Quercus chapmanii and Quercus geminata

(Dr. Alvin Chapman was an early Florida botanist. Geminata refers to twins because the acorns are often paired.)

Fagaceae

Florida sugar sand scrub is an unforgiving plant environment, with extreme exposure to sun and wind,  sterile over-drained white sand for soil,  intense drought exposure, and overall nasty conditions.  Harsh conditions call for extreme adaptations, which are especially interesting to compare when the adapting plants are closely related.  In human affairs, it is interesting to compare how different siblings adapt to the same upbringings.  In oak affairs, same thing, do two related oaks in the same environment adapt in the same ways, or different?   Chapman’s Oak and Sand Live Oak are both in the White Oak section of the broader category of Oaks (the genus Quercus).   They are fairly closely related, although each has more-closely related species.    They both are abundant in local scrub habitats.

Hot, dry, “thirst” is arguably the main “problem,” but look they handle it differently.    Chapman’s Oak avoids facing the driest times by dropping its leaves for about two months and waiting it out, just like trees in northern climates wait out the freezing winter.  It is a drought avoider.   By contrast, Sand Live Oak is a drought tolerator, keeping its leaves all year and toughing out the dry season, drawing more heavily on deep soil water during the thirsty springtime months.

That basic difference in leafiness styles ties in with additional differences.   Chapman’s Oak has to move a lot of water fast when rainy times return to make new leaves, so there’s no surprise it has the more porous stems allowing easy water passage.   Because its leaves are disposable, it can’t invest in the long-lived durable tough leaves found on Sand Live Oak.   The relatively thick heavy Sand Live Oak leaves stand up to  hot dry sun when Chapman’s flimsy foliage is absent.

Failing to make tough leaves creates a problem for Chapman’s, documented by botanists Andrew Tweel and Eric Menges in 2008.   The comparatively delicate leaves on Chapman’s Oak are vulnerable to insect or mite damage, making it necessary for that species to invest in an alternative security system… tannins to poison pests.  

Studying these two species (and the intermediate “Scrub Oak” Q. inopina) they found a trade-off where each species has one defense or the other, tough or toxic.