Which of these is a distinguishing feature of the longleaf pine?

The longleaf pine (Pinus palustris) is a pine native to the Southeastern United States, found along the coastal plain from East Texas to southern Maryland, extending into northern and central Florida. It reaches a height of 30–35 m (98–115 ft) and a diameter of 0.7 m (28 in). In the past, before extensive logging, they reportedly grew to 47 m (154 ft) with a diameter of 1.2 m (47 in). The tree is a cultural symbol of the Southern United States, being the official state tree of Alabama and the unofficial state tree of North Carolina.

Longleaf pine is highly pyrophytic (resistant to wildfire). Periodic natural wildfire selects for this species by killing other trees, leading to open longleaf pine forests or savannas. New seedlings do not appear at all tree-like and resemble a dark-green fountain of needles. This form is called the grass stage. During this stage, which lasts for 5–12 years, vertical growth is very slow, and the tree may take a number of years simply to grow ankle high. After that, it makes a growth spurt, especially if no tree canopy is above it. In the grass stage, it is very resistant to low intensity fires because the terminal bud is protected from lethal heating by the tightly packed needles. While relatively immune to fire at this stage, the plant is quite appealing to feral pigs; the early settlers' habit of releasing swine into the woodlands to feed may have been partly responsible for the decline of the species.