Whilst not unusual, the laburnum is a poisonous tree that is native to the mountainous areas of Southern Europe. It is a member of the Fabaceae (the pea and bean family).

The laburnum is a deciduous tree,  that has trifoliate leaves i.e. each leaf consists of three leaflets (see featured image).  

In the Spring, the tree is covered with pendulous clusters of bright yellow flowers.  These flowers are responsible for one of its names -  the golden chain tree (jpgs below).   When fertilised, a pod develops which contains black seeds when ripe.

The pod and seeds are poisonous (as are other parts of the tree).  The toxin is a quinolizidine alkaloid - cytisine.   Cytisine acts on the nicotinic receptors in the brain, and when ingested can cause 

• tachycardia (elevated heart rate) and hypertension (high blood pressure)
• vomiting and diarrhoea
• Confusion and dizziness
• Convulsive movements

The laburnum and its associated hybrids are much cultivated as ornamental trees in parks and gardens - often on pergolas.  Its growth habit can result in a multi-stemmed tree, but the tree is often pruned to a single trunk - with a smooth green bark.  Gardeners are often advised to remove the seed pods after flowering - as they are the most poisonous part of the plant (and pod and seed formation / maturation drain resources from other processes).