which the radiation is intercepted by the crop (dimensionless);k, radiation extinc-tion coefficient (dimensionless); LAI, leaf area index (dimen-sionless); PAR, photosynthetically active radiation (400 – 700 nm) (MJ m−2_); _S

t, integral of incident solar radiation (MJ m−2_); _t

l, thermal leaf area coefficient (DDTBXm2 leaf m−2 ground);Wh, above ground dry matter (t DM ha−1).

* Corresponding author. Fax: +353-1-6081147. E-mail address:[email protected] (J.C. Clifton-Brown)

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sion efficiency of light energy in photosynthesis which is 40% higher than C3 plants (Monteith,

1978). For C4 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 C3plants (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 C4

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,Miscanthushas 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 (Wh)

is the product of the integral of incident solar radiation (St), the fraction of radiation which is

intercepted by the canopy (ei) and the efficiency

with which intercepted radiation is converted into biomass (ec), so that

Wh=St·ei·ec (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 -teusis just one of several clones of theMiscanthus

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 cloneM.×giganteuswas planted in June 1990 at Cashel, Co. Tipperary, Ireland (Grid reference: 07°50%_{W 52°39}%_{N; 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 m2 _{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 Miscanthusplants 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

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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 91 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 ments at the base of the canopy and two measure-ments above the canopy were used to calculate the proportion of radiation intercepted (ei) 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°05%N 7°47%W) and 43 km east (Kilkenny, 52°40%N 7°16%W) 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 (tl) was obtained by

regression of LAI on accumulated degree days above a base temperature (DDTBX) 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 ei and the

LAI according to Eq. (2).

k=(exp(ei−1))/LAI (2)

Fig. 1. The relationship between the product of leaf length and width and leaf area (cm2_{) 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.

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Table 1

The 23 meteorological stations from which data was useda

Station County Latitude Longitude Altitude (m) Yield (t DM ha−1₎

Mean S.E.M.

54°39%N 6°13%W

Aldergrove Antrim 68 19.6 0.7

54°04%N 7°47%W

Ballinamore Leitrim 80 16.8 0.9

54°30%N 8°10%W 36

Donegal 18.6

Ballyshannon 0.5

54°53%N 6°58%W 216 18.0 0.8

Banagher Derry

54°14%N 10°00%W 9

Mayo 19.5

Belmullet 0.7

Offaly

Birr 53°05%N 7°53%W 70 17.4 0.9

Monaghan

Bryansford 54°13%N 5°57%W 85 20.1 0.9

53°43%N 8°59%W 69

Mayo 16.5

Claremorris 0.9

Monaghan

Clones 54°11%N 7°14%W 87 18.3 0.7

51°51%N 8°29%W 151

Cork 25.5

Cork Airport 0.8

53°26%N 6°14%W 68

Dublin Airport Dublin 22.5 0.7

52°05%N 7°40%W 14

Waterford 22.4

Dungarvan 0.9

Cork

Fermoy 52°10%N 8°16%W 52 18.4 1.1

53°17%N 9°4%W 11

Galway 22.1

Galway (UCG) 0.5

52°40%N 7°16%W 63

Kilkenny Kilkenny 18.9 1.1

55°22%N 7°20%W 20

Donegal 18.2

Malin Head 0.5

53°31%N 7°21%W 108

Mullingar Westmeath 18.7 0.8

53°55%N 9°34%W 11

Mayo 20.7

Newport 0.7

52°52%N 6°55%W 58

Oak Park (Carlow) Carlow 19.9 0.9

52°15%N 6°20%W 23

Wexford 24.6

Rosslare 0.8

52°41%N 8°55%W 3 23.1

Shannon Airport Clare 0.5

51°29%N 9°26%W 16

Cork 25.6

Sherkin Island 0.9

Kerry

Valentia Observ. 51°56%N 10°15%W 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 (ec) 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 maximum/minimum 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),

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has been widely used to estimate solar radiation from records of sunshine duration (McEntee, 1980), whereQ=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; anda

andb are constants. In this studyQaandNwere 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

3.1. Miscanthus growth and climate in 1994 and

1995

In 1994 the length of the growing season was 191 days. Air temperatures (Fig. 2a) were close to

average while radiation was below average (Fig. 2b). Rainfall from May to September was 357.4 mm. The maximum soil moisture deficit reached 50 mm in June (Fig. 2c). In 1995, the growing season was 133 days (2 months shorter than in 1994). Rainfall between May and September was 201.1 mm. Air temperatures during the growing season were higher than normal in 1995 and, due to the lower rainfall and higher temperatures dur-ing the growdur-ing season, the cumulative soil mois-ture deficits in September were greater than 250 mm (Fig. 2c).

Shoot densities in both 1994 and 1995 were highest in June (Fig. 3a) and decreased at the beginning of July as the canopy reached a LAI (Fig. 3b) sufficient to intercept 95% of the incident radiation (Fig. 3c). In 1994, plant growth lead to steady biomass accumulation until the first frost in autumn (day 273) (Fig. 3d). Measurements of the standing dry matter after the growing season (peak) and before harvest in December 1994 showed that yields decreased by about 25% (Fig. 3d). Drought conditions in 1995, which developed from the beginning of July (day of year 190), caused premature leaf senescence and a reduction in LAI (Fig. 3b). However, LAI did not decline sufficiently to reduce ei (fraction of radiation

in-tercepted by the crop) (Fig. 3c). The severe water deficit in 1995 halted significant increases in above ground dry matter (Fig. 3d) in August.

3.2. Model parameterisation

3.2.1. Thermal leaf area coefficient and radiation interception

Although in 1994 there was no indication that water stress limited canopy development, the wa-ter stress that developed in 1995 arrested leaf expansion in early August (relationship up to 7 August 1995 was LAI=0.0104×DDTB10; r

2 =

0.99) and measurements after 7 August 1995 are omitted from the model parameterisation.

The inset graph in Fig. 4a shows the effect of changing the base temperature for calculation of degree days on the correlation (r2_{) between LAI}

and accumulated degree days. The highest corre-lation coefficient was found with a base of 10°C

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(r2

=0.97) and this is shown in Fig. 4a. Here, from the regression, LAI=0.0102×DDTB10.

For both years a LAI of above 3.0 was sufficient to intercept 90% of incident radiation (Fig. 4b). The radiation extinction coefficient (k) from Eq. (1) for this canopy was calculated to be 0.6890.031.

3.2.2. Radiation use efficiency

Radiation use efficiency (ec) of the crop was

derived from the relationship between above ground dry matter and intercepted radiation (Fig. 4c) but excluding data when the soil moisture deficit was greater than 150 mm (August 1995) when all plant available water was extracted (150

Fig. 2. (a) Monthly values for 1994 and 1995 of mean maximum and minimum temperatures (°C), (b) mean monthly incident PAR (MJ m−2_{) for 1994 and 1995 and the monthly values of the previous 10 year period 1984 – 1993. (c) Total monthly, long term mean} precipitation, 1950 – 1980, monthly soil moisture deficit, and cumulative soil moisture deficit for 1994 and 1995. Climate data was obtained from Kilkenny and provided by Met E´ ireann. The unshaded area indicates the period of the growing season in the 2 years. SM – S=xis the sum of precipitation between May and September.

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Fig. 3. Growth parameters forM.×giganteusat Cashel, Co. Tipperary measured in the 1994 and 1995 growing seasons: (a) shoot density, (b) leaf area index, (c) proportion of radiation interception (ei) and (d) above ground dry matter. Error bars=91 S.E.M. (n=40 in (a), (b), (d) andn=8 in (c)).

3.3. Model outputs 3.3.1. Validation

Validation of the model with independent data obtained in Essex, UK is shown in Fig. 5 (Beale and Long, 1995). Radiation interception by the canopy is well predicted by the model. However,

Fig. 4. (Caption o6erleaf)

mm is the approximate moisture storage capacity for soils in this region (FAO, 1995)). In 1994 and 1995 ec was 2.4 and 2.3 g MJ−1, respectively

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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 withecvalues indicated) and a measured point near the end of the season in Essex, E. UK.

when an ec of 3.3 g MJ

−1 _{derived at Essex is}

used, is close to that measured.

3.3.2. Annual6ariation

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 (DDTB10) 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

/North 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 (DDTB10) for M.×giganteus in 1994 and 1995 at Cashel. Error bar=91 S.E.M. (n=40). Inset graph shows the influence of base temperatureXon the correlation coefficient r2 _{of the relationship. (b) Relationship between} radiation interception coefficient (ei) and green leaf area index (LAI) forM.×giganteusin 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=91 S.E.M. of % interception (n=8) and LAI (n=40), respectively. (c) Relationship between above ground dry matter (DM) ofM.×giganteus and intercepted PAR at Cashel, Co. Tipperary. The slope of the regression is the average radiation use efficiency (ec) for the crop for both the 1994 and 1995 growing seasons (S.E.M. of slope=0.075, n=16). Error bar=91 S.E.M. (n=40). Data for 1995 was only used when the soil moisture deficit was less than 150 mm.

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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 ecis 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

inM.×giganteuscontinues 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). Herek, the extinction coeffi-cient, for M.×giganteusat Cashel was estimated

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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 forM.×giganteuscalculated 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 (ec)

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

-teusgrowing 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 anecof 3.3 g MJ

−1_PAR

(Vleeshouw-ers, 1998). The proposed consistency of the empir-ical coefficient (ec) has received some strong

criticism (Demetriades-Shah et al., 1992) because if Monteith’s proposal is correct (Monteith, 1977), thenecshould 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 ec.

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 ecobserved 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 ec

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4.2. Model output

4.2.1. Validation of output

of 2.4 g MJ−1 _{PAR is used (Fig. 5). However, if}

the ec 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 onecin this type

of model (Demetriades-Shah et al., 1992; Reddy, 1995). Variability inecsuggests 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 6_{ersus actual} har6_{estable 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 6_{ariation in yield}

The model has been parameterised using a well established crop (4th and 5th season following planting). TheMiscanthusstand 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

(1) The model predictions show that the highest yield ofMiscanthusexpected in the cool temperate climate of Ireland is 26 t DM ha−1 _year−1_{. The}

highest predicted yields for the island of Ireland are in the south-west and the lowest yields are in the north-east.

(2) Substantial interannual variation in yields are expected, mainly as a consequence of the changes in the length of the frost free period. Interannual variation in incident radiation has less of an effect on predicted yields than interan-nual variation in air temperatures.

(3) Rainfall in Ireland is normally non-limiting

for Miscanthus growth. To account for yield

losses due to water deficits in dry years, further development of the model is required.

(4) Identical estimates of radiation use effi-ciency (ec) in the 1994 and 1995 growing seasons

were obtained at the trial site in spite of the difference in climatic conditions between the

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years. Theec values calculated here appear to be

suitable for use in the model predictions of yield in Ireland, but there is evidence that ec varies

from one geographical region to another. The reasons for this variation is not clear and requires further investigation.

Acknowledgements

The authors wish to acknowledge the assistance of Henry and Peter Clifton-Brown, Donal McGovern and Tom Kent in the field. We thank Clive Beale for the provision of validation data and Barbara Eusterschulte for discussion on the manuscript. This work was partly funded by the EU under contract number AIR – CT92 – 0294.

References

A,ngstro¨m, A., 1924. Solar and atmospheric radiation. Q. J. R. Meteorol. Soc. 50, 121 – 125.

Azam-Ali, S.N., Crout, N.M.J., Bradley, R.G., 1994. Perspec-tives in modelling resource capture by crops. In: Monteith, J.L., Scott, R.K., Unsworth, M.H. (Eds.), Resource Cap-ture by Crops. Nottingham University Press, Nottingham, pp. 125 – 148.

Baker, N.R., Long, S.P., Ort, D.R., 1988. Photosynthesis and temperature, with particular reference to effects on quan-tum yield. In: Long, S.P., Woodward, F.I. (Eds.), Plants and Temperature. Company of Biologists, Cambridge, pp. 347 – 375.

Beale, C.V., Long, S.P., 1995. Can perennial C4 grasses attain high efficiencies of radiant energy conversion in cool cli-mates? Plant Cell Environ. 18, 641 – 650.

Beale, C.V., Long, S.P., 1997. Seasonal dynamics of nutrient accumulation and partitioning in the perennial C4 grasses Miscanthus×giganteusandSpartina cynosuroides. Biomass Bioenergy 12, 419 – 428.

Beale, C.V., Bint, D.A., Long, S.P., 1996. Leaf photosynthesis in the C4-grass Miscanthus×giganteus, growing in the cool temperate climate of southern England. J. Exp. Botany 47, 267 – 273.

Clifton-Brown, J.C., Jones, M.B., 1997. The thermal response of leaf extension rate in genotypes of the C4-grass Miscan-thus: an important factor in determining the potential productivity of different genotypes. J. Exp. Botany 48, 1573 – 1581.

Collins, J.F., Cummins, T., 1996. Agroclimatic Atlas of Ire-land. AGMET, Dublin.

Demetriades-Shah, T.H., Fuchs, M., Kanemasu, E.T., Flitcroft, I., 1992. A note of caution concerning the

rela-tionship between cumulated intercepted radiation and crop growth. Agric. Forest Meteorol. 58, 193 – 207.

FAO, 1995. Digital soil map of the world and derived soil properties. Version 3.5, Rome, Italy.

Gallagher, J.N., Biscoe, P.V., 1979. Field studies of leaf growth 3. Barley leaf extension in relation temperature, irradiance, and water potential. J. Exp. Botany 30, 645 – 655.

Greef, J.M., Deuter, M., 1993. Syntaxonomy ofMiscanthus×

giganteusGREEF et DEU. Angewandte Botanik 67, 87 – 90.

Jamieson, P.D., Brooking, I.R., Porter, J.R., Wilson, D.R., 1995. Prediction of leaf appearance in wheat: a question of temperature. Field Crops Res. 41, 35 – 44.

Jones, H.G., 1992. Plants and Microclimate. Cambridge Uni-versity Press, Cambridge.

Lewandowski, I., Kicherer, A., 1997. Combustion quality of biomass: practical relevance and experiments to modify the biomass quality ofMiscanthus×giganteus. Eur. J. Agron-omy 6, 163 – 177.

McEntee, M.A., 1980. A revision of the equation relating sunshine hours to radiation income for Ireland. Ir. J. Agric. Res. 19, 119 – 125.

McMurtrie, R.E., 1993. Modelling of canopy carbon and water balance. In: Hall, D.O., Scurlock, J.M.O., Bolhar-Nordenkampf, H.R., Leegood, R.C., Long, S.P. (Eds.), Photosynthesis and Production in a Changing Environ-ment. A Field and Laboratory Manual. Chapman and Hall, London, pp. 220 – 231.

McVicker, I.F.G., 1946. The calculation and use of degree-days. J. Inst. Heating Ventilating Eng. 14, 256 – 283. Monsi, M., Saeki, T., 1953. U8ber den lichtfaktor in den

Pflanzengesellschaften und seine Bedeutung fu¨r die Stoff-produktion. Jpn. J. Botany 14, 22 – 52.

Monteith, J.L., 1977. Climate and the efficiency of crop pro-duction in Britain. Philos. Trans. R. Soc. B281, 277 – 294. Monteith, J.L., 1978. Reassessment of the maximum growth

rates for C3 and C4 crops. Exp. Agric. 14, 1 – 5. Numata, M., 1979. Distribution of grasses and grasslands in

Asia. In: Numata, M. (Ed.), Ecology of Grasslands and Bamboolands in the World. Dr. Junk, The Hague, pp. 92 – 102.

Penman, H.L., 1948. Natural evaporation from open water, bare soil and grass. Proc. R. Soc. A193, 120 – 145. Reddy, J.S., 1995. Over-emphasis on energy terms in crop

yield models. Agric. Forest Meteorol. 77, 113 – 120. Schapendonk, A.H.C.M., Stol, W., van Kraalingen, D.W.G.,

Bouman, B.A.M., 1998. LINGRA, a sink/source model to simulate grassland productivity in Europe. Eur. J. Agron-omy 9, 87 – 100.

Tayot, X., Chartier, M., Varlet-Grancher, C., Lemaire, G., 1995. Potential above-ground dry matter production of Miscanthus in north-central France compared to sweet sorghum. In: Chartier, P., Beenackers, A.A.C.M., Grassi, G. (Eds.), Biomass for Energy, Environment, Agriculture and Industry. Elsevier, Oxford, pp. 556 – 564.

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Van der Werf, H.M.G., Meijer, W.J.M., Mathijssen, E.W.J.M., Darwinkel, A., 1993. Potential dry matter pro-duction of Miscanthus sinensis in The Netherlands. Ind. Crops Prod. 1, 203 – 210.

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giganteusin the Netherlands. In: Kopetz, H., Weber, T., Palz, W., Chartier, P., Ferrero, G.L. (Eds.), Biomass for Energy and Industry-Proceedings of the 10th European Bioenergy Conference. CARMEN, Wu¨rzburg, pp. 1017 – 1019.

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when an ec of 3.3 g MJ

−1 _{derived at Essex is} used, is close to that measured.

3.3.2. Annual6ariation

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 (DDTB10) 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 Midlands/North 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

that measured. The yield prediction of the model

Fig. 4. (a) Relationship between leaf area index and degree days above 10°C (DDTB10) for M.×giganteus in 1994 and 1995 at Cashel. Error bar=91 S.E.M. (n=40). Inset graph shows the influence of base temperatureXon the correlation coefficient r2 _{of the relationship. (b) Relationship between} radiation interception coefficient (ei) and green leaf area index (LAI) forM.×giganteusin 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=91 S.E.M. of % interception (n=8) and LAI (n=40), respectively. (c) Relationship between above ground dry matter (DM) ofM.×giganteus and intercepted PAR at Cashel, Co. Tipperary. The slope of the regression is the average radiation use efficiency (ec) for the crop for both the 1994 and 1995 growing seasons (S.E.M. of slope=0.075,

n=16). Error bar=91 S.E.M. (n=40). Data for 1995 was only used when the soil moisture deficit was less than 150 mm.

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J.C.Clifton-Brown et al./Industrial Crops and Products12 (2000) 97 – 109 105

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 ecis 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

It has recently been shown that leaf expansion inM.×giganteuscontinues 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

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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 forM.×giganteuscalculated 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).

4.1.4. Radiation use efficiency

The average radiation use efficiency (ec) 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

-teusgrowing 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 anecof 3.3 g MJ

−1_PAR

(Vleeshouw-ers, 1998). The proposed consistency of the empir-ical coefficient (ec) has received some strong criticism (Demetriades-Shah et al., 1992) because if Monteith’s proposal is correct (Monteith, 1977), thenecshould 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 ec. 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 ecobserved 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 ec are observed.

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J.C.Clifton-Brown et al./Industrial Crops and Products12 (2000) 97 – 109 107

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 aecvalue of 2.4 g MJ−1 _{PAR is used (Fig. 5). However, if} the ec 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 onecin this type of model (Demetriades-Shah et al., 1992; Reddy, 1995). Variability inecsuggests 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 6_{ersus actual} har6_{estable 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.

4.2.3. Potential annual 6_{ariation in yield}

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

5. Conclusions

(1) The model predictions show that the highest yield ofMiscanthusexpected in the cool temperate climate of Ireland is 26 t DM ha−1 _year−1_{. The} highest predicted yields for the island of Ireland are in the south-west and the lowest yields are in the north-east.

(3) Rainfall in Ireland is normally non-limiting for Miscanthus growth. To account for yield losses due to water deficits in dry years, further development of the model is required.

(4) Identical estimates of radiation use effi-ciency (ec) in the 1994 and 1995 growing seasons were obtained at the trial site in spite of the difference in climatic conditions between the

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years. Theec values calculated here appear to be suitable for use in the model predictions of yield in Ireland, but there is evidence that ec varies from one geographical region to another. The reasons for this variation is not clear and requires further investigation.

Acknowledgements

References

A,ngstro¨m, A., 1924. Solar and atmospheric radiation. Q. J. R. Meteorol. Soc. 50, 121 – 125.

Beale, C.V., Long, S.P., 1995. Can perennial C4 grasses attain high efficiencies of radiant energy conversion in cool cli-mates? Plant Cell Environ. 18, 641 – 650.

Beale, C.V., Long, S.P., 1997. Seasonal dynamics of nutrient accumulation and partitioning in the perennial C4 grasses

Miscanthus×giganteusandSpartina cynosuroides. Biomass Bioenergy 12, 419 – 428.

Beale, C.V., Bint, D.A., Long, S.P., 1996. Leaf photosynthesis in the C4-grass Miscanthus×giganteus, growing in the cool temperate climate of southern England. J. Exp. Botany 47, 267 – 273.

Clifton-Brown, J.C., Jones, M.B., 1997. The thermal response of leaf extension rate in genotypes of the C4-grassMiscan

-thus: an important factor in determining the potential productivity of different genotypes. J. Exp. Botany 48, 1573 – 1581.