The rise of e-books and declining store sales have led to layoffs at Powell’s Books, the landmark bookstore in Portland, Ore. The store’s management said on Tuesday it would lay off 31 employees as part of its “response to the unprecedented, rapidly changing nature of the book industry.” Ryan Takas, the union representative for the employees who were laid off, said that the 31 employees make up more than 7 percent of the store’s 400-person unionized work force. Mr. Takas said that Powell’s had reduced its work force by about 40 employees in the last year through attrition, but still felt that layoffs were necessary because of a decline in sales of new books and a rise in health care costs. It was the first round of layoffs since the store’s workers formed a union in 2000.