I have some large poison sumac trees...

Like its cousins poison ivy and poison oak, poison sumac contains copious amounts of an oil called urushiol, which causes blistering, itching, and general misery in those who are exposed to it. Any attempt to cut down the tree and grub out the roots carries a mighty high risk of exposure. Old-fashioned ways of killing poison sumac include spraying brine on the leaves and shoots to kill them, or pouring kerosene or motor oil on the roots (not recommended, since the entire area would be contaminated). If you choose to cut down the trees and grub out the roots, be sure to do it in the cold season, when there are no leaves on the trees, and wear total protective gear. All saws and hoes must be washed in large amounts of water when you’re done, to remove the urushiol. Don’t burn the wood or roots, as urushiol can be carried in smoke. Maybe the best solution is to call your county extension agent and get advice on the best herbicide for poison sumac. Don’t feel too guilty about using a herbicide; the tree wages its own chemical warfare on whoever touches it!