Depending on the type of trees and the width there is a possibility to split these areas into sections and use like a shelter belt where you plant out extra shrub growth between trees for a thick coverage which acts as a protection for stock to adhere from cold winds in winter. The length and width will vary. This then enables you to clear when you don't need which in turn allows new gras growth to return. We have used this method with a small section of our farm - cattle love it now and the sheep head there when it gets cold. The other suggestion is carbon farming - forestry management. If you hop onto Carbon Farmers of Australia website you will find x 5 carbon management/ carbon netural management practices currently in place. One is specific for forestry. With a managment practice - (I can help as I've written these for clients - or you can get someone else) you can manipulate your area to plant more trees which are then harvested at a set time - usually 15-20yrs out from establishement for forestry needs I.e.: paper harvest. There are some trees that do grow quicker that are planted for a specific market. Hope this helps.