International Paper Co., the world’s largest paper company and one of the biggest private landowners in the U.S., has hired an outside firm to verify that its forest-management practices are environmentally sound, the company will announce today. Pressure is growing on the timber and paper industry to adopt sustainable logging practices, and International Paper said that more than 100 of its customers, including Home Depot and McDonald’s, have asked whether its products meet various environmental criteria. In part, the retailers are acting in response to pressure from environmental groups, which have conducted publicity campaigns and protests pushing for independent forest certification.