following the answers

Dear María,

Although not exactly what you're looking for, these references may be of some interest to you:

Bulcke et al. (2009). "X-ray tomography as a tool for detailed anatomical analysis".

Applied on a small number of species representing hard- and softwood, as well as temperate and tropical wood species; sampled from pith to bark.

Hietze et al. (2015). "High-resolution densitometry and elemental analysis of tropical wood".

It seems that the paper doesn't include images of bark, but the authors refer that they've also measured bark. Perhaps you could contact them?

All the best,

Vera

I would recommend to contact the German Institute Thünen in Hamburg, they clearly have what you need.

https://www.thuenen.de/en/topics/forest-management-and-wood-use/