For this wedding I made both the wedding cake and the grooms cake (a busy feat for sure, but the wedding cake was a simple cake, so I decided I could do it).  This cake was a yellow cake with chocolate buttercream and the wedding cake was a three tiered, roughly frosted, red velvet with cream cheese frosting.
I started by baking all the cakes...9 layers and a small anniversary cake for the wedding cake and three 12 inch and two 10 inch yellow cakes for the tree stump cake.  
I stacked and filled the 5 cakes with chocolate buttercream.  I was concerned that there would be too much unsupported height with this cake, so I added an 8 inch cake board and some dowels to hold up the top two layers.  Then I let my husband start carving.  I think my husband is much more artistic than I am and trusted his vision of a more realistic shape.  So he went to work cutting the basic shape.
We built out the roots just a bit with some of the scraps and buttercream to hold it together...
After getting the shape how we wanted it, I covered the entire cake with buttercream frosting.  I used less cocoa in the lighter frosting (because the full chocolate recipe was just too dark for the finished product).  The problem with this was that I didn't think the frosting tasted as good as I wanted, so I used a darker frosting between the layers.  Then I decided to go ahead and throw a layer of dark frosting on there as well, then texture the bark over that.  This gave the cake a light/dark contrast that I liked.  Had I planned it better, I would have just dirty iced it with the dark frosting, but I didn't decide to use the dark until later...
Now I had a few ideas about how I wanted to do the inside of the stump.  I'd seen some done with swirling buttercream but I just didn't like how they looked.  The best ones I saw online were done with fondant, nice and smooth.  Originally my plan was to use a light brown fondant and paint darker swirls on it.  I tried this and didn't like the results.  I just couldn't get it to look like tree rings.  Then I thought maybe i could experiment with coloring the actual fondant.  So I took a light brown fondant and put some food coloring on it, then I started to stretch it like taffy.  I pulled it, and folded it in half, then pulled it again.  I did this until I started to see the colors coming through.  Then I flattened it out with my hands taking care to keep the swirls going in the right direction.  From there I rolled it with a pin and came up with these.  I made a couple for some perspective and ended up using the darker one.  To get the shape, I traced paper over the actual top of the cake and cut it out to use as a guide.
Then I covered the entire cake with the darker, better tasting, buttercream and put the fondant on top.
Next I started adding the lighter buttercream for texturing.  I intentionally didn't cover every surface because I wanted some of the darker color to come through.  I piped on blotches of frosting and used a spatula to texture it...
When I was done with the texturing it looked like this...
Next came the carving of the initials.  My original idea was to pipe in a lighter frosting after I had carved it, but it just didn't pop.  I wasn't happy with how it looked.  However, because I had that darker frosting below the light, I found that when I used a round tool to 'carve' the initials, the darker frosting showed up and it looked great.  So I went with that (in my typical, fly by the seat of my pants fashion).  I did have to fill in a bit of dark frosting here and there, but for the most part it worked out on it's own.
Finally I pulled out my trusty air compressor and air brush gun and I added some more brown highlights to give it even a bit more contrast.  I also dipped some strawberries in chocolate and placed them around the base of the cake to look like acorns.  Overall, I was really happy with how it turned out and it was a big hit at the reception.  :-)