How did longleaf pine forests become dependent on fire?

Figure 1. Longleaf pines, along with many other species, not only adapted to frequent fires but became dependent on them. This savanna was burned two years before the photograph was taken. (Photograph by the author. More about the photograph)

“Fire-dependent forest” seems like an oxymoron — a combination of apparently contradictory terms put together to produce what seems to be a paradox. For southeastern pine savannas, though, the term fire-dependent defines the dominant ecological characteristic of the community. It also defines the feature that was overlooked when this community declined from covering more than 93 percent of the coastal plain in the precolonial era to less than 2 percent of that area today — a decline that was called “one of the major social crimes of American history” by two ecologists who reported in 1931 on the destruction of North Carolina’s pine forests. This decline was caused by greed-driven overharvesting and a lack of knowledge of the sustaining role that fire plays in these ecosystems. The decline is slowly being reversed in places like Camp Lejeune.

To understand how pine forests became fire-dependent requires that we know how the coastal plain habitat developed, how the climate changed in southeastern North America during the last post-glacial period, and the evolutionary adaptations of longleaf pines to the resultant frequent-fire regimes. The habitat of the southeastern coastal plain developed during repeated flooding by rising and retreating sea levels. (This sequence is explained in the Coastal Plain Wetlands field trip.) When the ocean covered the coastal plain, waves and currents flattened the sea floor. When sea level retreated, this flat landscape was exposed to the air and cut into large sections by rivers running through it. As the climate warmed after the last glacial period, the coastal plain dried out and became susceptible to lightning-ignited fires. These fires swept across the coastal plain, eliminating all species that could not tolerate them.

Many species disappeared, including most of the oaks, gums and poplars that make up the southern deciduous forests that have come to dominate the coastal plain in the current era of fire suppression. Longleaf pines, though, not only became resistant to these fires but became dependent upon them. The longleaf pines developed a life cycle in which seeds were shed only after the cones had been warmed by fire. Seedlings stayed in a low-growing “grass stage” that survived as fires swept over them, then underwent a growth spurt that put their fire-sensitive needles above the flames. Mature trees developed fire-resistant bark.

Longleaf pines were not the only species that became well-adapted to the frequent fire regime of the coastal plain. The main fuel of these fires was wire grass, a species with underground roots that produce shoots and seeds immediately after losing their above ground parts to fire. Running blueberry, another characteristic plant of the pine savannas, developed a ground-hugging morphology that keeps it below the fires throughout its life cycle. Many other species also adapted to the frequent fire regimes, so many that pine savannas developed the greatest small-scale species richness of any community in temperate North America.

Unsurprisingly, the era of fire suppression that led to the decline of the pines also led to the decline of these other species. As a result, longleaf savannas have over 100 rare species of plants and animals — more than any other community in the state. Several of these rare species are listed as threatened or endangered by state and federal agencies. Almost all of these are found in Camp Lejeune. Some will be seen on this field trip.

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