Strangler Fig

Ficus aurea

Moraceae

As a kid on Florida vacations I found it scary that a malevolent tree strangles innocent others like the living garrote of the Green World.  A more mature perspective is less ghastly, and of course the Ficus (Fig) does not actually strangle anything; it merely exploits its host tree for a cheap perch in the sun, eventually growing over and around the host, competing for light and eventually root space.  I’ll bet the host generally lives a long and prosperous life despite its hitchhiker.

Figs on Ficus aurea (by JB)

In a recent blog Mistletoe was cast as a smart parasite for supporting its host.  Strangler Figs, by contrast, can be indifferent to host’s fate because in time the Stranglers stand on their own two feet.  Sorta like the guy whose wife works in Wal Mart to pay for his Med School until he graduates and runs off with a nurse.

The young Strangler Fig sitting in a tree sometimes looks like a parasitic Mistletoe early on, and some observers have attributed the Ficus with a propensity for parasitism.  However, tree biologist Peter Tomlinson emphasizes the exploitation to be merely epiphytic.  But when the host dies does the strangler benefit nutritionally from the host’s decay?  (Probably not)

Forest tree babies struggle for light under taller canopy trees, so each forest tree species needs a coping adaptation to survive its shaded youth.  Many evolve large food-filled nuts to sustain the sapling until it rises high enough to compete effectively or until  a canopy gap opens.  Figs have a different plan:  they form lots of tiny bird-dispersed “seeds” (technically achenes) carried by birds to lodge in nooks and crannies high and bright on matrue trees.  Then they grow bassakwards from canopy downward to the ground.  Many additional Ficus species have similar tendencies.

Strangler Figs are Ficus aurea, one of 750 Ficus species worldwide.    The other Florida native is the Bearded Fig (Ficus citrifolia) almost restricted to the southern tip of Florida and the Caribbean.  Its whiskerish dangleroots reputedly account for the island name Barbados, translated as “bearded.”  Florida is home to numerous cultivated Figs, some of them escaped nuisances.   These garden Figs include the Banyan, Bo Tree, Counciltree, Cuban Laurel Fig,  Edible Fig, India Rubbertree, and more.

The “fruit” (the fig) is a swollen stem with a hollow cave inside. The cave is lined with tiny male and female flowers followed by seedlike fruits.  Pollination is by itsy bitsie teenie weenie wasps who enter through a portal at the end of the fig.  Stranger Fig has just one species of pollinating wasp (Pegoscapus mexicanus), which is perhaps why it does not (or not often) hybridize with Bearded Fig, which has its own wasp pollinator.

With variation among species, the general pattern is that female wasps enter the fig “fruit” and lay their eggs into the ovaries of specialized female flowers.  Male wasps hatch forth from the eggs inside those flower ovaries and proceed to fertilize the immature female wasps while the girls are still confined within their Fig flowers.  How do they do that?  The motivated guys chew their way through the flower ovary wall to the females, who later use those chew-holes to escape.

Upon exiting its flower-ovary but still inside the fig chamber, the pregnant female wasp packs pollen into a specialized pocket on her body.  Then she flies out to go a different fig to transfer pollen and lay eggs.  She must deliver pollen reliably because no pollen = no flower ovary growth, and no ovary = no nursery for her babies. Something to try: Bust open Ficus aurea “fruits” and find the little wasps inside.

One or more interloper wasp species use Stranger Figs as brood chambers without contributing to pollination.   One such sneaky pete  (Anidarnes bicolor) injects its eggs from the outside of the fig, positioning them to mature on the inside relying upon successful pollination by the proper pollinating wasp.

Have you ever seen a Strangler Fig germinated on the ground?  Yes, but not that often.  The seeds require a sustained moist substrate, most often in Cabbage Palm boots.

Starting out as epiphytes and living years on high before rooting in the ground, Strangler Figs contort around the host, which is why Bonsai enthusiasts like them.  In this connection Ficus wood structure is unusual.  Now we enter the speculation zone with a visual aid.  The wood photo is a cross section…think of viewing a stump surface through a powerful microscope. (The image comes from the University of North Carolina wood collection.)

Strangler Fig wood with alternating light (storage tissue) and dark (true wood) horizontal bands. (The vertical bands are “wood rays.” The roundish openings are large water-conducting pipes.)

Strangler fig and related species have something unusual:  bands of living storage tissue (axial wood parenchyma) layered in broad bands in the otherwise dead wood.  (Normal wood is predominantly dead water-pipe and support cells.)   The oddly abundant living storage tissue is conspicuous as those light-colored horizontal bands alternating with the darker bands of proper dead wood.  The living bands sequester water and starch, not a normal wood function in other plants.  Why do they do this?

Most epiphytes face a “dry” life trapped above the ground using their roots to cling to the host instead of accessing groundwater and storing starch like conventional roots.   Epiphytes throughout the plant world consequently develop diverse coping adaptations:  succulence, animal symbioses, suspended animation, the expandable pseudobulbs and sponge-covered roots of orchids, the tanks of epiphytes, elaborate scales and hairs, and more.   The weird and plentiful storage tissue seems to be the Ficus answer to the basic epiphyte lifestyle challenge.  There’s just a wee bit of “cactus” built in.

Open fig from Ficus aurea, showing tiny orange “seeds’ (which are technically the fruits. The entire fig is technically a hollow open stem. The hollow seed-lined space is the “cave’ mentioned in the text. You can find dead wasps in there. (Photo by JB, fingers Evan Rogers, Evan Rogers by GR)