an early thought was white poplar but not convinced.
are the stems 'downy'.

It doesn't look white enough on the undersides of the leaves to be a white poplar, to me. I was thinking a plane tree seedling (Platanus).

I guess it's just a tree seedling although it does grow medium fast tree seeds usually are slow aren't they?

many trees have very fast growth rates in the first few years to get leaves up and out of completion with other plants. so its not unusual.

In my climate, Chinese elm grows about a foot the first year, while white popinac can grow more than six feet!

Ok thanks