"A group of trees is a forest."
Translation:Aro de arboj estas arbaro.
In English a forest doesn’t have to have any trees. It was, historically, an area set aside for hunting. For example: http://www.visit-exmoor.co.uk/experience/exmoor-forest
https://adventuresinesperanto.wordpress.com/2011/10/24/clutch-of-eggs/ explains it well:
How many soldiers are there? – Grupo da soldatoj (A group of soldiers)
What kind of group is that? – Grupo de soldatoj (A group of soldiers)
The sentence here falls into the second category. We aren't particularly interested in the quantity of the trees (then da would be used) but more in the kind of group we're looking at.