Shaded by hemlocks 80 feet tall, East Rock Park is a cool summer refuge for New Have, Connecticut residents who prefer quiet woods to mobbed beaches. In winter, the trees lend the city a touch of green, a reminder that spring will eventually come. But like Eastern hemlocks throughout the region, the ones here are dying, turning from green to yellow to brown. In the summer, East Rock could eventually become a sweltering, impenetrable thicket if the cool shade provided by the hemlocks disappears. In winter it could take on the same dull gray as the rest of New Haven.