Unless, the staircase is wobbling. You do not have a safety issue, just cosmetics. You can remove the top tread, clean it up and re-install it tighter to the riser tp address the gap. The crack in the side board can be filled and refinished. You can use a color wax stick to fill it in.

It’s normal for a tree to deteriorate over time, I don’t think it poses a serious security risk. The crack can be cleaned and filled with sawdust, or with a suitable piece of wood (in both cases, the glue must be used). Then sandpaper and varnish should be used to give the step the appearance of the surrounding tree.

How do you glue sawdust Michael Stokes?

Thank you everyone, can someone link a video for me on how to remove a stair tread. I see a lot of videos on how to install but they skip the removing part.

Remove the scotia molding under the over hang. Use two pry bars, start at a corner of either side and start prying carefully at the front edge. Work your way towards the opposite sides, then work your way towards the back of the treads. It will pop right out.

Remove the nails, clean it up and re-install it.