Fire plays a key role in many ecosystems of the southeastern U.S. Longleaf pine (Pinus palustris) and Table Mountain pine-pitch pine (P. pungens–P. rigida) forests along with other ecosystems – including oak (Quercus) forests, grasslands, and spruce-fir (Picea-Abies) forests – illustrate the range of fire effects and plant persistence strategies in the American South. Fire history research reveals that fires and fire-associated vegetation were common before the fire exclusion of the past century. Both lightning and anthropogenic ignitions (caused by American Indians or European settlers) contributed to burning, but their relative importance is debated. The humid climate constrains burning, especially by lightning-ignited fires, which often occur during moist conditions. Studies of fire climatology indicate the importance of dry conditions (e.g. drought years and relatively dry areas) for widespread burning in this humid region. Landscape fragmentation also influences burning. In the past some fires also likely grew much larger than today because they were unimpeded by roads, farms, and other barriers.