With no public access, and no public roads running through it, the forest has become one of the biggest pristine parcels remaining in the area, according to the Kernans and the many environmental experts and consultants they have hired to make a case against the pipeline. The pipeline, the forest’s protectors say, would cause irreversible ecological damage by leaving the property vulnerable to invasive species of plants and insects, and alter the hydrology of the wetlands.

Bernd Blossey, an invasive plant species expert at Cornell University’s Department of Natural Resources, said that unfragmented forests and wetlands were rare in New York State. Pipeline construction, he said, would “compromise the immune system” of this forest’s ecology.

“We have to protect these places,” Dr. Blossey said.

Constitution Pipeline officials said they had gone to great expense to minimize any negative environmental impact the project might have.

More than a dozen routes for crossing the Kernan property were explored before the least intrusive one was chosen, said Christopher Stockton, a spokesman for Constitution Pipeline, a partnership of four companies. And instead of cutting a much wider path for construction — other stretches along the pipeline route required up to 120 feet — the passage through the Kernans’ property had been reduced to 75 feet across, he said.

Mr. Stockton said Constitution had also created “a robust management plan approved by state and federal agencies specifically designed to address concerns related to invasive species.”

Of the hundreds of landowners affected by the pipeline, about 85 percent have accepted payments in exchange for easement agreements, leaving about 100 who have not agreed to deals.