In an effort to prevent the spread of invasive species such as emerald ash borer, the Minnesota Department of Natural Resources has rules and suggestions around the use of firewood at state parks and forests. Presence of the ash borer was first confirmed in Minnesota in 2009. Minnesota has the largest concentration of ash trees in the country. The No. 1 rule is only DNR-approved firewood may be used or brought onto parks, said Jennifer Teegarden, DNR Forestry outreach specialist. Kiln-dried, unpainted and unstained lumber is allowed. The best bet, Teegarden said, is to buy bundles of firewood upon arrival at the campground. Also, wood-sellers approved by the state are listed online on the DNR website. The state recommends keeping a receipt handy for proof of purchase. Federal lands and private campgrounds also have rules about firewood — campers should be aware of them before embarking on their trip.