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Strangling a Live Oak

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Strangling a Live Oak
This 41-inch DBH live oak was declining while all the other protected trees near the construction 17 years ago were doing fine

This 41-inch DBH live oak was declining while all the other protected trees near the construction 17 years ago were doing fine

We had an unusual experience on a root crown excavation the other day on a 41-inch DBH live oak located near a couple houses built 17 years ago on a wooded pennisula that sticks out into the Intracoastal Waterway at the south end of the Guana-Tolomato Wildlife Preserve north of St. Augustine. The construction impacts were minor on all the trees in the area and there have been no signs of construction-related problems on the trees with the exception of the largest live oak — the 41-inch DBH tree. It has struggled. Foliage is sparse and getting sparser and some foliage in the crown is beginning to turn epicormic. The trees have been left in their natural state with no turf added and the natural grade covered by a variety of native understory plants.

When checking the tree a couple weeks before doing the root crown excavation, I noticed the root flare was not evident but visibility around the base of the tree was limited by thick undergrowth of a number of plants but predominantly soapberry (Sapindus saponaria). I scheduled the return visit for two weeks later and asked that the undergrowth be cleared away from the base of the tree for a distance of about 10 feet to allow for the root crown excavation with the Air Spade. The subsoil was laden with a thick layer of oyster shells either because we were on an ancient Indian midden or dredging from the 1950’s work on the Intracoastal Waterway.

When we started clearing the soil and oyster shells from around the base of the trunk to perform the root crown excavation, we found soapberry roots had encircled the entire trunk at three levels. The tree was being strangled. Before work began the client had begun to suspect strangulation even though the

The soapberry roots completely encircled the live oak trunk at three levels.

The soapberry roots completely encircled the live oak trunk at three levels.

circling, strangling roots were not yet visible. He observed the ring of understory soapberry saplings growing near the base of the tree and actually inquired in an email to me if the soapberry could be like the Florida strangler fig (Ficus aurea) of south Florida. I suspected the soapberry tree was allelopathic, which means it exudes a chemical that inhibits the growth of nearby plants. We were both correct. We found the soapberry roots had completely grown around the trunk of the live oak. And two research studies done in Brazil had shown that soapberry was allelopathic.

 

Strangler soapberry5

Note the deep groves in the root crown where the soapberry roots had been doing some severe girdling of the live oak trunk.

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