300.000 litres of heating oil – this princely sum is what it takes to heat the administrative building at the experimental agricultural research centre in Grub. The Bavarian State Research Centre for Agriculture (LfL) decided to use fuels from renewable energy sources for heating, and to replace the heating oil with woodchips. But where were they to get the woodchips? The State Research Centre’s own forest does not have the capacity to supply the necessary quantities. The solution was to set up short-rotation plantations (SRP) on more than ten hectares of grassland and arable land.
A golden opportunity to set up a research project!
The Bavarian State Institute of Forestry (LWF) thus promptly set up a research project. In Bavaria there are species-poor grassland areas in many places that have been intensively farmed up to now, and that are now available for the establishment of new short-rotation plantations. The pressure from competing vegetation is very high on these areas, however, and soil cultivation and the use of herbicides are the norm. On the areas of the State Research Centre for Agriculture (LfL) there was now an opportunity to test how SRPs could be implemented on former grazing land, without soil cultivation and without herbicides. At the same time, suitable clones for the grassland sites were to be identified and their biomass yield evaluated.
On a total of 48 plots, three cultivation systems were tested using four poplar clones. In addition, as well as conventional SRPs with soil cultivation and the use of herbicides, rods were planted on some areas (one-year-old shoots) for comparison.
The different systems
On the control area K, the soil was not cultivated. Despite the high level of competition from understorey vegetation, the cuttings initially sprouted. After just a few weeks, they were however overgrown with grass (Fig. 1a). This was a serious threat to the development of the cuttings, because of root competition and the lack of light.
In the foil system F, compostible foil was deployed with the aim of holding back the grass in the planting rows, thus protecting the cuttings from competition from the other vegetation. It was necessary to till between the planting rows in order to weigh the foil down sufficiently at the sides with earth (Fig. 1b). Otherwise, some of the cuttings would almost certainly have been lost under the foil. All in all, this method proved to be very work-intensive.
With the strip tilling system S, the rows of plants were tilled using a small tiller to a depth of approx. 20 cm (Fig. 1c). This process was carried out on approx. 35 % of the grassland. Depending on how compacted the soil is, it must be tilled one or more times. The tilling procedure delays the growth of the competing grass vegetation. Whether the poplar cuttings survive or not depends on the extent to which they get a sufficient head start during this "competitionless phase".
Fig. 1a: Control area - a poplar cutting on a control plot has to fight to survive (Picture: J. Neuner).
Fig. 1b: Foil systems - the foil has to be weighed down with soil to allow the young plants to sprout freely (Picture: J. Neuner).
Fig. 1c: The tilling of strips on grassland (Picture: J. Neuner).
Out with the competitor vegetation!
Even in the first vegetation period it became apparent that the establishment of short-rotation plantations on grassland is unlikely to succeed without soil cultivation or the use of herbicides. Given that an average of just 25 % of the cuttings sprouted on the control plots (Fig. 2, left), we cannot assume that the establishment of the plantation under these conditions will be successful. At an average of 51cm in height, the poplars on the control plots were also the smallest (Fig. 2, right). The prospects of success may be improved if the area is mowed early and several times per year, or if mulching plates are used when the plantation is established. The costs and effort involved in establishing a short-rotation plantation without any soil cultivation on grassland seem very high however.
The percentage of cuttings sprouting is considerably better with Variant S. Here, an average of 72 % of the cuttings survived. However, the average growth to an average of 63 centimetres in height was only marginally higher than with the control system (K). The tilling of strips ensured that the cuttings had a good planting bed, but the competing grass was only held back for a relatively short time. After three to five weeks, the competition from understorey vegetation was already considerable. It is helpful to mow or to mulch between the planting rows at least once in the first vegetation period.
The foil system was convincing because of the height growth that was achieved: The average height of 146 cm after the first vegetation period was relatively close to the values achieved by the ploughed and harrowed SRP system. With the foil it is possible to hold back the competition vegetation for a sufficient length of time, and the poplar cuttings are not limited in their growth. It is not necessary to cut the vegetation around the plants. The disadvantages of this system are the extensive work required and the costs.
Fig. 2: The percentages of rods sprouting (left) and average heights (right) of the K, F and S systems in comparison with conventional SRP, rods and alders in the first vegetation period.
On the comparison plots
The poplars grew considerably better on the ploughed plots treated with herbicides (conventional SRP), with between 89% and 97% of cuttings sprouting, and cuttings growing to an average height of 214 centimetres in one year.
One alternative is to plant rods from one-year old shoots. These may well be considerably more expensive and more work to plant, but they can no longer be overgrown by the understorey vegetation. There is no risk of grazing damage by game to the rods either, although damage may be caused by fraying (roe deer bucks rubbing the velvet off their antlers) where there are high numbers of game. After the first vegetation period, the (one-year old) rods reached the satisfactory height of an average of 204 centimetres. Depending on the site, a good alternative is to use poles (2-3 year old rods).
The planted grey and black alder trees also grew well. These are of particular interest as native tree species suitable for the site on grassland sites on former moor soils.
Summary
The initial results of the Institute of Forestry’s project show that the establishment of SRPs on grassland is unlikely to succeed without soil cultivation and the use of herbicides. The tilling of strips or use of foil has a positive effect on the development of the SRP. After the first vegetation period, the conventionally established SRP was however considerably more successful. As an alternative to the poplar cuttings, rods or indigenous alder species can be planted.