In the course of my work, I meet many pets. Most are friendly dogs. But last week, I had a visit involving goats.
The client was concerned about her coast live oak that was not looking so well. She expressed concern that maybe it was because her goats had chewed the bark.
I arrived with my clipboard and phone message note clipped to it, ready to investigate.
The client assured me the goats are harmless. “They won’t do anything to you” she said.
The goats eyed me close up, a new visitor.
At about 18-inch trunk diameter, the tree is still fairly young but established enough to withstand some abuse. Over all, the tree looked a little weak in terms of new shoot elongation and the amount of expanding new leaves it should have at this time of year.
Fencing around the trunk created a protection zone just large enough for me to move around and examine the trunk. The owner had put it up after the damage was done a few years ago.
Looking at all sides of the trunk, I could see what I consider to be a minor wound that had fully closed.
In comparison, I have seen numerous trees bashed by vehicles, crushing and tearing off the bark and cambial zone tissue about halfway around the trunk.
Except where the trunk or main roots are actually broken, it never seems to result in a visible loss of vigor.
A large wound can be an entry point for decay fungi, which can cause cavities in trunks over a period of years, depending on the particular decay fungus, the vigor and the species of the tree. Some are weak “compartmentalizers” and some, including live oaks, are strong. They have an inherent ability to respond to wounds and close them off — compartmentalize them.
Some of the twigs looked like they might have a disease, so I cut a small sample for a closer look.
One of the goats got interested and almost ate it right out of my hand. Another sneaked up behind me and stole my paper phone message note. After I noticed it was gone, I later found it on the ground partly eaten.
Stepping back to consider the whole situation I could see that about half of the root zone is in a county road shoulder and the other half is in the pen where the goats are constantly roaming. The bare soil surface is ground to fine dust.
Compare that to the understory in an oak woodland that is covered with a layer of fallen leaves, twigs, and naturally composted organic matter.
Those are the conditions to which oaks are adapted. And the trees help to generate the very conditions they need for survival.
This principle was documented in a study printed in California Agriculture (April 2003, Dahlgren Horwath, Tate, Camping: Blue oaks enhance soil quality in California oak woodlands).
The researchers measured various qualities of soil in blue oak root zones compared to closely adjacent soils outside of the root zones: bulk density, pH, microbial biomass carbon, cation exchange capacity, nitrogen and phosphorus.
The study abstract states: “Blue oaks create islands of enhanced soil quality and fertility beneath their canopy. The quality of soil beneath the oak canopy is considerably better than that of the grasslands adjacent to the trees. We found evidence of improved soil quality under blue oaks for physical, chemical, and biological soil properties… Removal of oak trees results in a rapid deterioration of soil quality with the majority of the loss occurring within 10 to 20 years after tree removal.”
In contrast to undisturbed oak woodlands, studies have shown that grazing animals compact soil unless they just pass through occasionally.
Compaction decreases soil pore space, slows water infiltration, reduces aeration, and reduces drainage.
Effects in compaction include reduced root growth, reduced activity of beneficial soil organisms, and increased susceptibility to root diseases.
I concluded the most likely cause of reduced vigor in the tree was simply compaction from goat traffic.
The solution: Put up a fence to keep the goats out from under the canopy, lay down a 4-inch deep layer of coarse wood chips, and allow litter fall from the tree to accumulate in a natural way.
In time, I expect the tree will heal the soil.