Ken asks a key question. Less than two inches and most any tractor with appropriately sized rotary cutter (brush hog) will handle it. Up to three inches is possible with medium duty brush hog and upwards of 35hp tractor. Over three inches and you are talking tree removal rather than mowing. Tree removal can be done a number of ways (contract it out, back hoe, chainsaw plus stump removal or grinding or just rotting, or grapple plus strong FEL and good size tractor to name some common options. Grapple techniques won't work with trees over about five or six inches max.
So, as Ken asks, what size and how many and what type of tree and slope/access etc?
let me start with a more complete dicription of what im up against. the pasture is fairly clear except for weeds(handle those later) the forest is five years of over grown brush and trees. every size known to man. plenty of spots that will need to be filled with dirt and rocks due to erosion making hay get from field to barn. every thing that it would take to turn a small forest into a fram. im retired and plan to be here in Gods country till the end so im lookingfor a tractor that will out last me (im 63) needs to handle a tractor freak