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Interaction of willow (Salix) clones grown in polyclonal stands in short rotation coppice
Biomassand Bioenergy Vol. 10. Nos S/6, pp. 307-31I. 1996
Copyright 0 1996published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0961-9534/96$15.00+ 0.00 PII: SO961-9534(96)00001-3
Pergamon
INTERACTION POLYCLONAL
OF WILLOW (SALIX) CLONES GROWN IN STANDS IN SHORT ROTATION COPPICE A. R. MCCRACKEN* and W. M. DAWSON~
*Applied Plant Science Division, Department of Agriculture for Northern Ireland, Newforge Lane, Belfast, Northern Ireland BT9 5PX TNorthern Ireland Horticulture and Plant Breeding Station, Loughgall, Co. Armagh, Northern Ireland BT6l 8JB (Received 30 May 1995; revised 22 December
199s; accepted 3 January 1996)
Abstract-Stool survival of Salix spp. clones is reduced when growing in polyclonal stands as compared to monoclonal stands. Stools growing in polyclonal stands show increased vigour with compensation of weak or dead stools from adjacent stools. In monoclonal stands the stools tend to be less vigorous but there is also less variation between stools. When a clone, which has become highly susceptible to rust infection, is included in the mixture, e.g. Salix burjarica Korso, it will die out completely within two years. Copyright 0 1996 published by Elsevier Science Ltd. Keyw0rdsSali.v;
biomass; clonal mixtures; short rotation coppice; yield.
1. INTRODUCTION
The potential for growing willow (S&X spp. ) at high density in short rotation coppice as a source of biomass for energy production is now well established.lm3 Wherever willow has been grown in the United Kingdom in monoclonal plantations there has been a severe and increasing problem with foliar rust disease, caused mainly by pathotypes of Melampsora epitea.4.5 In the most severe cases this has resulted in premature and complete defoliation, e.g. Salix burjatica Korso, which can no longer be grown in commercial plantations.6 The problem has been less severe, and not increasing, under the continental climatic conditions experienced in Sweden (Ramstedt, personal communication). The use of fungicides for the control of rust disease in willow plantations is not a viable option for economic, environmental and practical reasons and alternative methods of reducing disease impact have been investigated. Willow grown in polyclonal plantations of intimate mixtures of up to six different clones has given positive yield responses. Increased yields were obtained from polyclonal stands when compared to either the mean yield of the component clones or the individual yields of any of the component clones grown in monoculture.’ However, not all clones had similar survival and growth patterns when compared to either
the same clone in monoculture or to other clones in the mixture. The data presented in this paper compare the survival, vigour and yield of a range of Salix spp. clones, both in monoclonal and polyclonal stands and offer explanations for the interactions occurring between individual stools. 2. MATERIALS
AND METHODS
2.1. Willow plantations 2.1.1. Loughgall site. Five clones of Salix: Salix x dasyclados, S. burjatica Korso, S. burjatica Germany, S. viminalis Bowles Hybrid and S. viminalis 683 were planted in monoclonal
and polyclonal plots of structured, intimate mixtures at a density of 20,000 cuttings ha - ’in 1986. Clones were planted in sets of five, repeating the same sequence. Both monoclonal plots and polyclonal plots were approximately 300 m* and were arranged in three randomised blocks. In the winter of each year after establishment all stools were cut back to ground level. 2.1.2. Castlearchdale site. Two large areas were planted with Salix at a density of 20,000 cuttings ha - ‘: Area 1: five clones were planted in 1987: S. x dasyclados, S. burjatica Korso, S. burjatica Germany, S. viminalis Bowles Hybrid and S. viminalis 683. Plots were established using 307
A. R. MCCRACKENand W. M. DAWSON
308
hardwood cuttings in either monoclonal plots or in structured, intimate mixtures as polyclonal plots. Stools were harvested during the winter of 1990/91 and 1993/94. Both monoclonal and polyclonal plots were a minimum of 2000 ml, planted in a single block. Area 2: six clones were planted in 1989: S. x dasyclados, S. x calodendron, S. burjatica Germany, S. viminalis Bowles Hybrid, S. viminalis 683 and S. mollissima undulata SQ83. Plots were established as both monoclonal and polyclonal stands. Minimum plot size was 2500 m2. Stools were harvested in the winter 1992193. 2.2. Survival Survival of stools at the Castlearchdale sites was recorded in Spring 1994. In Area 1 stools were flushing following their second three-year harvest. In Area 2 stools were beginning their second year of growth from stools first harvested as three-year old material in 1992/93. In addition to stools which were dead or missing, stools where growth was weak or competition from surrounding stools had overgrown them were also considered not to have survived. 2.3. Vigour Vigour of individual stools was assessed during the winter 1994/95 at the Loughgall site. In monoclonal plots every stool in each of three randomly chosen rows was assessed. Each row had between 40 and 55 stools. The vigour of every stool in each of the polyclonal plots was assessed. The vigour of each individual stool was assessed on a O-4 scale, where: 0 = dead; 1 = still living, very few weak stunted shoots; 2 = growth, one or two large healthy shoots; 3 = vigorous growth, three to four large healthy shoots; and 4 = vigorous growth, several large healthy shoots. The purpose of assessing vigour was to make a judgement on the health and growth potential of stools and was not necessarily to measure potential yield. Comparisons of vigour were made between stools of the same clone growing in either monoclonal or polyclonal plots.
2.4. Yield At Loughgall in the monoclonal plots, ten consecutive stools were selected at random, in each of three rows. Each stool was harvested individually, weighed and a dry matter yield was determined. In polyclonal plots, 40 consecutive stools were selected at random in each of three rows. This sample should therefore have comprised 10 stools of each of the component clones. As before, each individual stool was harvested, weighed and a dry matter yield was determined. Stools were placed in size classes according to their individual dry matter yield as follows: 0= 1I 2= 3= 42
dead, 250 g, 251-550 g, 551-850 g, 851 g.
The percentage number of stools in each size class was calculated. The yield contribution, from each size class from individual clones, to the total yield was calculated. Total yield was calculated to a standard 50 m2. Analysis of variance was used to compare differences between yields in monoclonal and polyclonal plots, and between yield in each of the size classes. 3. RESULTS
3.1. Survival When stool survival on the polyclonal plots was assessed in Area 1 at Castlearchdale, only four clones had survived, S. burjatica Korso having completely died out. Differences in the survival of the four remaining clones were not significant (P < 0.05). Stool survival was between 77% and 82% producing a density of 13,000 stools ha - ‘. Survival of clones growing in monoclonal plots was not significantly (P < 0.05) different between clones. Neither was there any significant (P < 0.05) difference between the clones grown in monoclonal or polyclonal plots. Survival was between 80 and 83% resulting in density in the monoclonal plots of approximately 16,000 ha-‘. In both cases these were recorded 7 years after planting and after 2 three-year harvests, In Area 2 at Castlearchdale survival of all six clones was significantly less (P < 0.05) in the polyclonal plots when compared to survival in the monoclonal plots (Fig. 1). The greatest
309
Interaction of willow clones
difference was in S. x calodendron in which 98% of stools survived in the monoclonal plots while there was only 35% survival in the polyclonal plots. The smallest difference, of less than 10% reduction, was recorded in S. viminalis 683. In the remaining clones, survival in the polyclonal plots was reduced to less than 60% (Fig. 1). Final stool density 5 years after planting and after a single harvest was approximately 13,000 ha-’ in the polyclonal plots and ranged from 16,000 ha - ’ to 19,500 ha-’ in monoclonal plots. At the Loughgall site, which had been harvested annually for 8 years, there were no significant differences (P c 0.05) in survival of clones, with the exception of S. burjatica Korso which had died out in the polyclonal plots within two years. This is similar to the situation experienced in Area 1 at Castlearchdale.
Vigour (0 - 4)
/ ?? Mono E!Poly
2.5
/
Germany
3.2. Vigour At the Loughgall site, the vigour of surviving stools in polyclonal plots was significantly (P < 0.05) greater than in the monoclonal plots. Differences were observed in all clones, with the greatest increase in S. viminalis Bowles
/
Bowles
/
/
‘663’ Dasyclados
Clone Fig. 2. Vigour (assessed on a @4) scale) of stools of four Salix clones grown in mono- and polyclonal stands. The error bar represents the least significant difference (P = 0.05) between means.
Hybrid and the least, although still significant (P < 0.05), in S. burjatica Germany (Fig. 2).
96 stool survival
3.3. Yields
Fig. 1. Stool survival of six Sulix clones grown in monoand polyclonal stands. Survival was recorded 5 years after planting and after a single 3 year harvest. The error bar represents the least significant difference (P = 0.05) between means.
There was a significantly (P < 0.05) higher percentage of small stools ( < 550 g) of all clones in monoclonal plots as compared with polyclonal plots. These differences varied between clones. In S. viminalis Bowles Hybrid 95% of stools in monoclonal plots were less than 550 g in contrast to the polyclonal plots where 65% were in this category. No stools of Class 4 ( > 850 g) were recorded in the monoclonal plots although 16% of the polyclonal stools were of this size. The smallest differences were recorded in S. burjatica Germany where the differences were respectively 99% and 88% (Fig. 3). When grown in polyclonal plots all clones produced significantly (P < 0.05) higher dry matter yields than in monoclonal plots (Fig. 4). The proportion of the yield contributed by large stools was significantly higher in polyclonal plots. Similar to the percentage number of stools in each class their yield contribution varied between clones (Fig. 4). The yield contributed from each class size varied between
A. R. MCCRACKEN and W. M. DAWSON
310
clones (Fig. 4) which was similar to the percentage number of stools in each class.
100 stool weight (Kg)
ImClass 1 ? ?Class 2 ??Class 3 W Class 4
4. DISCUSSION
In Area 1 at Castlearchdale S. burjutica Korso was included in the original planting. However, almost 100% of stools of this clone in the mixtures had died by the first harvest cycle, four years from planting. Similarly at Loughgall, under an annual harvesting regime all stools of S. burjutica Korso had died within two years. In adjacent monoclonal plots of S. burjatica Korso survival was over 80% although growth and vigour were weak and dry matter yields unacceptably low. Yield reductions and poor stool survival in the mixtures were attributable to severe attacks by foliar rust caused by Melampsora epitea.6 In mixtures, stools of S. burjatica Korso weakened by the rust could not compete with adjacent stools and so quickly died out. In the same way, in Sweden, mortality of S. viminalis stools in mixtures was greater with increased susceptibility to rust. This was also attributed to increased competitive effects.’ Consequently it can be concluded that where a clone has become highly susceptible to
gowles
Germany
683
Dasyclados
Clones Fig. 4. Equivalent dry material yields (kg) of 100 stools of each of four Salix clones grown in mono- and polyclonal stands. The sections of each column represent the proportion of the total yield contributed by stools in a particular class size. Error bar ‘a’ represents the least significant difference (P = 0.05) between columns, and error bar ‘b’ represents the least significant difference (P = 0.05) within columns.
%
?? Dead ?? Class I
0 Class 1 H Class 2 3 W Class 4
I
,I
I
I
!
I
100
80
80
20
iowles
Germany
683
Dasyclados
Clone Fig. 3. Percentage number of stools in class sizes (see text for definition of classes) of four Salix clones grown in monoand polyclonal stands. The error bar represents the least significant difference (P = 0.05) between means of stools in each class (M = monoclonal, P = polyclonal).
rust through contact with the pathogen over a number of years no benefits accrued from its inclusion in mixtures. At the Castlearchdale sites total yield from polyclonal stands was always greater than the mean yield of its constituent clones grown in monoclonal stands.B,g These increased yields were obtained from plots where significantly reduced survival .had been recorded. Densities of 13,000 stools ha -I in polyclonal plots produced an increase of 20% in dry matter yields compared with densities of between 16,000 and 19,500 ha-’ in monoclonal plots. In monoclonal plots there was uniform competition between stools resulting in all stools being of similar small size. In contrast, in polyclonal plots, stools of different clones interacted in such a way that there was a higher proportion of large, vigorous stools which resulted in increased yield despite the lower survival. Yield compensation at these reduced densities was thus able to overcome the loss of stools or clones through disease or competition.‘O These data further indicate that the yield of individual stools of all clones, except S. x dasycludos is
Interaction of willow clones
significantly higher when grown in polyclonal as compared to their yield in monoclonal stands.* It is recognised that density effects can influence yield patterns. In consequence current trials investigating monoclonal and polyclonal stands have been planted at densities of 10,000, 15,000 and 20,000 ha-‘. Lower densities in monoclonal stands may give shoots with higher biomass yield. However, management considerations particularly in terms of weed control are important. In another trial in Northern Ireland total yields of Salk x dasyclados and S. mollissima undulata SQ83 were lower, even at reduced densities (unpublished data). This was due to the difficulty of obtaining adequate weed control. In Sweden, in harvest cycles of three years or more, densities of 10,000 to 20,000 ha - ’ did not affect biomass production in S. uiminalis.” Indeed it has been reported that the dependence of yield on planting density disappears at higher densities and becomes weaker at low densities in later rotations.” The major benefit of growing willow in polyclonal stands is its effect in reducing the impact of foliar rust. Disease onset is delayed, build-up is reduced and disease levels at the end of the season are significantly lower than in monoclonal stands.” In the experiment described in this paper mixtures were structured mixtures of clones, i.e. each set of four or six clones had been planted in the same order. This meant that every stool was surrounded by stools of different clones but that these clones were always the same, irrespective of which part of the plantation they were in. In more recent trials in Northern Ireland mixed plantations have been planted completely randomly. Results suggest that if a clone is severely affected by disease or other factors which result in this loss of vigour and even death of stools then, in structured mixtures, neighbouring clones can compensate and eventually grow to fill the space left by the missing stools. Hence yields from mixed plantations are greater than monoclonal plantations despite a significant reduction in the number of stools surviving. It is possible to structure mixtures in other configurations. The two most common are lines and mosaics. As the name suggests, in line mixtures each line within the mixture is composed of a single clone. This means that in one direction stools of a single clone are adjacent to stools of the same clone while in the other direction there may be some distance
311
and several barrier clones before the same clone is encountered. In addition line mixtures create a overall striped appearance which may be unacceptable environmentally. In mosaics small squares are planted using stools of the same clone. While there may be some advantages from both of these systems in terms of planting, the benefits from compensation are lost. This is particularly the case in situations such as those experienced in Northern Ireland where individual clones, e.g. S. burjatica Korso, have not survived in mixtures due to susceptibility to disease. Furthermore, in a completely random system the maximum benefits of disease dilution and the consequent slowing of spread of disease are achieved. In contrast it can be speculated that in the more organised line and mosaic structures pathogen spread could be rapid. REFERENCES 1. G. H. McElroy and W. M. Dawson, Biomass from short rotation- coppice willow on marginal land. Biomass 10. 225240
(1986).
2. M. Carter, The positive apphcation of set aside, in Proc. Oxford Farming Conference (1990).
3. W. M. Dawson, Some aspects of the development of short-rotation coppice willow for biomass in Northern Ireland. Proc. R. Sot. Edinburgh Sect. 98B, 193-205 (1992).
4. A. R. McCracken and W. M. Dawson, Clonal response in Salix to Melampsora rusts in short rotation coppice plantations. Eur. J. For. Pathol. 22, 19-28 (1992): 5. M. H. Pei. D. J. Rovle and T. Hunter. Identitv and host alternation of some willow rusts (Melampsora spp.) in England. Mycol. Res. 97, 845-851 (1993). 6. W. M. Dawson and A. R. McCracken, Effect of Melampsora rust on the growth and development of Salix burjatica ‘Korso’ in Northern Ireland. Eur. J. For. Pathol. 24, 32-39 (1994).
7. T. Verwijst, Influence of the pathogen Melampsora epitea on intraspecific competition in a mixture of Sali.\ciminalis clones. J. Veg. Sri. 4, 711-722 (1993). 8. W. M. Dawson and A. R. McCracken, The performance of polyclonal stands in short rotation coppice willow for energy production. Biomass and Bioenergy 8, 1-S (1995).
9. A. R. McCracken and W. M. Dawson, Experiences in the use of mixed-clonal stands of Salk as a method of reducing the impact of rust diseases. Norwegian J. Agr. Sci. Sutwl. 18. 101-109 (1994).
10. E. Willkbrand,‘S. Ledin and T.Verwijst, Willow coppice systems in short rotation forestry: effects of plant spacing, rotation length and clonal composition on biomass production. Biomass and Bioenergy 4, 323-33 I (1993).
11. E. Willebrand, T. Verwijst and J. Richardson, Population dynamics of willow coppice systems and their implications for management bf short-rotation forests. For. Chron. 69, 699-704 (1993). 12.A. R. McCracken and W. M. Da&on, Growing short rotation coppice willow (Salk) in mixtures as a method of reducing the impact of rust disease caused by Melampsora epitea var. epitea. Eur. J. For. Path. (submitted) (1996).
Copyright 0 1996published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0961-9534/96$15.00+ 0.00 PII: SO961-9534(96)00001-3
Pergamon
INTERACTION POLYCLONAL
OF WILLOW (SALIX) CLONES GROWN IN STANDS IN SHORT ROTATION COPPICE A. R. MCCRACKEN* and W. M. DAWSON~
*Applied Plant Science Division, Department of Agriculture for Northern Ireland, Newforge Lane, Belfast, Northern Ireland BT9 5PX TNorthern Ireland Horticulture and Plant Breeding Station, Loughgall, Co. Armagh, Northern Ireland BT6l 8JB (Received 30 May 1995; revised 22 December
199s; accepted 3 January 1996)
Abstract-Stool survival of Salix spp. clones is reduced when growing in polyclonal stands as compared to monoclonal stands. Stools growing in polyclonal stands show increased vigour with compensation of weak or dead stools from adjacent stools. In monoclonal stands the stools tend to be less vigorous but there is also less variation between stools. When a clone, which has become highly susceptible to rust infection, is included in the mixture, e.g. Salix burjarica Korso, it will die out completely within two years. Copyright 0 1996 published by Elsevier Science Ltd. Keyw0rdsSali.v;
biomass; clonal mixtures; short rotation coppice; yield.
1. INTRODUCTION
The potential for growing willow (S&X spp. ) at high density in short rotation coppice as a source of biomass for energy production is now well established.lm3 Wherever willow has been grown in the United Kingdom in monoclonal plantations there has been a severe and increasing problem with foliar rust disease, caused mainly by pathotypes of Melampsora epitea.4.5 In the most severe cases this has resulted in premature and complete defoliation, e.g. Salix burjatica Korso, which can no longer be grown in commercial plantations.6 The problem has been less severe, and not increasing, under the continental climatic conditions experienced in Sweden (Ramstedt, personal communication). The use of fungicides for the control of rust disease in willow plantations is not a viable option for economic, environmental and practical reasons and alternative methods of reducing disease impact have been investigated. Willow grown in polyclonal plantations of intimate mixtures of up to six different clones has given positive yield responses. Increased yields were obtained from polyclonal stands when compared to either the mean yield of the component clones or the individual yields of any of the component clones grown in monoculture.’ However, not all clones had similar survival and growth patterns when compared to either
the same clone in monoculture or to other clones in the mixture. The data presented in this paper compare the survival, vigour and yield of a range of Salix spp. clones, both in monoclonal and polyclonal stands and offer explanations for the interactions occurring between individual stools. 2. MATERIALS
AND METHODS
2.1. Willow plantations 2.1.1. Loughgall site. Five clones of Salix: Salix x dasyclados, S. burjatica Korso, S. burjatica Germany, S. viminalis Bowles Hybrid and S. viminalis 683 were planted in monoclonal
and polyclonal plots of structured, intimate mixtures at a density of 20,000 cuttings ha - ’in 1986. Clones were planted in sets of five, repeating the same sequence. Both monoclonal plots and polyclonal plots were approximately 300 m* and were arranged in three randomised blocks. In the winter of each year after establishment all stools were cut back to ground level. 2.1.2. Castlearchdale site. Two large areas were planted with Salix at a density of 20,000 cuttings ha - ‘: Area 1: five clones were planted in 1987: S. x dasyclados, S. burjatica Korso, S. burjatica Germany, S. viminalis Bowles Hybrid and S. viminalis 683. Plots were established using 307
A. R. MCCRACKENand W. M. DAWSON
308
hardwood cuttings in either monoclonal plots or in structured, intimate mixtures as polyclonal plots. Stools were harvested during the winter of 1990/91 and 1993/94. Both monoclonal and polyclonal plots were a minimum of 2000 ml, planted in a single block. Area 2: six clones were planted in 1989: S. x dasyclados, S. x calodendron, S. burjatica Germany, S. viminalis Bowles Hybrid, S. viminalis 683 and S. mollissima undulata SQ83. Plots were established as both monoclonal and polyclonal stands. Minimum plot size was 2500 m2. Stools were harvested in the winter 1992193. 2.2. Survival Survival of stools at the Castlearchdale sites was recorded in Spring 1994. In Area 1 stools were flushing following their second three-year harvest. In Area 2 stools were beginning their second year of growth from stools first harvested as three-year old material in 1992/93. In addition to stools which were dead or missing, stools where growth was weak or competition from surrounding stools had overgrown them were also considered not to have survived. 2.3. Vigour Vigour of individual stools was assessed during the winter 1994/95 at the Loughgall site. In monoclonal plots every stool in each of three randomly chosen rows was assessed. Each row had between 40 and 55 stools. The vigour of every stool in each of the polyclonal plots was assessed. The vigour of each individual stool was assessed on a O-4 scale, where: 0 = dead; 1 = still living, very few weak stunted shoots; 2 = growth, one or two large healthy shoots; 3 = vigorous growth, three to four large healthy shoots; and 4 = vigorous growth, several large healthy shoots. The purpose of assessing vigour was to make a judgement on the health and growth potential of stools and was not necessarily to measure potential yield. Comparisons of vigour were made between stools of the same clone growing in either monoclonal or polyclonal plots.
2.4. Yield At Loughgall in the monoclonal plots, ten consecutive stools were selected at random, in each of three rows. Each stool was harvested individually, weighed and a dry matter yield was determined. In polyclonal plots, 40 consecutive stools were selected at random in each of three rows. This sample should therefore have comprised 10 stools of each of the component clones. As before, each individual stool was harvested, weighed and a dry matter yield was determined. Stools were placed in size classes according to their individual dry matter yield as follows: 0= 1I 2= 3= 42
dead, 250 g, 251-550 g, 551-850 g, 851 g.
The percentage number of stools in each size class was calculated. The yield contribution, from each size class from individual clones, to the total yield was calculated. Total yield was calculated to a standard 50 m2. Analysis of variance was used to compare differences between yields in monoclonal and polyclonal plots, and between yield in each of the size classes. 3. RESULTS
3.1. Survival When stool survival on the polyclonal plots was assessed in Area 1 at Castlearchdale, only four clones had survived, S. burjatica Korso having completely died out. Differences in the survival of the four remaining clones were not significant (P < 0.05). Stool survival was between 77% and 82% producing a density of 13,000 stools ha - ‘. Survival of clones growing in monoclonal plots was not significantly (P < 0.05) different between clones. Neither was there any significant (P < 0.05) difference between the clones grown in monoclonal or polyclonal plots. Survival was between 80 and 83% resulting in density in the monoclonal plots of approximately 16,000 ha-‘. In both cases these were recorded 7 years after planting and after 2 three-year harvests, In Area 2 at Castlearchdale survival of all six clones was significantly less (P < 0.05) in the polyclonal plots when compared to survival in the monoclonal plots (Fig. 1). The greatest
309
Interaction of willow clones
difference was in S. x calodendron in which 98% of stools survived in the monoclonal plots while there was only 35% survival in the polyclonal plots. The smallest difference, of less than 10% reduction, was recorded in S. viminalis 683. In the remaining clones, survival in the polyclonal plots was reduced to less than 60% (Fig. 1). Final stool density 5 years after planting and after a single harvest was approximately 13,000 ha-’ in the polyclonal plots and ranged from 16,000 ha - ’ to 19,500 ha-’ in monoclonal plots. At the Loughgall site, which had been harvested annually for 8 years, there were no significant differences (P c 0.05) in survival of clones, with the exception of S. burjatica Korso which had died out in the polyclonal plots within two years. This is similar to the situation experienced in Area 1 at Castlearchdale.
Vigour (0 - 4)
/ ?? Mono E!Poly
2.5
/
Germany
3.2. Vigour At the Loughgall site, the vigour of surviving stools in polyclonal plots was significantly (P < 0.05) greater than in the monoclonal plots. Differences were observed in all clones, with the greatest increase in S. viminalis Bowles
/
Bowles
/
/
‘663’ Dasyclados
Clone Fig. 2. Vigour (assessed on a @4) scale) of stools of four Salix clones grown in mono- and polyclonal stands. The error bar represents the least significant difference (P = 0.05) between means.
Hybrid and the least, although still significant (P < 0.05), in S. burjatica Germany (Fig. 2).
96 stool survival
3.3. Yields
Fig. 1. Stool survival of six Sulix clones grown in monoand polyclonal stands. Survival was recorded 5 years after planting and after a single 3 year harvest. The error bar represents the least significant difference (P = 0.05) between means.
There was a significantly (P < 0.05) higher percentage of small stools ( < 550 g) of all clones in monoclonal plots as compared with polyclonal plots. These differences varied between clones. In S. viminalis Bowles Hybrid 95% of stools in monoclonal plots were less than 550 g in contrast to the polyclonal plots where 65% were in this category. No stools of Class 4 ( > 850 g) were recorded in the monoclonal plots although 16% of the polyclonal stools were of this size. The smallest differences were recorded in S. burjatica Germany where the differences were respectively 99% and 88% (Fig. 3). When grown in polyclonal plots all clones produced significantly (P < 0.05) higher dry matter yields than in monoclonal plots (Fig. 4). The proportion of the yield contributed by large stools was significantly higher in polyclonal plots. Similar to the percentage number of stools in each class their yield contribution varied between clones (Fig. 4). The yield contributed from each class size varied between
A. R. MCCRACKEN and W. M. DAWSON
310
clones (Fig. 4) which was similar to the percentage number of stools in each class.
100 stool weight (Kg)
ImClass 1 ? ?Class 2 ??Class 3 W Class 4
4. DISCUSSION
In Area 1 at Castlearchdale S. burjutica Korso was included in the original planting. However, almost 100% of stools of this clone in the mixtures had died by the first harvest cycle, four years from planting. Similarly at Loughgall, under an annual harvesting regime all stools of S. burjutica Korso had died within two years. In adjacent monoclonal plots of S. burjatica Korso survival was over 80% although growth and vigour were weak and dry matter yields unacceptably low. Yield reductions and poor stool survival in the mixtures were attributable to severe attacks by foliar rust caused by Melampsora epitea.6 In mixtures, stools of S. burjatica Korso weakened by the rust could not compete with adjacent stools and so quickly died out. In the same way, in Sweden, mortality of S. viminalis stools in mixtures was greater with increased susceptibility to rust. This was also attributed to increased competitive effects.’ Consequently it can be concluded that where a clone has become highly susceptible to
gowles
Germany
683
Dasyclados
Clones Fig. 4. Equivalent dry material yields (kg) of 100 stools of each of four Salix clones grown in mono- and polyclonal stands. The sections of each column represent the proportion of the total yield contributed by stools in a particular class size. Error bar ‘a’ represents the least significant difference (P = 0.05) between columns, and error bar ‘b’ represents the least significant difference (P = 0.05) within columns.
%
?? Dead ?? Class I
0 Class 1 H Class 2 3 W Class 4
I
,I
I
I
!
I
100
80
80
20
iowles
Germany
683
Dasyclados
Clone Fig. 3. Percentage number of stools in class sizes (see text for definition of classes) of four Salix clones grown in monoand polyclonal stands. The error bar represents the least significant difference (P = 0.05) between means of stools in each class (M = monoclonal, P = polyclonal).
rust through contact with the pathogen over a number of years no benefits accrued from its inclusion in mixtures. At the Castlearchdale sites total yield from polyclonal stands was always greater than the mean yield of its constituent clones grown in monoclonal stands.B,g These increased yields were obtained from plots where significantly reduced survival .had been recorded. Densities of 13,000 stools ha -I in polyclonal plots produced an increase of 20% in dry matter yields compared with densities of between 16,000 and 19,500 ha-’ in monoclonal plots. In monoclonal plots there was uniform competition between stools resulting in all stools being of similar small size. In contrast, in polyclonal plots, stools of different clones interacted in such a way that there was a higher proportion of large, vigorous stools which resulted in increased yield despite the lower survival. Yield compensation at these reduced densities was thus able to overcome the loss of stools or clones through disease or competition.‘O These data further indicate that the yield of individual stools of all clones, except S. x dasycludos is
Interaction of willow clones
significantly higher when grown in polyclonal as compared to their yield in monoclonal stands.* It is recognised that density effects can influence yield patterns. In consequence current trials investigating monoclonal and polyclonal stands have been planted at densities of 10,000, 15,000 and 20,000 ha-‘. Lower densities in monoclonal stands may give shoots with higher biomass yield. However, management considerations particularly in terms of weed control are important. In another trial in Northern Ireland total yields of Salk x dasyclados and S. mollissima undulata SQ83 were lower, even at reduced densities (unpublished data). This was due to the difficulty of obtaining adequate weed control. In Sweden, in harvest cycles of three years or more, densities of 10,000 to 20,000 ha - ’ did not affect biomass production in S. uiminalis.” Indeed it has been reported that the dependence of yield on planting density disappears at higher densities and becomes weaker at low densities in later rotations.” The major benefit of growing willow in polyclonal stands is its effect in reducing the impact of foliar rust. Disease onset is delayed, build-up is reduced and disease levels at the end of the season are significantly lower than in monoclonal stands.” In the experiment described in this paper mixtures were structured mixtures of clones, i.e. each set of four or six clones had been planted in the same order. This meant that every stool was surrounded by stools of different clones but that these clones were always the same, irrespective of which part of the plantation they were in. In more recent trials in Northern Ireland mixed plantations have been planted completely randomly. Results suggest that if a clone is severely affected by disease or other factors which result in this loss of vigour and even death of stools then, in structured mixtures, neighbouring clones can compensate and eventually grow to fill the space left by the missing stools. Hence yields from mixed plantations are greater than monoclonal plantations despite a significant reduction in the number of stools surviving. It is possible to structure mixtures in other configurations. The two most common are lines and mosaics. As the name suggests, in line mixtures each line within the mixture is composed of a single clone. This means that in one direction stools of a single clone are adjacent to stools of the same clone while in the other direction there may be some distance
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and several barrier clones before the same clone is encountered. In addition line mixtures create a overall striped appearance which may be unacceptable environmentally. In mosaics small squares are planted using stools of the same clone. While there may be some advantages from both of these systems in terms of planting, the benefits from compensation are lost. This is particularly the case in situations such as those experienced in Northern Ireland where individual clones, e.g. S. burjatica Korso, have not survived in mixtures due to susceptibility to disease. Furthermore, in a completely random system the maximum benefits of disease dilution and the consequent slowing of spread of disease are achieved. In contrast it can be speculated that in the more organised line and mosaic structures pathogen spread could be rapid. REFERENCES 1. G. H. McElroy and W. M. Dawson, Biomass from short rotation- coppice willow on marginal land. Biomass 10. 225240
(1986).
2. M. Carter, The positive apphcation of set aside, in Proc. Oxford Farming Conference (1990).
3. W. M. Dawson, Some aspects of the development of short-rotation coppice willow for biomass in Northern Ireland. Proc. R. Sot. Edinburgh Sect. 98B, 193-205 (1992).
4. A. R. McCracken and W. M. Dawson, Clonal response in Salix to Melampsora rusts in short rotation coppice plantations. Eur. J. For. Pathol. 22, 19-28 (1992): 5. M. H. Pei. D. J. Rovle and T. Hunter. Identitv and host alternation of some willow rusts (Melampsora spp.) in England. Mycol. Res. 97, 845-851 (1993). 6. W. M. Dawson and A. R. McCracken, Effect of Melampsora rust on the growth and development of Salix burjatica ‘Korso’ in Northern Ireland. Eur. J. For. Pathol. 24, 32-39 (1994).
7. T. Verwijst, Influence of the pathogen Melampsora epitea on intraspecific competition in a mixture of Sali.\ciminalis clones. J. Veg. Sri. 4, 711-722 (1993). 8. W. M. Dawson and A. R. McCracken, The performance of polyclonal stands in short rotation coppice willow for energy production. Biomass and Bioenergy 8, 1-S (1995).
9. A. R. McCracken and W. M. Dawson, Experiences in the use of mixed-clonal stands of Salk as a method of reducing the impact of rust diseases. Norwegian J. Agr. Sci. Sutwl. 18. 101-109 (1994).
10. E. Willkbrand,‘S. Ledin and T.Verwijst, Willow coppice systems in short rotation forestry: effects of plant spacing, rotation length and clonal composition on biomass production. Biomass and Bioenergy 4, 323-33 I (1993).
11. E. Willebrand, T. Verwijst and J. Richardson, Population dynamics of willow coppice systems and their implications for management bf short-rotation forests. For. Chron. 69, 699-704 (1993). 12.A. R. McCracken and W. M. Da&on, Growing short rotation coppice willow (Salk) in mixtures as a method of reducing the impact of rust disease caused by Melampsora epitea var. epitea. Eur. J. For. Path. (submitted) (1996).