Not sure if this is a scientific answer but I have one in particular White Oak (and probably others I know I've seen) that has been a 3 trunked tree since I've owned the place (long time). Each trunk is growing at an angle to the ground and the combined crown covers the area of a large mature tree. Surrounded by other types of trees it looks rather healthy, Each trunk has to be around 16" diam. (guessing).

Curious, why would that shorten a trees life?

Actually, I don’t know if it does shorten the life expectancy. I have read something to that effect, but it was more of an opinion rather than stated as scientific fact. It does seem such trees are more subject to rot and splitting, but that is also anecdotal.

I am asking in very general terms, but the reason I ask refers to a yard grown tree.

If I had to guess, I'd say my tree was a tree that was cut off and sprouted multiple at the base. The tree shows signs of a long rotted away stump, more like a hole, at the center of the trunks. No signs of rot or anything. In nature anything is possible though.

Depends alot on the structure & wood-durability. Many white-type oaks make reasonably good forks in the multi-trunk. Black-type oaks not so good, but I see those that eventually lose one of the trunks & callous-over pretty well. Other black oaks lose a trunk and rot begins in the base.

Same general issues for pretty much any multi-trunked tree. I see multi-trunked white pines (that were prb'ly "weeviled" at an early age) that formed reasonable forks & seem fine -- once saw an enormous, ancient white pine w/such multi-trunking.

This is a red-type oak, and not known as a long lived tree. It’s healthy, but I suspect there’s more decay where the two stems adjoin than what can be seen. I try to keep debris cleaned out, but this probably has no bearing on its health.

I haven’t had an arborist offer an opinion, but even if they did recommend cabling, I’d be reluctant to do it. The larger of the two stems leans towards the house, which is 35’-40’. The canopy might land on the roof, but I don’t lose any sleep over the prospect of that happening.

Yes and no. In northern Louisiana, with Eastern Red Cedar, a twin-trunked tree will eventually split in 25-50 yrs, The surviving trunk usually falls within 2-3 yrs. following the split.

In Live Oak trees, it seems to make no difference. If conditions are right these trees live into the hundreds of years, sometimes as multi-trunked specimens.

maackia, your red-type oak looks OK to me. If possible, every now & then take a stiff hooked wired & try to clean the debris out between the trunks.

How about it loses half its life expectancy?

There are many 40-60 year old (healthy) Basswood on my property that are in much worse shape than that you pictured with many more stems. Their typical life expectancy is 75 to 100 years I believe.

I don’t know the age of this tree, but it pre-dates the house, which was built in the early 1960’s. My best guess is it’s around 75 years. This is the one tree on our property that would be devastating to lose. Its location, maturity, shading, and beauty make it indispensable. I’m simply trying to understand the condition of this tree and do whatever I can to help it stay healthy. I sometimes have to fight my inclination to “help,” and leave well enough alone.

Thank you all for helping me think through this.

maackia, one unfortunate thing that happens to such trees is one trunk becomes dominate & the others might die from lack of sun. This then starts the rot at the base. With your oak's trunks pretty much equal, this prb'ly won't occur.