The image of a horse standing under the shade of a majestic old tree is a calming one, but the reality is a difficult one to maintain. Not only do you have issues with how the shade affects the nutritional content of the grass underneath it, but you also have practical issues concerning the fence around the pasture, as well as the health of the horse. When building a horse enclosure near or around trees, you have to keep the safety of the horse in mind as you plot how close the trees can be to the fence.
Falling Trees and Upended Roots
Recent videos of trees falling over during Hurricane Matthew bring to light two big considerations when plotting fences vs. trees. One is the effect if the tree falls over; even if you don't live in hurricane country, torrential rains can quickly saturate soil and let shallow-rooted trees topple over, crushing nearby fences. Tree height -- not just what it's like now but also its maximum possible height -- need to be considered before you place a fence near that tree, or vice versa.
The other problem the video shows is that the tree doesn't just break in half and fall that way. The roots actually tear out of the ground. While the tree will likely be taller than the radius of the roots, you still have to be sure that you keep fences away from the reach of the roots. If the tree then falls over and the roots are torn out of the ground, they won't take the fence with them.
Two more issues involve falling branches. One, obviously, is that an overhanging branch can fall and crush the fence, allowing horses to escape. You need to both keep the fence outside of the tree canopy radius and regularly trim nearby trees to ensure loose branches can't break off in high winds and land on the fence.
There is another issue, though, and that is impaling. Unfortunately, a fallen branch can impale a horse if the horse tries to jump over it and misses or if the horse runs into it by accident.
Dead Trees and Dead Horses
Then there is the consideration of tree type and health, and this one goes both ways. Horses can eat bark off trees, especially young trees, causing the tree to eventually die. Any tempting trees need to be outside the enclosure and far enough away that the horse can't stretch its neck and reach the bark. Trees that are inside the enclosure need to be from species that are not very tempting to horses -- and the trees need to be mature. Because trees take so long to mature (even fast-growing trees can take several years), you shouldn't plant new trees inside the enclosure. You can change the enclosure boundaries to include a nearby mature tree, though, as long as the distance issues are taken into account. However, you'll have to watch that tree carefully and clean up any fallen branches immediately to keep the horses safe.
Some tree species are toxic to horses, too, which makes it doubly important to keep outside trees far away from the fence and do your research. If you find that a stand of trees near the enclosure is of a toxic species, you have to either move the enclosure -- leaves can too easily blow into the enclosure and be eaten by the horse -- or remove the stand of toxic trees. Try to plant new, nontoxic trees in their places so that you can still have the benefit of a windbreak and noise screen.
It may be of great benefit to you to work with a fencing company and arborist to identify all the trees around the planned enclosure. These companies can help you change the perimeter if possible and make the horses safe, comfortable, and protected. Visit websites like http://www.rapsfence.com for more information.