Acer cissifolium  

Acer cissifolium (Siebold & Zucc.) K. Koch (Japan) – A very rare escape from cultivation. Two self-sown mature (fruit-bearing) individuals were found growing on the border of river Durme near Lokeren in 2014 (still present in 2017).
This Japanese species is much reminiscent of the American invasive tree Acer negundo. Both may have been intermixed which means that A. cissifolium may be more widespread but overlooked. It differs in having all leaves 3-foliolate (vs. usually 5-foliolate), leaflets are distinctly petiolate (vs. often nearly sessile) with minutely ciliate margins (vs. smooth margins). Shoots also tend to become darker (brown from the second year onwards) and are hairy, not glabrous.
Acer cissifolium is relatively rarely grown as an ornamental tree. Records of escaped individuals are, apparently, exceptional. Wilmore (2000) reports it from Yorkshire.

Selected references

Wilmore G.T.D. (2000) Alien plants of Yorkshire. Nats. Union, Bradford.