This depends on how much effort you want to make. You won't stabilise the wall with cement, a tree is much stronger than a little bit of cement. Besides this will look worse not better!
If you like the stones and you like hard work, why not pull them out, mound it up with some top soil, bringing it out slightly, then re-lay the stones and turn it into a bed/rockery. This will create a bit of soil above the tree roots and leave some moisture for a few plants.
You could use ferns to block up the gaps. Conifers are mega thirsty so you'll have your work cut out growing something there (hence the mound).
Thanks Bob, I may do this, my only concern is that removing the stones may undermine the trees. What do you think?
Thanks Bob, I may do this, my only concern is that removing the stones may undermine the trees. What do you think?
I really can't see that being a problem, there are no roots visible. I suspect they've gone down and/or in the other direction. Besides, if they fall over you can plant something else that you prefer!
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Besides, if they fall over you can plant something else that you prefer!