Effect of spacing, parental genotype and harvesting cycle on biomass production in two half-sib progenies of Robinia pseudoacacia L.

This study investigated the influence of spacing, parental genotype and harvesting cycle on woody biomass production in open-pollinated families of the locally selected black locust (Robinia pseudoacacia L.) clones ‘Tsarevets’ and ‘Srebarna’ at specific site conditions. A Nelder experiment with 16, varying along the spokes, nearly-square spacings and two black locust families arranged in alternating sectors, was established in North Central Bulgaria. Alternative harvesting options (‘harvesting cycles’), consisting of one, two or three successive coppicings, were applied and the amounts of dry shoot dendromass collected per plant were accumulated to calculate and analyse the total yield of woody biomass, achievable within 2-, 3- and 4-year periods (‘production period’). The one-coppicing cycle proved superior for both families, regardless of the length of the production period. Biomass yield in the family of ‘Srebarna’ exceeded that of ‘Tsarevets’ at the wider spacings, when 1 and 2 year-old shoots were harvested, while dendromass production of ‘Tsarevets’ was substantially higher in the 4-year-old shoots. The woody biomass yield per plant increased with spacing, and growth tended to saturation at the lowest densities, enabling derivation of optimal planting densities. The total dendromass yield per hectare was maximized at growing space of around 0.5 m2 per plant for the 2-year production period, while spacing of 1.2–2 m2 was required for production periods of 3 and 4 years. If the results from this experiment can be reliably scaled up in practice then short rotation crops of ‘Srebarna’ and ‘Tsarevets’ families at comparable site conditions should be able to produce annual biomass yields of 9–11 Mg.ha−1.year−1 within a 4-year period, given the optimal planting densities and harvesting regimes prescribed.