Oaks of North Florida and the Lower Suwannee Region

In my 24 years in north Florida, I’ve become fascinated by our diversity of oaks. Close relatives usually compete, with only the most successful competitors able to persist. So, how can our 17 species of oaks all manage to live together? The August 2020 Scientific American article “How Oak Trees Evolved to Rule the Forests of the Northern Hemisphere” provides some answers based in part on recent evidence from molecular genetics. Fortuitously, one of the authors, Dr. Jeannine Cavender-Bares, based her extensive research here in north Florida.

All American oaks belong to one of two groups; red oak species have bristles on tips of immature leaves and acorns taking two years to mature; white oaks lack leaf bristles and their acorns mature in a single year. The groups split long ago in geological time. Part of the diversity we see results from red and white species adapting differently to the same conditions, so pairs can often occur together without competing directly. Think of dog-like (Canidae) and cat-like (Felidae) carnivores. Wolves and cougars use the same food resources and coexist, as do foxes and bobcats. The distantly related pairs avoid direct competition because they adapted to shared environments in different ways and use available resources differently.

The following table shows pairings of the red and white oaks that co-occur in our area.

Species of Oaks, Genus Quercus, in the Northern Florida Peninsula
Habitats	Species Commonly Occurring Together		Notes:
	Red Oaks	White Oaks
Flatwoods/Sandhill/Scrub	myrtle oak Q. myrifolia	sand live oak Q. geminata	medium-size trees
	bluejack oak Q. incana	Chapman’s oak Q. chapmanii	small trees, shrubs, “clone domes”
	running oak Q. pumila	dwarf live oak Q. minima	ground level shrubs
Dry Hammock	sand laurel oak Q. hemispherica southern red oak Q. falcata	bluff oak Q. austrina1	large trees
	turkey oak Q. laevis	post oak Q. stellata2	small to medium-size trees
Mesic Hammock	swamp laurel oak Q. laurifolia Shumard’s oak Q. shumardii1	swamp chestnut oak Q. michauxii southern live oak Q. virginiana	large trees
Pioneers3	water oak Q. nigra	sand post oak Q. margarettae	large to medium-size trees

¹prefer limestone soils.

²prefer clay or marl soils.

³invade pine flatwoods in the absence of fire, often accompanied by Q. hemisphaerica.

 

Note that oaks also avoid competition by different growth forms; large trees, small trees, and shrubs deal with fire differently, and occupy different habitats. For example, three closely related white oaks coexist by adapting to different fire and drought regimes. Southern live oak (Q. virginiana) favors moister areas where fires are rare, sand live oak (Q. geminata) favors drier areas with infrequent but devastating fires, and dwarf live oak (Q. minima) occurs in places with frequent low intensity fires.

Thus, moisture, drought, and fire, and adaptations unique to the red oak and white oak lineages help our 17 species of oaks to avoid competition and persist together. To learn more, download the Scientific American article (https://www.scientificamerican.com/article/how-oak-trees-evolved-to-rule-the-forests-of-the-northern-hemisphere/).

Photos and Captions:

Epiphytes on a Live Oak Branch. Like most live oaks this tree near the Dennis Creek trailhead on the Shell Mound unit of the Lower Suwannee National Wildlife Refuge harbors epiphytes on its branches. Seen here are resurrection fern (Pleopeltis polypodioides) and Spanish moss (Tillandsia usneoides). Other oak trees commonly bear oak mistletoe (Phoradendron leucarpum). Unlike parasites, epiphytes don’t take nutrition from trees, and don’t significantly harm their hosts.

Leaves of Southern Live Oak and Sand Live Oak. The cluster on the left is from southern live oak and that on the right from sand live oak. Leaves nearest the three o’clock position are turned over to show their undersides. Southern live oak leaves are flat and wide, unlike the narrower, recurved (curled downward on the long axis and curled under on the short axis) leaves of sand live oak. Southern live oak thrives in moist sites whereas sand live oaks occur in drier, more open places. Cup-like hollows on the undersides of sand live oak leaves create dead air spaces, trapping humidity and preventing water loss. Upper and lower surfaces of southern live oak leaves are green, but the whitish undersides of sand live oak leaves result from microscopic projections that retard evaporative moisture loss.

Dwarf Live Oak. Unlike its close relatives, this species rarely grows more than knee high, adapting to frequent fires by having most of its biomass underground and sending up sprouts after fires. This cluster is growing along the refuge’s Tram Ridge Trail.