Shortleaf and loblolly pine are native to the southeastern U.S., where they have coexisted and occasionally hybridized for millennia. Historically, hybrids were rare. In the 1950s, hybrids made up just 3 percent of the pines in shortleaf stands, but since then their numbers have skyrocketed, and in some stands, half of the shortleaf pines are hybrids. Forest Service scientists discovered that fire strongly selects against hybrids. They studied mixed-species stands at the Tall Timbers Research Station in northwest Florida, where some stands had not been burned for decades. Most of the young trees in these stands were hybrids or loblolly pine. However, in stands that had been burned every two years, young trees were almost exclusively pure shortleaf pine. It remains to be seen how frequent the fires must be to control hybrids, but the results of most shortleaf pine restoration efforts recommend burning on a three-year cycle to achieve optimal under- and mid-story conditions. This also may be near-optimal for maintaining the integrity of the genetics of the dominant over-story species.