Biomassand BiomnergyVol. 8. No. I. pp. l-5, 1995 ‘II 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0961-9534195 $9.50 + 0.00
THE PERFORMANCE OF POLYCLONAL STANDS IN SHORT ROTATION COPPICE WILLOW FOR ENERGY PRODUCTION W. M. DAWSON* and
A. R. MCCRACKEN?.
*Northern Ireland Horticulture and Plant Breeding Station, Loughgall, Co. Armagh, Northern Ireland BT61 8JB, U.K. tPlant Pathology Research Division, Department of Agriculture for Northern Ireland, Newforge Lane, Belfast, Northern Ireland BT9 5PX, U.K. (Received 17 March 1994; revised received I JUIV 1994; accepted 8 July 1994) Abstract-The benefits of growing Salix spp. as a short rotation coppice in polyclonal (mixed) stands as compared to monoclonal stands are described. Increased yields are obtained from the polyclonal stands when compared to either the mean yield of component clones or the individual yields of any of the component clones grown in monoculture. Different component clones contribute different degrees to the increased yield. Salix viminalis Bowles Hybrid and S. Burjatica Germany showed the greatest positive response. In contrast, S. dasyclados performed poorly in polyclonal stands, both in stool survival and in growth of individual shoots. Some of the reasons for the improved growth in polyclonal plots and their practical benefits are discussed and recommendations are made.
coppice; clonal mixtures; Mix
it became clear that because of the clonal nature of the crop, it was particularly vulnerable in Willow grown at close spacings (lO,OOO-20,000 monoclonal plots to development and rapid build up of serious disease problems. The ha-‘) harvested on a short rotation (3-5 years) absence of genetic diversity in the crop plants can produce significant amounts of stemwood even from marginal agricultural land-12 t DM exerts a strong selection pressure on potential ha - ‘, annually equivalent to approximately 228 disease organisms, particularly in this case where foliar rusts were involved and where the casual GJ ha-‘. ’ organism, Melampsora epitea has been shown to Increasing interest is being shown in this crop exist in at least eight different pathotypes.3,4 in the United Kingdom in response to two major Salk burjatica Korso, first introduced to the world issues. Firstly, in the short term since this programme in 1974, showed great potential crop is grown on a short cycle it can be considered resulting in a 2.0 ha monoclonal site being at worst a carbon-dioxide-neutral fuel, consumestablished in 1983. This stand having estabing at least as much carbon dioxide as is released on its combustion. Consequently, there is no net lished and grown well suffered a severe attack of addition to atmospheric carbon dioxide levels. rust in 1985, producing almost total defoliation by mid August. This clone has been attacked with Secondly, short-rotation coppice cropping provides an opportunity for large-scale farm varying degrees of severity every year since.s’6 have been recorded in diversification, playing a significant role in Similar trends S. mollissima undulata SQ83 introduced in 1983 changing EC policies and providing an alternate use for land taken out of food production. It has and first showing severe rust levels in 1992. Fungicides have been shown to have a been estimated that l-l.5 Mha of agricultural land in the United Kingdom may be surplus to significant controlling effect on the disease.’ food production by the year 2000, rising to 5.5 However, their use on short-rotation coppice Mha by 2010.2 This represents 5-30% of land cannot be considered a viable approach for economic, environmental and practical reasons. used for agricultural production in the U.K. Consequently, an alternative disease control Research and development work on the crop began at the Northern Ireland Horticultural and strategy was required and the possibility of Plant Breeding Station in the mid 1970s and introducing diversity and hence stability into the system was investigated using polyclonal or covers many aspects of its agronomy. However, 1. INTRODUCTION
W. M. DAWSONand A. R. MCCRACKEN
mixed stands. This paper presents data on the effect on yield of growing a number of clones in polyclonal stands compared with the same clones grown monoclonally. 2. MATERIALS AND METHODS
2.1. Establishment Three semi-commercial areas of Salix coppice, each of 3.3 ha, were established in consecutive years beginning in 1987 on the Department of Agriculture for Northern Ireland’s Grassland Experimental Farm as Castlearchdale, Co. Fermanagh (54”28’N 7”43‘W). The plots were established on unimproved grassland sites on heavily gleyed mineral soils with impeded drainage. All sites were between 60 and 70 m above sea level (Fig. 1). The three sites were established to a standard protocol-25 cm hardwood cuttings were inserted at 1.0 m x 0.5 m spacing on ground which had been ploughed the previous autumn. Residual herbicide (Simazine) was applied post planting. The established cuttings were cut back at the end of the first growing season and translocated and residual herbicides (aminotriazole and simazine) applied. From that initial cut-back, each plantation entered a 3 year cycle with the first 3 year harvest being taken in the winter of 1990 from the site established in 1987. Four clones were used on each site (Salix viminalis Bowles Hybrid, Salix viminalis 683, Salix x dasyclados and Salix burjatica Germany) and they were planted both as monoclonal plots and intimate mixtures (polyclonal).
Site I was established in 1987 as a single replicate block with a minimum plot size of 0.6 ha. Sites II and III, planted in 1988 and 1989, each had two replicate blocks and the minimum plot size was 0.25 ha. These large plot sizes were considered necessary to ensure that monoclonal plots of a specific clone were unaffected by surrounding plots and hence to give a soundly based practical evaluation of the concept of polyclonal planting.
2.2. Total yields Annual samples of 20 stools chosen at random were taken in the dormant season in 1988, 1989 and 1990 from each of the four monoclonal plots on sites I, II and III. At the same time, 20 stool samples were taken from the polyclonal plots comprising five stool samples of each of the constituent clones. In each year, therefore, samples were taken from 1, 2 and 3 year old stools in both monoclonal and polyclonal plots. Mean dry weight per stool was calculated. No attempt was made to differentiate between the contribution made by the individual component clones in the mixture. Collected data fell into three groups of equivalent aged material-l year old material from site I in 1988, from site II in 1989 and site III in 1990, and similarly for 2 and 3 year old material. Equivalent aged material was analysed separately using standard analysis of variance, treating the single replicate on site I and the two replicates on sites II and III as five replicates of equivalent aged material.
Fig. 1. Aerial photograph of the three semi-commercial blocks of S&K at Castlearchdale, Co. Fermanagh
The performance of polyclonal stands
2.3. Component yields in polyclonal plots In 1992 the contribution to the overall yield from the polyclonal plots of the four component clones was differentiated in sites I and III. In site I, on each monoclonal and on the polyclonal plot, two sub-plots of 200 stools were marked out. In site III, two plots were similarly marked out in each of the two replicate blocks. From these plots the tallest shoot from each of 10 randomly chosen stools was taken at various intervals from May 1991 to October 1992 over two growing seasons. When recording began in May 199 1, material harvested from site I was the regrowth from stools harvested in March 1991, whilst that from site III was beginning its second year of growth. Mean dry weight per shoot was calculated. This enabled comparisons to be made between individual clones growing in both monoclonal and polyclonal stands. Data was subjected to a normal analysis of variance. 3. RESULTS
3.1. Total yields Year 1. After 1 year’s growth, the mean dry matter yield from polyclonal plots (kg DM) was not significantly different from the mean of the four constituent clones grown in monoclonal stands. Yield from the polyclonal plots was greater than the yield from S. viminalis Bowles Hybrid, S. x dasyclados and S. burjatica Germany grown in monoclonal stands, but these differences were not statistically significant. In
contrast, S. viminalis 683 produced a greater yield as a monoclonal stand, but again this difference was not statistically significant. Year 2. The mean dry matter yield from the polyclonal plots was significantly greater than the mean yield from the constituent clones grown as monoclonal stands and also significantly greater than the individual yields of each of the component clones. Dry matter yield from the mixture exceeded its components, mean by 43.6% and the highest individual monoclonal yield by 37%. There were no significant differences between yields from any of the monoclonal stands. Year 3. The significantly higher yield from the polyclonal plots compared with the mean of the monoclonal plots or the individual yields of the component clones grown in monoclonal stands, recorded in year two was maintained in year three. Polyclonal yields were 37% greater than the mean from the monoclonal stands. In the monoclonal plots the highest yield was obtained from S. burjatica Germany and this was significantly higher than S. viminalis Bowles Hybrid and S. viminalis 683 (Fig. 2). 3.2. Component yields Bowles Hybrid. Dry matter yields were greater from polyclonal than from monoclonal plots throughout the first two years. The differences were not significant (P = 0.05) however, until towards the end of the second year of growth (74 weeks). This trend continued, and at the end of the third year of growth there was a large and
Time of harvest Fig. 2. Mean stool dry matter (kg) of four Mix clones grown as monoclonal or polyclonal stands after 1, 2 or 3 years’ growth. Horizontal line represents the mean stool dry matter of the component clones grown monoclonally. (Error bar ‘a’indicates LSD between polyclonal stands and mean of component clones grown in monoclonal stands. Error bar ‘b’indicates LSD between polyclonal stands and individual component clones grown in monoclonal stands.)
W. M. DAWSONand A. R. MCCRACKEN
significant (P = 0.1) yield difference between monoclonal and polyclonal plots. This improvement represented approximately 20% more yield for polyclonal as compared with monoclonal plots (Fig. 3A). Germany. A similar pattern was observed although significant differences (P = 0.01) were observed much earlier. Early in the second year (56 weeks) these dry matter yields were significantly greater (P = 0.01) in polyclonal plots, a situation repeated after 69 weeks (P = 0.05), 74 weeks (P = 0.05) and 128 weeks (P = 0.001); by the late recording date after 3 years’ growth and just before harvest the improvement was almost 51% (Fig. 3B). 683. Significantly greater yields were observed from polyclonal plots after 69 weeks (P = O.Ol), 74 weeks (P = 0.05) and 128 weeks (P = 0.01) (Fig 3C). Dasyclados. With the exception of a small significant (P = 0.005) difference early in the second year of growth (56 weeks), no benefit accrued from growing Dasyclados in polyclonal plots. Even after 3 years’ growth there was no difference between the two plots in terms of dry weight of individual shoots. Furthermore, survival of Dasyclodos in polyclonal stands was very poor (unpublished data) (Fig. 3D).
Cl Monoclonal I Polyclonal
i 600 I. 400
!! 1000 G 500
200 0t 30we&r
? ?Monoclonal ??Polyclonal
1111000 6 e 800 u 600
2 600 8
(1) 56 weeks
Period of growth
Period of growth
Period of growth
in monoclonal stands. The use of fungicides as a disease control strategy in short-rotation crops for energy is not an option for environmental, economic and practical reasons. Therefore, alternative methods of reducing disease impact are required. Disease impact has been reduced in other crops including cereals, potato, grasses and various tree crops by the use of variety mixtures.s Following the serious rust infections experienced in 1985 and 1986, particularly in large monoclonal plantings of Salk burjactica Korso, the use of mixtures as a control strategy was initiated. Furthermore, the problems associated with growing variety mixtures of cereals where the specification of the end product is critical do not apply to willow grown as an energy crop, since the objective is simply to maximise the production of wood. When crops are grown in mixtures, those mixtures usually yield as well as, if not better than, their components and often exceed the yield of the best components At least part of this increased yield is due to disease restriction.
(Melampsora spp.) infections have been experienced when Salix spp. clones have been grown
Period of growth
Fig. 3. Mean shoot dry matter(g) of four Sdix clones grown in monoclonal and polyclonal stands measured on five occasions. Periods of growth measured from time of coppicing. Numbers in brackets represent site. (ns = no significant difference, * = P > 0.05, **= P > 0.01, *** = P > 0.001.)
The performance of polyclonal stands
Disease restriction can function at three levels. These three levels are: (i) the influence of lower densities of susceptible plants; (ii) the barrier effect of disease resistant plants; and (iii) the induced resistance due to non-virulent pathogen biotypes.’ Differences in yield between monoclonal and polyclonal stands do not become evident until the end of the second year of growth. Differences recorded at this time were maintained throughout the third growing season. When the contribution of component clones was identified, it was apparent that clonal response varied. Where increases in yield were recorded, these were only becoming apparent at the beginning of the second year of growth. Willebrand et al. reported increased yield for polyclonal stands in all harvests of a 3 year rotation cycle.” These differences were evident in 1 and 2 year rotation cycles. Similar patterns were obtained in the results reported in this paper. However, at least one clone, Salix x dasyclados, did not show any positive response to being grown in polyclonal stands. There is strong evidence to indicate that clonal response in Mix spp. is site specific.lol” However, we consider that the evidence presented in this paper along with that from SwedenlO clearly indicates that the differential between monoclonal and polyclonal yields is maintained irrespective of site, although the relative response of component clones may differ between sites. 5. RECOMMENDATIONS We would therefore recommend the use of polyclonal stands where commercial planting of short rotation coppice willow for energy is being considered for three main reasons:
(1) yields from polyclonal stands are consistently higher than either the mean of clones or even the highest yielding clone grown as monoclonal stands; (2) even where a single component clone fails to make a positive contribution, the yield of the polyclonal stand is still better than the corresponding monoclonal stand; (3) polyclonal stands reduce the problems associated with the rapid build up of disease in a clonally propagated crop. REFERENCES 1. G. H. McElroy and W. M. Dawson, Biomass from short rotation coppice willow on marginal land. Biomuss 10,
225-240 (1986). 2. M. Carter, The positive application of set aside. Proc. Oxford Farming Conf.
3. A. R. McCracken and W. M. Dawson, Clonal response in Sahx to Melampsora rusts in short rotation coppice plantations. Europ. J. Forest Parhol. 22, 19 (1992). 4. M. H. Pei, D. J. Royle and T. Hunter, Identity and host alternation of some willow rusts (Melampsoru spp.) in Ennland. Mvcol. Res. 97. 845-851 (1993). 5. W.-M. Da&on, Some aspects of the development of short-rotation coppice willow for biomass in Northern Ireland. Proc. R. Sot. Edin. 98B, 193-205 (1992). 6. A. R. McCracken and W. M. Dawson, The effect of Melampsora sp. rust on the growth and development of Salix in N. Ireland. Proc. Joinf Meeting of International Energy Agency-Task 5 Activity Group (ed. V. Steenackers and P. Smets), Geraardsbergen, pp. 15-26 (September 1990). 7. W. M. Dawson and A. R. McCracken, Effect of Melampsora rust on the growth and development of Salix burjatica ‘Korso’ in Northern Ireland. Europ. J. Forest Pathol. 24, 32-39 (1994).
8. M. S. Wolfe, The current status and prospects of multiline cultivars and variety mixtures for disease resistance. Ann. Rev. Phyropathol. 23, 251-273 (1985). 9. K. M. Chin and M. S. Wolfe, The spread of Erysiphe graminis f. sp. hordei in mixtures of barley varieties. Pfanr Pathol. 33, 89-100 (1984).
10. E. Willebrand. S. Ledin and T. Verwiist. Willow counice systems in short rotation forestry: effects of ‘piant spacing, rotation length and clonal composition on biomass production. Biomass Bioenergy 4, 323-331 (1993).
Il. C. P. Mitchell, A. D. Wightman, J. B. Forbes-Robertson and R. Ennion, Establishment and monitoring of large scale trials of short rotation forestry for energy. ETSU contract report B1171, p. 71 (1989).