Question
QUESTION: I have a dead oak tree in my yard.  We rented some land to a man who needed to keep two horses there.  They stayed about 3 months.  (The oaks where at least 50 years old)  The neighbors are telling me that the horses killed one of my oaks and the other one is dying.  Now with what I have read, my question is did the horses standing and walking under and on the roots kill the tree?

ANSWER: If the ground under the trees is bare and harder than the other areas I would say the horses standing and walking under the trees for shade caused compaction of the soil.

Compaction destroys soil structure, thus increasing density, carbon dioxide concentrations (plant roots need oxygen to live and grow) and heat build-up. Additionally, it creates surface runoff rather than allowing water to penetrate the roots. Compaction subsequently decreases the amount of large pore space available, as well as oxygen in the soil, water penetration, and nutrient influx.

When compaction increases soil density, root elongation is inhibited, causing poor development of root systems essential for summer survival. This damage is more severe in drier, heavier soils.

Plant roots need oxygen to survive, and as the density of a compacted soil increases, carbon dioxide and other toxic gasses do not readily move from the root system. Their concentration can build up to the point that they actually become toxic to the root.

Compaction is very much a surface phenomenon affecting mainly the top four inches of soil. Compacted soils do not allow rapid water penetration, causing increased runoff. This means that more irrigation is necessary to adequately soak compacted areas to get water to the root-feeding zone during times of drought stress.

Compacted soils are hotter in the summer and colder in the winter because of the conductivity of tight soil particles. Lower temperatures in the spring could result in less root growth, delayed green-up and even winter-kill.

Porosity of compacted soil is less. Both the numbers of pores and their size are decreased. Small pores in soil are usually filled with water, so water begins to replace air in a compacted soil. In the absence of air, plant root cannot actively absorb nutrients, causing plant decline. Pathogenic fungus organisms thrive in higher soil temperatures in the presence of a lack of oxygen. Thus, the probability of summer disease problems is increased in a compacted soil. Weeds that can persist in low oxygen soils can gain the competitive edge over desirable grasses and take over.

If the soil was wet when the horses were under the trees this adds to the compaction. And yes the roots were damaged and this shows up in the foliage dying.

---------- FOLLOW-UP ----------

QUESTION: The problem is around one tree the horses (2) have not been here since February,  They usually return in the winter and spend a large amount of their time in the barn.  Six inches Around one tree from base the ground is harder then anywhere else.  However I would that was more likely made by our goat.  
If this is still an issue can I solve it by adding soil around the tree?  Or having water run in the area for long periods of time?

Answer
Application of a mulch layer is a low cost method that can be benefit trees on compacted sites, even in the absence of other treatments. Apply a mulch layer about 2 inches thick near the trunk and out to the drip line. Never place mulch directly against the trunk. Reapply mulch each year as necessary. Over the years the mulch will protect against additional compaction and give organic matter for soil development. Do not expect rapid results. However, such treatments will greatly aid trees during periods of drought.

Drill-hole soil replacement
This method utilizes a drill bit 2 to 4 inches in diameter and 8 to 10 inches deep, to place holes at 1.5- by 1.5-foot spacings around a tree. The holes are filled with a combination high quality compost, soil inoculants and fertilizer. Roots next to the drilled holes will respond to this by producing fine roots that grow into the drill holes. This treatment has a relatively small effect on the overall soil bulk density because the number of 6 inche deep, 2-inch-diameter holes used on a 1000 square foot area would only replace about 5 cubic feet of soil. A 4-inch-diameter hole would replace about 20 cubic feet of soil. While better it only accounts for a small portion of the total area. Some hold that this technique is less effective than other techniques.

Restrict any further animals around the tree. Trees subject to compaction are susceptible to many pests that would not ordinarily affect healthy plants. Frequent monitoring and treatment of pest problems before they build up to damaging levels is important while the plants are responding to treatment. Further, soil compaction around established trees already may have damaged roots irreparably, so it is best to inspect the tree before starting treatment. There is no use investing in a tree that is already doomed.

After starting any of these treatments, it is best to keep traffic off the treated area. Don't counteract your good efforts by failing to prevent further compaction.