several possibilities..check drainage..white pine does not do great in very wet areas..check trunks for signs of Blister-rust, if cankers are present excise them and treat with tree paint...if needles are browning at the tips of branchs and moving inward it may be affected by air pollution...if tips of branches are browning from top of trees downward it may be nematodes. Other problems are more readily observed i.e. white pine aphid, scale. webworm, etc.