horses eating plants affects development of forest garden?

The land im using for creating a forest garden was abandoned for a lot of years .
My neighbour used to bring his horses to my land so they can eat the plants in my land. The thing is that he was doing this for years and the horses always destroyed everything in my land and didnt let the plants to grow.
When i started my forest i told him to not to bring his horses to my land anymore.
For first time in years i let the plants from my land to develop without disturbance, but i noted that most of these plants (disturbed before by horses) was growing with illness and not fine .

For example i planted a new pacae in a new place of the forest ,a new place (new plants) that has a lot of new vegetation and pacae is growing very fast and it looks very vigorous.
But the pacae growing that was disturbed permanently by the horses is growing very slowly and with illness and i can see the same for other plants disturbed by horses.

Horses affects plants? In which way does horses affects plants?
do you think i should kill all the disturbed plants by horses and replant from new? or is it possible to heal theses plants? what do you think?

Horses will definitely affect the forest garden. They browse on tender and small branches and leaves, this will stunt the early growth of trees and plants and cause them to grow in ways they would not have if they were unmolested. Personally, I would not destroy the damaged plants, rather I would use them as an experiment. With those damaged plants I would try some pruning methods to see if I could stimulate growth that would help the plants to regenerate good health. There's a learning experience here, in my opinion, and there's no reason that things will not grow around the damaged plants.