AusBonsai

might be a good place to ground layer perhaps

I think it is probably time to get some definitions so we are all talking about the same things.
I believe that aerial roots are those that start to grow above ground in the air and grow down until they reach the soil. Figs and Metrosiderus are a couple that I know that can do this.

Elms do not start roots growing above the ground. Both the examples shown in this thread are trees that have been potted up much higher than before with roots that were already growing under the ground now beginning above ground level. As long as the tip of the roots are still in the soil they can continue to grow as we see here.
Bonsai growers often pot up trees slightly higher so the roots are visible. If done properly this can make the tree look like a much older tree. If done badly it just looks crappy. There is a bonsai style called exposed roots that represents an old tree growing where erosion has exposed the roots. The roots in this style of bonsai should ADD to the whole and make it look better. Each exposed root needs to flow harmoniously from the base of the trunk to the soil and look like it supports the tree. Movement and arrangement of roots should also harmonise with the flow and movement of the trunk above. All that is not actually easy to achieve which is probably why we don't see very many exposed root style bonsai at shows.

I am now wondering if these aerial roots are depleting it.

Roots above ground don't 'deplete' a tree. If your tree

hasn’t done much in recent years

it is far more likely to be an issue with how it is being looked after. Maybe not enough fertiliser, water, light, etc or needs repotting (rootbound trees do not grow very well because they can't get enough water or nutrients and will eventually die).
One issue that roots like this can cause is reverse taper. The trunk will thicken most between the lowest branch and the closest root. Thickening will be slightly less below the 1st root and above the 1st branch because a bit less food and energy will be passing there.
Removing unwanted roots is so simple - just cut them off close to the trunk. The tree has plenty of other roots to maintain it and will hardly even notice they are gone.

shibui wrote:I think it is probably time to get some definitions so we are all talking about the same things.
I believe that aerial roots are those that start to grow above ground in the air and grow down until they reach the soil. Figs and Metrosiderus are a couple that I know that can do this.

Elms do not start roots growing above the ground. Both the examples shown in this thread are trees that have been potted up much higher than before with roots that were already growing under the ground now beginning above ground level. As long as the tip of the roots are still in the soil they can continue to grow as we see here.

Thanks for the clarification shibui, my example Elm is the first Bonsai I acquired, at the trade table of a local group's annual show about 6 weeks ago. Not much was known about the tree. Shortly after, I joined the group as they were so friendly and happy to share their knowledge and experience. Needless to say I am pretty inexperienced and am voraciously trying to absorb as much knowledge around the subject as my aging (and sometimes overwhelmed) brain will allow!

Plenty of good people at YVBS
Your tree may well be a zelkova AKA Japanese grey bark elm. It should be a good one to learn with. The roots on your tree do not look too out of place so could be left as is or if you decide you don't like that look there are other options to change that.

Welcome to bonsai and also to Ausbonsai and I hope you enjoy the journey.

shibui wrote:Plenty of good people at YVBS
Your tree may well be a zelkova AKA Japanese grey bark elm. It should be a good one to learn with. The roots on your tree do not look too out of place so could be left as is or if you decide you don't like that look there are other options to change that.

Welcome to bonsai and also to Ausbonsai and I hope you enjoy the journey.

Thanks shibui.