FREEPORT, Texas — Before Houston and its suburbs were built, a dense forest naturally purified the coastal air along a stretch of the Texas Gulf Coast that grew thick with pecan, ash, live oak and hackberry trees.

It was the kind of pristine woodland that was mostly wiped out by settlers in their rush to clear land and build communities. Now one of the nation’s largest chemical companies and one of its oldest conservation groups have forged an unlikely partnership that seeks to recreate some of that forest to curb pollution.