The USNA's site has similar information, although they don't mention the other two species by name.
Quoting:Pine tree (genus Pinus) state tree of ARKANSAS
Arkansas has designated the “pine tree” as its official state tree. The 1939 resolution adopting pine cites the utility of pine timber resources as a great source of wealth for the state, and that pine is a renewable resource that will continue to be important to Arkansas in the future. There are four species of pine native to Arkansas. Among them, the loblolly pine, also known as the Arkansas pine, often is cited as the state tree of Arkansas. This would be a good choice, given its significance as a timber tree, but it is entirely unofficial. In the National Grove of State Trees another Arkansas native, the shortleaf pine (Pinus echinata) has been planted to represent Arkansas. This too is a good choice, being an important timber tree, widely distributed across most of Arkansas (more common in Arkansas than loblolly pine). The other two pines native to Arkansas are much rarer, and less suitable candidates.
Shortleaf pine (Pinus echinata) is grown in the National Grove of State Trees to represent Arkansas.
Look for: medium to large pine tree; needles 3-5 inches long, in groups of 2-3, flexible and with a persistent sheath at the base; egg-shaped cones two inches long with scales that remain flexible, each with a small sharp central prickle (called an umbo).