The southern live oak (Quercus virginiana) is my favorite species of tree. I love the sprawling canopy, usually covered in silvery-gray Spanish moss, and the way they grow in shady groves that cool off their native habitat–the hot sea islands and lowlands of southeastern North America. The southern live oak is specially adapted to grow on the edge of maritime forests where their sturdy limbs can reach over adjacent fresh or saltwater marshes. Before Europeans modified the environment, most live oaks occurred on the edges of watery habitats. Unless a natural gap occurred within the forest, the limbs of most individual trees sprawled in one direction over water, enabling them to capture sunlight that other species of trees couldn’t reach. But man cleared the original maritime forests and replanted live oaks in groves where the limbs of widely spaced trees could sprawl in all directions. The limbs evolved to withstand strong sea winds, and when Europeans first colonized the region they were quick to make use of the solid wood for ship-building. Today, the tree is planted as an ornamental, and its abundant acorn production feeds wild hog, deer, squirrel, and other wildlife.
There are 7 species of live oaks including the southern, the sand (Q. geminata), the dwarf or runner (Q. minima), the Cuban (Q. sagraeana), the Texas (Q. fusiformis), the Baja (Q. bradegeei), and the encina or Central American (Q. oleoides). All of these species grow at low elevations in well drained soils. They are evergreen trees that require cross pollination. Live oaks will occasionally hybridize with other species of white oaks and with other species of live oaks in ranges where they overlap. The genetic evidence suggests white oaks first evolved 28 million years ago, and the live oak group (a subsection within the white oak family) diverged from other white oaks 11 million years ago.
The Seven Sisters live oak in Mandeville, Louisiana is the largest and oldest known live oak in the world. It is estimated to be 1500 years old.
Sand Live Oak (Quercus geminata) in a stand of open pine savannah. It is more fire tolerant than Quercus virginiana.
Dwarf live oak (Q. minima). This species is fire dependent. Its root system is much larger than the shrub itself.
The ranges of southern, sand, and dwarf live oak overlap; but they don’t hybridize often because they flower at different times and they prefer different habitats. The southern live oak is a large tree that is fire intolerant. This species can only grow successfully in environments where it is protected from fire such as islands surrounded by salt marsh and inlets or hardwood hammocks surrounded by swamp. The sand live oak is fire tolerant, and it can grow in pine savannahs subject to frequent fire. The runner live oak is a small shrub that is fire dependent. It has an extensive root system underground and will re-sprout after a ground fire, but it can’t grow in shade.
Scientists believe live oaks evolved in southern North America because most species are frost tolerant. The encina or Central American live oak lost this frost tolerance after it colonized the tropics. The genetic evidence suggests the Cuban live oak is derived from the Central American species, but scientists don’t know how the ancestor of the Cuban species made it to the island. Acorns likely rafted across the Caribbean protected from the salt water in a clump of vegetation.
The genetic evidence suggests the ancestor of the Baja live oak was formerly much more abundant, but the opening of the Sea of Cortez isolated this species. It shared a common ancestor with the closely related Texas live oak.
Cavender-Bares, J.; et. al.
“Phylogeny and Biogeography of the American Live Oaks (Quercus subsection virentes); a Genomic and Population Genetics Approach”
Molecular Ecology 24 2015