Weyerhaeuser Co. said it will buy Plum Creek Timber Co. for about $8.4 billion in stock, creating a $23 billion real estate investment trust focused on the timber industry.

Under the merger agreement, Plum Creek stockholders will get 1.6 shares of Weyerhaeuser for each share they own, the companies said in a statement Sunday. That’s 13.8 percent more than the 30-day volume-weighted average price ratio, according to the statement.

When the transaction is complete, Weyerhaeuser shareholders will own 65 percent of the combined company.

Seattle-based Plum Creek’s shares closed on Friday at $40.29, giving it a market value of $6.99 billion. Federal Way, Washington-based Weyerhaeuser closed at $30.40, for a market capitalization of about $15.5 billion.

Rick Holley, currently Plum Creek’s chief executive officer, will become non-executive chairman and Weyerhaeuser CEO Doyle Simons will retain that title for the combined companies, according to the statement.

Weyerhaeuser owns or controls almost 7 million acres of timberland, mostly in the U.S. , while Plum Creek has more than 6 million acres in 19 states.