Initial Spacing of Poplars and Willows Grown As Arable Coppice

INITIAL SPACING OF POPLARS AND WILLOWS GROWN AS ARABLE COPPICE

ETSU B/W2/00336/REP

Contractor

Silviculture & Seeds Research Branch Forestry Commission Research Agency

Prepared

A Armstrong & C Johns

The work described in this report was carried out under contract as part of the New and Renewable Energy Programme, managed by the Energy Technology Support Unit (ETSU) on behalf of the Department of Trade and Industry. The views and judgements expressed in this report are those of the contractor and do not necessarily reflect those of ETSU or the Department of Trade and Industry

Figures 1 - 8 NOT AVAILABLE ELECTRONICALLY

Summary i

List of Figures and Tables iii

1.0 INTRODUCTION 1

2.0 EXPERIMENT DESIGN 1

3.0 SITE DETAILS 2

3.1 Wishanger site 2

3.2 Downham Market site 4

4.0 HARVESTING 8

5.0 RESULTS 10

5.1 Wishanger 10

5.2 Downham Market 11

6.0 DISCUSSION 14

7.0 CONCLUSIONS 19

8.0 RECOMMENDATIONS 19

Acknowledgements 21

References 22

Appendix 23

Experiment Plan 24 EXECUTIVE SUMMARY

Two clones of poplar and two clones of willow were grown at two sites, on a three year cutting cycle, at six different square spacings, between 0.8 metres and 1.5 metres.

The two willow clones 'Bowles hybrid' and Dasyclados' were planted at both sites. The poplar clones Populus interamericana 'Beaupre' and Populus trichocarpa 'Columbia River' were planted at Wishanger in Hampshire. The poplar clones Populus interamericana 'Boelare' and Populus trichocarpa 'Trichobel' were planted at Downham Market in Norfolk.

The highest yield of 17.55 oven dry tonnes per hectare (odt/ha/annum) was obtained from 'Bowles hybrid', at the closest spacing, grown on a water meadow adjacent to the River Wey at Wishanger.

The highest yield for all clones at both sites was achieved at the closest spacing (in this first rotation). There was a significant linear effect. One of the most interesting observations was that when comparing the gradient of the linear relationship, within species, the gradient was steeper for the higher yielding clone. This was particularly so for the willows. This would suggest that higher yielding clones are more tolerant of crowding, or, that upright Salix viminalis make better use of close space than the more spreading Salix dasyclados. The new Salix viminalis x Salix schwerinnii hybrids should therefore also be responsive to closer spacing. The same effect was observed for the poplars at Wishanger only, but the difference was not as dramatic. There was a suggestion from the highest yielding poplar plots that

i optimum yield may still be obtained at the currently recommended spacing of 1.0 metre x 1.0 metre.

An initial close spacing ensures rapid canopy closure and maximises yield production by allowing optimal absorption of available radiation, whilst wider spacings in the early establishment phase do not fully utilise available space. An additional benefit to the grower is the rapid suppression of competing weed growth and reduced herbicide usage with cost and environmental benefits.

When comparing the yield from 'Bowles hybrid' 1.0 metre spacing with the yield from 0.8 metre spacing at Wishanger it can be seen that the penalty at the end of the three year rotation for opting for the wider spacing would be 146 odt on a 10.0 hectare field.

These trials were planted in a square design and the findings may not hold true for rectangular spacings currently being favoured. In a twin row design of alternatively 1.5 metre and 0.75 metre between rows the distance within the row would have to be reduced to 0.59 metre to increase the stocking to an equivalent 0.8 metre square spacing (15,000 stools/hectare). This may result in a competition effect whilst not making maximum use of space and light in the wider (1.5 metre) row spacing.

ii LIST OF FIGURES and TABLES

Figure 1 Wishanger 5 Layout 3

Figure 2 Downham Market layout and location of poor growth 5

Figure 3 Foliar nutrient and metal levels at Downham Market 6

Figure 4 Foliar nutrient and metal levels at Downham Market 7

Figure 5 Poplar yield against spacing at Wishanger 9

Figure 6 Willow yield against spacing at Wishanger 9

Figure 7 Willow yield against spacing at Downham Market 13

Figure 8 Poplar yield against spacing at Downham Market 13

Table 1 Wishanger 5 Yield 10

Table 2 Downham Market Yield 12

Table 3 Wishanger Block Yield 16

Table 4 Downham Market Block Yield 17

in 1.0 INTRODUCTION

This report covers the period January 1992 - April 1997, and summarises the data from the first harvests at the Wishanger and Downham Market sites.

Previous Forestry Commission experiments under contract to ETSU for DTI have investigated the effect of initial spacing on yield over two and four year rotations. The results

(Tabbush, 1993 and Armstrong, 1996) have suggested that a spacing of 1.0 metre results in higher yields than that for a 2.0 metre spacing over both two and four year rotations. The difference in yield from Populus interamericana 'Beaupre' and 'Boelare' compared with

Populus trichocarpa 'Trichobel' is less pronounced at 2.0 metre spacing than at 1.0 metre spacing. The results from Nelder plots were examined by Tabbush (1993) and Armstrong

(1996). Tabbush found that both in the first four year rotation and the second four year 2 rotation Populus interamericana 'RAP' had optimum yield at 1.42 m /stool (1.2 m equivalent square spacing). Armstrong found that in the first four year rotation of Populus interamericana 'Beaupre' optimum yield occurred at 1.38 m2/stool (1.18 m equivalent square spacing). The first four year rotation of Populus interamericana 'Boelare' produced optimum 2 yield at 0.62 m /stool (0.78 m equivalent square spacing) although there was little difference 22 in yield between 0.62 m /stool and 1.37 m /stool (0.78 m and 1.17 m equivalent square spacing). These similar findings were not observed for Populus trichocarpa 'Trichobel' 2 which achieved optimum yield at 3.02 m/stool (1.74 m equivalent square spacing). The

Nelder design is limited, and the findings needed to be confirmed in experiments with plots of square spacing.

2.0 EXPERIMENT DESIGN

At two sites, two different clones of two species (poplar and willow) were tested at six different spacings on a three year rotation in a randomised block design with four replicates.

Each species was treated separately. Square spacings of 0.8 m, 1.0 m, 1.15 m, 1.3 m, 1.4 m and 1.5 m were tested. Plot size was 7 x 7 trees to include a 1 row buffer surround giving a

1 5 x 5 assessment plot (25 stools). Plots were planted either side of a central base line (Figure 2).

Wishanger site:

For poplar: 2 clones (Populus interamericana 'Beaupre' and Populus trichocarpa 'Columbia River') x 6 spacings = 12 treatments x 4 randomised blocks = 48 plots

For willow: 2 clones (Salix caprea x cinerea x viminalis 'Dasyclados' and Salix viminalis 'Bowles hybrid') x 6 spacings = 12 treatments x 4 randomised blocks = 48 plots

Downham Market site:

For poplar: 2 clones (Populus interamericana 'Boelare' and Populus trichocarpa 'Trichobel' x 6 spacings = 12 treatments x 4 randomised blocks = 48 plots

For willow: 2 clones (Salix cap x cin x vim 'Dasyclados' and Salix viminalis 'Bowles hybrid') x 6 spacings = 12 treatments x 4 randomised blocks = 48 plots

Data were analyzed by analysis of variance (ANOVA) using GENSTAT 5 (Genstat 5 Committee, 1987)

3.0 SITE DETAILS

3.1 Wishanger site

Both species were planted adjacent to the river Wey at Wishanger estate in north Hampshire in April 1992 (Figure 1). Two distinct site types were identified. The willow were planted on

Figure 1 - Wishanger plot layout NOT AVAILABLE ELECTRONICALLY

2 a water meadow on an alluvial soil whilst the poplars were planted on ex-arable land on Lower Greensand Folkestone beds, Shirrell Heath 1. association. Within these areas there were still differences in the soils. The willows were blocked such that block 1 closest to the poplars was on an interface between the alluvial soil and the greensand. Blocks 2 and 3 were on the better alluvial soil and block 4 was on heavier land with seasonal water logging. Blocks 1 and 2 of the poplar experiment were more sheltered than blocks 3 and 4 which were exposed to cold easterly winds. Block 3 and in particular block 4 were more stony than blocks 2 and 3.

The site lies at 60 masl with an annual rainfall of 705 mm. Previous land use was cereal production.

The site was kept weed free by repeated applications of contact and residual herbicides, prior to and after stumping back in January 1993. First year survival was excellent: 'Beaupre' - 97%, 'Bowles hybrid' - 97%, 'Columbia River' - 95% and 'Dasyclados' - 95%.

3.2 Downham Market site

Both species were planted on what appeared to be a very homogenous site that had been under continuous arable production. The willows were planted to the south of the site and poplars to the north (Figure 2). The soil is Downham association resting on ferruginous Lower Greensand.

The site lies at 15 masl, with an annual rainfall of 630 mm. Previous land use had been cereal production.

The site was kept weed free by repeated applications of contact and residual herbicides, prior to and after stumping back in January 1994. Survival was excellent for all clones (>99%). In the first year abnormal growth was observed in one block of poplars. Herbicide and soil

3 Figure 2 - Downham Market plot layout and location of areas of poor growth NOT AVAILABLE ELECTRONICALLY

Figure 3 - Foliar nutrient and metal levels at Downham Market NOT AVAILABLE ELECTRONICALLY

Figure 4 - Foliar nutrient and metal levels at Downham Market NOT AVAILABLE ELECTRONICALLY experts were consulted but no conclusive reasons could be found for the poor growth, but it was most probably a result of micro nutrient deficiency or imbalance. There were large differences in foliar metal levels between the undamaged Block 1 and the very badly damaged Block 3, this block was excluded from the analysis (Figures 3 and 4). The area affected is shown in Figure 2. Similar problems were observed on adjacent land.

The soil conditions at Downham Market are very similar to the soil conditions found at the poplar plots at Wishanger.

4.0 HARVESTING

The Wishanger site was harvested in March 1996 and Downham Market in March 1997. At Wishanger each plot was cut and weighed green. All diameters had been measured for a separate study. This tally of diameter distribution was used to select 20 stems in the same ratio as they existed in the plots. The twenty stems were chipped and thoroughly mixed and a sample taken which was weighed green and then oven dried to provide a green:dry weight ratio. This ratio was then applied to the total green plot weight to give a plot dry weight. Dry tonnes/ha/annum were then calculated using the following formula:

Dry tonnes/ha/annum = Plot dry weight(kg) x No stools/ha at plot spacing x 1 x 1 No stools per plot x rotation x 1000

4 The landowner at Downham Market assisted in the harvest in exchange for the resultant chips. It was therefore decided to chip and weigh all material from each plot after removal of the guard rows. The chips were thoroughly mixed and a sample taken which was weighed green and then oven dried to provide a green:dry weight ratio. Yield was then assessed using the same formula as at Wishanger.

Figure 5 - Mean PoplarYield Against Spacing at Wishanger Feb ’96 NOT AVAILABLE ELECTRONICALLY

Figure 6 - Mean WillowYield Against Spacing at Wishanger Feb ’96 NOT AVAILABLE ELECTRONICALLY

5.0 RESULTS

5.1 Wishanger

The highest yields were obtained in the willow area on the alluvial soil adjacent to the River Wey. This site would have had better moisture retention and higher fertility than the poplar site (Jarvis et al, 1984). The poplars planted on the greensand could have suffered from poor moisture availability especially in view of the dry summer of 1995. It is interesting to note that Block 1 of the willows, closest to the poplar, produced the lowest yield (Table 3). Block 4 gave a yield marginally better than Block 1 which could be attributable to the seasonal waterlogging observed. The mean yield (average across all spacings) of 12.5 oven dry tonnes per hectare annum (odt/ha/annum) from 'Bowles hybrid' was significantly (p<0.001) greater than the yield of 8.26 odt/ha from 'Dasyclados' (Table 1). Within clones the effect of spacing was significant, with a highly significant linear effect (Figure 6). The yields from the poplar plots were considerably lower than for the willow plots.

5 Poplar (odt/ha/annum) Willow (odt/ha/annum) Spacing/area Beaupre Col River Spacing/area Bowles Dasyclados Hybrid 0.80/0.64 9.11 6 94 0.80/0.64 17.55 9 80 1.00/1.00 8 06 6 59 1.00/1.00 12.68 8.75 1.15/1.32 6 88 5.53 1.15/1.32 12.65 9.11 1.30/1.69 5 08 5.52 1.30/1.69 10.02 7 80 1.40/1.96 5.52 4.07 1.40/1.96 12.22 7 89 1.50/2.25 4 96 3 93 1.50/2.25 9 87 6 18 Mean 6 60* 5.43 Mean 12.50*** 8 26

Table 1 Wishanger level of significance for difference between means (* p<0.05, ** p<0.01, *** p<0.001)

In the poplar trial 'Beaupre' had the highest mean yield at 6.60 odt/ha/annum which was significantly (p<0.05) greater than the 5.43 odt/ha/annum mean yield from 'Columbia River'. The highest poplar yields were obtained in Blocks 1 and 2 (Table 3), which were more sheltered and less stony than Blocks 3 and 4. Within clones the effect of spacing was significant, with a highly significant linear effect. The gradient was steeper for the more productive clone, 'Beaupre' (Figure 5).

The highest yield for all clones was achieved at the closest spacing of 0.8m and the lowest yield was obtained at the widest spacing tested of 1.5m. In between these spacings there is a significant linear effect although for example the yield for 'Bowles hybrid' at 1.4m spacing is greater than for the 1.3m spacing (Table 1). The most productive clone, 'Bowles hybrid', had a steeper gradient than 'Dasyclados'.

The highest yield (17.55 odt/ha/annum) for all treatments was obtained by 'Bowles hybrid' at 0.80m spacing. Yield dropped off at 1.0m spacing although there was very little difference in yield between 1.0m and 1.4m spacing.

6 5.2 Downham Market

The highest yield was obtained for 'Bowles hybrid'. The mean yield of 8.57 odt/ha/annum was significantly (p<0.001) greater than the yield of 6.94 odt/ha/annum from 'Dasyclados' (Table 2). 'Bowles hybrid' consistently gave higher yields at all spacings over 'Dasyclados'. Within clones the effect of spacing was highly significant, with the gradient being steeper for the most productive clone, Bowles hybrid (Figure 7).

Poplar (odt/ha/annum) Willow (odt/ha/annum)

Spacing/area Boelare Trichobel Spacing/area Bowles Dasyclados Hybrid 0.80/0.64 10.67 9 18 0.80/0.64 11.13 8 84 1.00/1.00 8 31 7.54 1.00/1.00 948 7 39 1.15/1.32 7 88 7.11 1.15/1.32 8.3 7.01 1.30/1.69 6.27 5 98 1.30/1.69 8.27 6 32 1.40/1.96 7.57 6 07 1.40/1.96 7.2 6 23 1.50/2.25 6 94 5 63 1.50/2.25 7.04 5 86 Mean 7.94* 6 92 Mean 8.57*** 6 94

Table 2 Downham Market level of significance for difference between means (* p<0.05, ** p<0.01, *** p<0.001)

Figure 7 - Mean WillowYield Against Spacing at Downham Market Feb ’97 NOT AVAILABLE ELECTRONICALLY

Figure 8 - Mean PoplarYield Against Spacing at Downham Market Feb ’97 NOT AVAILABLE ELECTRONICALLY

7 The two blocks either side of 'Boelare' in Block 3, which was abandoned, had a lower mean yield than Block 1 (Table 4). This may indicate that these neighbouring blocks were still being affected by the problems encountered in Block 3. It could also explain the poor yield of 4.7 odt/ha/annum from 'Boelare' at 1.0 metre spacing in Block 4 (Table 4 and Figure 3). 'Boelare' gave the highest yield between the two poplars. The mean yield of 7.94 odt/ha/annum was significantly (p<0.05) greater than the yield of 6.92 odt/ha/annum from 'Trichobel' (Table 2). 'Boelare' consistently gave higher yields at all spacings over 'Trichobel'. The effect of spacing was significant (Figure 8).

6.0 DISCUSSION

When the same clones of willow, as used at Wishanger, were planted on a drier site at Downham Market, similar to the poplar area at Wishanger, 'Bowles hybrid' still produced the highest yield, although there was not much difference in mean yield between species or clones. Although different clones of poplar were used at the two sites it is important to note that 'Beaupre' is a full sister to 'Boelare' and 'Columbia River' is the male parent of the male clone 'Trichobel', as such it would be expected that performance would be comparable. On this basis there is a suggestion that the yield from poplar has not been compromised by the higher accumulated temperature and the lower annual rainfall at the Downham Market site. Similarly there is a suggestion that willow, in particular 'Bowles hybrid' has responded to the higher fertility and greater moisture availability of the alluvial soil found at the Wishanger site.

At both sites the highest yields were obtained for both species and all clones at the closest spacing of 0.8 metres and the lowest yields at the widest spacings of 1.5 metres. These findings are supported by Armstrong (1996) in a study of Populus interamericana 'Beaupre' and 'Boelare' and Populus trichocarpa 'Trichobel' tested at spacings of 1.0 metres and 2.0 metres where the 1.0 metre spacing consistently gave a significantly higher yield than the wider spacing over rotations of two and four years. An initial close spacing ensures rapid canopy closure and maximises yield production by allowing optimal absorption of available

8 radiation, whilst wider spacings in the early establishment phase do not fully utilise available space. An additional benefit to the grower is the rapid suppression of competing weed growth and reduced herbicide usage with cost and environmental benefits.

Heilman et al. (1972) in a study with Populus trichocarpa at spacings of 0.3, 0.6 and 1.2 metre spacings found that in the first two year rotation the closest spacing 0.3 x 0.3 metres resulted in the highest yield whereas in the second two year rotation, when yields were greater, the highest yield was achieved at the 0.6 metre spacing. The greatest increase in yield between the first and second year rotations was observed at the 1.2 metre spacing whilst the second rotation resulted in a decrease in yield for the 0.3 metre spacing. This suggests that once stools are established and yield increases the effect of spacing becomes less critical and that for a vigorous crop the spacing could be wider without adversely affecting yield. In the highest yielding block of poplar at Wishanger which was Block 1 for both 'Columbia River' and 'Beaupre' the highest yield occurred at 1.0 metre spacing not 0.8 metre. In the other three blocks where yield was not so great 0.8 metre gave the highest yield (Table 3). At Downham Market this finding was not fully reinforced although 'Boelare' in Block 1, which had the highest mean yield, also achieved the highest yield at 1.0 metre spacing (Table 4).

In the willow plots at Wishanger the highest yielding plots for Bowles hybrid (Blocks 2 and 3) had the highest yield at the closest spacing (Table 3) and had the best linear relationship. In the lower yielding blocks the relationship is less clear. In the Dasyclados plots although there was a significant linear effect (p<0.05) in two blocks (Block 1 and Block 4) there was little difference in yield between 0.8 metre spacing and 1.4 metre spacing (Table 2). In the other two blocks there was a general downward trend in yield with increased spacing. Willebrand et al. (1993) found that densely spaced stands of willow exhibited a higher production than wider spaced stands during the first harvests under the shortest rotation periods. In later harvests of short cycles (1 to 3 years) and harvests of longer cycles (>3 years) yield was not affected by spacing.

9 Table 3 Wishanger Block yields Spacing Block 1 Block 2 Block 3 Block 4 0.8 9.0 11.0 9.0 8.0 1.00 12.0 7.0 7.0 5.0 1.15 9.0 6.0 5.0 8.0 1.30 8.0 5.0 4.0 3.0 1.40 8.0 5.0 3.0 5.0 1.50 7.0 5.0 4.0 4.0

mean 8.83 6.5 5.3 5.5

Beaupre plot yields by block (odt/ha/annum). Spacing Block 1 Block 2 Block 3 Block 4 0.8 9.0 7.0 7.0 5.0 1.00 10.0 6.0 6.0 4.0 1.15 6.0 7.0 5.0 4.0 1.30 6.0 9.0 4.0 4.0 1.40 5.0 4.0 5.0 2.0 1.50 5.0 3.0 4.0 3.0

mean 6.8 6.0 5.2 3.7

Columbia River plot yields by block (odt/ha/annum). Spacing Block 1 Block 2 Block 3 Block 4 0.8 15.0 21.0 17.0 17.0 1.00 12.0 14.0 15.0 10.0 1.15 12.0 14.0 13.0 12.0 1.30 8.0 13.0 11.0 9.0 1.40 12.0 13.0 11.0 13.0 1.50 8.0 12.0 10.0 10.0

mean 11.2 14.8 12.8 11.8

Bowles hybrid plot yields by block (odt/ha/annum). Spacing Block 1 Block 2 Block 3 Block 4 0.8 8.0 10.0 14.0 7.0 1.00 5.0 13.0 10.0 7.0 1.15 8.0 11.0 11.0 6.0 1.30 6.0 11.0 7.0 7.0 1.40 8.0 9.0 7.0 7.0 1.50 5.0 7.0 7.0 6.0

mean 6.7 10.2 9.3 6.7

Dasyclados plot yields by block (odt/ha/annum).

10 Table 4 Downham Market Block yields Spacing Block 1 Block 2 Block 3 Block 4

0.8 9.7 11.2 - 10.9 1.00 10.5 9.6 - 4.7 1.15 8.6 7.1 - 7.8 1.30 6.3 6.1 - 6.3 1.40 8.4 6.4 - 7.8 1.50 6.9 7.1 - 6.6

mean 8.4 7.9 - 7.4

Boelare plot yields by block (odt/ha/annum). Spacing Block 1 Block 2 Block 3 Block 4

0.8 7.1 9.1 - 11.2 1.00 7.4 7.2 - 7.8 1.15 7.2 6.2 - 7.7 1.30 5.4 6.3 - - 1.40 6.5 5.0 - 6.6 1.50 6.2 4.4 - 6.2

mean 6.6 6.4 - 7.9

Trichobel plot yields by block (odt/ha/annum). Spacing Block 1 Block 2 Block 3 Block 4 0.8 8.5 14.2 8.3 13.2 1.00 8.8 9.5 8.7 10.7 1.15 7.7 8.0 8.4 8.9 1.30 6.9 8.9 9.2 7.9 1.40 6.8 7.4 6.2 8.2 1.50 7.0 7.3 5.3 8.3

mean 7.6 9.2 7.7 9.5

Bowles hybrid plot yields by block (odt/ha/annum). Spacing Block 1 Block 2 Block 3 Block 4 0.8 8.6 10.2 7.9 8.1 1.00 7.1 7.8 6.6 7.8 1.15 6.7 7.1 5.8 8.2 1.30 5.6 6.9 5.5 7.1 1.40 6.2 6.2 5.5 6.9 1.50 5.3 5.4 6.0 6.6

mean 6.6 7.3 6.2 7.4

Dasyclados plot yields by block (odt/ha/annum).

11 For both species at Wishanger and willow at Downham Market the gradients of the linear relationship are much steeper for the more vigorous clones. This was not observed for the poplar clones at Downham Market and this could be attributed to the adverse conditions encountered in Block 3. The finding that the more vigorous clones responded better to close spacing is surprising. It could be argued that for the willow clones the more upright habit of 'Bowles hybrid' could be responsible for the more efficient use of close spacing. This argument could not be applied to the poplar clones whose growth habit is similar. Recent observations of the growth habits (very tall and upright) of Salix viminalis x schwerinnii hybrids would suggest that these hybrids may lend themselves to much closer spacings than those currently recommended.

The economic implications of these findings are important. If using a highly productive clone on a moist and fertile site, like 'Bowles hybrid' at Wishanger, at the end of the first three year rotation the crop would be standing at 17.55 odt/ha x 3 year rotation = 52.65 odt when planted at 0.8 metre square spacing. At a 1.0 metre spacing the crop would be standing at 12.68 odt/ha x 3 year rotation = 38.04 odt. On a ten hectare site opting for a 1.0 metre spacing over a 0.8 metre spacing would result in a loss of 146 odt on the first harvest. The increased yield would not cost much more to harvest than the lower yield, as such the harvest cost per odt would also be reduced by opting for the closer spacing.

It must be remembered that both these trials were planted in a square matrix and that the findings may not hold true for the rectangular spacings currently being favoured to allow machinery access. The current rectangular spacing is based on a twin row design of alternatively 1.5 metre and 0.75 metre between rows and 0.9 metre within the row to give a stocking of (almost) 10,000 stools/hectare. To increase the stocking to an equivalent 0.8 metre square spacing (15,000 stools/hectare) the distance within the row would have to be reduced to 0.59 metre. This very close spacing within the row may result in a competition effect whilst still not making maximum use of space and light in the wider row spacing (1.5 metre). Rectangular spacing may therefore not give the increased yield found with

12 square spacings. In a study comparing square spacings with rectangular spacings of energy crops of Eucalyptus Mohinder and Raturi (1988) found that higher yields resulted from the square spacings in the early years, although they suggested that these differences may disappear with age.

7.0 CONCLUSIONS

1. In the first three year rotation of Populus trichocarpa 'Beaupre' and 'Boelare' and Populus interamericana 'Columbia River' and 'Trichobel' the highest yield was obtained at a spacing of 0.8 metres, on more productive areas there was a suggestion that a 1.0 metre spacing may result in a higher yield. In the first three year rotation of the willow clones 'Bowles hybrid' and 'Dasyclados' the highest yield was obtained at a spacing of 0.8 metre. There is an economic penalty in the first rotation for opting for a 1.0 metre spacing over a 0.8 metre spacing.

2. Higher yielding clones, especially willow clones, were more responsive to closer spacing, and were therefore more tolerant of crowding.

3. The highest yield was obtained from the willow clone 'Bowles hybrid' on a moist site.

4. This research suggests that the interactions between clone, site, design, spacing and cutting cycle is a very complicated one. Further research is required into the interactions between: site, clone, spacing, design and cutting cycle.

8.0 RECOMMENDATIONS

The results from this experiment, on two sites with six different spacings and two clones of willow and two clones of poplar, indicate that to optimise yield of wood fuel in the first rotation a planting density 15,000 stools/ha should be used. The yield increase in later

13 rotations is likely to be less. The benefits will be at a maximum when the planting design is as square as possible. It is possible to calculate the cost effectiveness of increased planting density for this particular example. On the assumption that cuttings cost 10p each 15000 cuttings would cost £500 more than 10000 cuttings. Planting costs would not be greatly different. This additional £500 cost would be offset by the likelihood of one less weeding being required due to early site capture. This is one less weeding later in the season when costs are more expensive as a manual application would likely be required, at a possible cost of £150/ha. This would give a net increase in establishment costs of £3 50/ha. In the Bowles hybrid example cited, yield would be 14.6 odt/ha greater for the close spaced crop. If the offered price for the produce was £30/odt then the return would be increased by £438/ha which more than offsets the increased establishment costs. The cost of cuttings, the price paid for wood chips and the yield increase on different sites will vary and therefore individuals should perform their own calculations to determine the cost effectiveness of increased planting density.

14 Acknowledgements

The work reported here was funded by DTI through ETSU and the encouragement offered by the officers of this unit especially Dr Kathryn Rushton and Dr Paul Maryan is gratefully acknowledged. The work put in by past project leaders of the Forestry Commission Research Division in particular Ian Willoughby is similarly acknowledged, as is the support offered by the head of Silviculture and Seeds Research Branch, Paul Tabbush.

The care and maintenance of these experiments has been entrusted to Forestry Commission field staff at Alice Holt and Thetford and thanks are given to Ralph Nickerson, Dave Hendrie and Dave West.

Harvesting and assessments were carried out by research workers at the above mentioned field stations, whose hard work, often in inclement conditions, is gratefully appreciated. In particular thanks must be given to Marvyn Butchers for his work in drying samples and recording results.

Acknowledgement is given to Tracy Houston who carried out the statistical analysis.

15 References

Armstrong, A.T. (1996). The Continuing Study of Poplars as an Energy Crop. Final Report to ETSU for DTI. on Contract B/W2/00402/00/00.

Genstat 5 Committee. (1987). Genstat 5 Reference Manual. Oxford: Clarendon Press.

Heilman, P.E., Peabody Jr., D.V., DeBell, D.S. and Strand, R.F. (1972). A Test of Close- Space, Short-Rotation Culture of Black Cottonwood. Canadian Journal of Forest Research. 2, 456-459.

Jarvis, M.G., Allen, R.H., Fordham, S.J., Hazelden, J., Moffat, A.J. and Sturdy, R.G. (1984). Soils and their use in South East England. Soil Survey of England and WalesBulletin No15. Harpenden.

Matthews, R. (1995) Non Destructive Assessment of Biomass Growth and Yield. Unpublished Forestry Commission report to ETSU for DTI.

Mohinder, Pal. and Raturi. D.P. (1988). Effects of plantation geometry on biomass production in energy plantations of Eucalyptus hybrids. Van-Vigyan. 26, Nos 3-4, 91 95.

Tabbush, P.M. (1993) Coppiced trees as energy crops. Final report to ETSU for DTI on Contract E/5A/1291/2285.

Willebrand, E., Ledin, S. and Verwijst, T. (1993). Willow Coppice Systems in Short Rotation Forestry: Effects of Plant Spacing, Rotation Length and Clonal Composition. Biomass and Bioenergy. 4, No.5, 323-331.

16 APPENDIX

17 Forestry Commission Research Division Silviculture (South) Branch

Forest Experiment Plan

Experiment Name: Downham Market 1 P93 (see also Wishanger 5 P92)

Project Number: 691 - Short Rotation Coppice

Background: Interest in Arable Energy Cropping using poplar and willow on short rotations of 2-5 years, and grown at close spacings of e.g. 1m x 1m, is increasing. This parallels a general increase in interest in Farm Forestry as a means of reducing agricultural overproduction.

Previous FC experiments, under contract to the Department of Energy, have investigated the effect of initial spacing on yield. The results (Potter, 1990 and 1991) indicate a critical relationship between yield and spacing in the range of ground area per stool of 0.8-1.6 square metres (0.9m x 0.9m to 1.3m x 1.3m square spacing). These results were obtained using Populus x interamericana ‘RAP’. It is likely that the optimum spacing will vary with leaf area index, and one would therefore expect Populus trichocarpa clones and willow cultivars to exhibit different optima.

Objectives: To establish the initial spacing which optimises the yield over the first two harvests for poplars and willows with contrasting leaf area index.

Experimental Treatments: Species:

Poplars and willows will be treated separately, so that each experiment will comprise either poplars or willows.

1. Poplars: PB - Populus x interamericana /‘Beaupre’ or ‘Boelare’ PT - Populus x trichocarpa ‘Trichobel’ or ‘Columbia River’

2. Willows: WC - Salix (Cap. Vim.) sericans? WV - Salix (Vim.) Bowles?

18 Spacing:

Area/stool Equiv. square spacing (m) (m) 0.64 0.80 1.-00 1.00 1.32 1.15 1.69 1.30 1.96 1.40 2.25 1.50

Design: For each experiment: 2 (species) x 6 (spacings) = 12 treatments 12 (treatments) x 4 (randomised blocks) = 49 plots

Plot size: 7 trees x 7 trees to include a 1 row buffer surround (25 trees assessment plot)

Statistical Analysis: Dry weight yields will be estimated from stem diameters, after establishing the way in which dry weight varies with stem diameter, for each species and diameter. It may be possible to establish a single relationship across all spacings, and to combine within “poplars ” and within “willows”.

Dry weight yields can be presented graphically with standard error bars, or tabulated with least significant differences.

An analysis of variance can be carried out, testing for linear and quadratic effects within each of the two species. If the responses are found to be quadratic it will be possible to estimate a maximum yield and the spacing at which this occurs. The ANOVA would be:

Source d.f Blocks 3 Between species 1

Within species (1) Spacing - linear 1 - quadratic 1 } ^ - deviations 3

Within species (2) Spacing - linear 1 - quadratic 1 } ^ - deviations 3

Residual 33 TOTAL 47

19 Basic Treatments: The areas must be completely cleared, sprayed with ‘Roundup’ at 3 1/ha, then ploughed and harrowed. Deer/rabbit fencing will be required. 25 cm cuttings will be inserted at accurately measure spacings in the period Jan-March (or later with cold stored material). Residual herbicide will then be applied before flushing. Thereafter weeds will be rigorously controlled as necessary.

In the following winter, the shoots will be cut back to a single stump, 1-2 cm above the ground.

Filler area will be planted at 1m x 1m spacing with native species in pure blocks, so that there will be some assessable plots of 6 trees by 6 trees. Species to be used are hornbeam, ash, lime, oak, hazel.

Requirements: 1. O.7ha (gross) of ground per experiment.

2. 1200 (net) cuttings of each clone for each experiment. For 2 poplar and 2 willow experiments altogether, 2,400 cuttings of each clone will be required:

3. Dee/rabbit fencing.

4. Access to agricultural machinery.

Assessments: 1. Experiment Report Forms 1-4 to be completed as appropriate.

2. Plots will be harvested 3 and 6 years after the maiden cut, and dry weight yield per plot assessed using a sampling method based on diameter measurements (see Alice Holt 148).

Duration: Review after 4 years.

Drafted by: P M Tabbush (Principal Silviculturist) 30 August 1991

Design approved: R C Boswell (Statistician) 23 September 1991

Distribution: Director (RD) 1 CRO 1 D Elgy 1 D Hendrie 1 File Project 691 1 Spare 2

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