Never too old to have seen it all—this is surely true for that which I saw during a National Holly Society conference tour near New Harmony Indiana, this past November. As an explanatory lead-in I’ve included an image of a conifer trunk showing how the wood of this tree’s trunk naturally surrounds the base of branches. This old arborvitae trunk dramatically exhibits how a tree’s branches and trunk interact.
Trunk wood grows a new layer each year covering wood of the branch, forming very strong “collars.” Collars don’t always result in strong unions, but that’s another topic. These layers of branch collars over the years are how branches remain flexible but keep attached to the tree, except in cases of excessive wind, ice or snow load. I compare tree branches with assembling Tinker Toys where a stick is inserted into another connecting piece. Branch tissue is essentially “inserted” into a tree’s trunk, meaning they are individual entities making up a whole. And, they are not truly inserted, but occur from a small bud, growing larger each year to become those massive overhead tree members we like to climb or swing on. Of course it’s more complicated than that but this description certainly provides an entirely different mental image when looking at a tree!
The second image shows something I’ve never seen before…but perhaps I should get out more often. What appears to be two smaller trunks growing from the base of a tulip-tree are actually individual sassafras trees. They are both living and have had the base of their trunks completely surrounded by the base of the tulip-tree. I guess what is most amazing to me is that they are still living. You would think the tulip-tree would give no advantage to other trees that are competing with it for water and nutrients; indeed, competition for root space alone. If the competition from the tulip-tree wasn’t enough to do-in the sassafras trees, it seems logical the constricting nature of the tulip-tree’s trunk tissue would shut off flow of water and nutrients to the sassafras trees by “girdling” their bases, a common problem with landscape trees. However, that doesn’t seem to be the case here since they have obviously been growing in this space for quite some time. I can’t believe this large old tulip-tree is so vigorous at this age to have done this engulfing quickly.
In the past, I’ve seen two trees growing very close to each other but keeping their individual entities separate. Two that come to mind were a white oak and white ash along the trail west of the Visitors Center. These were very large, mature trees pressed so closely together at their bases most thought it was one tree with two trunks. A closer inspection of their bark showed enough difference that most people, when challenged whether these were the same type tree, were able to tell they weren’t the same.
Two plants of different genera seldom graft naturally (I say seldom because “never” is no longer in my vocabulary) whereas branches of similar plants can self-graft if pressed tightly together during their development. I’ve seen branches of beech, maples, oaks and yews (Taxus) just to name a few that had naturally grafted together. It’s made me wonder whether it would be practical to intentionally graft branches in young developing trees to provide internal support between trunks. Nah. It’s difficult enough just getting proper pruning done…
by Mike Ecker, Director of Horticulture