In large pot, bring potatoes, rutabagas and enough salted water to cover to a boil. Reduce heat and simmer 20 minutes, or until potatoes and rutabagas are fork-tender; drain.
Transfer to large bowl. Using hand masher or potato ricer, mash potato mixture until smooth. In pot over medium-high heat, warm buttermilk and butter for 1 minute, or until hot.
Stir hot buttermilk mixture into potatoes. Fold in chives; season with salt and pepper, if desired.