Black Locust

Robinia pseudoacacia

General info

Black Locust is a hardwood deciduous medium size tree with leguminous properties associated with root nodules formations beneficial to strong bacterium interactions. It is the second most planted hardwood tree globally after eucalyptus spp, accounting for significant proportions of both Hungary’s and the Mediterranean region’s tree species. It is native to a small region of the South East of the United states but has been naturalised across temperate regions including Europe, and Asia and in several instances has been considered an invasive species.

Black locust is often considered weedlike as it can spread through underground suckers and shoots and, as a pioneer species, is often first to establish in areas of damage or canopy openings in environments. It can outcross or undergo selfing to provide adaptability for a successful pioneer species alongside asexual sucker strategies. When it undergoes asexual reproduction, mother and daughter plants are integrated sharing resources through a common root system. It is commonly utilised in plantations for short rotation coppicing alongside reclamation of polluted soils such as surrounding mines. They produce large white flowers attractive to honeybees.

The species has undergone a significant amount of cultivation enhancement through breeding, largely for wood timber property enhancement, though Kiscsalai and Ulloi varieties have focused on energy plantation benefits and other co-benefits such as frost resistance. Its wood is well known for being heavily resistant to fungal decay.

Cultivation and agronomy

Generally, a good pioneer species which is shade intolerant and requires a lot of sun. Black locust thrives in moist loamy well drained soils but can tolerate dry nutrient poor soils also. It can tolerate a range of pH’s across European plantations (3.2–8.8) with an optimum range of 5.5–7.0. Optimum conditions ensure the productivity of root-associated nitrogen fixing bacteria, which can help to fix between 75–150 kg of N ha-1 yr-1. It can tolerate a range of altitudes, though it grows best between 600–700 m above sea level.

Whilst it does have some pest and pathogens of note, it is largely considered to be a heavily resistant species with its most damaging native pathogen (the locust borer Megacyllene robiniae) having not made its way to Europe. It can achieve up to 14 Mg ha-1 yr-1 according to certain studies with variability in output depending on rotational strategies employed and successive years of production and maturity.

Utilised normally on a 1–3-year rotation period, it can grow to heights of 10m within 5 years with coppice growth from stool shoots having been demonstrated maximums of 4.9 m a year in certain studies. The volume growth from coppice stands has been shown to range between 10 – 16 m3 ha-1 y-1 as early as 5–10 years into cycles. However, over time, if regeneration of stands is performed from stool shoots, a decreasing trend of wood production is observed, necessitating the need for regenerating plantations via seed or root sucker alternatives periodically. This leads to many strategies currently employing 2–3 coppice cycles, or at most 5 cycles in some studies, before stools are removed and replanted from other sources.