Using Black locust for Energy or Valuable Timber Production

Using Black locust (Robinia pseudoacacia) for Energy or Valuable Timber Production

Excellent growing and wood properties make Robinia pseudoacacia the ideal tree for outdoor use, or short rotation biomass energy plantations.

The wood is used for construction and in the furniture industry; the major criteria being a shape of stem as long and straight as possible. The utilisation of black locust for energy production requires a high volume of standing timber. A cultivation trial near Riedenthal in the Weinviertel (Lower Austria) provides information on the types of black locust clone best suited for this purpose.

The trial plot was established in the years 1988 and 1990 by the then Department of Silviculture of the Federal Research and Training Centre for Forests, Natural Hazards and Landscape (BFW) with clone provenances from Hungary and Austria.

Yield, site suitability and form properties were assessed. In 1988 the Hungarian varieties Nyírségi and Apalachia were planted together with ten Austrian clones. In 1990 the trial area was enlarged: the two Hungarian clones Zalaí and Jászkíséri were added to the already mentioned clones. Planting was made at a spacing of 2 m between rows and 2 m within rows.

In 1996, a first survey was made on the most promising clones of the Tulln and Hungarian varieties. Although the trial was intended to continue for a period of 40 years, the area had to be cleared ahead of time due to road construction works. Unfortunately, this was before the Department of Genetics of BFW could collect record dbh data and form properties for all the varieties.

The Hungarian varieties Appalachia, Jászkiséri and Nyíségi (see Graph above) seem to be most suitable for valuable timber production, since between 40 and 55 % of the trees are straight-boled. Considering also the stems with slight sweeps, which can be used for tool handles, vine or fruit tree poles, the share of converted timber of these clones amounts to 60-70 %.

Of the clones selected in Austria, only the clone Tulln-83/10 reaches a similar result, with 65 % of straight and slightly crooked boles – although only 20 % are completely straight-boled. All other clones have no noteworthy share of straight boles. Sometimes, the share of strongly crooked boles amounts even to 85 %.

From the growth performance (see Graph bellow) we can see that the Tulln clones Tulln-81/62 and Tulln-81/83 take the lead followed by the Hungarian variety Jászkiséri. They reach a dbh of 14,7 and 14,2 cm on average at an age of 17 years. The variety with the worst performance is Tulln-82/55, with a dbh of 11 cm. The performance of the other Hungarian and Tulln clones is somewhere in between (12,8 to 13,5 cm). This tendency was already visible in 1996 when measuring the trees at an age of six and eight years. As in 1996, tree height was measured in addition to dbh allowing calculation of the biomass production of individual varieties. For conversion of dbh and tree height into oven dry mass (in t/ha) conversion factors were used, which were determined as a mean value of some Hungarian varieties (according to Rédei & Veperdi, 2005): a form factor of 0,85, a density (humid) of 0,895 t/m3 and a drying factor of 0,682.

After eight years, the best clones produce up to 90 t of biomass/ha (see Table), while the worst clones only produce half of this. The annual increment is between 6 and 11 t/ha. This means a fuel value of 4200 litres of heating oil at best, and of 2200 litres at the worst.

It must be pointed out that the spacing of 2 x 2 m does by no means utilise the full potential of Robinia as regards energy wood production. Hungarian trials have revealed that, considering a five year short rotation period, the highest increments per hectare could be achieved with 1,5 x 0,3 m spacing. This shows the enormous potential of black locust for energy wood production: great increment performance, low moisture content, high natural regeneration ability and suitability for the most diverse sites. In addition, black locust’s role in fixing atmospheric nitrogen makes it best suited for marginal sites and otherwise non-productive land (Führer 2005).

Mention should be made of the fact that at the moment there is no approved source material available in Austria for the production of forest reproductive material (see Council Directive 1999/105/EG and FVG 2002). Forest owners who are interested in black locust plantations for the production of valuable timber will have to use approved varieties from neighbouring countries (from Hungary in particular).

Forest reproductive material for the establishment of short rotation plantations on agricultural land does not fall under the Austrian Act on Forest Reproductive Material. Accordingly, the above described Austrian varieties can be used for energy wood production without restriction of any kind. So as to secure the success of such biomass plantations, interested land owners should make sure, that the chosen planting stock originates from highly performing clones based on expert opinion.

Károly Rédei & Irina Veperdi (2005): Robinienwirtschaft in Ungarn: III. Robinienenergieholzplantagen. Forst und Holz, 60: 468-469.
Ernõ Führer (2005): Robinienwirtschaft in Ungarn: I. Die Robinie im praktischen Waldbau. Forst und Holz, 60: 464-466.

Establishment and management of short rotation plots
Folder “Bewirtschaftung von Kurzumtriebsplantagen”