Hi
Could you please let us know what town you are in, I've tried to do some rainfall stats research and working on Lansing it appears to be a rather dry year.
After the wetter and cooler winter period trees put on a lot of foliage for spring/summer season. If there's a hot dry spell trees can shed some of the foliage ... but close attention has to be paid to the leaves being shed, if there's no sign of disease such as spots,chewing, fungus/webs etc it could be a natural response for the tree. The shedding you have described is at the natural abscission zone of the petiole, the tree's preferred place, so if the leaf appears healthy it may be that there isn't enough stored energy, water and nutrients to support an overgrown lush canopy ... natural thinning.
Good health and vigor is the answer, trees have a good chance of fighting off problems if they're healthy. I don't know if it's practical to water, organic fertilize and mulch if you are talking about a forest or large area.
It sounds like the condition is common to most of the trees so it's more likely to be a climatic response.
I found some links that may help you out, check these.
http://www.mortonarboretumphc.org/f... or Develop Early Fall Color August 2002.pdf
http://web1.msue.msu.edu/msue/imp/modzz/00001706.html
http://cahe.nmsu.edu/ces/yard/1997/030397.html
And yeah, a picture is worth a 1000 words. Pics of the area, soil, etc help to.
Keep us posted.