The biomass production of three poplar clones in relation to intercepted solar radiation
R. Milne a, M. Sattin b'l, J.D. Deans a, P.G. Jarv is b a n d M . G . R . C a n n e l l a
aInstitute of TerrestriaI Ecology, Bush Estate, Penicuik, EH26 OQB, UK bInstitute of Ecology and Resource Management, University of Edinburgh, Mayfield Rd., Edinburgh,
EH9 3JU, UK
(Accepted 25 February 1992)
ABSTRACT
Milne, R., Sattin, M., Deans, J.D., Jarvis, P.G. and Cannell, M.G.R., 1992. The biomass production of three poplar clones in relation to intercepted solar radiation. For. Ecol. Manage., 55: l-14.
The biomass production, in relation to intercepted solar radiation energy, of the poplar clones, cultivars 'Beaupr6', 'Fritzi Pauley' and 'Robusta', is compared for stands, grown in containers from cuttings, over one season in Scotland. The 'Beaupr6' stand produced 8 t ha - 1 of woody biomass, while the 'Fritzi Pauley' and 'Robusta' stands produced 7 t ha - ' and 5 t ha - 1 respectively. Although it had the smallest biomass production 'Robusta' had the largest solar radiation conversion ratio, i.e. 0.59 g M J - l, while 'Beaupr6' produced 0.56 g M J - ~ and 'Fritzi Pauley' 0.47 g M J - '. The lower production of 'Robusta' was due to late canopy development reducing the total amount of solar radiation intercepted.
The results suggest that, of the three clones compared, 'Beaupr6' would be the most useful for short- rotation energy forestry in Europe for three reasons. First, it develops its canopy early in the season enabling the capture of the large amount of solar radiation available at that time. Second, it has a high ratio of conversion from solar radiation to dry matter. Third, stem growth continues into November, even in the UK.
I N T R O D U C T I O N
Comparisons of the biomass production of clones of Populus have been carried out extensively with the objective of selecting those which would max- imise pulp and fuel wood from short rotation coppices (e.g. Ceulemans, 1990). Here we compare the woody biomass production of three clones of poplar grown in containers from cuttings over one season. The dry matter produc- tion of the trees is analysed in relation to intercepted solar radiation energy
Correspondence to: R. Milne, Institute of Terrestrial Ecology, Bush Estate, Penicuik, EH26 0QB, UK. 'On leave from: Centro per lo Studio dei Diserbanti C.N.R., Istituto di Agronomia, Universi ta Degli Studi, 35100 Padova, Italy.
using a description, introduced by Monteith ( 1977 ), often termed 'light-use efficiency' or 'growth efficiency' but here termed the 'solar radiation conver- sion ratio'. The concept has already been used, for example, by Eckersten and Nilsson ( 1990 ) for Salix viminalis, CanneU and co-workers ( 1987, 1988 ) for Salix and Populus and discussed by Jarvis and Leverenz (1983) for Pinus radiata. The concept assumes that dry mass production is linearly related to the amount of incoming solar radiation absorbed by the crop. Cannell et al. (1988) showed that, although the solar radiation conversion ratio in Salix and Populus clones studied were similar, Salix developed its canopy sooner, absorbed more energy and partitioned less dry matter to roots. These features enabled the Salix clone to produce twice as much woody biomass in one sea- son as the Populus clone produced in another season, despite incoming solar energy being about 10% less.
The objective of this investigation is to determine the influence of canopy structure, solar radiation conversion ratio and partitioning of dry matter in determining the production of stem wood in three Populus clones, cultivars 'Beaupr6', 'Fritzi Pauley' and 'Robusta'.
MATERIALS AND METHODS
The principle of the analysis is that dry matter production of tree crops is directly proportional to the rate at which incoming solar radiation energy is absorbed, i.e.
dW - - < I (1) dt
where W is the dry matter per unit ground area (g m-2) and I is the absorbed energy flux (J s- 1 m-2) . The solar radiation reflected from the canopy is only 5-10% of incoming energy and is usually ignored, allowing energy intercepted to be used in place of that absorbed. The proportionality will not be accurate when there is significant leaf or plant death. The biomass accumulated over a given time (B, g m - : ) is therefore given by
B= ~ efSodt (2)
where So is the total incoming solar radiation energy (MJ m -2 ) , f i s the frac- tion of that radiation which is intercepted by the canopy and e (g MJ- 1 ) is the solar radiation conversion ratio. Over a period of measurement a constant value for E is assumed and is given by
B e - - - (3)
f fSodt
SOLAR RADIATION AND BIOMASS PRODUCTION IN POPLARS 3
Values of e can be calculated for total biomass or individual components of the stand, particularly for woody biomass.
Here, the values of e for total and woody biomass were estimated for the three Populus clones, 'Beaupr6' (Populus trichocarpa Torr. & Gray V- 235 X Populus deltoides Bartr. S. 1-173, an Interamerican clone), 'Fritzi Pau- ley' (P. trichocarpa Torr. & Gray V-235) and 'Robusta' (P. del- toides × Populus nigra, a Euramerican clone also known as Populus euramer- icana (Dode) Guinier).
Uniform plots of each clone were established on level ground at 185 m above sea level at Bush Estate, near Edinburgh, Scotland, (56°51 'N, 3 ° 12'W) in spring 1989 by the following methods. Eighteen hundred 10 dm 3 pots were filled with potting compost comprising loam, peat and 2-3 m m grit in the proportions 7: 3 : 4, by vol. To each pot, 50 g of a slow release fertiliser was added and thoroughly mixed into the compost. This fertiliser provided an additional 8 g of N, 1.75 g of P and 5 g of K per pot, as well as all other necessary mineral elements.
Single cuttings were inserted into each pot (600 per clone) before setting the pots out in three separate clonal blocks (40 X 15 pots) to form a single block of 40 X 45 pots. A min imum of five plants formed a perimeter strip around the plants which were to be harvested, and an additional strip, eight plants wide, separated plants designated to be harvested from adjacent clones. The pots were 287 m m in diameter and were laid out touching each other, giving a density of 12.14 plants per square metre.
The variability of growth potential between individual plants in the differ- ing clones was limited by the following procedure. Length, mid-point diame- ter and dry mass were recorded for 20 cuttings from each clone. The volume of each cutting was calculated and a regression line fitted which estimated the dry mass of a cutting from its volume. Each cutting that would produce a harvested plant was selected to be within one standard error of the mean cut- ting volume of the clone. This initial volume was used in the regression to estimate the initial dry mass of the cutting, which was then taken into account in estimating dry mass increments. Cutting lengths did not vary much be- tween clones (20-25 cm) but the diameter, and hence the volume and mass, differed substantially between clones. Mid-point diameter averaged 9.1 mm, 8.9 m m and 7.3 m m for 'Fritzi Pauley', 'Beaupr6' and 'Robusta', respectively, while weights per unit volume were about 0.43 g cm -3, 0.33 g c m -3 and 0.33 g c m - 3, respectively.
All plants were set out on capillary matting in their blocks on 13 April 1989. Plants which failed to develop were replaced on 25 May 1989 by others, which had known initial cutting dimensions, and which had been similarly main- tained adjacent to the experimental block. After setting out the plants, the capillary matting was soaked by a fixed network of hoses and the cuttings were watered from above to enable capillary rise of water from the matting to
4 R. MILNE ET AL.
the pots. The capillary matting was maintained at full water capacity through- out the study, and during periods of hot dry weather additional water was supplied to the pots from overhead sprinklers. As the plants grew, a green net barrier was progressively raised around the perimeter of the experimental plot. The net was 50% permeable to light and wind and reduced edge effects while providing protection from wind damage.
To estimate the increase in dry matter of the stands, ten pre-selected plants were taken from each clone block on 11 July, 14 August, 4 September, 2 Oc- tober and 6 November, all 1989. On each occasion, the space left in the block was filled by moving pots in from positions nearer the edge of the block. For each sampled plant the following characteristics were measured: height, root dry mass, cutting dry mass, dry mass and area of leaves and dry mass of stem in each of the height intervals, less than 0.3 m, 0.3-0.55 m, 0.55-0.80 m, 0.80-1.05 m, 1.05-1.30 m, 1.30-1.55 m and greater than 1.55 m. Mean val- ues of these variables for the ten sample plants were then calculated for each harvest date. The mean increment in cutting dry mass was calculated by al- lowing for the original dry mass of individual cuttings. The total plant and stem only dry mass increments were calculated from these means, for the pe- riods from the start of the experiment to each harvest. After the start of leaf fall in any height interval, the peak leaf dry mass achieved in that interval was used in subsequent calculations of total dry mass production.
Solar radiation flux was measured using 1.0 m long tube solarimeters (wavelength range 300-3000 nm ). Two solarimeters were placed beneath each of the clones at the position of the final sample plants and one in open ground nearby. A dome solarimeter was placed beside this tube solarimeter to pro- vide measurements which could be used to make adjustments for the non- uniform angular response of the tube. The signals from all solarimeters were recorded by a data logger and stored as hourly totals which were transferred to a mainframe computer using a portable computer. These hourly totals were subsequently used to produce weekly and inter-harvest totals of intercepted solar radiation for each of the clones. From visual observation, 5 June was taken as the starting day for growth and solar radiation energy totals were accumulated from this date.
For each clone it was assumed that e was a constant throughout the season and for all plant samples. The biomass data and the measured accumulated intercepted solar radiation could therefore provide estimates of E over five different t ime periods, each starting on 5 June. The value of e up to the first harvest was atypical however, because of partial canopy development (par- ticularly for 'Robusta' ), and hence the mean of the values calculated over the periods ending at the remaining four harvests was taken as the estimate of e for that clone over the season.
S O L A R R A D I A T I O N A N D B I O M A S S P R O D U C T I O N I N P O P L A R S 5
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6 R. MILNE ET AL.
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Fig 2. Dry matter components of poplar clones (a) 'Beaupr6', (b) 'Fritzi Pauley', and (c) 'Ro- busta' at Bush Estate near Edinburgh in 1989. Each point is the average from ten plants and is expressed as a mass per unit ground area using container spacing (0.29 m×0.29 m).
SOLAR RADIATION AND BIOMASS PRODUCTION IN POPLARS 7
RESULTS
Weather
Figure 1 shows the mean max imum and min imum air temperature, the mean wind run per day, the total rainfall and the total incoming solar radia- tion for each week starting on 5 June 1989 and finishing at the last harvest date on 6 November. The early part of the period was generally fine and warm, except for 1 week at the beginning of July, with incoming solar radiation reaching a peak daily total of 22 MJ m-2 After the second harvest the weather deteriorated with strong winds and heavy rain. Although these adverse con- ditions did not recur, solar radiation and temperature were now falling and growth rate did not recover to the peak achieved before the second harvest.
Crop growth and canopy development
'Beauprr ' and 'Fritzi Pauley' were similar in their patterns of growth while 'Robusta' was slow to develop initially and produced less total dry matter by the end of the season (Fig. 2 ). By that time, 'Beauprr ' and 'Fritzi Pauley' had a maximum height of 2.4 m while 'Robusta' was 1.7 m. The canopies of 'Beauprr' and 'Fritzi Pauley' developed at the same rate to maximum leaf area indices of 5.6 and 5.1, respectively, by the second harvest (14 August) (Fig. 3 ). The canopy of 'Robusta ' started growing later but grew very quickly to overtake the others and reach a maximum leaf area index of 6.2 by 14 August. The nitrogen content of sample leaves taken from the tops of the har-
7 O Beaupre • Fritzi Pauley v Robusta
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Fig. 3. Growth of leaf area of poplar clones (a) 'Beauprr', (b) 'Fritzi Pauley', and (c) 'Robusta' at Bush Estate near Edinburgh in 1989. Each point is the average from ten plants and is ex- pressed as a leaf area index, i.e. leaf area per unit ground area, using container spacing (0.29 m×0.29 m).
8 R. MILNE ET AL.
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Fig. 4. Vertical development of leaf canopy in poplar clones (a) 'Beaupr6', (b) 'Fritzi Pauley', and (c) 'Robusta' at Bush Estate near Edinburgh in 1989. Each point is the average of the leaf area in vertical height intervals of ten plants as described in the text and is plotted at the height of the section mid-point. The values are expressed as leaf area index, i.e. leaf area in each inter- val per unit ground area, using container spacing (0.29 m×0.29 m). The harvest on 6 Novem- ber is not shown as there were few leaves on any clone at that time.
SOLAR RADIATION AND BIOMASS PRODUCTION IN POPLARS 9
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Fig. 5. Relationship between dry matter per unit ground area and intercepted solar radiation in poplar clones (a) 'Beaupr6', (b) 'Fritzi Pauley', and (c) 'Robusta' at Bush Estate near Edin- burgh in 1989. The value at each harvest is the average of ten plants and excludes the initial mass of the cutting but includes (in the total values) the increase resulting from growth in the cutting. The zero point for growth, estimated to be 5 June, is included for reference. The error bars are the SEM and for the earlier harvests are smaller than the symbol for the mean.
10 R. MILNE ET AL.
TABLE 1
Percentage allocation after one season of accumulated dry matter in three clones of poplar growing in containers at Bush Estate, Edinburgh, S.E. Scotland
1The value for the cutting is the increment in dry matter above the mass at planting.
TABLE2
Dry matter production (g m-2 ) and solar radiation conversion ratio, ~, (g M J-~ ) for (a) stem wood and (b) total dry matter for poplar clones 'Beaupr6', 'Fritzi Pauley' and 'Robusta' , grown over one season from cuttings in containers at Bush Estate, Edinburgh, S.W. Scotland. The data relate to the periods from 5 June to each harvest date. Seasonal values are calculated as the mean of the values found over these different periods, but exclude the value for the period up to the first harvest
Harvest date 'Beaupr6' 'Fritzi Pauley' 'Robusta '
Dry matter Conversion Dry matter Conversion Dry matter Conversion production ratio production ratio production ratio ( gm -2) (gMJ - l ) ( g m -2) (gMJ - l ) ( g m -2) (gMJ - l )
(a) Stem wood dry matter 11 July 82 0.38 64 0.35 17 1.03 14 August 377 0.51 312 0.45 246 0.65 4 September 442 0.47 386 0.41 280 0.48 2 October 714 0.64 484 0.45 497 0.65 6 November 765 0.62 669 0.55 505 0.58
(b) Total dry matter 11 July 339 1.58 271 1.50 113 7.02 14 August 850 1.15 893 1.30 713 1.88 4 September 1016 1.09 963 1.08 765 1.32 2 October 1424 1.27 1135 1.05 1156 1.51 6 November 1526 1.23 1405 1.17 1216 1.39
Seasonal mean 1.19 + 0.04 1.15 + 0.06 1.53 + 0.12
vested plants did not fall below 2%, even at the last harvest, and was generally about 4% of dry mass. With the adverse weather conditions in late August leaves were lost and the leaf area index of each clone fell continuously for the rest of the season. The vertical development of the canopies is shown in Fig. 4. At the first two harvests the canopy was distributed with a peak leaf area density below the top of the crop while later the lower leaves were progres- sively lost (particularly during the wet and rainy period of late August) so
SOLAR RADIATION AND BIOMASS PRODUCTION IN POPLARS 1 1
that by the third and fourth harvests the maximum leaf density was at the top of the canopy. At the last harvest ( 6 November) there were few leaves left on the plants of any clone.
Dry matter production and intercepted solar radiation
The accumulated dry matter and intercepted solar radiation for each of the clones at each harvest is shown in Fig. 5. By the last harvest 'Beaupr6', 'Fritzi Pauley' and 'Robusta' had accumulated, respectively, 1526_ + 122 g m -2, 1405 ___ 155 g m -2 and 1216 + 158 g m -2 of dry matter in total while the stem components of these were 765 ___ 77 g m -2, 669 ___ 80 g m -z and 505 + 86 g m -2 (all errors quoted in this paper are Standard Error of Mean (SEM) and 1 g m - 2 = 0.01 t h a - l ) . Allocation of dry matter to the growth of the cutting, to the roots, stems and leaves by the last harvest is given in Table 1, which shows that 'Beaupr6' and 'Fritzi Pauley' put similar fractions of 50 -+ 1% and 48 -+ 2% of the dry matter into stem wood while less (42 _+ 3%) went into the stems of 'Robusta'.
The seasonal mean values of solar radiation conversion ratio, E, for total dry matter were I. 19 _+ 0.04 g M J - 1 for 'Beaupr6', 1.15 + 0.06 g MJ- 1 for 'Fritzi Pauley' and 1.53 _+ 0.12 g MJ-1 for 'Robusta' (Table 2 ). The ratios for con- version of intercepted solar radiation to stem wood did not follow the same pattern across the clones as for total dry matter, since 'Beaupr6' had a value of 0.56 ___ 0.04 g MJ- 1 and 'Robusta' 0.59 ___ 0.04 g MJ- 1 but 'Fritzi Pauley' had the low value of 0.47 ___ 0.03 g MJ-1 (Table 2). The values relevant to total dry matter up to the first harvest were high compared with the later harvests, because the solarimeters were not fully covered by the sparse canopy present at that time. An analysis of variance of e, for both total and stem wood dry matter, relevant to the other four harvests showed that, at the 95% level, there were no significant harvest effects, but that there were significant differ- ences between clones.
D I S C U S S I O N
The clone 'Beaupr6' produced the largest amount of dry matter while 'Ro- busta' produced the least. The solar radiation conversion ratio was however greatest in 'Robusta' ( 1.53 g M J-1, Table 2) with 'Beaupr6' and 'Fritzi Pau- ley' being about 25% lower. The reason for the poor production of 'Robusta' lies in the lower fractional interception both on an instantaneous and an ac- cumulated basis (see Table 3 ). The canopy of 'Robusta' developed later than the others and, although reaching a higher peak leaf area index, had a lower fractional interceptance when fully developed. There was also more variation from sample to sample in 'Robusta', as can be seen from the pattern of total dry matter recorded in Fig. 5. The 'Robusta' cuttings were smaller than those
12 R. MILNE ET AL.
TABLE3
Fractional interception of solar energy and leaf area index for poplar clones 'Beaupr6', 'Fritzi Pauley' and 'Robusta ' growing in containers at Bush Estate near Edinburgh in 1989. The harvest interception values are for periods of 1 week centred on the harvest date while the seasonal totals refer to the period from 5 June to 6 November. All interception was measured at the position which was to be the final harvest. Leaf area index, i.e. leaf area per unit ground area using container spacing (0.29 m x 0 . 2 9 m ), is the average from ten plants
Harvest date 'Beaupr6' 'Fritzi Pauley' 'Robusta '
Fractional Leaf area Fractional Leaf area Fractional Leaf area interception index interception index interception index
11 July 0.76 2.8 0.68 2.0 0.20 1.0 14 August 0.86 5.6 0.88 5.1 0.85 6.2 4 September 0.84 3.3 0.87 3.4 0.87 4.2 2 October 0.81 2.8 0.80 2.7 0.80 3.3 6 November 0.61 0.0 0.64 0.7 0.48 0.0
Seasonal total 0.64 0.62 0.45
of the other clones and an analysis of covariance showed that the difference in size of cuttings had a significant effect on growth. However, there were still significant clonal differences in growth after removal of this cutting size ef- fect. 'Beaupr6' and 'Fritzi Pauley' were similar in their dry matter production with the former producing 10% more in total.
Allocation of the total dry matter was also similar in 'Beaupr6' and 'Fritzi Pauley' with about 50% going to stems by the end of the season. 'Robusta' allocated only 42% of total dry matter to stems. Barigah et al. (1990) found more allocation to roots in field-planted 'Robusta' at Orsay, France than in the cultivar 'Raspalje', a poplar clone similar in origin to 'Beaupr6'. Here, however, the dry matter not allocated to stems in 'Robusta' was not all found in the roots, but also contributed to the larger growth of the cutting. The frac- tion of accumulated dry matter allocated to leaf dry mass over the season was similar in the three clones.
The growth and dry matter allocation of'Robusta' reported here are similar to those reported previously (Cannell et al., 1988 ) for the P. trichocarpa clone, numbered cultivar ' 1562' by Scott Pauley, grown in containers under similar soil conditions. That clone had a total dry matter production of 12 t ha- 1 of which 42% was in the stems and over the period up to the end of September the solar radiation conversion ratio was 1.50 g MJ- 1 for total and 0.72 g MJ- l for stem dry matter compared with 1.51 g MJ -~ and 0.65 g MJ -~ for 'Ro- busta' here. 'Robusta' and the ' 1562' clone were also similar in that no further dry matter was added to the stems after the end of September as compared with the 'Beaupr6' and 'Fritzi Pauley' which both showed increased stem mass at the last harvest on 6 November. Barigah et al. ( 1990 ) also found that 'Ro-
SOLAR RADIATION AND BIOMASS PRODUCTION IN POPLARS 13
busta' did not perform as well as 'Beaupr6' and 'Fritzi Pauley' in a trial with 0.8 × 0.8 m 2 spacing near Paris (48 ° 50' N) where it might have been expected that the warmer conditions, compared with Scotland, in spring, would have allowed 'Robusta' to develop its canopy earlier and hence produce more growth.
Impens et al. (1990) recorded seasonal average solar radiation conversion ratio for stem wood production in the second growing season at Gent (51 °02 'N) in a 0 .8×0.8 m 2 spacing trial of 0.75 g MJ -1 for 'Beaupr6', 0.57 g M J- 1 for 'Fritzi Pauley' and 0.47 g M J - 1 for 'Robusta', suggesting that solar radiation conversion ratio depends on soil and weather conditions and will not be a constant for a particular clone. They did, however, find that 'Ro- busta' at their site had a shorter canopy duration than 'Beaupr6' and 'Fritzi Pauley' in agreement with the results reported here for the same clones grown in containers at Edinburgh.
Sheppard (L.J. Sheppard, personal communication, 1991 ) measured the solar radiation conversion ratio for stem wood production in unirrigated 0.35 ×0.35 m 2 field plots near Edinburgh for eight Populus clones (Table 4) and found values of 0.43 g MJ-~ for 'Robusta' and 0.44 g M J - l for Scott Pauley No. '1562'. Ceulemans (1990) quotes averages of 0.70 g MJ -~ for Interamerican clones, 0.63 g M J - 1 for P. trichocarpa clones and 0.47 g M J - for 'Robusta' in a review of studies in the USA and Europe. Taken together these results suggest that, under good growing conditions like those used here, 'Robusta' can improve its solar radiation conversion ratio but that this is in- sufficient to improve the stem production, because the light intercepted re- mains less than in other clones.
The results reported here, with those of Impens et al. (1990) and Barigah et al. ( 1989 ), suggest that 'Beaupr6' is a good clone for short-rotation energy
TABLE 4
Solar radiation conversion ratio to stem wood for eight 3-year-old P. trichocarpa clones growing at 0.35 m × 0.35 m spacing under field conditions after coppicing in March 1984 (L.J. Sheppard, per- sonal communication, 1991 ). The conversion ratios refer to the period June to August 1984
Clone Stem wood production Stem wood conversion ratio ( g m -2) (gMJ -~)
forestry in northern Europe, with a seasonal mean solar radiation conversion ratio of 0.56-0.75 g M J-~, seasonal light interception reaching 64% in a can- opy that develops early and has stem growth continuing into November, even in northern latitudes.
ACKNOWLEDGEMENTS
Bob Astles, his staff at Edinburgh University, and Ray Ottley and Frank Harvey at I.T.E. are thanked for their work in planting out, and later in meas- uring, the trees.
REFERENCES
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Cannell, M.G.R., Milne, R., Sheppard, L.J. and Unsworth, M.H., 1987. Radiation interception and productivity of willow. J. Appl. Ecol., 24:261-278.
Cannell, M.G.R., Sheppard, L.J. and Milne, R., 1988. Light use efficiency and woody biomass production of poplar and willow. Forestry, 61: 126-136.
Ceulemans, R., 1990. Genetic variation in functional and structural productivity determinants in poplar. Thesis Publishers, Amsterdam, 100 pp.
Eckersten, H. and Nilsson, L-O., 1990. Light absorption and willow production in Southern Sweden, a case study. In: Burley, J. (Editor), Proceedings of Division 2 of the XIXth IUFRO World Congress, Montreal, Canada, August 1990. IUFRO Secretariat, Vienna, Austria, pp. 59-67.
Impens, I., Mau, F., van Hecke, P., Chen, S. and Ceulemans, R., 1990. Clonal differences in solar radiation conversion efficiency of closely spaced poplar stands. In: G. Grassi, G. Gosse and G. dos Santos (Editors), Biomass for Energy and Industry. Vol. 1., Elsevier, London, pp. 439-445.
Jarvis, P.G. and Leverenz, J.W., 1983. Productivity of temperate deciduous and evergreen for- est. In: O.L. Lange, P.S. Nobel, C.B. Osmond and H. Ziegler (Editors), Encyclopedia of Plant Physiology, Vol. 12D, Physiological Plant Ecology IV, Springer, Berlin, pp. 234-280.
Monteith, J.L., 1977. Climate and the efficiency of crop production in Britain. Philos. Trans. R. Soc. London, Ser. B, 281: 277-294.
