What is this tree or shrub seedling?

My first guess would be a Sorbus but the leaves are very serrated.

I'd say it was definitely an Ash of some kind, just not sure which - yet! Looks quite like Fraxinus excelsior, but am not sure because of the bronze tint to the leaves. There should be between 9 and 13 leaflets and they usually fall whilst still green.

Would have been nice to see the plants stem and not the bamboo cane presumably holding it up :-)

I'm going to plump for Sorbaria.

gorgeous colours green with a deep purple
x x x

I don't think it's a Fraxinus excelsior - the leaves on our trees are not serrated like these.

I'd agree with Fractal.

It certainly looks like Sorbaria but I have never seen one that rich in colour.

got me looking--- one seemed near and thats Sorbus reducta seems gorgeous autumn colour

Must admit sorbus reducta looks far more like it in leaf shape. When I checked images of sorbus hupehensis the leaf shape looked very different, but I'd put that down to the difference between immature and mature leaves. Certainly the seed supplied was for Sorbus hupehensis so I guess it must be that.

time will tell but it does look lovely wharever it is

This is very quick growth for a tree seedling. Is the cane in the top part of the picture supporting anything, Bertie, or does it rise above the plan?

The cane rises above the plant, Bulbaholic. The original seedling got a bit twisted so I tied it to the cane to straighten it out. It's only about eight inches high so far. I will overwinter it in the polytunnel and plant it out in the spring. Looks like a nice little tree from what I've read, but fairly short-lived.

We had a pot of S. recucta at work many, many years ago and it really was a suckering shrub of a thing. It may have grown taller if it had been planted in open ground. Couldn't remember the exact form of the leaves so this was not what I thought it would be. It does look as though it will get good autumn colour too.