Please contact your county's Extension Service office to ask how to contact the Extension Agent for your area who assists owners of small woodlands.
S/he will have specific suggestions to help you obtain the best value from your woodland, be that selling lumber and/or developing wildlife habitat.

Locate your county's Extension Service office with this interactive map:
http://www.csrees.usda.gov/Extension/

Here is a link that might be useful: Locate your county's Extension Service office

Top pruning vill not help, it will only give the trees a strange shape. The best thing you can do is exactly what you are already doing, spreading the work over several years. You can also take a walk in the area next winter after each major snowfall and shake of wet, heavy snow from the trees.

what they said ...

with the thought that no one ferts a forest ...

thats the whole point of the humas mat under the trees... self creating a good soil under the trees.. and that usually includes how a fire processes such ...

besides... you said yourself.. its a light issue... so how would one expect a fert to overcome lack of light ...

they are trees.. not children.. they do not need to be fed.. nor educated.. nor clothed.. thank god.. lol ...

a good soil test.. might help you address any deficiencies in the soil ... and that can be had thru the extension office ....

ken