You say for "tax reasons". Income tax return? Property tax? Inheritance tax? Depending on what you are dealing with will matter how how to value it (or if it even matters whether or not to assign a discrete value).

For those with experience in apple orchards and their likely life/value (that's not me), some additional information would be of use. What rootstock? What varieties of apple? General condition of the trees? Do you have information on recent production?