In certain parts of the agricultural USA areas you must have severe soil erosion as the hedgerows reduces solid erosion and acts as a filtered wind break. In certain parts of France, you can see miles upon miles of sweeping agricultural lands with very few hedgerows.
As as a rule of thumb, the more species of shrubs and trees, within a given length of hedge, the older the hedge is. In theory, if you have 10 different species, then the hedge is approximately 1000 years old which is often evidenced by old maps dating back to the 15 century.
